The microwave spectrum of the trans and gauche isomers of the normal and two isotopic species (CH3CH2SD and CH2DCH2SH) of ethyl mercaptan has been studied. Identification of the spectrum of the trans isomer of the normal and sym-CH2DCH2SH molecules has been extended up to J=19, enabling preliminary centrifugal distortion constants to be determined. From a and c dipole transitions full sets of rotational constants of the gauche isomer of all species have been determined. For the normal molecule they are A=28 746.37 MHz, B=5294.85 MHz, and C=4846.96 MHz. The tunneling frequency of the thiol top between its two equivalent gauche configurations has been measured directly for the normal species (1754.09 MHz), sym-CH2DCH2SH (1790.05 MHz), and CH3CH2SD (70.40 MHz). Identification has been made of excited thiol torsion satellites in both isomers. Dipole moments of all configurations for each species have been determined using second-order Stark effects. For the trans isomer of the normal molecule they are μa=1.06±0.03 D, μb=1.17±0.03 D,μc=0, μ=1.58±0.04 D; for the gauche isomer of the normal molecule they are μa=1.48±0.02 D, μb=0.19±0.10 D, μc=0.59±0.02 D, μ=1.61±0.05 D. An approximate structure, which has been fit to the data, indicates that the thiol top rotates about an axis with the CS bond tilted away from the axis by 3.16 °±0.05 °, while & CSH=95.23 °±0.50 ° and the angle of the axis with the CC bond is 111.71 °±0.10 °. The barrier to methyl internal rotation in the trans isomer was found to be 1156±30 cm−1. Splittings observed in the gauche ground state of the normal and sym-CH2DCH2SH molecules have been effectively reproduced on the basis of structural effects and tunneling. Relative intensity measurements show the gauche isomer to be lower in energy with the difference between the lowest states (trans − gauche) equal to 142±15 cm−1 for the normal species and 138±30 cm−1 for CH3CH2SD. Using the measured energy differences of the ground and excited states, the first three Fourier coefficients of the thiol torsional potential were found to be V1 = −65 cm−1, V2 = −135 cm−1, and V3 = 440 cm−1. Evidence of coupling between the methyl and thiol internal rotors was also investigated.

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The nuclear electric quadrupole hyperfme structure in the vibrational ground state of cis -propylene imine has been measured. The quadrupole coupling constants of the 14N nucleus with respect to the principal inertial axes of the molecule were found to be Xaa = -1.14 ± 0.06, Xbb = + 2.01 ± 0.05, and Xcc = -0.88 ± 0.07 MHz.