Energy Levels for Internal and Over-All Rotation of Two-Top Molecules. I. Microwave Spectrum of Dimethyl Silane

Abstract

A method is given for analyzing barrier‐dependent ``tunnel effect'' splittings in rotational spectra of molecules with two equivalent tops. The perturbation treatment is an extension of the ``principal axis'' method for single‐top molecules, and is limited to torsional states in which the two principal torsional quantum numbers are identical. Tables of perturbation coefficients are given from which the effects of top‐top coupling on the rotational spectrum may be calculated. The microwave spectra of six isotopic species of dimethyl silane have been investigated in the region 8 to 25 kMc. Changes in moments of inertia with isotopic substitution yield the following structural parameters: $$\begin{array}{cccc}\mathrm{SiC}& \text{1.867±0.002\hspace{0.17em}}\mathrm{A}& \mathrm{CSiC}& {110}^{\circ }{59}^{\prime }{\text{±10}}^{\prime }\\ \mathrm{SiH}& \text{1.483±0.005\hspace{0.17em}}\mathrm{A}& \mathrm{HSiH}& {107}^{\circ }{50}^{\prime }{\text{±20}}^{\prime }\\ \mathrm{CH}& \text{1.095±0.005\hspace{0.17em}}\mathrm{A}& \mathrm{HCH}& {108}^{\circ }{0}^{\prime }{\text{±20}}^{\prime },\\ & {\text{2θ=110}}^{\circ }{59}^{\prime }{\text{±1}}^{\circ },& & \end{array}$$ where 2θ is the angle between the symmetry axes of the methyl groups. From splittings in the rotational spectrum the angle 2θ is found to be 110°50′±20′. Analysis of splittings for (CH₃)₂SiH₂, (CH₃)₂SiD₂, and (CH₃) (CD₃)SiH₂ indicates that coupling between tops is very small. Assuming no coupling, the barrier to internal rotation is 1647±3 cal/mole. From Stark effect measurements the dipole moment is found to be 0.75±0.01D.