Filename: b405115g

Back | Show only these items: | First observation of the rotational spectrum of the bromomethyl radical, CH2Br

1
First observation of the rotational spectrum of the bromomethyl radical, CH₂Br

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

The first high resolution rotational spectrum of the bromomethyl radical in the gas phase has been observed.

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

The radical was produced in a fast flow system by hydrogen abstraction of CH₃Br by Cl or F atoms obtained from a 2450 MHz microwave discharge in Cl₂ or in CF₄, respectively.

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

The rotational, quartic centrifugal distortion constants have been obtained as well as the electron spin–rotation interaction constants.

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

The small positive inertial defect, Δ₀ = 0.032 amu Å² deduced from the rotational constants of the CH₂⁷⁹Br species clearly indicates that the radical is planar in its ground vibronic state.

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

It has been well established that halogen atoms and radicals play an important role in catalytic reactions contributing in the stratospheric ozone destruction.^1–4

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

In particular, many studies have been devoted to the role of chlorine.^5–9

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

Until recently, the chemistry of bromine-containing molecules and the adverse impact of bromine on the atmosphere have received little attention although there has been accumulating evidence for more than two decades that reactive bromine is also involved in the depletion of the ozone layer.^10–12

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

The concentration of bromine in the stratosphere is much lower than that of chlorine.

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

However several studies have demonstrated that bromine is much more destructive to the stratospheric ozone than chlorine.

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

Indeed, Garcia and Solomon have recently reported that bromine could be up to 100 times more efficient than chlorine, on an atom-per-atom basis.¹³

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

Methyl bromide is considered to be the major source of bromine in the atmosphere.¹⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

CH₃Br is released into the atmosphere from both natural and anthropogenic sources.¹⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

The main known removal process of this halomethane occurs via hydrogen abstraction by its reaction either with the hydroxyl radical or with chlorine, leading to the formation of the monobromomethyl radical, CH₂Br.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

This radical reacts further with other open shell species to release Br atoms for secondary reactions with ozone.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

The monobromomethyl radical is therefore a key transient for which it is essential to obtain accurate information from high-resolution spectroscopy.

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

Such information is crucial to allow kinetic and photochemical laboratory studies in order to achieve a better understanding of the role played by bromomethane in the processes leading to the destruction of stratospheric ozone.

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

In addition to its importance in atmospheric chemistry, the study of CH₂Br is also of importance in order to compare its structure with those of its well-studied fluorine- and chlorine-analogues, CH₂F¹⁶ and CH₂Cl.^17,18

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

These studies have revealed that CH₂F is nearly planar whereas CH₂Cl is planar, the difference being attributed to the larger electronegativity of fluorine.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

CH₂Br was first detected in electron impact experiments with CH₂Br₂.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

An early matrix infrared study has suggested that this elusive species has a planar structure because of the large positive quartic term in the out-of-plane bending potential function.²⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

This conclusion was later supported by EPR investigations.²¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Davies et al. have recently reported the detection of this radical in the gas phase by far infrared laser magnetic resonance.²²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

They have also interpreted the spectra in terms of a planar structure.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

However, recent ab initio calculations²³ indicate that CH₂Br is most probably very slightly non-planar, if not planar.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

Depending on the structure, the ground electronic state is either ²A' (non-planar) or ²B₁ (planar).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

This shows that the planarity still needs to be confirmed by high-resolution spectroscopic studies.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot4

Furthermore, high quality ground state constants will make the analysis of vibrational or electronic spectra of this radical significantly easier, and identification of CH₂Br in such experiments more definitive if the ground state constants agree to within error with the millimeter wave values.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot5

In this letter, we report the detection and the identification of the first high resolution rotational spectrum of CH₂Br in the gas phase.

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

The millimeter wave (mmw) spectrometer used has been described in detail elsewhere.²⁴

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

It was adapted for the production of the CH₂Br radical.

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

Briefly, it consists of a single-pass absorption system comprising a tunable mmw source, an absorption cell and a detector.

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

The mmw radiation was obtained from two phase-locked backward wave oscillators, one emitting in the 160–260 GHz frequency region (Istok), the other one oscillating between 418–470 GHz (Thomson C.S.F.).

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

The detection of the absorption signals was achieved with a helium-cooled InSb hot electrons bolometer (Q.M.C.

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

Instruments).

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

For improved sensitivity, source modulation at 70 kHz was used together with lock-in detection at twice this frequency, resulting in a second-derivative lineshape.

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

Frequency scanning, data acquisition, signal processing and frequency measurement were controlled using a PC; the errors on the frequency measurement were estimated to be less than 50 kHz.

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

CH₂Br was generated in a fast flow system by hydrogen abstraction of monobromomethane by chlorine or fluorine.

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

The 1 m long absorption cell in Pyrex was equipped with a coil for creating a magnetic field to confirm that the detected lines were paramagnetic by on/off measurement.

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

At one end a vertical Pyrex crosspiece was used to connect to a partially choked diffusion pump below the cell and to a pressure gauge mounted above the cell.

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

At the other end, a horizontal Pyrex crosspiece was used to introduce CH₃Br vapour and Cl₂ or CF₄ separately.

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

This method allowed in situ production of the reactive species.

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

The outer end of each crosspiece was sealed by a Teflon window mounted at Brewster angle in order to improve transmission and to reduce standing waves.

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

Cl₂ diluted with He (2:100) or CF₄ mixed with Ar (CF₄:Ar = 1:10) were used as a source for atomic chlorine or fluorine, respectively.

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

Cl or F atoms were produced by a microwave discharge in a small quartz tube placed at the entrance of the second crosspiece and surrounded by a cavity fed by radiation from a magnetron at 2450 MHz.

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

The power of the microwave discharge was kept as low as possible (≈20 W).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The optimised partial pressure for CH₃Br was 7 × 10⁻³ mbar, and the total pressure was 1.5 × 10⁻² mbar.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

Before initiating our search for the spectra, we decided to extend the measurement of the spectra of CH₂³⁵Cl and CH₂³⁷Cl (natural abundances: ³⁵Cl: 75.77%, ³⁷Cl: 24.23%) in the 418–470 GHz frequency region.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

We have used the molecular constants published by Endo et al¹⁷. to predict the spectra falling within this range.

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

The analysis of the spectra is now in progress for publication.

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

The observation of the spectral lines of the monochloromethyl radical has allowed us to optimize the experimental conditions in order to maximize our chance to detect transitions of the bromomethyl radical.

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

In particular, we found that the use of Cl₂ led to a much clearer spectrum than the use of CF₄.

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

We have afterwards guided our search for CH₂Br according to the rotational constants deduced from the ab initio structure²³ of this radical in the ground state together with the centrifugal distortion constants obtained for CH₂Cl.¹⁷

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

The calculated A, B and C rotational constants are listed in Table 1.

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

The ab initio B + C value of CH₂⁷⁹Br, 22 304 MHz,²³ agrees with the value reported by Davies et al., 22 305 MHz.²²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

More importantly, this group has also assigned the main rotational quantum number N = 20 ← 19 to the unresolved lines lying at 14.88 cm⁻¹ (446 091 MHz).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

This frequency falls in a range where our spectrometer has its highest sensitivity.

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

Bromine has two stable isotopes of nearly equal abundances (⁷⁹Br: 50.69%, ⁸¹Br: 49.31%).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

The spectra of the two isotopomers should then be detected with the same intensity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

Assuming a planar structure, the dipole moment of CH₂Br lies along the C₂ symmetry axis, which is the axis of the least moment of inertia.

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

The rotational spectrum should hence display the characteristic a-type pattern of a slightly asymmetric prolate rotor (κ = −0.996).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

In addition, owing to the electron spin–rotation interaction, each rotational level is split into two, leading to a fine structure in the spectrum.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

As a first estimate of the fine splittings, we have used for the prediction the ε_ii constants of CH₂Cl¹⁷ (see Table 1).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

The two components of the N = 20 ← 19, K_a = 0 lines of CH₂⁷⁹Br were predicted at 444597 and 444680 MHz.

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

The four K_a = 1 components were calculated to lie at 440736, 440772, 450320 and 450767 MHz.

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

The corresponding calculated set of transition frequencies for the CH₂⁸¹Br species was shifted to the lower frequencies by about 1600 ± 40 MHz.

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

We then carried out the search for the N = 20 ← 19 lines of CH₂Br by scanning the 439–452 GHz frequency range.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

The observed spectrum appeared so congested that we proceeded in two steps: at first we scanned using the experimental conditions described above.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Subsequently we repeated the measurement by applying a magnetic field along the cell in order to both distinguish only the transitions sensitive to a magnetic field and to eliminate the numerous and often strong absorption lines of CH₃Br.

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

This procedure led to the detection of more than 40 unidentified paramagnetic lines, most of them lying in the 443–447 GHz frequency region.

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

These lines could also be observed using CF₄ for the production of fluorine instead of chlorine, but they disappeared when CH₃Br was replaced with CH₃Cl.

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

At this stage, the chemical tests and the comparison with the ab initio calculations support strongly the detection of the two isotopomers of CH₂Br.

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

Moreover, the majority of the transitions appeared as partly resolved doublets or triplets, sometimes as singlets.

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

This hyperfine structure could be due to the nuclear spins of bromine (I(⁷⁹Br) = I(⁸¹Br) = 3/2) and of hydrogen (I(H) = ½).

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

By comparison with the predictions, we were quickly able to recognize two sets of lines of nearly equal intensity, each set displaying an identical pattern.

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

The average difference between the transitions of these two sets is 1609 MHz, a value very close to the calculated one.

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

A stick spectrum of the detected lines is shown on Fig. 1.

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

However, the electron spin–rotation interaction led obviously to a strong mixing of the various K_a components.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

It was therefore not possible to readily recognize a typical a-type R branch pattern of a prolate asymmetric top, preventing us from assigning the spectrum.

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

We have consequently extended the measurement in the lower frequency region in order to observe the various K_a components of the N = 19 ← 18 series, focusing our search on the ⁷⁹Br species.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

This led to the detection of 22 new paramagnetic lines in the 422–425 GHz frequency span.

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

In addition, some K_a = 1 lines for both N series and for the two isotopic species turned out to be difficult to detect because they fall in frequency regions where strong transitions of CH₃Br were observed.

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

These lines were finally observed after a more careful search.

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

An example of a K_a = 1 transition obtained by using either chlorine or fluorine is given in Fig. 2.

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

The observation of the K_a = 1 transitions proved to be decisive in the identification of the spectrum of CH₂⁷⁹Br.

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

Indeed, after several attempts, a least squares fit of the K_a = 0, 1 and 2 components of the N = 20 ← 19 and N = 19 ← 18 series of CH₂⁷⁹Br allowed us to assign the K_a = 3–6 components successfully.

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

The K_a = 0 lines of CH₂⁷⁹Br have been observed at 445248 and 445465 MHz.

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

The four K_a = 1 components lie at 441317, 441377, 450320 and 450767 MHz.

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

The identification of the spectrum of CH₂⁸¹Br was then straightforward.

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

Repeating this procedure eventually led to the measurement of 59 rotational lines for the CH₂⁷⁹Br isotopic species corresponding to N = 9, 10 and 19 ≤ N ≤ 21, K_a ≤ 6 in the 160–470 GHz frequency range.

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

For the CH₂⁸¹Br isotopomer, only 42 transitions corresponding to N = 9, 10 and 19 ≤ N ≤ 21, K_a ≤ 6 have been measured.

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

The average frequencies of the hyperfine components have been used for the fit.

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

These frequencies were fitted to a Watson A-reduced Hamiltonian in the I^r representation.²⁵

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

Predictions and fittings were made using Pickett's programs SPFIT and SPCAT.²⁶

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

The adjusted rotational, centrifugal distortion and fine structure constants for the two isotopic species are listed in Table 1 together with the CH₂³⁵Cl parameters for comparison.

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

The ab initio rotational constants are in good agreement with those obtained from the fit.

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

Moreover, the experimentally determined B + C value of CH₂⁷⁹Br, 22327.3 MHz, is very close to the one reported by Davies et al. (22305 MHz).

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

In addition, this group has noticed when studying the radical by LMR spectroscopy that all K_a components except K_a = 1 are very closely spaced²² (within 0.16 cm⁻¹, i.e. 4800 MHz approximately, for the N = 30 ← 29 transition).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

Fig. 1 shows that our identification confirms their observation.

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

100

The centrifugal distortion constants of CH₂Br given in Table 1 are of the same order of magnitude than the ones of CH₂Cl.

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

101

Their signs are also identical.

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

102

Finally, the ε_aa spin–rotation interaction constant is about four times larger in CH₂Br than in CH₂Cl.

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

103

Such a difference was already observed between the CH₂Cl and CH₂F radicals.¹⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

104

The fact that reaction of CH₃Br with Cl or F atoms led to the observed transitions together with the relatively good agreement between the experimental and ab initioA, B and C constants for both isotopic species enables us to ascertain that we have observed the rotational spectrum of the monobromomethyl radical.

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

105

This is the first high-resolution spectroscopic identification of this species.

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

106

Work directed toward the analysis of the hyperfine structure is in progress.

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

107

The small positive value of the inertial defect, Δ₀ = 0.032 amu Å² proves the planarity of CH₂Br in the ground vibronic state.

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