Juan C. López

The millimetre-wave spectrum of oxetane

Abstract The millimetre-wave spectrum of oxetane in the ground state and the first seven vibrational excited states of the ring-puckering motion has been measured in the frequency range 80–250 GHz. A small vibrational dependence of the centrifugal distortion constants Δ J and δ J has been observed. This dependence is well accounted for in terms of the ring-puckering potential function and the vibrational dependence of the rotational constants. Measurements of the electric dipole moment for the ground state and the first four ring-puckering excited states are also reported.

Pseudorotation in tetrahydrofuran⋯HF heterodimer

Seven isotopomers of the hydrogen-bonded heterodimer tetrahydrofuran⋯HF have been investigated in the frequency range 6–18 GHz using molecular-beam Fourier transform microwave spectroscopy. The rotational spectra of C4H8O⋯HF and C4H8O⋯DF exhibit small tunneling splittings, which were not observed for C4D8O⋯HF and the four single 13C isotopomers. These observations analyzed in terms of symmetry considerations allow us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit of the complex and not to HF inversion. The spectroscopic parameters of the complex have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to the twisted ring form, with HF lying on the plane bisector to the COC ring angle.

Rotational spectrum, internal rotation barrier and ab initio calculations on 1‐chloro‐1‐fluoroethane

The microwave spectrum of 1‐chloro‐1‐fluoroethane (CFC 151‐a) has been studied in the frequency region 8–250 GHz using waveguide Fourier transform, Stark, and source modulation spectrometers. Accurate rotational, quartic, and sextic centrifugal distortion and quadrupole coupling constants have been obtained from a global fit for the ground, v17=1 (Cl–F skeletal bending mode), and v18=1 (CH3 torsional) vibrational states of the 35Cl isotopomer and for the ground state of the 37Cl isotopomer. The larger off‐diagonal element of the χ tensor was also determined for the 35Cl isotopomer. Assignment of the v17=1 and v18=1 states was confirmed by the presence of small A–E internal rotation splittings in the v18=1 state, in agreement with ab initio calculations, but in contradiction with previous assignment of the microwave spectrum by Thomas et al. [J. Chem. Phys. 61, 5072 (1974)]. The barrier height for the internal rotation of the methyl group was determined to be 3814(11) cal/mol, and compared with the result ...

Theoretical study of the structure and ring inversion in trimethylene sulphone

Abstract The structure and ring-puckering potential function of trimethylene sulphone have been obtained within the framework of ab initio SCF calculations. The importance of including polarization functions for the sulphur atom is verified. Computations have shown that trimethylene sulphone is a puckered four-membered ring with a double minimum potential function, which is in agreement with microwave data. Several flexible models for describing the ring-puckering motion have been tested and predicted molecular parameters are compared with experimental values.

Electric dipole moments of thietane in excited states of the ring puckering vibration: an experimental and ab initio study

Abstract The electric dipole moment in the ground state ( v p = 0) and the first five excited states ( v p = 1 … 5) of the ring puckering vibration of thietane have been determined from Stark shifts of rotational transitions. The results are: 〈0| μ a |0〉 = 1.87583(16) D, 〈1| μ a |1〉 = 1.87341(18) D, 〈2| μ a |2〉 = 1.89759(28) D, 〈3| μ a |3〉 = 1.88688(29) D, 〈4| μ a |4〉 = 1.90036(18) D, 〈5| μ a |5〉 = 1.88596(59) D. The dependence of these values on v p shows the zig-zagging behaviour typical of modes with double minimum potentials. A combined analysis of the ground and first excited states yielded also a precise value for the transition moment, 〈0| μ c |1〉 = 0.24023(49) D. A potential and electric dipole moment function has been derived from ab initio calculations, using MP2 and the 6–31G∗∗ basis set. Expectation values of the dipole and transition moments were determined from these data. Absolute values are about 5% in error, but the variation with vibrational state is reproduced excellently by the theoretical values.

The microwave spectrum of pentafluoroethane

Abstract The rotational spectrum of pentafluoroethane in the ground, first five excited states of the CF3 torsion vibration (ν18) and the first excited state of the CF2 rocking vibration (ν17) has been analyzed in the 12–72xa0GHz region using Stark modulation and waveguide Fourier transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants have been obtained for each observed vibrational state. No splittings due to the internal rotation of the CF3 have been observed. The electric dipole moment has been determined from Stark effect measurements to be μa=0.237(9), μc=1.540(6) D and μ T =1.558(6) D (μb=0 by symmetry). The results of MP2/6-311+G(2d,p) ab initio calculations on the structure of pentafluoroethane are also reported.

ROTATIONAL SPECTRUM AND STRUCTURE OF THE OXETANE-ARGON VAN DER WAALS COMPLEX

Abstract The rotational spectrum of the Van der Waals complex oxetane–argon has been observed using a molecular beam–Fourier transform microwave spectrometer in the frequency range of 5–18.5 GHz. The complex has C s symmetry with the argon atom located above the plane of oxetane, 3.57 A from the center of mass of the ring. The line joining the Ar atom with the center of mass of oxetane makes an angle of ∼10° with the normal to the oxetane plane.

The microwave spectrum of 2(5H)-thiophenone

Abstract The microwave spectrum of 2(5H)-thiophenone has been analysed in the frequency range 26.5–40 GHz. R-branch and Q-branch transitions allowed by μa and μb have been assigned for the ground state, the first four excited states of the ring-bending vibration, νB, and the first excited state of the ring-twisting mode, νT. Relative intensity measurements give the vibrational separations uB = 1 ← 0 and vT = 1 ← 0 as 136 ± 15 and 295 ± 28 cm−1, respectively. The observed value for the quantity Δ0 = I0c − I0a − I0b and the variation of the rotational constants with vB are consistent with a planar ring equilibrium configuration and an essentially quadratic potential energy function for the ring bending mode. The electric dipole moment components determined from Stark-effect measurements are μa = 4.12 ± 0.03 D and μb = 1.71 ± 0.08 D leading to a μTotal = 4.46 ± 0.05 D. A computer-controlled microwave spectrometer is also presented.

The rotational spectrum of 2-indanone

The rotational spectrum of 2-indanone has been analysed in the 26.5-40 GHz frequency region. The ground state, the first five excited states of the five-membered ring bending mode and the first excited state of the ring twisting and butterfly modes have been assigned and analysed. From the ground state inertial defect and the variation of the rotational parameters with the ring bending quantum number a strictly planar C2v equilibrium conformation has been established. A one-dimensional potential function of the quadratic-quartic type V(X) = 3.3(7) (X4 + 11.66(15)X2)cm-1 has been derived for the bending vibration.

Conformation and internal rotation in 2-nitrofuran and 2-nitrothiophene by microwave spectroscopy

Abstract Results of a microwave investigation of the molecules 2-nitrofuran and 2-nitrothiophene are reported. Transitions in the ground state, torsional excited states and out-of-plane and in-plane excited states of the bending vibration of the nitro group have been assigned. The rotational parameters are consistent with a planar equilibrium configuration for both molecules. The torsional frequency has been derived from relative intensity measurements and inertial defect values. The barrier hindering internal rotation has been calculated to be 17kJ mol−1 and 19kJ mol−1 for 2-nitrofuran and 2-introthiophene, respectively.