Filename: b314320a

Back | Show only these items: | The rotational spectrum of thiophene⋯HBr and a comparison of the geometries of the complexes B⋯HX, where B is benzene, furan or thiophene and X is F, Cl or Br

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The rotational spectrum of thiophene⋯HBr and a comparison of the geometries of the complexes B⋯HX, where B is benzene, furan or thiophene and X is F, Cl or Br

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

The ground-state rotational spectra of three isotopomers C₄H₄S⋯H⁷⁹Br, C₄H₄S⋯H⁸¹Br and C₄H₄S⋯D⁷⁹Br of a weakly bound complex formed by thiophene and hydrogen bromide have been observed in the gas phase by means of a pulsed-jet, Fourier-transform instrument.

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

Each spectrum was analysed and fitted to give rotational constants A₀, B₀ and C₀, centrifugal distortion constants Δ_J, Δ_JK, Δ_K, δ_J and δ_JK and the components χ_aa, χ_bb – χ_cc and χ_ab of the bromine nuclear quadrupole coupling tensor.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

A detailed analysis of the spectroscopic constants established that the geometry of the complex is of the face-on type.

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

The Br atom of HBr is located close to the perpendicular drawn through the centre of mass of the thiophene ring and the H atom of HBr lies between the Br atom and the ring.

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

The angle α_az made by the HBr internuclear axis z with the a-axis has the two possible values ±9.83°.

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

The preferred structure is that generated when the positive value of the angle is chosen and has the HBr sub-unit pointing in the direction of the S atom of thiophene.

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

The determined geometrical parameters are r(S⋯H) = 2.728(3) Å, φ = 116.0(2)° and θ = 7.08(4)°, where φ is the angle made by the S⋯H internuclear line with the local C₂ axis of thiophene and θ is the angular deviation of the S⋯H–Br nuclei from collinearity.

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

Introduction

Molecules B that exhibit both non-bonding (n) and π- bonding electron pairs are dealt with by the third part of some rules put forward in 1982 to account for the observed angular geometries of hydrogen-bonded complexes B⋯HX, where B is a simple Lewis base and HX is a hydrogen halide.¹

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

Part 3 of the rules states that, in such a complex, the n-pair is definitive of the angular geometry.

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

Furan⋯HCl evidently obeys this rule because it was shown² to have a planar geometry of C_2v symmetry, with the HCl lying along the C₂ axis of the furan subunit so as to form a hydrogen bond to O. The experimental geometry of this complex is shown, drawn to scale, in Fig. 1, which also includes diagrams of related molecules.

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

The furan⋯HCl complex was detected and characterised by means of its rotational spectrum, as observed by pulsed-nozzle, Fourier-transform microwave spectroscopy.

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

In this technique the complexes are produced by supersonic expansion and therefore only the lowest energy conformer in its zero-point state is usually detected.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met1

Thus, the zero-point state of the most stable form of furan⋯HCl has an angular geometry predicted by the rules.

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

Subsequently, it was shown,³ by using the same experimental method, that the related complex furan⋯HF has an angular geometry of the same type as that of furan⋯HCl.

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

A recent investigation⁴ of the rotational spectrum of the complex furan⋯HBr led to an unexpected result, namely that it was not isomorphous with the HCl and HF complexes of furan.

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

Instead, the observed angular geometry of furan⋯HBr resembles that of benzene⋯HBr,⁵ in the sense that the Br atom lies above the centre of the face of the aromatic ring and the H atom of HBr appears to form a hydrogen bond to the π electrons system in each case.

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

This is clear from the structures shown in Fig. 1, which compares the experimental results for furan⋯HCl, furan⋯HBr and benzene⋯HBr.

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

The series of complexes pyridine⋯HX, where X is F, Cl or Br, all have the C_2v geometry,^6–8 as expected from the rules, that is the HX molecule lies along the C₂ axis of pyridine and forms a hydrogen bond to N. The hydrogen bond is reasonably strong in each case and indeed there is evidence of a significant contribution of the ionic form C₅H₅NH⁺⋯Br^– to the valence-bond description of the complex with HBr.⁸

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

A reason why the pyridine⋯HX series is homogeneous in its angular geometry while the furan⋯HX series is not becomes apparent when the molecular electric dipole µ and quadrupole moments Θ of the heterocyclic aromatic molecules pyridine and furan are compared.⁹

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

In pyridine, both µ and the component Θ_zz of Θ, where z is the C₂ axis, are large and negative, corresponding to a prominent nonbonding electron pair on N. In furan, µ is smaller and Θ_zz is close to zero, suggesting that the n-pair in furan is more involved in the ring.

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

It has been proposed that the change from the C_2v to the face-on geometry when HX is HBr arises from a reduced contribution of electrostatic factors coupled with an increased dispersion effect.

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

The former occurs because the electric dipole moment of HBr¹⁰ is smaller than that of HF¹¹ or HCl¹² while the latter is expected to increase in the order HF < HCl < HBr.

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

Both changes are in a direction that favours a face-on geometry, as observed in benzene⋯HBr.

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

For thiophene, µ is even smaller and the component Θ_zz is positive.

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

This suggests that the non-bonding electron pair formally associated with the heteroatom is progressively withdrawn into the ring as we pass from pyridine, through furan, to thiophene and therefore that thiophene should behave more like benzene.

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

The fact that the complexes of thiophene⋯HF¹³ and thiophene⋯HCl¹⁴ involve the π type, face-on arrangement is consistent with these conclusions deduced from the charge distributions of the heteroaromatic molecules and indicates that thiophene⋯HBr should also have the face-on geometry.

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

We report here an investigation of thiophene⋯HBr by means of its ground-state rotational spectrum observed by the pulsed-jet, Fourier transform (FT) method.

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

The aim of the investigation is to identify and characterise this complex and, in particular, to establish whether its angular geometry is isomorphous with those of thiophene⋯HF and thiophene⋯HCl, as predicted by the arguments rehearsed earlier.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

Experimental

The rotational spectrum of thiophene⋯HBr was observed with a pulsed-jet, Fourier transform microwave spectrometer based on the original design of Balle and Flygare.¹⁵

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

Recent modifications¹⁶ have been outlined elsewhere.

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

A fast-mixing nozzle¹⁷ was used in conjunction with the spectrometer to preclude any possible reaction between thiophene and hydrogen bromide that might have occurred when these two substances were mixed in a conventional, stainless steel stagnation vessel in the common way.

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

Thiophene vapour above a liquid sample held at room temperature was flowed continuously through the central glass (0.3 mm internal diameter) capillary of the mixing nozzle into the evacuated chamber containing the Fabry-Pérot cavity.

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

The flow rate was adjusted to yield a nominal pressure of ca. 10^–4 mbar in the chamber.

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

A mixture consisting of HBr and argon in the partial pressure ratio 2∶100 and held at a stagnation pressure of 3 bar was pulsed into the outer stainless steel tube of the mixing nozzle at a rate of 5 Hz by means of a Series 9 solenoid valve (Parker Hannifin).

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

The outer tube of the mixing nozzle is concentric and approximately coterminous with the glass capillary, an arrangement that enabled thiophene⋯HBr complexes to be formed at the interface of the thiophene and HBr/Ar gas flows.

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

Such complexes were rotationally polarized with 1 µs pulses of microwave radiation of appropriate frequency and the subsequent free-induction decay at rotational transition frequencies was processed as described elsewhere.¹⁶

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

Individual Br nuclear quadrupole hyperfine components in observed transitions of the isotopomers based on HBr had a full-width at half height of ca. 20 kHz, which led to 2 kHz as the estimated accuracy of frequency measurement.

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

Thiophene and HBr gas were obtained from Aldrich and used as received.

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

DBr was prepared by the exchange of HBr gas with D₂O adsorbed on the walls of the stainless steel stagnation vessel.

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

To saturate the walls of the vessel with D₂O, the vessel was heated to ca. 70–80 °C and then the vapour from above a warmed sample of the liquid D₂O was brought into contact with the vessel walls while they cooled.

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

The vessel was then reheated and the HBr gas added.

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

After a period to allow exchange of D for H, the required partial pressure of argon was added.

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

In this way a sample of DBr with ca. 70 atom% D was produced.

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

Results

Spectral assignment and analysis

The search for the rotational spectrum of thiophene⋯HBr was guided by predictions of transition frequencies made using a face-on model of the complex based on thiophene⋯HCl.

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

This model was constructed by taking the observed angular geometry¹⁴ of thiophene⋯HCl, but with the distance r(S⋯Cl) increased by the difference between the van der Waals radii of Br and Cl to give a rough estimate of the distance r(S⋯Br).

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

After a short search, a spectrum that required both HBr and thiophene was detected.

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

Identification of the observed spectrum with the complex thiophene⋯HBr was reinforced through the observation of nuclear quadrupole hyperfine patterns characteristic of the presence of a Br nucleus in the molecular source of the spectrum.

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

The first transitions assigned in the rotational spectrum of thiophene⋯HBr were of the R branch type allowed by the a component of the electric dipole moment.

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

Each transition consisted of several hyperfine components arising from Br nuclear quadrupole coupling.

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

An initial set of spectroscopic constants obtained by fitting these a-type transitions was then used to predict b-type transitions, which were subsequently observed and readily assigned on the basis of their frequencies and their Br nuclear quadrupole hyperfine structure.

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

Observed frequencies and assignments of hyperfine components in a- and b-type rotational transitions of each of the three isotopomers C₄H₄S⋯H⁷⁹Br, C₄H₄S⋯H⁸¹Br and C₄H₄S⋯D⁷⁹Br are recorded in Table 1.

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

For each isotopomer, observed frequencies were fitted by a non-linear, least-squares analysis with the aid of the program SPFIT, developed and made available by Pickett.¹⁸

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

The Hamiltonian H constructed for this purpose was of the form H = H_R + H_Q + H_SR where H_R is the rotational energy operator for a semi-rigid asymmetric rotor and H_Q = –¹/₆Q_Br:▽E_Br is the operator describing the energy of interaction of the Br nuclear electric quadrupole moment Q_Br with the electric field gradient ▽E_Br at Br.

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

The weaker, magnetic mechanism for coupling I and J, namely Br spin–rotation coupling, contributed in a minor but just detectable way to observed frequencies and hence the appropriate operator H_SR = I·M·J, where M is the familiar spin–rotation coupling tensor, was added to the Hamiltonian, as shown in eqn. (1).

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

The matrix of H in the coupled basis I + J = F was diagonalised in blocks of the quantum number F.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

The forms of the elements of H_R, H_Q and H_SR in this basis are well known.

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

For H_R, the Watson A reduction¹⁹ was employed.

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

The spectroscopic constants obtained in the final cycle of the fit are given in Table 2 for each isotopomer investigated.

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

It should be noted from Table 2 that only three independent components of the Br nuclear quadrupole hyperfine coupling tensor χ_αβ = eQ∂²V/∂α∂β (namely, χ_aa, χ_bb–χ_cc, and χ_ab) were required to fit the spectra with a rms error similar in magnitude to the estimated error of frequency measurement.

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

Values of the other two off-diagonal elements χ_ac and χ_bc were not determinable; attempts to fit them gave values near to zero and smaller in magnitude than their associated standard errors.

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

An equilibrium geometry of C_2v symmetry in which the HBr molecule forms a hydrogen bond to the sulfur atom and lies along the C₂ axis of the thiophene sub-unit is immediately precluded by the fact that χ_ab is non-zero and by the observation of b-type transitions.

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

In that case, all three off-diagonal elements χ_ab, χ_ac and χ_bc of the Br hyperfine coupling tensor in the principal inertial axis system would be zero, as would the component µ_b of the electric dipole moment.

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

The contribution from Br spin–rotation coupling to the observed frequencies allowed only the diagonal components M_bb of the tensor M to be determined.

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

Only quartic centrifugal distortion constants were necessary to give satisfactory rms deviations σ of the fits.

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

It should be noted that four of the five quartic distortion constants had the expected positive sign but that Δ_K is negative.

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

This is a real effect, true for all three isotopomers investigated.

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

Residuals Δν = ν_obs – ν_calc from the final cycles of the least-squares fits are included in Table 1 while the rms deviations σ of the fits are given in Table 2.

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

The values of σ for all three isotopomers are close to the estimated error of frequency measurement (2 kHz).

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

Symmetry of thiophene⋯HBr

The magnitude of the rotational constant A₀ in each of the three isotopomers of thiophene⋯HBr investigated is such that a geometry of C_2v symmetry for the complex can be ruled out immediately.

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

If the HBr sub-unit were to lie along the C₂ axis of thiophene, A₀ of the complex should be almost identical in value to that of the free thiophene molecule.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

A comparison of the values of the A₀ for each of the isotopomers of the complex (Table 2) with that of thiophene²⁰ (Table 3) shows that the change in A₀ is large and clearly HBr cannot lie along the C₂ axis of thiophene.

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

Thus, the highest symmetry that thiophene⋯HBr can have is that associated with the point group C_s.

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

Such a conclusion is consistent with the observation of b-type transitions and a non-zero value of the component χ_ab of the Br nuclear quadrupole coupling tensor.

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

We also note from Table 2 that the change in the rotational constant A₀ when either ⁸¹Br or D is substituted in the HBr sub-unit of the isotopomer thiophene⋯H⁷⁹Br is less than 2 MHz in each case.

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

This observation means that the HBr sub-unit must lie almost along the principal inertia axis a of the complex.

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

Moreover, the A₀ values of these isotopomers are not very different from the rotational constant C₀ of free thiophene²⁰ (see Tables 2 and 3).

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

Therefore, in the complex, the Br atom sits on or close to the local c axis of the free thiophene molecule.

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

Support for the assignment of a geometry of C_s symmetry, in which the HBr subunit lies in the molecular symmetry plane, comes from the values of the planar moment P_c of the three isotopomers of thiophene⋯HBr investigated.

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

P_c is defined in eqn. (2) and depends only on the principal-axis co-ordinates c_i of the atoms i.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod9

2P_c = –(I_a + I_b – I_c) = 2∑m_ic2iValues of P_c for each of the isotopomers of the complex are included in Table 2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod9

We note that this quantity is essentially invariant among thiophene⋯H⁷⁹Br, thiophene⋯H⁸¹Br and thiophene⋯D⁷⁹Br.

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

Moreover, P_c is similar in magnitude to the corresponding planar moment P_b of thiophene itself (see Table 3).

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

The invariance of P_c to isotopic substitution in the HBr subunit provides strong evidence that HBr lies in the ab principal inertial plane and that this plane is the molecular symmetry plane.

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

The near-identity of P_c of the complex and P_b of thiophene indicates that the geometry of thiophene is not significantly perturbed by complex formation.

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

The observation of smaller changes in the rotational constants B₀ and C₀ between the H⁷⁹Br and D⁷⁹Br isotopomers than between the H⁷⁹Br and H⁸¹Br isotopomers (see Table 2) suggests (but does not prove-see below) that the H atom of HBr lies closer to the thiophene subunit than does Br.

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

However, it is advisable to be cautious in using D substitution to locate the H atom of HBr within the complex.

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

This atom will necessarily contribute little to the equilibrium principal moments of inertia, and hence to the equilibrium rotational constants.

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

On the other hand, the large changes in zero-point motion that accompany D for H substitution at the hydrogen-bond H position in weakly bound complexes tend to increase the rotational constants.

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

Accordingly, the changes in the rotational constants of thiophene⋯H⁷⁹Br when D is substituted for the H in the HBr sub-unit carry little information about the exact location of this atom within the complex.

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

Some progress with the location of the H atom is possible via the value of the off-diagonal element χ_ab of the Br nuclear quadrupole coupling tensor, as discussed in Section 3.3

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

Quantitative geometry of thiophene⋯HBr

The qualitative model of the complex proposed in the preceding section has the Br atom lying on or close to the c axis of the thiophene sub-unit and the H atom of HBr lying between the face of the thiophene ring and the Br atom.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

The geometry is therefore of the face-on type shown in Fig. 2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod10

To determine the geometry of thiophene⋯HBr, we shall assume that thiophene²⁰ and hydrogen bromide²¹ are negligibly changed in geometry (the free-molecule geometries are given in Table 3 for convenience) when they are subsumed into the complex.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

In that approximation, the quantitative geometry of the complex may be described by means of the values of three quantities φ, θ and r(S⋯H), as defined in Fig. 2.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod10

The angle φ, the angular deviation θ of the S⋯H–Br nuclei from collinearity and the distance r(S⋯H) were then determined in a non-linear least-squares fit to the nine principal moments of inertia of the isotopomers C₄H₄S⋯H⁷⁹Br, C₄H₄S⋯H⁸¹Br and C₄H₄S⋯D⁷⁹Br as follows.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod11

In view of the difficulty in locating the H atom of HBr through its contributions to the rotational constants, as discussed in Section 3.2, an alternative method of establishing the position of this atom was necessary.

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

It was demonstrated some time ago^22,23 that a good approximation to the value of the angle α_az between the a axis and the HX internuclear axis z in a complex of C_s symmetry, such as thiophene⋯HBr, is given in terms of the components of the Br nuclear quadrupole coupling tensor χ_αβ by the expression α_az = ½ tan^–1{–2χ_ab/(χ_aa – χ_bb)}If the distance r(H–Br) is unchanged from the free molecule, it follows that the angle α_az provides the location of H, once the position of Br is fixed by the principal moments of inertia.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod12

The necessary values of α_az calculated from eqn. (3) for each of the three isotopomers of thiophene⋯HBr investigated are given in Table 4.

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

100

The angle α_az was used in the least-squares fit in the following way.