Microwave Spectrum, Dipole Moment, Structure, and Internal Rotation of Dimethyl Sulfide

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Abstract
The microwave spectra of five isotopic species of dimethyl sulfide are reported. Changes in rotational constants with isotopic substitution yield the following structural parameters: CS 1.802 A; CSC=98°52′; CH 1.901 A; HCH=109°34′; 2θ=104°22′, where 2θ is the angle between the symmetry axes of the methyl groups. The equilibrium conformation of both methyl groups is the staggered one, i.e., staggered with respect to the adjacent CS bond axis. From Stark effect measurements the dipole moment of dimethyl sulfide is found to be 1.50±0.01 debye. Fine structure in the ground-state rotational spectrum of (CH3)2S and an excited torsional state of CH3SCD3 has been resolved and analyzed. This fine structure results from coupling of internal and over-all rotation and is affected by top-top coupling terms in the kinetic and potential energy portions of the Hamiltonian. Neglecting only the potential energy coupling terms, the (CH3)2S and CH3SCD3 splittings yield as the barrier to internal rotation 2132±6 and 2118±3 cal/mole, respectively. Estimates of the potential energy coupling parameters are made. They are found to be an order of magnitude smaller than the main term of ∼2100 cal/mole in the Fourier expansion of the potential energy.