Microwave spectrum of tert-butyl mercaptan

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Abstract
The spectrum of the ground vibrational state of t‐butyl mercaptan from 8 to 40 GHz has been measured and assigned using a computer controlled spectrometer. The spectrum consists of many long Q‐branch series arising from the perpendicular component of the dipole moment and also parallel R‐branch groupings at intervals of B+C. The molecule is a near prolate symmetric top with energy level splittings of several gigahertz due to the internal rotation about the C–S bond. A computer program utilizing the IAM in a form suited for the intermediate to low barrier case has been written and used to obtain a satisfactory fit of the observed spectrum to the simple rigid top‐rigid frame Hamiltonian with six adjustable parameters: A, B, C, ρ, β, and Δ0. The barrier obtained (1742 cal/mole) is significantly larger than the previously determined microwave value for methyl mercaptan and is also greater than the recent far infrared estimates for t‐butyl mercaptan. Measurements of the Stark effect for several transitions has permitted determination of the magnitudes and (because of interference effects between the internal rotation and Stark perturbations) the relative sign of the two components of the dipole moment. The direction of the dipole moment vector thus obtained is consistent with the only physically reasonable choice in this molecule.