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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 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 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.

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 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.

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 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.