Semiclassical model calculations of weak, strong, and short H-bonds

The OH potentials of the complexes [Formula: see text] are calculated by an ab initio MO-LCAO-SCF-CISD method with a 6-31G** basis set for 30 different [Formula: see text] distances. The resulting OH vibration levels are calculated by the so-called semiclassical Planck–Sommerfeld phase integral. The experimental H-bond effects on vibration spectra, and their anomalies, could be established by calculations of H-bond energies, OH frequencies, anharmonicities, the Badger–Bauer rule, correlation between OH and [Formula: see text] distances, isotopic effects, and the Ubbelohde effect. For very short H-bonds new properties could be predicted. There seem to exist three classes of H-bond: (1) weak or medium-strong, (2) strong, and (3) very short. The simple phase integral method could demonstrate the anomalous effects of strong H-bonds solely by the transition from two separated minima to one, without necessarily assuming tunnel effects. Key words: strong H-bonds, IR spectra and theory, tunnel effects.