Knowledge Bank

Tunneling splitting patterns, selection rules, and effective B values for $K=0$ rotational levels of the methanol-proton-donor and water-proton-donor forms of the hydrogen-bonded methanol-water complex have been predicted using: group-theoretical considerations, a modified internal axis method, rough estimations of barrier heights, approximate descriptions of possible tunneling paths, and the assumption that $K=0$ levels can be treated by themselves. The tunneling paths considered consist essentially of internal rotations of the methyl and hydroxyl groups of the methanol molecule in the complex, overall rotation of the water molecule in the complex, and combinations of these motions. It is found that the tunneling components of all a-type microwave transitions in both complexes obey bottom-bottom and top-top selection rules, so that none of the tunneling splittings can be observed directly. Also, statistical weights give the same intensity ratios in both forms of the complex. The main difference between spectra of the two forms is the fact that one asymmetric rotor $K=0$ a-type line splits into four tunneling components in the methanol-donor form and into eight components in the water-donor form of the complex.