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

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Abstract
Ab initio calculations at the MP2/6-31+G(d,p) level of theory have been carried out to determine the equilibrium structures and vibrational 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 possibilities, including traditional Y-H...N, proton-shared Y...H...N, and ionic Y-...H+-N hydrogen bonds. The type of hydrogen bond in a complex is related to the proton affinity of the substituted pyridine and the nature of the hydrogen halide. Plots of normalized distance and force constant changes versus the proton affinity of the substituted pyridines 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 axis. The shift to lower frequency of this band in complexes with traditional hydrogen bonds increases as the proton affinity of the substituted pyridine increases. A shift of greater than 40% relative to the free HY frequency is a spectroscopic signal of the presence of a proton-shared hydrogen bond.

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