Knowledge Bank

The high-resolution Fourier transform spectrum of $CH3OH$ has been investigated in the $1400-1650cm-1$ region, containing the $CH3$-bending fundamental bands. Twenty-two perpendicular $\Delta K=+1$ subbands have been identified so far, with origins ranging from 1490 to $1570cm-1$ for transitions $K=2\leftarrow 1$ up to $11\leftarrow 10$ for various torsional symmetries. Assignment of the subbands to the $\nu 4$ in-plane ($A\prime$) or $\nu 10$ out-of-plane ($A\prime \prime$) asymmetric methyl-bending modes is not yet clear, but the one subband so far observed with resolved K-doublet structure suggests c-type selection rules consistent with a $\nu 10$ vibrational assignment. The pattern of the $K$-reduced torsion-vibration energy $\tau$-curves is inverted compared to the normal 1- dimensional picture for $n=0$ torsional levels, in agreement with prediction based on fitting torsional variation of ab initio $CH3$-bending frequencies to a local mode model. However, the periodicity of the curves is unusual and significantly different from the ground state. The vibrational energy for the bending mode is $1481cm-1$, and the mean B-value is $0.008cm-1$ higher than that of the vibrational ground state.