What a difference a decade makes: progress in ab initio studies of the hydrogen bond

This article provides a summary of our studies of hydrogen-bonded complexes during the decade of the 90's. These studies began with systematic investigations of the methodological dependence of the computed structures and binding energies of these complexes. The MP2/6-31 + G(d,p) level of theory was identified as the minimum level required to obtain reliable structures, while reliable energetics required larger polarized split-valence basis sets that include diffuse functions. While the experimental frequency shift of the A-H stretching band upon formation of an A-H-B hydrogen bond could also be reproduced at MP2/6-31+G(d,p) for a variety of hydrogen-bonded complexes, significant discrepancies were observed for others, including complexes of HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines. Resolving these discrepancies became the primary focus of our work, and redefined our research efforts. We solved a model two-dimensional nuclear Schrodinger equation to obtain anharmonic dimer- and proton-stretching frequencies, modeled matrix effects with external electric fields, and characterized hydrogen bond types as traditional, proton-shared, and ion-pair. We were able to resolve the observed discrepancies between theory and experiment, and explain the rather disparate effects of matrices on the IR spectra of closely related complexes. We also initiated studies of the NMR properties of the chemical shift of the hydrogen-bonded proton, and the A-B spin-spin coupling constant across the A-H-B hydrogen bond. We demonstrated the dominance of the Fern-ii-contact term for determining coupling constants in complexes with N-H-N, N-H-O, O-H-O, and Cl-H-N hydrogen bonds, and the distance dependence of this term. We also showed that the IR anharmonic proton-stretching frequency and the NMR spin-spin coupling constant are spectroscopic fingerprints of hydrogen bond type, which provide information about intermolecular distances in hydrogen-bonded complexes. (C) 2001 Elsevier Science B.V. All rights reserved.