Knowledge Bank

Fourier transforms spectroscopy has been used to study the weakly allowed torsional spectrum of methyl silane. Between $330-380cm-1$, 347 lines in the $(v6=2\leftarrow 0)$ band were measured at low pressure with a resolution of $0.015cm-1$. Between $150-360cm-1$ significant structure and partially resolved features have been observed at moderate pressure with a resolution of $0.15cm-1$. From the low resolution spectrum, 13 Q branch lines for $(v6=3\leftarrow 1)$ and $(v6=4\leftarrow 2)$ bands were assigned. A Hamiltonian which included rotational, torsional, and distrotion effects to sixth power in angular momentum was used to carry out a simultaneous fit of the infrared transitions, microwave frequencies, and molecular beam splittings. From the best fit, 20 effective rotation, torsion, and distortion parameters were obtained. The torsional dependence of the electric dipole moment originates from the vibration-torsion-rotation interactions with the nearby excited vibrational states. To degree two in angular momentum this dependence is characterized by 4 parameters. The two even'' moments $\mu_{0}^{T}$ and $\mu_{2}^{T}$ are determined from the difference in the Stark shift of the microwave transitions $(J=1\leftarrow 0)$ in the $v_{6}=0$ and 2 torsional states. The two odd'' dipole moments $\mu 1T$ and ($\mu 1T-\mu \perp T$) were determined from the lineshape analysis of ($v6-1\leftarrow 0$) band of the low resolution data. We show that for methyl silane the centrifugal distortion dipole moments $\mu J0$ and $\mu K$ measured from the beam spectrum by Ozier and Meerts should be reinterpreted as linear combinations of the centrifugal and torsional dipole moments. If $\mu J0$ and $\mu K$ are fixed at zero, it then becomes possible to calculate $\mu 1T$ and $\mu 1T-\mu \perp T$ from the beam data. The results obtained are in agreement with the values determined from the lineshape analysis. We therefore conclude that the torsion distortion effects make a much larger contribution than centrifugal distortion to the variation with the state of the dipole moment in $CH3-SiH3$.