Knowledge Bank

High-resolution FTIR spectra have been obtaqined of the weak in-plane methyl-rocking band of the $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CD3OH$ isotopomer of methanol. This band lies below the strong $C-O$ stretching band, and has a quite different structure with two distinct Q-branch peaks at low resolution. The spectrum is clearly that of a parallel band, somewhat surprisingly, with hte origins of the various torsion-rotastion series being relatively widely spread. This speading immediately indicates substantial chnages in molecular constants in going from ground to excited state, and accounts for the twin peaks as accidental piling up of individual Q series. The spreading greatly eses the task of assignment, and numerous P, Q and R branch series have been identified and analyzed for the $n=O$ ground torsional state. The patern of the $n=O$ torsional energy levels in the excited methyl-rocking state deduced from the series origins suggests an increased to rsional barrier height, but is not consistent with the usual Hamiltonian model. However, several IR series have been identified for the $n=1$ first excited torsional state with origins aboout $50cm-1$ above those of the $n=O$ series, supporting a substantial increaqse in barrier. The mean value of $V3$ obtained by fitting the $n=1$ origins is $483cm-1$, a 31% increase over the ground state vaqlue of $370cm-1$. The general features of interest in the rocking band will be discussed, along with the evidence supporting the significant increase in barrier height.