Microwave spectrum of two top molecules in the excited states dimethyl ether, dimethyl sulfide, and dimethyl silane

Microwave spectra of dimethyl ether, dimethyl sulfide, and dimethyl silane in the torsionally excited states have been measured. The methyl internal rotations of these molecules were analyzed from the observed multiplets and from the reported multiplets of transitions. The method developed for ethyl silane in the previous paper was extended to equivalent two top molecules. For equivalent two top molecules, apparent barriers of methyl internal rotations obtained from the experiments were corrected by the kinetic and potential cross terms. They are V₃=2645.8±10.0, 2632.4 ± 42.9, 2146.0 ± 13.8, 1651.5 ± 10.1, 1648.0 ± 13.7, and 1649.9 ± 11.8 cal/mol for (CH₃)₂O, (CD₃)₂O, (CH₃)₂S, (CH₃)₂SiH₂, (CH₃)₂SiD₂, and (CH₃)₂SiHD, respectively. The potential cross terms, V₁₂(1−cos3α₁)(1−cos3α₂) terms are negligible for the three molecules, while V^′₁₂sin3α₁sin3α₂ terms are also very close to zero except those for (CH₃)₂O and (CD₃)₂O which are small but not negligible (V^′₁₂=−124.4,−158.0 cal/mol). The investigations were extended to those of non-equivalent two top species and the corrected barriers of the methyl tops, V₃, are obtained to be 2615.6 ± 8.6 and 2155.0 ± 15.2 cal/mol for CH₃OCD₃ and CH₃SCD₃. The corrected barrier, V₃(CD₃) of CH₃OCD₃, is obtained to be 2634.4 ± 7.1 cal/mol, while that of CH₃SCD₃ cannot be solved due to the lack of the data available.