Vibrational Coupling Pathways in the $\nu _{3}$ CH Stretch Fundamental Region of Methanol as Revealed by Coherence-Detected FTMW-IR Spectroscopy

Rotational state-selection on the methanol E species transitions 2$_{0}\leftarrow $3$_{-1}$ and 2$_{1}\leftarrow $3$_{0}$ is used to record the infrared (IR) spectra of the connected rotational levels of jet-cooled methanol, CH$_{3}$OH and CH$_{3}$OD. The observed spectra of CH$_{3}$OH contain twelve interacting vibrational bands in the interval 2755-2855 cm$^{-1}$, whereas a direct state count gives 14 vibrations in this interval grouped into 6 tiers by coupling order. The isotope dependence of the spectra confirms that the dominant coupling pathway is a third order coupling of the CH stretch to a combination of the COH bend and an HCH bend.