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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 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 γ-butyrolactone has been reinvestigated in order to determine small vibrational splittings associated with the ring inversion. For the ground vibrational state the inversion splitting has been measured directly as 30.3 MHz by the observation of the µc rotation–inversion transitions. The inversion splitting in the first excited state of the two ring puckering vibrations has been determined through analysis of a type Coriolis interactions. In the first excited state of the 148 cm–1 vibration the inversion splitting is 609 MHz while for the 225 cm–1 it is 2930 MHz. These inversion splittings indicate that the 148 cm–1 vibration is associated with a single minimum potential function and the 225 cm–1 vibration with a double minimum potential function but that there is some interaction between the two vibrations. A one-dimensional reduced potential function has been derived for the 225 cm–1 vibration and gives a barrier to ring inversion of 8.0 kJ mol–1(670 cm–1).

The conformation and ring inversion of γ-butyrolactone have been studied using ab initio and flexible model computations. The potential energy surface has been computed using STO-3-21G orbitals and complete geometry optimization as a function of the ring puckering coordinates. The computed equilibrium geometry has a ring in which all of the atoms with the exception of the β carbon are nearly coplanar. The computed inversion barrier is 6.1 kJ mol–1. Analytical expressions, as functions of the correct symmetry of the ring puckering coordinates, have been obtained for the potential energy surface and internal coordinates by least-squares fits to the computed molecular energies and geometries. These data have been used as the input for flexible model computations of the vibrational frequencies, inversion splittings and rotational constants. By using a simple uniform scaling of the potential energy surface good agreement with the observed vibrational frequencies and inversion splittings is obtained for a barrier to ring inversion of 7.9 kJ mol–1. The changes in the rotational constants are reasonably well reproduced for excitation of the vibration which is associated with a double minimum potential and less successfully for the other ring puckering vibration.

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 microwave spectrum of 5,6-dihydro-2H-thiopyran has been observed between 12.4 and 40 GHz. Rotational and quartic centrifugal distortion constants have been obtained for the ground and seven excited vibrational states. The fundamental wavenumbers of the two lowest vibrations have been obtained from relative intensity measurements as ν36= 134 (12) cm–1 and ν35= 219 (18) cm–1. The electric dipole moment of the ground vibrational state has been determined from Stark effect measurements as [in D, 1 D (Debye)≈ 3.335 64 × 10–30 C m], µa= 1.35 (1), µb⩽ 0.1, µc= 0.74 (2), with a total dipole moment of µ= 1.54 (2). The rotational constants and relatively large µc component of the electric dipole moment are consistent with 5,6-dihydro-2H-thiopyran having a twisted or half-chair conformation. A molecular structure has been derived from the ground vibrational state rotational constants and planar moments of inertia using the method of predicate observations. The equilibrium twist angle of the ring is determined as τ= 32.5 (1.5)°.

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.

The microwave spectra of adamantan-1-ol and [2H1]adamantan-1-ol have been observed in the frequency range 8–40 GHz at temperatures of 340–350 K. The spectra consist of strong series of µa R-branch bands and in the case of adamantan-1-ol a weaker series of µb Q-branch bands. The separation of the µb Q-branch bands is almost twice that expected for a rigid rotor. An analysis using the internal axis method to treat the internal rotation of the hydroxy group and which assumes the molecule to be a symmetric rotor gives the approximate formula, v=(A–text-decoration:overlineB+w)(2K+ 1), for the observed µb Q-branch bands. The parameter w arises from the internal rotation part of the Hamiltonian. The barrier to internal rotation in adamantan-1-ol is estimated to be V3= 4.9 (4) kJ mol–1[410 (30) cm–1]. The error in V3 arises largely from the uncertainty in the moment of inertia of the hydroxy group about the internal rotation axis.

The microwave spectra of 1-adamantamine, [2H1]1-adamantamine and [2H2]1-adamantamine have been observed at room temperature in the frequency range 8–40 GHz. The spectra consist of both µa R-branch and µc Q-branch bands. The µa R-branch bands of the three isotopic species appear as doublets and this is attributed to the inversion of the amine group. A simple treatment of the internal rotation and inversion in which the molecule is regarded as a symmetric rotor shows that six series of ΔK=+1 µc Q-branch bands should be associated with the ground vibrational state. The band frequencies are shown to follow approximately the formulae v=a(2K+1) or v=a(2K+1)+b, where the constants a are the sum of the rotational constant A–text-decoration:overlineB and contributions from the internal motions. The formulae are verified experimentally for [2H2]1-adamantamine. The barrier to internal rotation in [2H2]1-adamantamine is determined to be V3= 9.0 (9) kJ mol–1[750 (70) cm–1] and the inversion splitting to be Δ°i= 1250 (30) MHz.

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.

The microwave spectra of the ground and five excited states of the ring puckering vibration of 2,5-dihydrothiophen S,S-Dioxide show this vibration to have a double minimum potential function with a barrier of ca. 100 cm–1 to planarity of the ring.

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 the ground and first three excited states of the inversion vibration of 2-aminopyrimidine-NH2 and for the ground and first excited inversion state of 2-aminopyrimidine-ND2 have been assigned. Vibration rotation interactions prevent an accurate determination of the amine group geometry but there is considerable evidence to indicate a greater tendency towards molecular planarity in 2-aminopyrimidine than in a number of other amine molecules. SCF computations using STO/3G orbitals support this suggestion.

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 three excited torsional states of C6H511BF2 and for the ground and first two excited torsional states of C6H510BF2 have been assigned. The variation of the inertial defect with torsional quantum number shows the molecule to be planar. The torsional frequency has been determined as ν= 49 cm–1 and the barrier to internal rotation as Y2= 1114 cm–1(13.3 kJ mol–1). With the assumption of a regular benzene ring (rC—C= 1.397 A and rC—H= 1.084 A) the geometry of the rest of the molecule is calculated to be rC—B= 1.551 A, rB—F= 1.330 A and FBF = 116.0°.

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 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 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.

The microwave spectrum of the normal isotopic species of p-fluoroanisole has been measured for the ground vibrational state and for the first excited torsional state about the Ar—O bond. The rigid rotor rotational constants were found to be: v= 0, A= 4883.87 MHz, B= 972.475 MHz, C= 815.540 MHz; v= 1, A= 4868.62 MHz, B= 972.675 MHz, C= 816.488 MHz. The values of Δ(Ia+Ib–Ic) for the two states (Δ°= 3.476 a.m.u. A2, Δ1= 4.413 a.m.u. A2) were found to be consistent with a planar framework of heavy atoms.