AB-INITIO THEORETICAL AND MATRIX-ISOLATION EXPERIMENTAL STUDIES OF HYDROGEN-BONDING .2. A THEORETICAL-STUDY OF DISTANCES, FORCE-CONSTANTS, AND VIBRATIONAL FREQUENCIES IN COMPLEXES OF HYDROGEN HALIDES AND 4-SUBSTITUTED PYRIDINES

Ab initio calculations at the MP2/6-31 + G(d, p) level of theory have been carried out to determine the equilibrium structures and vibration al spectra of three series of complexes involving the hydrogen halides HF, HCl, and HBr, and a set of 4-substituted pyridines. The hydrogen bonds in these complexes span the range of hydrogen bonding possibilit ies, including traditional Y-H ... N, proton-shared Y ... H ... N, and ionic Y-... H+-N hydrogen bonds. The type of hydrogen bond in a compl ex is related to the proton affinity of the substituted pyridine and t he nature of the hydrogen halide. Plots of normalized distance and for ce constant changes versus the proton affinity of the substituted pyri dines exhibit breaks which correlate with the three types of hydrogen bond. The infrared spectra of all complexes exhibit very intense bands associated with motion of the hydrogen-bonded proton along the Y-N ax is. The shift to lower frequency of this band in complexes with tradit ional hydrogen bonds increases as the proton affinity of the substitut ed pyridine increases. A shift of greater than 40 % relative to the fr ee HY frequency is a spectroscopic signal of the presence of a proton- shared hydrogen bond.