An Infrared Spectroscopic Study of Hydrogen Bonding Interaction. Structural Studies of Proton-donating and -accepting Powers

Abstract

Abstract The relationship between the proton-donating and proton-accepting powers and the structures of various proton donors and acceptors was studied in the liquid phase by means of an infrared spectrophotometer. The proton-donating powers of the donors were evaluated by means of the ratios of the O–H shifts of the donors (RO–H) to those of methanol in various proton acceptor solutions. The proton-accepting powers of the acceptors were evaluated by means of the division of the O–H shifts by the proton-donating powers of the donors used. The proton-donating powers increased with the decrease in the pKa or the ionic RO–H bond dissociation energies and with the increase in the electron affinities of RO radicals, i. e., t-butanol<methanol<<trimethylsilanol<phenol<p-nitrophenol. In a homologous series, the proton-accepting powers increased with the decrease in the ionization potentials. In the case of the aromatic ethers with two proton accepting sites, two characteristic O–H vibrational bands were observed. The difference in the protonaccepting powers of the vinyl and ethyl compounds with the same substituent was observed, and the π-electron-mobility index, which was considered to be a measure of the energy level of the π-electrons of the vinyl group, was defined and discussed.