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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.

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.

The ground-state rotational spectra of two isotopomers, (CH 2 ) 2 32 S·HC 14 N and (CH 2 ) 2 32 S·HC 15 N, of a complex formed by thiirane with hydrogen cyanide were analysed to give rotational constants, centrifugal distortion constants and 14 N-nuclear quadrupole coupling constants. The spectroscopic constants were interpreted in terms of a geometry of the complex characterised by the angle φ = 89.2(9)° made by the S·H internuclear axis with the thiirane heavy atom plane at S and a nonlinearity θ≈43° of the S·H-C hydrogen bond. This large nonlinearity is contrasted with that in (CH 2 ) 2 O·HCN and the difference discussed in terms of a simple model of the hydrogen bond.

Ground-state rotational spectra of four isotopomers CH 3 NC…HCCH, CH 3 NC…HCCD, CH 3 NC…DCCH and CH 3 NC…DCCD of a dimer of methyl isocyanide and acetylene have been observed in a pulsed jet. The properties r (C…HC) and k σ determined from the observed spectroscopic constants are compared with the corresponding quantities for the isomer CH 3 CN…HCCH.

The rotational spectrum of the argon—thiirane van der Waals molecule has been observed using pulsed-nozzle, Fourier-transform microwave spectroscopy. Transitions due to μa and μb components of the electric dipole moment have been observed and used to derive rotational and quartic centrifugal distortion constants. The molecule has Cs symmetry with a distance R = 3.79 pm from the centre of mass of thiirane to the argon atom and an angle θ = 98° between this vector and the bisector of the CSC bond angle of the thiirane molecule. An upper limit of 10 kHz is placed on the ground vibrational state inversion frequency in Ar…S(CH2)2.

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.

The ground-state rotational spectra of the isotopomers H315⋯35Cl2, H315N ⋯37Cl35Cl, H315N ⋯35Cl37Cl, H314, N ⋯35Cl2 and H2DN ⋯35Cl2 of a transient complex of ammonia and chlorine have been observed by using a pulsed-nozle, FT imcrowave spectrometer incorporation a fast-mixing nozzle; the spectroscopic constants alliw the conclusion that the complex has C3v symmetry, with the nuclei in the order shown, and is of the outer type defined by Mulliken.

The rotational spectrum of α-angelicalactone has been analyzed in the frequency range 18.0–40.0 GHz. The internal rotation barrier of the methyl group has been determined in the ground and four vibrationally excited states from the A-E splittings. The results indicate a possible rotational between the methyl torsion and the ring puckering mode. The ground state rotational parameters are consistent with a planar ring skeleton. The dipole moment components obtained from Stark displacements are μa = 3.16(1) D and μb = 2.59(2) D with a total value of 4.08(2) D.

The electric dipole moment of ethylene carbonate has been determined from Stark-effect measurements in its microwave spectrum as 5.35 ± 0.15 D. The breakdown of second-order perturbation theory made it necessary to use a full matrix diagonalization treatment in order to obtain consistent results. Anomalous Stark effects are observed for the M= 1 and 2 components of the 303â†� 202 transition and are analysed in terms of changes in the rotational wavefunctions. The high electric dipole moment and the electronic structure of ethylene carbonate are discussed using ab initio wavefunctions.

Direct l doublet transitions have been observed for the 01 1 0 state of the 35 Cl 12 C 14 N and 37 Cl 12 C 14 N isotopic species of cyanogen chloride and the 79 Br 12 C 14 N and 81 Br 12 C 14 N isotopic species of cyanogen bromide in the frequency range 4–22 GHz. The l -doubling constants ( q 0 , q 1 , q 2 ) and the asymmetry parameters ( ηeQq z ) of the halogen nuclear quadrupole coupling tensors have been derived. The constants q 1 have been calculated form the vibrational force fields of Whiffen for the four cyanogen halides using the formulation of Watson. Agreement with the observed values is found to be 10% or better and for cyanogen fluoride and cyanogen chloride the isotopic shifts in q 1 are also well predicted.

The quadratic force field of diborane has been computed using the force method and an STO 4—31 G orbital basis.

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 microwave spectrum of 2-methyl-4,5-dihydrofuran has been observed in the frequency range 8–60 GHz. Spectra of the ground and first two states of the ring-bending vibration have been assigned. The variation of the rotational constants with vibrational state confirm the potential function derived by Carreira and Lord from the far-infrared spectrum. The molecule has a bent-ring conformation with a dihedral angle of ∼20°. A-E splittings due to the internal rotation of the methyl group have been observed and there is evidence for an interaction between the methyl torsion and the ring-puckering vibration. The electric dipole moment has been determined as μa = 0.47D, μb = 0.76D and μ = 0.90D.

Ab initio computations have been performed on aniline and N -methylaniline using standard STO-3G orbitals to analyze the inversion barrier and the degree of non planarity in the two molecules. Our analysis indicates that the lowering of thei barrier in N -methyl-aniline with respect to aniline is due to hyperconjugation.

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.

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.

Some low-J μa R-branch lines of the E torsional substate of the ground vibrational state of methyl thiolformate were assigned. Calculations using the Internal Axis Method (IAM) and the Principal Axis Method (PAM) reproduce the observed line frequencies within a few MHz. The barrier to internal rotation of the methyl group is determined to be 1.78 ± 0.03 kJ mole−1.

The microwave spectrum of N,N-dimethylaniline shows the presence of low vibrational states associated with the inversion of the NMe2 group and its torsion about the N-aryl bond. The μa 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 = Ia - Ib + Ic and Δc = Ia+Ib-Ic indicate values for the CH3NCH3 angle, α, of 114° and for the angle between the ring plane and that containing the two N-CH3 bonds, φ, of 27°. The barrier to inversion is in the range 1–3 kJ mol−1

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.

Microwave spectroscopic measurements on ethylene sulphide have been extended to allow a complete determination fo substitution structure parameters. These are similar to r o -parameters previously reported, with the exception that the C-C distance is slightly shorter at 1.484 A.

SCF computations using STO/3G orbitals have been made for methyl formate, ethyl formate and propargyl formate in order to investigate rotational isomerism in the last two molecules. The results are in good agreement with those of IR and microwave studies.

The microwave spectrum of 1,3,5-trithiane has been observed at a temperature of ca. 360 K in the frequency range 26–70 GHz. Only the spectrum of the symmetric top chair conformer is identified. Spectra of the ground vibrational state and some excited states involving the two lowest degenerate vibrations are assigned.

The microwave spectrum of N,N-dimethylaniline shows the presence of low vibrational states associated with the inversion of the NMe2 group and its torsion about the N-aryl bond. The μa 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 = Ia - Ib + Ic and Δc = Ia+Ib-Ic indicate values for the CH3NCH3 angle, α, of 114° and for the angle between the ring plane and that containing the two N-CH3 bonds, φ, of 27°. The barrier to inversion is in the range 1–3 kJ mol−1

The microwave spectrum of methyl chloroformate vapour has been analysed and found to be consistent only with a planar heavy atom skeleton with the methyl group cis to the carbonyl group. Both µa and µb lines were observed. The three-fold barrier to internal rotation of the methyl group was calculated from splittings in µb Q branch lines to be 1250 ± 30 cal mol–1(5230 ± 126 J mol–1). The 35Cl nuclear quadrupole constants were measured from the µa 2â†�1 transitions to be Xaa=–58.8 ± 0.2 MHz and Xbb= 31.1 ± 0.5 MHz.

Rotational constants are reported for 31 previously unmeasured isotopic species of methylmercury chloride, bromide and iodide. The constants for CH3HgI do not substantiate earlier measurements, and a new set of internally consistent constants for 15 isotopic species of methylmercury iodide is presented. Assignment of the spectra of the bromide and iodide is facilitated by the smallness of the effects of excitation of the bending mode, νs on the rotational constant.Substitution lengths for the C—Hg bonds in these substances are derived for the first time. The accuracy of the Hg placement is insufficient for complete methyl group structures to be derived from the constants for CD3 species, but a range of acceptable methyl group structures can be defined for the chloride and bromide, and an accurate Hg—I distance in methyl mercury iodide can be derived from first moment considerations. These structures are compared with those derived by least squares fitting procedures using arrays of rotational constants.

The microwave and infrared spectra of propargyl formate have been studied in the respective ranges 40–15 GHz and 4 000–50 cm–1. Evidence is presented for the coexistence of rotational isomers, one of which (probably the more stable) is shown to have a planar arrangement of heavy atoms with the alkyl group s-cis with respect to the carbonyl bond and with the acetylenic group directed away (trans) from the carbonyl group. The dipole moment was measured in dilute benzene solution at 297 K to be 1.94 ± 0.02 D.

The microwave spectra of three isotopic species (C6H511B35Cl2, C6H511B35Cl37Cl and C6H510B35Cl2) of phenylboron dichloride have been observed. The spectra of the first four excited torsional states of C6H511B35Cl2 and the first two excited torsional states of C6H511B35Cl37Cl have been assigned, the variation of the inertial defect with torsional quantum number gives a torsional frequency of 37 ± 2 cm–1 and a barrier to internal rotation of V2= 13.6 ± 0.8 kJ mol –1. The rs structure of the BCl2 group is rB–Cl= 176.0 ± 0.8 pm and ClBCl = 121.7 ± 0.5°. With the assumption of regular benzene ring (rC–C= 139.7 pm and rC–H= 108.4 pm) a least squares fitting procedure to the observed rotational constants gives the following structural parameters rB–Cl= 176.0 ± 0.8 pm, rB–C= 155.6 ± 1.4 pm and ClCl = 121.8 ± 0.4°.

The pyrolysis products of some aromatic amines have been identified using microwave spectroscopy. Aniline is found to produce 1-cyanocyclopenta-1,3-diene, N-methylaniline a mixture of 1-cyanocy-clopenta-1,3-diene and benzonitrile, N,N-dimethylaniline produces benzonitrile, and N,N-dimethylpyridine-2-amine produces pyridine. In the pyrolysis of N-methylaniline the proportion of 1-cyanocyclopenta-1,3-diene to benzonitrile is estimated to be 4 : 1 from the integrated relative intensities of the low-resolution microwave band spectra of the two substances.

Microwave spectra of the ground and first excited vibrational states of the normal and amine deuterated species of 3-aminopyridine and 4-aminopyridine have been assigned and the structure of the amine group has been determined in these two molecules. The structure of the amine group in the aminopyridines has been derived from ab initio molecular orbital calculations using single determinant molecular orbital wave functions constructed from standard minimal basis sets of contracted Gaussian orbitals. The experimental and theoretical results are found to be in broad agreement with each other.

Microwave spectra of the normal and amine deuterated isotopic species of m-fluoroaniline have been observed. The structure of the amine group determined using the normal isotopic species as the parent molecule is rN–H= 1.00 A(assumed), ∠ HNH = 115.0°, ϕ= 36.2°(ϕ is the angle between the bisector of the HNH angle and the extension of the CN bond) and is similar to that in aniline. The structure of the amine group determined using m-fluoroaniline-ND2 as the parent molecule is rN–H= 1.00 A(assumed), ∠ HNH = 117.5°, ϕ= 42.0°.

The microwave spectrum of methyl chloroformate vapour has been analysed and found to be consistent only with a planar heavy atom skeleton with the methyl group cis to the carbonyl group. Both µa and µb lines were observed. The three-fold barrier to internal rotation of the methyl group was calculated from splittings in µb Q branch lines to be 1250 ± 30 cal mol–1(5230 ± 126 J mol–1). The 35Cl nuclear quadrupole constants were measured from the µa 2â†�1 transitions to be Xaa=–58.8 ± 0.2 MHz and Xbb= 31.1 ± 0.5 MHz.