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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.

The microwave spectrum of phthalan has been observed in the frequency range of 18–70 GHz. The spectra of the ground and ten excited vibrational states have been assigned and rotational and quartic centrifugal distortion constants have been obtained from semirigid rotor fits to the observed line frequencies. The microwave spectrum is not consistent with either the C2v equilibrium conformation deduced from the far infrared spectrum or the C2 equilibrium conformation obtained from the single vibronic level fluorescence spectra. The microwave data indicates that the bending of the five membered ring is a very anharmonic vibration with a double minimum potential function with a low barrier and that phthalan therefore has a Cs equilibrium conformation. The previous spectroscopic data can be reinterpreted in a manner consistent with this and gives a barrier height of 38 cm-1. The twisting of the five membered ring is a reasonably harmonic vibration and its fundamental wave-number is ∼ 180 cm-1. The microwave dat...

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 conformation and ring-puckering vibration of 2-methoxyloxetane have been studied using microwave spectroscopy and ab initio computations. The microwave spectra of the ground and first four excited states of the ring-puckering vibration have been observed in the frequency range 8–40 GHz and the rotational and quadratic centrifugal distortion constants have been determined. Vibrational energy separations have been obtained from relative intensity measurements. The electric dipole moment (in D) of the ground vibrational state has been determined from Stark effect measurements as μ a = 0.0187 (5), μ b = 1.852 (2), μ c = 0.08 (3), and μ t = 1.854 (3). The vibrational energy separations and the vibrational dependence of the rotational constants suggest that the ring-puckering vibrational has an asymmetric single minimum potential function and also gave a revised assignment of the far infrared spectrum (J. Mol. Struct. 56 (1979) 157). A combined fit to the rotational constants, vibrational energy separations and far infrared vibrational frequencies has been used to determine a reduced potential for the ring puckering vibration. The partial derivatives of the rotational constants with respect to the reduced ring-puckering coordinate show that the equilibrium conformation has the methyl group in the equatorial position. Ab initio computations of the ring-puckering potential function have been made using 6-31G * orbitals and full geometry optimization. These computations also show the molecule to have an asymmetric single minimum potential function with an equatorial equilibrium conformation.

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.

Stark effect measurements in trimethylene sulphone have been analysed to give a value of 0.23 D for the μb electric dipole transition moment connecting the v=O and v=1 states of the ring-puckering vibration. The μa component of the electric dipole moment has been determined in the v=0 to v=3 states of this vibration.

The microwave spectrum of 3-methyloxetane has been observed in the frequency range 12.4–40.0 GHz in order to investigate the conformational behaviour of this molecule. The spectra of the ground vibrational state, the first five excited states of the ring-puckering vibration and four other excited vibrational states have been assigned and their rotational and quartic centrifugal distortion constants have been obtained. The ring-puckering vibrational energy separations obtained from microwave relative intensity measurements do not appear to be consistent with those predicted from a ring-puckering potential function obtained from far infrared spectroscopy. A revised assignment of the far infrared spectrum gives a potential function consistent with the microwave relative intensity measurements. This potential function is preferred because of its superior ability to account for the ring-puckering vibrational dependence of the rotational and centrifugal distortion constants. Although this is a slightly asymmetric double minimum potential function, the top of the barrier lies below the lowest vibrational level and therefore in its ground vibrational state 3-methyloxetane effectively has a planar ring. The four excited states which do not belong to the ring-puckering progression based on the ground vibrational state are assigned to a ring-puckering progression based on the first excited state of the methyl torsion. The first two members of this progression show relatively large changes in their rotational constants relative to the ground vibrational state indicating a strong interaction between the ring-puckering and methyl torsion vibrations. The electric dipole moment has been determined from Stark effect measurements as μa = 1.910 (5) D, μc= 0.77 (2) D and μ = 2.06 (1) D for the ground vibrational state. The μa component and the total electric dipole moment decrease with excitation of the ring-puckering vibration.

A two-dimensional ring puckering potential energy surface has been derived for ethylene carbonate using ab initio and flexible model computations. The surface is expressed as a mixed Taylor—Fourier series expansion of the generalized ring puckering coordinates ( q , φ). The ab initio molecular energies have been used to determine the degree of truncation of the series which gives an accurate representation of the surface in the region of q , φ space spanned by data from microwave spectroscopy. The ab initio geometries have been used for the kinetic energy part of the flexible model computations. By scaling the coefficients of the ab initio potential it is possible to obtain reasonably good agreement with the observed inversion splitting, fundamental puckering vibrational frequencies and most of the variation in the rotational constants with excitation of the ring puckering vibrations. This potential function has a twisted C 2 symmetry equilibrium conformation with a puckering amplitude of 27 pm. The lowest energy path to ring inversion passes through the planar ring conformation where there is a barrier of 2.7 kJ mol − (229 cm −1 ).

The ring puckering in butadiene sulphone has been investigated using microwave spectroscopy and ab initio computations. Microwave spectra of the ground and first eight excited states of the ring‐puckering vibration have been observed. A two state analysis of the vibration–rotation interaction for the v=0 and v=1 states gives an energy separation between these states of 4.97 (3) cm−1. This separation and the vibrational dependence of the rotational constants have been used to derive the reduced potential function V(X)=4.7(X4−6.5X2) for the ring‐puckering vibration. This potential function gives a barrier to ring inversion of 50 (11) cm−1 and an equilibrium dihedral angle of ≊20°. Ab initio computations using STO 3‐21G* orbitals and full geometry optimization give a barrier height of 88 cm−1 and an equilibrium dihedral angle of 19.2°. The ab initio computations predict structural relaxation including a rocking of the sulphone group during the ring‐puckering motion. This is supported by calculations of the v...

An attempt has been made to account for the ring-puckering vibrational dependence of the quartic centrifugal distortion constants of 3-methylthietan. The calculations indicate revised assignments for the excited vibrational states in the microwave spectrum and for some of the transitions in the far-infrared spectrum of the ring-puckering vibration. The resulting potential function has a slightly higher barrier to ring inversion for the more stable equatorial conformer and a larger energy difference between the equatorial and axial conformers than that proposed previously.

The microwave spectrum of 4-methylenecyclohexene has been observed in the frequency range 12–40 GHz and the spectra of the ground and nine excited vibrational states of a single conformer have been assigned. Relative intensity measurements give 104(27) cm −1 and 161(33) cm −1 for the two lowest vibrational frequencies. The microwave spectrum is consistent with a high barrier to ring inversion.

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.

For the past few years in the Microwave Spectroscopy Laboratory in the University of Valladolid we have concentrated on the study of ring-puckering in four and five membered ring molecules. Experimentally, we have extended our frequency coverage into the millimeter wave range in order to determine rotational and centrifugal distorsion constants with improved precision. The increased frequency coverage is also very useful in the study of Coriolis interactions which is used to determine vibrational energy separations and vibration-rotation interaction parameters. Our theoretical approach to these problems has drawn very heavily on the work of Gwinn and his collaborators on the vibrational dependence of the rotational constants and on that of Creswell and Mills for the vibrational dependence of the centrifugal distortion constants. Aspects of these problems will be discussed using molecules studied in our laboratory as examples.

The microwave spectrum of 4-methylenecyclohexene has been observed in the frequency range 12–40 GHz and the spectra of the ground and nine excited vibrational states of a single conformer have been assigned. Relative intensity measurements give 104(27) cm −1 and 161(33) cm −1 for the two lowest vibrational frequencies. The microwave spectrum is consistent with a high barrier to ring inversion.

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 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 microwave spectrum of 2,3-dihydrofuran has been reinvestigated and measurements for the ground and first five excited states of the ring puckering vibration have been extended to higher frequencies and rotational quantum numbers in order to study the vibrational dependence of the rotational and centrifugal distortion constants. The ring puckering potential function derived by Green from the far infrared spectrum does not reproduce the vibrational dependence of the rotational constants well. A slightly different potential function is derived which gives a reasonable fit both to the far infrared spectrum and the rotational constants. This changes the barrier to ring inversion from 1.00 kJ mol −1 to 1.12 kJ mol −1 . The vibrational dependence of the centrifugal distortion constants is accounted for satisfactorily by the theory developed by Creswell and Mills. An attempt to reproduce the vibrational dependence of the rotational and centrifugal distortion constants using the ring puckering potential function and a simple model for this vibration has very limited success.

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.

A re-investigation of the microwave spectra of 2,5- and 2,3-di-hydrothiophene was performed. For the 2,5 isomer lines up to J = 40 were measured for the ground and first five vibrationally excited states of the ring-puckering mode. The centrifugal distortion constants in the ground and excited states show the expected behaviour for a single mininum potential function. For 2,3-dihydrothiophene evidence was found for a Coriolis interaction between the ground vibrational state and the first excited state of the ring-puckering vibration. A preliminary analysis of this effect places the vibrational energy separation ΔE 01 at 348 ± 4 MHz.

The conformation of ethyleneurea has been investigated using microwave spectroscopy and ab initio computations. The molecule is found to have a C 2 symmetry equilibrium conformation with a non-planar ring and non-planar geometries at the nitrogen atoms. Both the inertial data and ab initio computations show that the amide hydrogen atoms occupy equatorial positions. The observation of inversion splittings on ground vibrational state μ a Q -branch lines indicates an inversion barrier of ∼6–8 kJ mol −1 . The computed energy difference between the planar ring molecule and the equilibrium conformation is 7.2 kJ mol −1 using a 6.31 G** orbital basis. A second conformer with C s symmetry is computed to have an energy of 0.2 kJ mol −1 less than that of the planar ring molecule.

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.

A reinvestigation of the microwave spectrum of N-methyl-p-fluoroaniline shows this molecule to have low lying excited vibrational states associated with the inversion and torsion of the HNCH3 group similar to N-methylaniline. The experimental data indicate a greater tendency towards a planar configuration at the nitrogen atom and a lower barrier to inversion compared to p-fluoroaniline.

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 N,N -dimethyl-4-aminopyridine has been observed in the temperature range 320–350 K. Effects due to the inversion and torsion of the NMe 2 group have been observed. The results show that the barrier to inversion and degree of non planarity are somewhat lower than in N,N -dimethylaniline. Some details of the computer controlled system used to obtain low resolution microwave spectra are given.

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 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 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 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.

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.

The14N and35Cl nuclear quadrupole coupling constants of the14N32S35Cl isotopic species of thiazyl chloride have been determined from an analysis of the hyperfine structure of sixμa andμb transitions in the microwave spectrum as (in MHz)χaa(35CL) = −39.00(3),χbb(35CL) = 23.84(4),χaa(14N) = −1.42(6),χbb(14N) = −4.16(6).

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 3-cyclohexen-1-one has been observed in the frequency range 18–40 GHz and transitions for the ground and 19 excited vibrational states have been assigned. Relative intensity measurements give the frequency of the two lowest vibrations as 74 and 147 cm−1. The molecule has a nonplanar ring and the molecular structure obtained from a molecular mechanics computation gives rotational constants in reasonable agreement with those obtained from the spectra.

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.