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The microwave spectra of gauche-ethanethiol and its isotopic species were studied. Most of the observed spectra for the species having the plane of symmetry exhibited doublet structures with large spacings due to the internal rotation of the mercapto group. The rs structure of the gauche isomer was determined from the observed moments of inertia. The gauche isomer whose dihedral angle τ(CCSH) is 61°45′±58′ has structural parameters close to those for the trans isomer except the angles around the carbon atom in the methylene group. The difference in angle values around the carbon atom in the methylene group between the trans and gauche isomers can be explained by the 3°7′ tilt of the methylene group towards the lone pair electrons on the sulfur atom. The direction of the dipole moment in the molecule was discussed on the basis of data given by Schmidt and Quade. The potential barrier of the mercapto internal rotation was obtained from splittings of the observed spectra. The Fourier coefficients of this barrier are V1=−207±67, V2=−386±27, and V3=1305±20 cal/mol. The spectra due to the excited methyl and mercapto torsional states were also measured for the trans and gauche isomers. The potential barrier of the methyl internal rotation for the trans isomer was found to be 3260±30 cal/mol from splittings of the spectra due to the first excited methyl torsional state.

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