TEST OF THE TORSION ROTATION HAMILTONIAN FOR SYMMETRIC TOPS USING THE MILLIMETRE WAVE SPECTRUM OF METHYL SILANE
Loading...
Date
1982
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
The standard torsion rotation $Hamiltonian^{1,2}$ for symmetric tops has been tested in methyl silane $(CH_{3}SiH_{3})$ by combining recent anticrossing molecular beam $measurements^{3}$ in the ground torsional state (v=0) with pure rotational spectra taken for v as high as 4. The earlier microwave data $set^{4}$ which consisted of $J=1\leftarrow 0$ and $2\leftarrow 1$ has been greatly extended by studying milimeter wave transitions for $J=4\leftarrow 3, 5\leftarrow 4$, and $13\leftarrow 12$. Improved measurements have been made several $J=1\leftarrow 0$ lines, including a molecular beam study of the (v=0) spectrum. An analysis of the 77 rotational frequencies for $v\leq 2$ and the 15 anticrossing splittings for v=0 yielded an excellent fit. Among the 13 rotational, and distortion constants determined were the effective rotational constant ${A_{0}}^{eff}=56,189.449(25) MHz$, the effective barrier height ${V_{3}}^{eff}=592.3371(70) cm^{-1}$, and the effective ratio of the moment of inertia about the symmetry axis for the methyl top to the corresponding moment for the entire molecule $\rho^{eff}=0.3518120(49)$. The determination of the true values of $A_{3}, V_{3}$, and $\rho$ is discussed. It has been shown that the conventional model for internal rotation in symmetric tops cannot simultaneously explain the data for levels $(v\leq 2)$ well below the barrier top and the data for levels $(v\geq 3)$ near or above the barrier top. The disagreement has been most clearly demonstrated for v=3. Here all the lines detected have been identified, but when a least squares fit is made to all data with $v\leq 3$, the calculated frequencies for v=3 typically differ from the observed values by many times the experimental error. Possible mechanisms for the failure of the model are suggested.
Description
$^{\ast}$Current address: Herzberg Institute of Astrophysics, National Research Council, Ottawa, Ontario, Canada. $^{1}$C.C. Lia and J.D. Swalen, Rev. Mod. Phys. 31, 841 (1959). $^{2}$R.M. Lees and J.G. Baker, J. Chem. Phys. 48, 5299 (1968). $^{3}$W.L. Meerts and I. Ozier, J. Mol. Spectrose. (to be published). $^{4}$E. Hirota, J. Mol. Spectrose, 43, 36 (1972).