David G. Lister

Direct l doublet transitions for the 0110 state of cyanogen iodide

Abstract Direct l doublet transitions for the 01 1 0 state of 127 I 12 C 14 N and 127 I 13 C 14 N were observed between 4 and 25 GHz using a computer-controlled Stark modulation spectrometer. Values of the l doubling constants and the asymmetry parameters of the 127 I nuclear quadrupole coupling tensors were derived. Some details of the construction of the spectrometer are also given.

Non-reactive interaction of ammonia and molecular chlorine: rotational spectrum of the ‘charge-transfer’ complex H3N⋯Cl2

The ground-state rotational spectra of five isotopomers of the ‘charge-transfer’ complex H3N⋯Cl2 have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The complex has C3v symmetry with the nuclei in the order H3N⋯Cl—Cl. A detailed analysis of the Cl nuclear quadrupole hyperfine structure in transitions of H315N⋯35Cl2, H315N⋯35Cl37Cl, and H315N⋯37Cl35Cl gave the rotational constant, Bo, the centrifugal distortion constants Dj and Djk, and the nuclear quadrupole coupling constants χ(Cli) and χ(Clo)(i = inner, o = outer) in each case. The distance r(N⋯Cli) was obtained by an rs-type method and an ro-type method and lies in the range 2.73 ± 0.03 A. A detailed analysis that allowed for bond shrinkage on isotopic substitution in the 35Cl2 subunit of H315N⋯35Cl2 gave the rs-type coordinates of Cli and Clo and hence the distance rs(Cl—Cl)= 2.00 A. This value is very close to that in free Cl2 and indicates only a slight perturbation of this subunit when the complex is formed. The relatively small intermolecular stretching force constant, kσ= 12.71(3) N m–1 determined from DJ and the weak perturbation of χ(Cli) and χ(Clo) from the value in free Cl2, reinforce this conclusion. The observed difference χ(35Clo)–χ(35Cli)= 13.99 MHz can be interpreted in terms of a transfer of 0.064e from Cli to Clo on formation of H315N⋯35Cl2. It seems likely that the molecular interaction is mainly electrostatic in origin and charge-transfer effects are small.

The microwave spectrum of N,N-dimethylaniline

Abstract The microwave spectrum of N,N -dimethylaniline shows the presence of low vibrational states associated with the inversion of the NMe 2 group and its torsion about the N-aryl bond. The μ a R -branch lines of the ground and First inversion states show small deviations from rigid-rotor behaviour which are consistent with an energy difference of a few cm −1 between these states. The quantities Δ = I a - I b + I c and Δ c = I a + I b - I c indicate values for the CH 3 NCH 3 angle, α, of 114° and for the angle between the ring plane and that containing the two N-CH 3 bonds, φ, of 27°. The barrier to inversion is in the range 1–3 kJ mol −1

Infrared and microwave spectra, molecular conformation and electric dipole moment of methyl thiolformate

Abstract The microwave spectrum (41-10 GHz) and the infrared spectrum (4000-50 cm −1 ) of methyl thiolformate have been obtained and analyzed. The spectra are consistent with a single molecular conformation having a planar array of heavy atoms and with the alkyl group cis to the carbonyl group. The measured rotational constants are: A , 11042.22 MHz; B , 5118.27 MHz; C, 3562.03 MHz (κ = −0.5839). No internal rotation doublets were observed in the microwave spectrum for the ground vibrational state, which implies that the barrier hindering internal rotation of the methyl group is either much larger or much smaller than the corresponding value for methyl formate. If the former is true then a lower limit of 10.5 kJ mol −1 may be placed on the barrier height. The dipole moment of methyl thiolformate was measured using the Stark effect to be 1.58 ± 0.05 Debyes ( μ A = 1.52 D; μ B = 0.43 D) for the vapor, and for dilute solutions in benzene at 295 K the value of 1.6 ± 0.1 D was found from capacitance measurements. SCF computations using minimal basis sets of STO 3G atomic orbitals and extended basis sets of STO 4.31G atomic orbitals have been carried out for methyl thiolformate and methyl formate. Energy differences between rotational isomers and estimates of barrier heights are given together with the calculated dipole moments.

The microwave spectrum and large-amplitude vibrations of N-methylaniline

Abstract Microwave spectra of the normal and amine-deuterated isotopic species of N -methyl-aniline in the ground and some low-lying excited vibrational states have been observed. The inertial defect indicates that the dihedral angle of the N-CH 3 , bond with respect to the ring plane is somewhat less than that for the N—H bonds in aniline. The position of the amino-hydrogen atom is very poorly determined by the isotopic substitution method because of large zero-point effects. The excited vibrational states are consistent with a double-minimum potential for the inversion of the HNCH 3 group and there is some evidence for a lower barrier than in aniline. The excited states of the HNCH 3 torsion indicate a barrier in the range 8 2 −1 to the internal rotation of this group. No splitting of the ground-state lines attributable to the torsion of the methyl group has been observed, which implies a barrier of V 3 > 8 kJ mol −1 .

The centimeter and millimeter microwave spectra of butadiene sulfone and α,α′-D4 butadiene sulfone

Abstract Microwave measurements on the ground and first eight excited states of the ring-puckering vibration of butadiene sulfone have been extended to millimeter wavelengths. The microwave spectra of the same vibrational states of α,α′-D 4 butadiene sulfone have been observed. For both isotopomers the Coriolis interaction between the v = 0 and v = 1 states has been analyzed to give the energy separation between these two states. These data and the variation of the rotational constants have been used to derive reduced potential functions for the ring-puckering vibration. The barrier to ring inversion is 49(2) cm −1 for butadiene sulfone and 44(2) cm −1 for the α,α′-D 4 isotopomer. The ring-puckering vibrational dependence of the quartic centrifugal distortion constants, including a small dependence of Δ J and δ J , has been accounted for.

Rotational isomerism, barrier to internal rotation and electric dipole moment of acrylic acid by microwave spectroscopy

Microwave spectra have been observed for the normal and acid deuterated isotopic species of the s-cis and s-trans rotamers of acrylic acid in the ground and a number of excited torsional states. Both rotameric forms are planar with the hydroxyl group adopting the cis conformation relative to the carbonyl group. The dipole moments determined from Stark effect measurements are [graphic omitted].The difference in zero point vibrational energy between the two rotamers and the 1 â†� 0 torsional frequencies have been determined from relative intensity measurements as E°s-trans—E°s-cis= 58 ± 20 cm–1, νs-cis= 105 ± 20 cm–1, νs-trans= 95 ± 20 cm–1. The potential function hindering the internal rotation is essentially two-fold with a barrier of 1340 ± 500 cm–1(16.0 ± 6.0 kJ mol–1) at an angle of 89 ± 4° from the s-cis configuration.

Microwave spectrum and ab initio computations for ethylene carbonate. Part 1.—Conformation and ring inversion

The conformation and ring inversion in ethylene carbonate have been investigated using microwave spectroscopy and ab initio computations. The vibrational satellite spectra of the normal isotopic species of ethylene carbonate and the rotational constants of Backvall et al.(Tetrahedron Lett. 1980, 21, 4985) for cis- and trans-[1,2-2H2]ethylene carbonate show the molecule to have a twisted C2 equilibrium conformation. The vibrational satellite spectra have been analysed in terms of independent ring-bending and twisting vibration. A potential function for the twisting vibration derived from the variation of the rotational constants with vibrational state, and vibrational energy separations obtained from relative intensities and Coriolis perturbations give an equilibrium twist angle of 19° and a barrier to ring inversion of 2.8 kJ mol–1. The twist angle has also been obtained from inertial data, and a value of 15° is obtained using two methods of calculation. Ab initio computations of the ring-puckering potential-energy surface have been made using STO-3G and STO-3-21 G orbitals and complete geometry optimization. The STO-3G computations predict a planar C2ν equilibrium geometry, but with the twisting vibration being lower in frequency and more anharmonic than the bending vibration. The STO-3-21G computations predict a C2 conformation with an equilibrium twist angle of 14° and barrier to inversion through the planar ring conformation of 1.1 kJ mol–1. The computed barrier to ring inversion by pseudorotation is 14.3 kJ mol–1.

IS PYRIDINIUM HYDROCHLORIDE A SIMPLE HYDROGEN-BONDED COMPLEX C5H5N...HCL OR AN ION PAIR C5H5NH+...CL- IN THE GAS PHASE ? AN ANSWER FROM ITS ROTATIONAL SPECTRUM

The ground-state rotational spectra of the three isotopomers C5H514N···H35Cl, C5H514N···H37Cl and C5H514N···D35Cl of a complex formed by pyridine with hydrogen chloride have been observed by using a fast-mixing nozzle in combination with a pulsed-nozzle, FT microwave spectrometer. Rotational constants A0, B0, C0, centrifugal distortion constants, ΔJ, ΔJK, δJ, δK and nuclear quadrupole coupling constants χaa(A) and χbb(A) − χcc(A) (where A = 14N or Cl) were determined in each case. A detailed interpretation of the spectroscopic constants led to the conclusion that the observed complex has a planar, C2v geometry, with the HCl subunit forming a hydrogen bond to N and lying along the C2 axis of pyridine. The nitrogen to chlorine distance was determined to be 2.999(2) A. The magnitudes of the nuclear quadrupole coupling constants χaa(A) and the intermolecular stretching force constants kσ in comparison with those expected in the hydrogen-bond C5H5N···HCl and ion-pair C5H5NH+···Cl− limits show that the extent of proton transfer from HCl to pyridine is small.

Rotational spectrum and molecular properties of the dinitrogen–chlorine monofluoride complex

The ground-state rotational spectra of the three isotopomers 14N2⋯35ClF, 15N2⋯35ClF and 15N2⋯37ClF of a complex formed by dinitrogen and chlorine monofluoride have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. The spectroscopic constants B0, DJ, χaa(A)(A =14Ni, 14No or Cl) and Mbb(Cl) are reported. The complex is shown to have a linear (or nearly linear) arrangement NoNi⋯ClF of the nuclei in the equilibrium conformation with r(Ni⋯Cl)= 2.920(2)A. The intermolecular stretching force constant, kσ= 5.00(5) N m–1, is implied by the centrifugal distortion constant DJ. Interpretation of the nuclear quadrupole coupling constants χaa(A) leads to the oscillation angles θav= cos–1〈cos2θ〉1/2= 17.8(5)° and ϕav= cos–1〈cos2θ〉1/2= 10(3)° for the N2 and ClF subunits, respectively. Additionally, the diffence χaa(Ni)–χaa(No) leads, on the basis of a simple model, to the conclusion that the polarisation of N2 attending complex formation is equivalent to the transfer of a fraction δ≈ 0.02 of an electronic charge from No to Ni. A comparison of the properties of four related complexes N2⋯YF and OC⋯YF, where Y = Cl or H, is presented.