Knowledge Bank

The standard torsion rotation $Hamiltonian1,2$ for symmetric tops has been tested in methyl silane $(CH3SiH3)$ by combining recent anticrossing molecular beam $measurements3$ in the ground torsional state (v=0) with pure rotational spectra taken for v as high as 4. The earlier microwave data $set4$ 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 $v\le 2$ 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 $A0eff=56,189.449(25)MHz$, the effective barrier height $V3eff=592.3371(70)cm-1$, 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 $A3,V3$, 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 $(v\le 2)$ well below the barrier top and the data for levels $(v\ge 3)$ 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 $v\le 3$, 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.