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Back | Show only these items: | Threshold photoelectron photoion coincidence study of the fragmentation of valence states of CF3–CH3+ and CHF2–CH2F+ in the range 12–24 eV

1
Threshold photoelectron photoion coincidence study of the fragmentation of valence states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺ in the range 12–24 eV

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Using tunable vacuum-ultraviolet radiation from a synchrotron source, threshold photoelectron photoion coincidence spectroscopy has studied the unimolecular decay dynamics of the valence electronic states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Threshold photoelectron spectra and fragment ion yield curves of CF₃–CH₃ and CHF₂–CH₂F have been recorded in the range 12–24 eV, electrons and ions being detected by a threshold electron analyser and a linear time-of-flight mass spectrometer, respectively.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

For the dissociation products of (CF₃–CH₃⁺)* and (CHF₂–CH₂F⁺)* formed via cleavage of a single covalent bond, the mean translation kinetic energy releases have been measured and compared with the predictions of statistical and impulsive mechanisms.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Ab initio G2 calculations have determined the minimum energies of CF₃–CH₃ and CHF₂–CH₂F and their cations, with their geometries optimised at the MP2(full)/6-31G(d) level of theory.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

The nature of the valence orbitals of both neutral molecules has also been deduced.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Enthalpies of formation of both titled molecules and all fragment ions and neutrals observed by dissociative photoionisation have also been calculated.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Combining all experimental and theoretical data, the fragmentation mechanisms of the ground and excited states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺ are discussed.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

The ground state of both ions, formed by electron removal from the C–C σ-bonding highest occupied molecular orbital, is stable only over a narrow range of energies in the Franck–Condon region; it dissociates by C–C bond cleavage with a small fractional translational energy release.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Low-lying excited states of both ions, produced by electron removal from F 2pπ nonbonding orbitals, show some evidence for isolated-state behaviour, with impulsive dissociation by cleavage of a C–F bond and a larger fractional translational energy release into the two fragments.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

For energies above ca.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

16 eV smaller fragment ions, often resulting from cleavage of multiple bonds and HF elimination, are observed; for both molecules with hν > 18 eV, CF–CH₂⁺ is the dominant fragment ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

New experimental values are determined for the enthalpy of formation at 298 K of CF₃–CH₃ (−751 ± 10 kJ mol⁻¹) and CHF₂–CH₂F (−671 ± 12 kJ mol⁻¹), with upper limits being determined for CF₂–CH₃⁺ (≤546 ± 11 kJ mol⁻¹) and CHF–CH₂F⁺ (≤663 ± 13 kJ mol⁻¹).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Introduction

An international effort is underway to replace chlorofluorocarbons (CFCs) with environmentally-acceptable alternatives,^1–3 and hydrofluorocarbons (HFCs) are likely to become the accepted CFC replacement in many industrial applications.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Specifically, several fluorinated ethanes are already being used, including 1,1 difluoroethane (R152a), 1,1,1,2 tetrafluoroethane (R134a) and pentafluoroethane (R125).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

1,1,1,2 tetrafluoroethane, for example, has been used for many years as a replacement for CF₂Cl₂ in air-conditioning systems of cars.⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

HFCs are being used as CFC alternatives because they lack the chlorine atoms which catalyse the depletion of ozone from the stratosphere.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

HFCs still pose an environmental threat as they contribute to global warming,⁵ but the presence of C–H bonds cause them to react faster than CFCs with OH radicals, thereby reducing their lifetime in the earth’s atmosphere.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

The rate constant at room temperature for the reactions of the two titled HFCs with OH is ca.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

1.2 × 10⁻¹⁵ (for CF₃–CH₃) and 1.6 × 10⁻¹⁴ (for CHF₂–CH₂F) cm³ molecule⁻¹ s⁻¹, respectively.^6,7

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

The atmospheric lifetime of these two trifluoroethanes will then be as low as ca.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

25 or 2 years, assuming an average OH concentration of 10⁶ molecule cm⁻³.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Removal by photoionisation and photodissociation processes in the mesosphere, therefore, is only likely to play a small role in the overall loss of these molecules from the atmosphere, and is unlikely to be dominant.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

However, a knowledge of the vacuum-UV (VUV) photochemistry of HFCs that might take place in this region of the atmosphere is needed, and might be important in the determination of the atmospheric lifetime.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Our group is investigating the decay dynamics of halocarbon and HFC cations containing at least two carbon atoms, using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy and synchrotron radiation as a tunable VUV photoionisation source.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj1

To date, we have studied saturated and unsaturated perfluorocarbons, C_xF_y⁺,^8,9 and three HFCs; pentafluoroethane,¹⁰ and the two isomers of tetrafluoroethane.¹¹

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

In this paper we report data for the two isomers of the trifluoroethane cation; CF₃–CH₃⁺ (R143a, labelled 1,1,1 in this paper) and CHF₂–CH₂F⁺ (R143 and labelled 1,1,2 here).

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Preliminary results for the 1,1,1 isomer have been reported elsewhere.¹²

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

Research carried out on these two isomers of trifluoroethane to date has focussed on the structure, conformational stability, and spectroscopy of the neutral molecule.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

These investigations include infrared and Raman studies,¹³ electron diffraction,¹⁴ and ab initio calculations.^15–19

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

A microwave spectrum has been reported only for the 1,1,1 isomer,²⁰ and recommended values for the structure, vibrational frequencies and standard enthalpy of formation for this isomer have been published.²¹

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

The only papers describing properties of the ionised trifluoroethanes report He I photoelectron and electron-impact mass spectrometric studies of the 1,1,1 isomer.^22,23

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

In the latter paper, appearance energies of the fragment ions were measured.²³

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

To our knowledge, there are no equivalent data for the 1,1,2 isomer.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

In this paper we describe the results of a TPEPICO study of CF₃–CH₃ and CHF₂–CH₂F from the onset of ionisation (ca.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

12 eV) up to 24 eV.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The threshold photoelectron spectrum (TPES) and state-selected fragmentation studies of the parent ions are presented.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Breakdown diagrams, yielding the formation probability of fragment ions as a function of photon energy, are obtained.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The mean translational kinetic energy releases for unimolecular fragmentation proceeding via a single-bond cleavage are determined, and compared with the predictions of statistical and dynamical impulsive models.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Enthalpies of formation at 298 K for the two neutral isomers and some fragment ions are also determined.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

These experimental results are complemented and compared with ab initio calculations of the structure of the two isomers of trifluoroethane, their ionisation energies, and the enthalpy of formation of several fragment ions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Theoretical and experimental methods

Computational methods

Using Gaussian 98, ab initio molecular orbital calculations have been performed for CF₃–CH₃ and CHF₂–CH₂F, both in their neutral ground states and in the ground states of the parent cations.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

Calculations have also been performed for fragments produced by VUV dissociative photoionisation (e.g.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

CF₂–CH₃⁺).

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

Structures for all species were optimised using the second-order Møller–Plesset theory (MP2) with the 6-31G(d) basis set, and all electrons were included at the MP2(full)/6-31G(d) level.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The MP2(full)/6-31G(d) structures were then employed for energy calculations according to the Gaussian-2 (G2) procedure.²⁴

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

This procedure involves single-point total energy calculations at the MP4/6-311G(d,p), QCISD(T)/6-311g(d,p), MP4/6-311G(d,p), MP4/6-311G(2df,p), and MP2/6-311G(3df,2p) levels.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

A small empirical correction is employed to include the high-level correlation effects in the calculations of the total electronic energies (EE).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The HF/6-31G(d) harmonic vibrational frequencies, scaled by 0.8929, are applied for zero-point vibrational energy (ZPVE) corrections to obtain the total energies at 0 K (E₀ = EE + ZPVE).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The enthalpies of formation at 298 K (Δ_fH°₂₉₈) for molecular species are calculated using total energies and the scaled HF/6-31G(d) harmonic frequencies, leading to predicted enthalpies of unimolecular reactions (e.g.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

CHF₂–CH₂F → CHF–CH₂F⁺ + F + e⁻).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

The agreement between G2 and experimental results is usually well within ±0.2 eV (or ±20 kJ mol⁻¹)²⁴.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

Experimental methods

The TPEPICO apparatus has been described in detail elsewhere.^11,25

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

Synchrotron radiation from the 2 GeV electron storage ring at the Daresbury Laboratory is energy-selected using a 1 m Seya monochromator equipped with two gratings, covering the energy range ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

8–40 eV.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The majority of the experiments for CF₃–CH₃ were performed using the higher-energy grating (range 105–30 nm (12–40 eV), blazed at ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

55 nm) with an optical resolution of 0.3 nm; this corresponds to an energy resolution of 0.035 and 0.140 eV at 12 and 24 eV, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

For CHF₂–CH₂F, most experiments used the lower-energy grating (range 150–60 nm (8–21 eV), blazed at ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

90 nm) with the same resolution.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

With the higher-energy grating, the effects of second-order radiation are insignificant for λ < 95 nm, and for the lower-energy grating insignificant for λ < 120 nm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The VUV radiation is admitted into the interaction region through a glass capillary, and the photon flux is monitored using a photomultiplier tube via the visible fluorescence from a sodium salicylate-coated window.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Threshold photoelectrons and fragment cations produced by photoionisation are extracted in opposite directions by a 20 V cm⁻¹ electric field applied across the interaction region, and detected by a single channel electron multiplier and microchannel plates, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The design of threshold electron analyser and time-of-flight mass spectrometer are described elsewhere.^11,25

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

Following discrimination and pulse shaping, signals from the electron and ion detectors pass to a time-to-digital converter (TDC) configured in the multi-hit mode and mounted in a PC.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The electrons provide the ‘start’, the ions the ‘stop’ pulses, allowing signals from the same ionisation process to be detected in delayed coincidence.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

TPEPICO spectra are recorded either continuously as a function of photon energy or at a fixed energy.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

In the scanning-energy mode, flux-normalized TPEPICO spectra are recorded as three-dimensional, false-colour maps of coincidence count vs. ion flight time vs. photon energy.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

A cut through the map at a fixed photon energy yields the time of flight mass spectrum (TOF-MS), which identifies the fragment ions formed in the dissociative photoionisation at that energy.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

Alternatively, a background-subtracted cut taken through the histogram at a fixed flight time, corresponding to a mass peak in the TOF-MS, gives an ion yield curve.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

In this mode of operation, the TOF resolution is degraded to 64 ns so that all the fragment ions are observed simultaneously.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

The threshold electron and total ion counts are also recorded, yielding the TPES and total ion yield curve, respectively.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

In the fixed-energy mode, time-of-flight spectra (later referred to as TPEPICO-TOF spectra) are measured at single energies corresponding to peaks in the TPES.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

Now a TOF resolution as high as the signal level permits is employed, typically 8 ns, and usually only one fragment ion is observed per spectrum.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

Fragment ions often have enough translational energy for the peaks comprising the TPEPICO-TOF spectra to be substantially broadened.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met10

From an analysis of the peak shape, it is then possible to obtain kinetic energy release distributions (KERDs) and hence mean kinetic energy releases, 〈KE〉_T.^26,27

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met10

The sample gases, CF₃–CH₃ and CHF₂–CH₂F, were obtained commercially (Fluorochem Ltd., UK), with a stated purity of >99% and used without further purification.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The operating pressure was ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

5 × 10⁻⁵ mbar, with a chamber base pressure of ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

5 × 10⁻⁸ mbar.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Energetics of the dissociation channels

The energetics of the dissociation channels of CF₃–CH₃⁺ and CHF₂–CH₂F⁺ into fragment ions are given in Table 1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The enthalpies of reaction at temperature T, Δ_rH°_T, associated with the unimolecular reaction AB → A⁺ + B + e⁻, where AB refers to the parent molecule, are determined by calculating the difference between the enthalpies of formation of products and reactants.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

We have used Δ_fH° data at 298 K for neutrals taken from the Janaf tables,²⁸ for ions from Lias et al.,²⁹ and these values, in units of kJ mol⁻¹, are shown in brackets in column 1 of the table.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

Where values different from these compilations are used, they are referenced later.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

The values used for the two parent molecules, −751 ± 10 kJ mol⁻¹ for CF₃–CH₃ and −671 ± 12 kJ mol⁻¹ for CHF₂–CH₂F, are discussed in Section 5.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

Our experiment measures appearance energies at 298 K (AE₂₉₈) of fragment ions, and some discussion is pertinent on how these data are measured and how they relate to Δ_rH°₂₉₈.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

Columns 2 and 3 of Table 1 give AE₂₉₈ and Δ_rH°₂₉₈ values for the major fragment ions; a major ion is defined as one produced by a single bond fission.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

The AE₂₉₈ of each fragment ion has been determined from the extrapolation of the linear portion of the ion yield to zero signal.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

At the optical resolution of our experiment, this is equivalent to the first onset of signal.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

No corrections have been made for exit channel barriers or kinetic shifts, and AEs determined in this way can only be regarded as upper limits.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

The procedure of Traeger and McLoughlin³⁰ has been used to convert the AE₂₉₈ into Δ_rH°₂₉₈.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

For the reaction AB → A⁺ + B + e⁻, Traeger and McLoughlin have shown that:As above, the upper limit for Δ_rH°₂₉₈ arises due to the fact that there may be an exit channel barrier and/or a kinetic shift; if both are zero, then the equality sign in eqn. (1) applies.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

This equation assumes the validity of the stationary electron convention that, at threshold, the electron has zero translational energy.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

If the last three terms in eqn. (1) are ignored, a significant error may be introduced in equating the measured AE₂₉₈ into an upper limit for Δ_rH°₂₉₈.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met14

The second and third terms on the right-hand side of eqn. (1), equivalent to H°₂₉₈ − H°₀ for A⁺ or B, contain contributions from translational (2.5RT), rotational (1.5RT) and vibrational (N_Ahν/[exp(hν/k_BT) − 1] per vibrational mode) motion, evaluated at T = 298 K. The error is greater the larger the number of vibrational modes, and hence the number of atoms in A⁺ and B. Vibrational frequencies of A⁺ and B are taken from standard sources.^28,31

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

If they are not available, they are estimated by comparison with isoelectronic molecules.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

For the minor fragment ions, defined as ions caused by a fission of multiple bonds, column 3 of Table 1 gives the values of Δ_rH°₂₉₈ calculated from the enthalpy of formation of products minus that of reactants, using the values in column 1.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met14

Column 2 shows the AE₂₉₈ of the minor ion, and we have not converted this value into an upper limit for Δ_rH°₂₉₈via the procedure of Traeger and McLoughlin.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

Comparison of the values in Columns 2 and 3 can suggest what neutral partner(s) form with the minor fragment ion.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

Theoretical results

Structure of CF₃–CH₃ and CF₃–CH₃⁺, and orbital character

The optimised geometries of CF₃–CH₃ and CF₃–CH₃⁺ (Table 2) have been obtained at the MP2(full)/6-31G(d) level.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

100

Both have a staggered C_3v symmetry, and the geometry for the neutral is very close to that from electron diffraction¹⁴ and microwave²⁰ studies.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

101

The highest occupied molecular orbital (HOMO) has mainly C–C σ character (Fig. 1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

102

Loss of an electron from this orbital yields the ground state of CF₃–CH₃⁺ which is predicted to have a lengthened C–C bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

103

The orbitals of next lowest energy (labelled HOMO − 1(a) and HOMO − 1(b) in Fig. 1) are degenerate π orbitals with a node on different C–C–H planes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

104

They are largely localised on the CH₃ group with some C–H bonding character, and give rise to two bands due to Jahn–Teller splitting following ionisation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

105

The next two orbitals, labelled HOMO − 2 and HOMO − 3, are mainly F 2pπ nonbonding in character.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

106

We note that ionisation from these orbitals will give rise to excited states of CF₃–CH₃⁺ which are expected to dissociate via F-loss to CF₂–CH₃⁺ + F, provided dissociation follows a rapid impulsive mechanism.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

107

At this level of theory, the main geometry changes upon ionisation from the HOMO of CF₃–CH₃ are a C–C bond length increase of 0.42 Å, a C–F bond length decrease of 0.06 Å, and a transition from non-planar to planar geometry for the CF₃ and CH₃ groups where the positive charge is localised.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

108

G2 energies are computed for the ground states of CF₃–CH₃ and CF₃–CH₃⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

109

From the difference, an adiabatic ionisation energy (AIE) of 12.51 eV at 0 K, 12.54 eV at 298 K, is obtained.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

110

The unfavourable Franck–Condon factors at the onset of the first photoelectron band will almost certainly lead to an experimental onset of signal which is significantly greater than this ab initio value.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

111

A vertical ionisation energy (VIE) of 13.92 eV was deduced from the ground state of CF₃–CH₃, using the G2 energy for CF₃–CH₃⁺ calculated with its geometry constrained to that of CF₃–CH₃.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Structure of CHF₂–CH₂F and CHF₂–CH₂F⁺, and orbital character

112

The minimum energy geometries of CHF₂–CH₂F and CHF₂–CH₂F⁺ have also been determined at the MP2(full)/6-31G(d) level (Table 3).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

113

For the neutral, the trans structure with a symmetry of C₁ is more stable, in agreement with the conclusions reported through analysis of infrared/Raman spectra and electron diffraction.^13,14

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

114

For CHF₂–CH₂F⁺, the main structural change after ionisation is an increase of 0.43 Å in the C–C bond length, a decrease of 0.07 Å in the C–F bond length, and an increase in the ∠FCF, ∠FCH and ∠HCH bond angles in both the CHF₂ and CH₂F groups.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

115

Thus, both the CHF₂ and CH₂F groups adopt a more planar structure upon ionisation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

116

There is also a small rotation about the C–C bond upon ionisation that leads to four atoms FCCF being approximately located in a plane.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

117

The AIE of CHF₂–CH₂F was calculated through the G2 energy difference between the ground state of the neutral molecule and its cation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

118

A value of 11.68 eV is then deduced at 298 K. As with CF₃–CH₃, the poor Franck–Condon factor in the threshold region will almost certainly lead to an overestimation of the adiabatic ionisation energy from the onset of signal in the threshold photoelectron spectrum compared to this ab initio value.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

119

The VIE of CHF₂–CH₂F was not determined.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

120

The structure of the neutral molecule and the five highest valence molecular orbitals (MOs) are shown in Fig 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

121

At this level of theory, the HOMO consists of mainly C–C σ bonding and C–H σ* antibonding character, consistent with the changes in geometry after ionisation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

122

The orbital of next highest energy (i.e. HOMO − 1) is a π* orbital localized on the CH₂F group, with C–H σ bonding character.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

123

The (HOMO − 2) orbital is a hybrid orbital largely localized on the CHF₂ group, consisting of C–H σ bonding and F 2pπ lone-pair character.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

124

Both of the next two higher excited valence orbitals (HOMO − 3 and HOMO − 4) are mainly composed of F 2pπ lone-pair nonbonding orbitals.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

125

The removal of an electron from this type of orbital is expected to result in C–F bond fission, i.e. fragmention to C₂H₃F₂⁺ + F, provided the dissociation follows an impulsive mechanism.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

126

Note that if this mechanism is operative, the electron density maps of the orbitals (Fig. 2) suggest that F loss from the (HOMO − 3) orbital should produce predominantly the isomer CHF–CH₂F⁺, whereas F loss from (HOMO − 4) will yield both CHF–CH₂F⁺ and CHF₂–CH₂⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Calculation of Δ_rH°₂₉₈ for dissociative photoionisation reactions

127

As described earlier, the enthalpies of formation at 298 K of both isomers of C₂F₃H₃ and all the neutral and fragment ions observed by dissociative photoionisation have been calculated.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met16

128

It is therefore possible to calculate the enthalpy of reaction at this temperature for all the observed reactions.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met16

129

For reactions involving production of a major fragment ion, these values are shown in column 4 of Table 1, and we should note that these G2 calculations refer specifically to reactants and products whose energies have been determined with optimised geometries.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Experimental results and discussion

Threshold photoelectron spectra of CF₃–CH₃ and CHF₂–CH₂F

130

The threshold photoelectron spectrum (TPES) of CF₃–CH₃ was measured in the range 13–22 eV at an optical resolution of 0.3 nm (Fig 3).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met17

131

The vertical ionisation energies of the six peaks, labelled as the X̃, Ã, B̃, C̃, D̃ and Ẽ states of the parent ion, are 14.56, 15.19, 16.03, 16.91, 19.00 and 20.23 eV, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

132

The onset of ionisation is 12.98 ± 0.04 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

133

A notable feature of this spectrum is that the ground electronic state of CF₃–CH₃⁺ partially overlaps that of the first excited state.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

134

This observation is different from CHF₂-CF₃, CF₃–CH₂F and CHF₂–CHF₂, and CHF₂–CH₃,^10–12 where these molecules all have broad but well-separated first photoelectron bands following electron removal from the HOMO.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

135

These four molecules also show an increase in the energy of the onset of ionisation as the number of fluorine atoms increases, an effect already noted by Sauvageau et al²². from He I photoelectron spectra.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

136

Furthermore, corresponding bands of higher energy in their TPES shift to higher energy with an increase in the number of fluorine atoms.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

137

These observations are consistent with the perfluoro effect, arising from the higher effective nuclear charge of a fluorine compared to a hydrogen atom and the corresponding stabilisation of the σ orbitals in fluorinated ethanes.³²

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

138

The anomalous behaviour of CF₃–CH₃, with its unusually high onset of ionisation giving rise to overlap of the first and second photoelectron bands, may arise due to the higher symmetry of this molecule, although the same behaviour is observed for CHF₂–CH₂F which has lower symmetry (see later).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

139

Note that the ab initio calculations described in Section 4 show that the energy difference between the HOMO of CF₃–CH₃ and the degenerate π orbitals is small, only 0.6 eV at the MP2/6-31G(d) level.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

140

As with all the other hydrofluorocarbons, electron removal from the strong C–C σ-bonding HOMO will yield a broad photoelectron band.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

141

Thus, the overlap of the first and second photoelectron bands in the (T)PES of CF₃–CH₃ is not surprising.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

142

The two bands centred around 16 and 17 eV in the TPES of CF₃–CH₃ are likely to be due to removal of an electron from the (HOMO − 2) and (HOMO − 3) molecular orbitals of F 2pπ lone pair character.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

143

The band positions reported by Sauvageau et al²². from the He I photoelectron spectrum of CF₃–CH₃ are similar to those of the TPES reported here, but the relative intensities of the bands are different.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

144

This difference could be due either to a change in the relative ionisation cross section between excitation at threshold and excitation above threshold with non-resonant radiation, or to autoionisation effects.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

145

As noted in our previous papers on small perfluorocarbons,^8,9 a comparison of the total ion yield and the integrated TPES can reveal the peaks in the TPES that arise via autoionisation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

146

There is excellent agreement between the integrated TPES and the total ion yield of CF₃–CH₃,³³ indicating that no autoionisation processes occur in this energy area.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

147

It is likely, therefore, that for CF₃–CH₃ the former explanation is correct.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

148

The onset of ionisation, 12.98 ± 0.04 eV, is 0.28 eV lower than that reported by electron impact ionisation,²³ but 0.44 eV higher than the ab initio calculation at 298 K (Section 4.1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

149

This latter difference must be due to the near-zero Franck–Condon factor at the onset of the first photoelectron band due to the large geometry change upon ionisation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

150

Combining the experimental onset with the value for Δ_fH°₂₉₈(CF₃–CH₃) of −751 ± 10 kJ mol⁻¹ (see later), we determine Δ_fH°₂₉₈(CF₃–CH₃⁺) < 501 ± 11 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

151

The threshold photoelectron spectrum of CHF₂–CH₂F was also measured with an optical resolution of 0.3 nm (Fig 4).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

152

The vertical ionisation energies of the eight observed peaks (or shoulders) in the range 12–25 eV are determined to be 13.03, 13.75, 14.96, 15.97, 17.51, 18.62, 19.17 and 22.26 eV, and these are labelled as ionisation to the X̃, Ã, B̃, C̃, D̃, Ẽ, F̃ and G̃ states of the parent ion.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

153

The band corresponding to the ground ionic state is relatively weak and broad, consistent with the large change in geometry following electron removal from the HOMO of C–C σ-bonding character.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

154

The next two bands, labelled Ã and B̃ at 13.75 and 14.96 eV respectively, are assigned to mainly C–H σ-bonding orbitals localised on the CH₂F and CHF₂ groups, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

155

Note that, as with CF₃–CH₃, the X̃ and Ã bands are not well separated.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

156

At higher energy the TPES of CHF₂–CH₂F shows two relatively narrow peaks centred at 15.97 and 17.51 eV, labelled C̃ and D̃.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

157

Narrow peaks in photoelectron spectra with unresolved vibrational structure often relate to the removal of a non-bonding electron, confirming that the (HOMO − 3) and (HOMO − 4) molecular orbitals of this molecule are essentially F 2pπ nonbonding in character.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

158

It is shown later that 15.97 eV corresponds to the photon energy leading to the maximum intensity of the fragment ion CHF–CH₂F⁺ produced by breaking a C–F bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

159

This phenomenon of isolated state-selected behaviour in the hydrofluorocarbon cations is relatively common, and has been observed by us and others in C₂F₄H₂⁺, C₂F₅H⁺ and C₂F₆⁺.^10,11,34

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

160

The observed onset of signal in the TPES occurs at 11.88 ± 0.04 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

161

This experimental ionisation threshold is 0.20 eV higher than the calculated AIE of CHF₂–CH₂F from the G2 calculation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

162

This difference is due to the very low Franck–Condon factor in the threshold region, and the observed ionisation threshold should only be regarded as an upper limit.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

163

Combining the observed IE with a value for Δ_fH°₂₉₈(CHF₂–CH₂F) of –671 ± 12 kJ mol⁻¹ (see later), we determine Δ_fH°₂₉₈(CHF₂–CH₂F⁺) < 475 ± 13 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

164

As with CF₃–CH₃, the excellent agreement between the integrated TPES and the total ion yield suggests that autoionisation is not an important process in this energy range.³³

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

165

We note no published HeI photoelectron spectrum exists to compare the relative peak intensities to the spectrum recorded under threshold conditions (Fig. 4).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

Scanning-energy TPEPICO spectra

Coincidence ion yields of CF₃–CH₃⁺

166

A TPEPICO spectrum in the scanning-energy mode was recorded for CF₃–CH₃ from 12–22 eV at a wavelength resolution of 0.3 nm and an ion TOF resolution of 64 ns.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

167

The parent ion and the fragments CF₃⁺, CH₃⁺, CF₂–CH₃⁺ and CF–CH₂⁺ were detected as the strongest five ions, and their yields are shown in Fig. 5.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

168

The parent ion appears weakly at the lowest energy, then with increasing energy the three major fragment ions CF₃⁺, CF₂–CH₃⁺ and CH₃⁺ are observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

169

These four ions are the main fragments within the energy range 12–17 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

170

At higher energy, an ion of mass 45 u, almost certainly CF–CH₂⁺, appears gradually and becomes the dominant fragment in the range 18–21 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

171

Very weak minor fragment ions are also observed with masses of 33 u (CH₂F⁺) and 64 u (CF₂–CH₂⁺), but their yields are not shown in Fig. 5.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

172

We comment that as with CHF₂-CF₃⁺,¹⁰ but unlike both isomers of C₂H₂F₄⁺,¹¹ we did not observe any signal due to CF₃–CH₂⁺, corresponding to C–H bond fission.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

173

Note that by using a TOF resolution of only 64 ns, a definitive determination of the number of hydrogen atoms in a fragment ion can be problematic, but we are confident of these assignments.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met19

174

Within the energy range of the ground ionic state, the cation CF₃–CH₃⁺ is observed with an appearance energy of 12.98 ± 0.04 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

175

This signal is relatively weak and appears over a narrow energy range, suggesting that the X̃ state of CF₃–CH₃⁺ is bound only for a small range of low vibrational levels in the Franck–Condon envelope.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

176

The slow rise of the ion yield in the threshold region is due to the small Franck–Condon factor at threshold.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

177

Electron impact studies have observed an ionisation threshold of 13.26 eV.²³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

178

Due to the electron energy resolution being significantly inferior, the results from our photon-impact experiment should be more accurate.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met19

179

The CF₃⁺ fragment ion has an AE₂₉₈ of 13.25 ± 0.05 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

180

This fragment is the most intense and dominates until 15.5 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

181

Using the procedure of Traeger and McLoughlin,³⁰ this value of AE₂₉₈ converts into an upper limit of 13.41 ± 0.05 eV for Δ_rH°₂₉₈ for the reaction CF₃–CH₃ → CF₃⁺ + CH₃ + e⁻ (Table 1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

182

C–C bond cleavage can also produce CH₃⁺ as the fragment ion, where we measure AE₂₉₈ to be 14.25 ± 0.05 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

183

As above, this value can be used to determine an upper limit of Δ_rH°₂₉₈ for the reaction CF₃–CH₃ → CH₃⁺ + CF₃ + e⁻ to be 14.41 ± 0.05 eV (Table 1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

184

G2 calculations predict Δ_rH°₂₉₈ for these two reactions to be very similar, 13.52 and 14.28 eV, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

185

Our experimental values can be used to determine an average Δ_fH°₂₉₈ for the parent neutral molecule.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met20

186

Using literature values of Δ_fH°₂₉₈ for CH₃,²⁸ CH₃⁺,³⁵ CF₃³⁶ and CF₃⁺,³⁷ a lower limit of −751 ± 10 kJ mol⁻¹ is determined for Δ_fH°₂₉₈(CF₃–CH₃).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

187

This value is in excellent agreement with −771 kJ mol⁻¹ from our G2 calculation, and independent theoretical values of −746 ± 2 and −755 from Chen et al²¹. and Zachariah et al.¹⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

188

By assuming that there are no exit channel barriers or kinetic shifts in either reaction we equate our lower limit value with the absolute value.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met21

189

Henceforth, therefore, we use Δ_fH°₂₉₈(CF₃–CH₃) = −751 ± 10 kJ mol⁻¹.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met21

190

The CF₂–CH₃⁺ fragment ion signal appears with a threshold of 14.10 eV ± 0.05 eV, corresponding to an upper limit of Δ_rH°₂₉₈ for the reaction CF₃–CH₃ → CF₂–CH₃⁺ + F + e⁻ of 14.26 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

191

The signal increases slowly, then rises rapidly from approximately 15.5 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

192

From 14.1–17.8 eV, the peaks in the ion yield of CF₂–CH₃⁺ match the bands in the TPES.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

193

An interesting point is that the emergence of the second threshold at 15.5 eV corresponds to the onset of the B̃-state band in the TPES.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

194

The ab initio calculation shows that this band is associated with the electronic state caused largely by electron loss from the (HOMO − 2) F 2pπ nonbonding orbital.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

195

The (HOMO − 3) orbital also has F 2pπ nonbonding character.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

196

The similarity of the ion signal with the shape of the B̃ and C̃ bands in the TPES suggest that CF₂–CH₃⁺ is produced directly via C–F bond cleavage by an impulsive mechanism from these electronics states of the parent cation without prior internal energy conversion to the ground state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

197

From the upper limit of Δ_rH°₂₉₈, we determine Δ_fH°₂₉₈(CF₂–CH₃⁺) ≤ 546 ± 11 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

198

We were not able to measure the kinetic energy release in the dissociation CF₃–CH₃⁺ → CF₂–CH₃⁺ + F (Section 6), but by analogy with the 1,1,2 isomer we can asssume that it may be considerable.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

199

It is likely, therefore, that the true enthalpy of formation of this ion is significantly lower than this value.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

200

A G2 calculation, for instance, predicts Δ_fH°₂₉₈(CF₂–CH₃⁺) to be 443 kJ mol⁻¹, and Lias et al²⁹. give an indirect value of 458 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

201

These data are therefore self-consistent, and suggest that the AE₂₉₈(CF₂–CH₃⁺) lies well above the thermochemical threshold energy of CF₂–CH₃⁺ + F + e⁻.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

202

The daughter ion is therefore likely to be formed with the release of significant kinetic energy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

203

We comment that, in principle, it should be possible to determine experimentally the absolute value of Δ_fH°₂₉₈(CF₂–CH₃⁺) by measuring the kinetic energy release into CF₂–CH₃⁺ + F as a function of photon energy, and extrapolating the linear graph to determine the photon energy at which the kinetic energy release would be zero.³⁸

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

204

This procedure would then yield the dissociative ionisation energy, i.e. Δ_rH°₀ for the reaction CF₃–CH₃ → CF₂–CH₃⁺ + F + e⁻.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

205

Unfortunately, whilst this experiment has been successfully employed for the ground electronic state of polyatomic cations which are repulsive in the Franck–Condon region (e.g.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

206

CF₄⁺ and SF₆⁺),³⁸ we have not been able to yield equivalent data for repulsive, excited electronic states.^10,11

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

207

Possible reasons for the failure of such experiments are described elsewhere.^10,11

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

208

We can, therefore, only confirm an experimental upper limit for Δ_fH°₂₉₈(CF₂–CH₃⁺) of 546 ± 11 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

209

Above 17 eV, minor ions are observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

210

The strongest is CF–CH₂⁺ which appears with a threshold of 17.1 ± 0.1 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

211

Energetically, this minor ion can only form with HF + F as neutrals (Table 1).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

212

As the direct three-body dissociation CF₃–CH₃ → CF–CH₂⁺ + HF + F + e⁻ seems unlikely, we propose a two-step mechanism to form CF–CH₂⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

213

The first step involves the loss of a F atom to produce CF₂–CH₃⁺, the second step (CF₂–CH₃⁺ → CF–CH₂⁺ + HF) proceeds via a tight transition state and HF elimination.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

214

The ion yield curves of CF–CH₂⁺ and CF₂–CH₃⁺ support this suggestion, since the increase of the CF–CH₂⁺ signal corresponds exactly to the decrease of the CF₂–CH₃⁺ signal.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

215

The second step will almost certainly involve a barrier in the exit channel, and could explain why the AE₂₉₈ of CF–CH₂⁺ bears no relation to the energy of the dissociation channel CF–CH₂⁺ + HF + F + e⁻; the former lies ca.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

216

1.5 eV higher in energy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

217

Such a three-body dissociation through a sequential two-step mechanism has already been suggested to explain the products of dissociative photoionisation of CFCl₂–CH₃³⁹ and CHF₂–CH₃.⁴⁰

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

218

Above 18 eV, CF–CH₂⁺ becomes the dominant ion fragment.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

219

Very weak signals due to the minor ions CF₂–CH₂⁺ (mass 64 u) and CH₂F⁺ (mass 33 u) are also observed above 16.5 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

220

In the latter case, the most likely accompanying neutral fragment is CHF₂, so these products can only form via both H- and F-migration across the C–C bond.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

Coincidence ion yields of CHF₂–CH₂F⁺

221

The TPEPICO spectrum in the energy range 11.8–24.0 eV was measured with an optical resolution of 0.3 nm.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met22

222

The ion yields are shown in Fig. 6.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

223

The parent ion appears at lowest energy from the ground ionic state.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

224

As the photon energy increases, a C–C bond fragmentation reaction takes place, followed at higher energy by cleavage of a C–F bond.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

225

This can be seen in the ion yields for CHF₂⁺, CH₂F⁺, and CHF–CH₂F⁺ or CHF₂–CH₂⁺.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

226

As with the 1,1,1 isomer, C–H bond cleavage is not observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

227

These four major ions are the dominant fragments until 16 eV, when a new reaction channel involving two or more bond cleavages opens, possibly with intra-molecular proton transfer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

228

The fragment CF–CH₂⁺ gradually becomes the dominant ion in the higher photon energy region, and we note that an ion of mass 45 u was also dominant with hν > 18 eV for 1,1,1 trifluoroethane (Section 5.2.1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

229

As for CF₃–CH₃, we have used the procedure of Traeger and McLoughlin³⁰ to convert the AE₂₉₈ of the major fragment ions, determined from an extrapolation of the linear portion of the ion yield to the baseline, into an upper limit for the enthalpy of the unimolecular reaction at 298 K, Δ_rH°₂₉₈, in order to determine unknown values of enthalpies of formation at this temperature.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met23

230

For the minor ions, we only compare AE₂₉₈(CF–CH₂⁺) with Δ_rH°₂₉₈ for the possible dissociation reactions to infer what the accompanying neutrals may be.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met23

231

From the onset of ionisation, 11.88 eV, up to ca. 12.5 eV, the parent ion forms exclusively, implying that low vibrational levels of the ground state of the parent cation are bound and lie below the first dissociation threshold.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

232

The parent ion intensity decreases sharply when the fragmentation channel to produce CHF₂⁺ becomes energetically allowed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

233

CHF₂⁺ has an AE₂₉₈ of 12.50 ± 0.04 eV, and is the predominant ion from ca. 12.8 to 16.0 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

234

The AE₂₉₈ can be converted into an upper limit of the enthalpy change for the reaction CHF₂–CH₂F → CHF₂⁺ + CH₂F + e⁻ at 298 K of 12.65 ± 0.04 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

235

This value is in excellent agreement with the value of Δ_rH°₂₉₈, 12.76 eV, derived by us from G2 calculations for the enthalpy of formation of reactants and products of this reaction.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

236

The other possible ionic product from cleavage of the C–C bond, CH₂F⁺, has an AE₂₉₈ of 13.19 ± 0.04 eV, corresponding to Δ_rH°₂₉₈ ≤ 13.34 ± 0.04 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

237

This latter value is only in reasonable agreement with our G2 calculation for the enthalpy change for the reaction CHF₂–CH₂F → CHF₂ + CH₂F ⁺ + e⁻ of 13.08 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

238

Both CHF₂⁺ and CH₂F⁺ form by cleavage of the C–C bond, and are the expected products for dissociation by removing a σ electron from the HOMO of CHF₂–CH₂F. As with the 1,1,1 isomer, the good agreement of both energies with theory implies no exit channel barriers or kinetic shifts in either fragmentation channel.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

239

Combining these experimental values of Δ_rH°₂₉₈ with literature values for the enthalpies of formation of CHF₂ (−237 kJ mol⁻¹), CH₂F (−33 kJ mol⁻¹), CH₂F⁺ (833 kJ mol⁻¹)²⁹ and CHF₂⁺ (604 kJ mol⁻¹),¹⁰ a refined, average enthalpy of formation at 298 K for CHF₂–CH₂F of −671 ± 12 kJ mol⁻¹ is deduced.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

240

This value is in excellent agreement with our G2 calculation, −680 kJ mol⁻¹, and other literature values in the range −656 to −665 kJ mol⁻¹.^16,41

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

241

At a photon energy of 14.51 ± 0.05 eV, corresponding to Δ_rH°₂₉₈ ≤ 14.65 ± 0.05 eV,³⁰ the signal from an ion of mass 65 u increases rapidly.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

242

This signal, corresponding to F-atom loss from the parent ion, approximately matches the drop in the ion signal of CHF₂⁺.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

243

It is not possible to differentiate the two isomers CHF–CH₂F⁺ or CHF₂–CH₂⁺ in the TOF-MS.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

244

G2 calculations predict Δ_rH°₂₉₈ to be 13.72 eV for the reaction CHF₂–CH₂F → CHF–CH₂F⁺ + F + e⁻ and 12.42 eV for the reaction CHF₂–CH₂F → CHF₂–CH₂⁺ + F + e⁻.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

245

Both channels are therefore open at the AE₂₉₈ threshold of 14.51 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

246

Formation of the other isomer with mass 65 u, CF₂–CH₃⁺, involves both fission of a C–F bond and H-atom migration, and seems unlikely.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

247

The energy of 14.51 eV is close to the onset of the (HOMO − 3) C̃ excited state of CHF₂–CH₂F⁺ centred at 15.97 eV, and the yield of this fragment ion follows closely the threshold photoelectron signal of the C̃ state.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

248

Molecular orbital calculations predict that the C̃ state of CHF₂–CH₂F⁺ is produced by electron removal from a F 2pπ non-bonding orbital localised predominantly on the CHF₂ group (Section 4.2).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

249

It seems likely, therefore, at least near threshold, that CHF–CH₂F⁺ is the dominant component and arises from the dissociation CHF₂–CH₂F → CHF–CH₂F⁺ + F + e⁻.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con18

250

Careful analysis of the ion yield shows a two-step increase, with a second threshold at ca.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

251

15.2 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

252

It is possible that this second threshold is due either to rapid dissociative ionisation from a different electronic state of the parent ion or to formation of a different isomer of C₂H₃F₂⁺; we note that ionisation from the (HOMO − 4) orbital followed by impulsive F-atom loss may lead to significant production of the isomer CHF₂–CH₂⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con19

253

From Δ_rH°₂₉₈ ≤ 14.65 ± 0.05 eV, we determine Δ_fH°2₉₈ (CHF–CH₂F⁺) ≤ 663 ± 13 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

254

However, since the dissociation CHF₂–CH₂F⁺ → CHF–CH₂F⁺+F has a considerable kinetic energy release (Fig. 7 and Section 5.3), it is likely that the absolute enthalpy of formation of this ion is significantly lower than this value.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

255

A G2 calculation, for example, predicts Δ_fH°₂₉₈(CHF–CH₂F⁺) to be 565 kJ mol⁻¹, and Lias et al²⁹. quote 543 kJ mol⁻¹ determined from the proton affinity of CHFCHF.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

256

As observed in the dissociative photoionisation of other fluorine-substituted ethanes,^10,11,34 a rapid impulsive mechanism involving cleavage of the C–F bond often occurs when the molecular orbital from which the electron has been removed has mainly F 2pπ lone pair character.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

257

If this mechanism is occurring, the fragment ion + F atom will have considerable translational kinetic energy, and can lead to a large difference between the observed dissociative ionisation threshold and the calculated energy of reaction.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

258

From the calculated electron densities of the molecular orbitals, and the fact that a large fraction of the available energy is deposited into translation kinetic energy of fragments in the CHF–CH₂F⁺ or CHF₂–CH₂⁺ + F decay channel (Table 4), it seems likely that CHF–CH₂F⁺ or CHF₂–CH₂⁺ is produced directly and impulsively from the C̃ and/or D̃ excited electronic states of CHF₂–CH₂F⁺ without prior internal conversion to the ground state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

259

These states of the parent ion are then showing the characteristics of isolated-state behaviour, a phenomenon which is expected in small cations but is unexpected in polyatomic cations with as many as eight atoms.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

260

At higher photon energies, the ion CF–CH₂⁺ (possibly with a very small component of ions with mass 44 and 46 u) gradually increases, and for photon energies above ca.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

261

17 eV this ion becomes dominant.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

262

This ion was also the dominant, minor ion for dissociative photoionisation of CF₃–CH₃ with hν > 18 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs32

263

The observed AE₂₉₈ of the ion, 16.21 ± 0.05 eV, is significantly higher than the only possible thermochemical reaction energy, 14.76 eV, for the three-body fragmentation CHF₂–CH₂F → CF–CH₂⁺ + F + HF + e⁻; fragmentation to CF–CH₂⁺ + F₂ + H + e⁻ at 19.07 eV, or even to CF–CH₂⁺ + 2F + H + e⁻ at 20.71 eV, are both forbidden energetically As with the 1,1,1 isomer, the increase of the CF–CH₂⁺ signal roughly matches the decrease of the C₂H₃F₂⁺ signal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res42

264

This may imply that CF–CH₂⁺ is formed via a two-step mechanism.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con21

265

First, a fluorine atom is lost from the C̃ or D̃ excited states of the parent ion through an impulsive mechanism as described above to form an isomer of C₂H₃F₂⁺, then formation of CF–CH₂⁺ occurs from C₂H₃F₂⁺via a tight transition state, an exit channel barrier, and HF elimination.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con21

266

We note that the difference between the AE₂₉₈(CF–CH₂⁺) and the energy of the dissociation channel CF–CH₂⁺ + HF + F + e⁻ is ca.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con21

267

1.5 eV, the same value as with HF + F elimination from the 1,1,1 isomer (Section 5.2.1).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con21

Kinetic energy releases

268

TPEPICO-TOF spectra at a resolution of 8 ns have been recorded for the major fragment ions at photon energies corresponding to the Franck–Condon maxima of the valence states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met24

269

These measurements include the parent ion spectra, where the peaks are predicted to be Gaussian in shape with a full width at half maximum (fwhm) proportional to (MT)^1/2/E, where M is the mass of the parent ion (84 u in this case), T is the temperature (298 K), and E is the extraction field from the interaction region (20 V cm⁻¹).^42,43

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met24

270

The observation of good fits to the experimental data for both CF₃–CH₃⁺ and CHF₂–CH₂F⁺³³ with the correct fwhm indicate that spatial focussing is operating correctly in the TOF mass spectrometer.⁴⁴

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs33

271

Fragment ions, however, often have enough translational energy for the TOF peak to be broadened from that expected for a thermal source.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met24

272

Analysis of the shape of such peaks allows a determination of the kinetic energy release distribution (KERD), and the total mean translational kinetic energy, 〈KE〉_T, associated with a particular single-bond dissociation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met24

273

For example, Fig. 7 shows the TPEPICO-TOF spectrum of CHF–CH₂F⁺ from the C̃ state of the parent ion CHF₂–CH₂F⁺ accessed at 16.02 eV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs34

274

A fit to the peak by a procedure described elsewhere^26,27 yields 〈KE〉_T = 0.82 ± 0.04 eV.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res43

275

The values of 〈KE〉_T are sometimes insensitive to the exact form of the KERD, and the error quoted is probably unrealistically low.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met24

276

〈KE〉_T can be divided by the available energy, E_avail, to determine the fraction of the available energy, f_T, being channelled into translational energy of the two fragments.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met25

277

E_avail is given by the photon energy plus the thermal energy of the parent molecule at 298 K minus the AE₂₉₈ of the daughter ion.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met25

278

Experimental values of f_T can be compared with those expected if the dissociation follows a pure statistical⁴⁵ or a pure impulsive⁴⁶ model.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met25

279

These two limiting models are described elsewhere.^10,47

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met25

280

(Note that if dissociation follows the modified-impulsive model,^47,48 values of f_T may be greater than those calculated for the pure-impulsive model.) Values of 〈KE〉_T and f_T are shown in Table 4, together with calculated values of f_T for these two models.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

281

Since some of the vibrational wavenumbers of the fragment ions are unknown, statistical values for f_T were calculated according to the lower limit value of 1/(x + 1), where x is the number of vibrational degrees of freedom in the transition state of the unimolecular reaction.⁴⁹

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met26

282

For CF₃–CH₃, spectra were measured for the dissociation reaction CF₃–CH₃⁺ → CF₃⁺ + CH₃ at photon energies of 14.50, 14.95, 18.99 and 20.16 eV.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met27

283

These energies correspond to the initial formation of the X̃, Ã, D̃ and Ẽ states of the parent ion.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met27

284

Spectra were also measured for CF₃–CH₃⁺ → CH₃⁺ + CF₃ at 16.00, 18.99 and 20.16 eV, corresponding to formation of the B̃, D̃ and Ẽ states of the parent ion.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met28

285

The experimental and predicted data are shown in the top half of Table 4.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

286

For dissociation to CF₃⁺, assuming dissociation always occurs to the ground electronic state of the fragments, low values of f_T are observed at all energies.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs35

287

The value of f_T at 14.95 eV may be anomalously high because there is a minor component of CF₂–CH₃⁺ signal in the CF₃⁺ peak.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

288

For dissociation to CH₃⁺, low values of f_T again are observed at all excitation energies.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs36

289

No measurements could be made for CF₃–CH₃⁺ → CF₂–CH₃⁺ + F at the peak of either the B̃ or C̃ states of the parent ion, because the fragment ion signal (65 u) shows weak blends from CF₂–CH₂⁺ (64 u) and CF₃⁺ (69 u).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs37

290

For CHF₂–CH₂F, spectra were also measured for the corresponding dissociation reactions at photon energies corresponding to initial formation of the electronic states of the parent ion.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met29

291

The same pattern for fractional translational energy release is observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs38

292

That is, low values of f_T, always less than 0.10, are observed at all energies for dissociation to either CHF₂⁺ + CH₂F or CH₂F⁺ + CHF₂.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs38

293

The values are close to that predicted for statistical decay.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

294

The signal at mass 65 u due to F-atom loss from dissociative photoionisation of CHF₂–CH₂F is now unblended because CF₃⁺ is not observed as a fragment ion.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs39

295

The large value of 〈KE〉_T, 0.82 ± 0.04 eV, when the molecule is excited into the C̃ state of the parent ion was noted in Section 5.2.2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res48

296

The corresponding value of f_T, 0.51, is almost exactly that predicted by the pure-impulsive dissociation model.⁴⁶

Type: Result | Advantage: None | Novelty: None | ConceptID: Res48

297

Although this pattern of low values of f_T for C–C bond and high values for C–F bond cleavage has been observed before in other HFC cations,^10,11 the absolute values of f_T should be treated with some caution.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22

298

They depend upon the values used for E_avail, which themselves depend on a precise determination of the AE₂₉₈ of the daughter ion.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22

299

Certainly for C–F bond fission, the AE₂₉₈ of the daughter ion may be much higher than the dissociation energy to fragment ion + F. Nevertheless, the high value of f_T for F-atom loss from CHF₂–CH₂F⁺ is consistent with isolated-state behaviour for the C̃ (and possibly D̃) states of the parent ion.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22

300

Dissociation then proceeds along a pseudo-diatomic exit channel of the potential energy surface of the initially excited state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22

301

The two atoms of the breaking C–F bond recoil with such force that a relatively large fraction of the available energy is converted into translational energy of the two fragments.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22

302

Although we were not able to measure f_T, we predict the same behaviour for the B̃ and C̃ states of CF₃–CH₃⁺.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con23

303

By contrast, the much lower values of f_T for C–C bond cleavage in both CF₃–CH₃⁺ and CHF₂–CH₂F⁺ suggest that the initially-excited state of the parent ion decays non-radiatively by internal conversion to the bound parts of the ground state, then dissociation occurs in a statistical manner from this surface.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con24

304

An alternative explanation is that the C–C bond does break in an impulsive manner, but the much lower values of 〈KE〉_T and f_T than either impulsive model suggests arise because one of the bond lengths or bond angles in the fragment ion is considerably changed from its value in the parent ion.⁵⁰

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con25

305

Such an intramolecular mechanism of fragmentation would result in the daughter ion and/or neutral partner having significant amounts of vibrational energy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con25

306

Since CF₃⁺, CH₃⁺, and presumably CHF₂⁺ and CH₂F⁺, are planar in the isolated ion but approximately pyramidal when located in the parent ion, this model could explain the low values of f_T for C–C bond cleavage, with the ν₂ umbrella bending mode of these daughter ions incorporating much of the available energy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con25

307

From the kinetic energy data alone, therefore, we are not able to distinguish these two mechanisms for how the central C–C bond breaks in either isomer of the trifluoroethane cation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con26

Conclusions

308

We have recorded the threshold photoelectron and threshold photoelectron photoion coincidence spectra of the two isomers of trifluoroethane, CF₃–CH₃ and CHF₂–CH₂F, in the range 12–24 eV.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con27

309

Ion yield curves have been determined, and the breakdown diagrams are shown elsewhere.³³

Type: Result | Advantage: None | Novelty: None | ConceptID: Res49

310

The mean translational kinetic energy releases into fragment ions involving a single bond cleavage from selected valence states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺ have been measured, and compared with the predictions of statistical and pure-impulsive models.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con28

311

Ab initio G2 calculations have determined the minimum energies of CF₃–CH₃ and CHF₂–CH₂F and their cations, with their geometries optimised at the MP2(full)/6-31G(d) level of theory.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con29

312

The nature of the valence orbitals of both neutral molecules has also been deduced.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con30

313

In addition, enthalpies of formation at 298 K of CF₃–CH₃, CHF₂–CH₂F, and the major and minor ions observed by dissociative photoionisation have been calculated at this level of theory.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con31

314

Combining experimental and theoretical data, the decay mechanism of the ground and low-lying excited valence states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺ have been discussed.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con32

315

Both molecules have ground electronic states of the parent cation which are stable only over a narrow range of energies corresponding to the lower vibrational levels.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con32

316

As the photon energy increases, the fractional yield of the parent cation decreases from unity, and C–C bond cleavage produces CF₃⁺ and CH₃⁺ from CF₃–CH₃, CHF₂⁺ and CH₂F⁺ from CHF₂–CH₂F. It is assumed that these four ions all turn on at their thermochemical threshold with no activation barrier in the exit channel.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con32

317

We have converted these energy thresholds into upper limits for the enthalpy of the corresponding reactions at 298 K,³⁰ and thus determined new values for the enthalpy of formation at this temperature of CF₃–CH₃ and CHF₂–CH₂F. Only a low fraction of the available energy, f_T < 0.1, is channelled into translational energy of the fragment ion and polyatomic neutral.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con33

318

At higher energy, C–F bond cleavage is associated with electron removal from the (HOMO − 2) and (HOMO − 3) 2pπ nonbonding orbitals of CF₃–CH₃.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con34

319

Impulsive dissociation from the B̃ and C̃ states of CF₃–CH₃⁺via C–F bond cleavage then leads to production of CF₂–CH₃⁺ + F. Although not measured experimentally, we infer that a much larger fraction of the available energy is now channelled into product translation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con35

320

Decay from the C̃ and D̃ states of CHF₂–CH₂F⁺, formed by electron removal from the F 2pπ nonbonding (HOMO − 3) and (HOMO − 4) orbitals of the neutral molecule, also leads to C–F bond cleavage, and production of either CHF–CH₂F⁺ or CHF₂–CH₂⁺ + F. Now a large value of f_T is measured experimentally, and confirms that these states display isolated-state behaviour and decay impulsively.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con36

321

It is also likely that the geometry of the daughter ion will not differ significantly from that of the corresponding group in CHF₂–CH₂F⁺.^46,47

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con37

322

For both molecules, the AE₂₉₈ of the daughter ion with mass 65 u lies significantly higher in energy than the thermochemical energy of the dissociation channel, so only an upper limit for the enthalpy of formation at 298 K of CF₂–CH₃⁺ and CHF–CH₂F⁺ or CHF₂–CH₂⁺ is determined.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con38

323

Several examples of minor fragment ions caused by more complicated unimolecular reactions are observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res50

324

For both isomers of trifluoroethane, CF–CH₂⁺ is observed, and indeed for hν >ca.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res51

325

18 eV this ion is the dominant fragment from dissociative photoionisation of both CF₃–CH₃ and CHF₂–CH₂F. A two-step mechanism, fluorine-atom loss then HF elimination, is suggested to explain its presence.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con39

Introduction

Theoretical and experimental methods

Computational methods

Experimental methods

Energetics of the dissociation channels

Theoretical results

Structure of CF3–CH3 and CF3–CH3+, and orbital character

Structure of CHF2–CH2F and CHF2–CH2F+, and orbital character

Calculation of ΔrH°298 for dissociative photoionisation reactions

Experimental results and discussion

Threshold photoelectron spectra of CF3–CH3 and CHF2–CH2F

Scanning-energy TPEPICO spectra

Coincidence ion yields of CF3–CH3+

Coincidence ion yields of CHF2–CH2F+

Kinetic energy releases

Conclusions

Structure of CF₃–CH₃ and CF₃–CH₃⁺, and orbital character

Structure of CHF₂–CH₂F and CHF₂–CH₂F⁺, and orbital character

Calculation of Δ_rH°₂₉₈ for dissociative photoionisation reactions

Threshold photoelectron spectra of CF₃–CH₃ and CHF₂–CH₂F

Coincidence ion yields of CF₃–CH₃⁺

Coincidence ion yields of CHF₂–CH₂F⁺