Vibrational spectroscopy of the hydrogen bond: an ab initio quantum-chemical perspective

The hydrogen bond has long been recognized as an important type of intermol ecular interaction. Its infrared (IR) spectroscopic signature is the shift to lower frequency and the increase in intensity of the A-H stretching band upon formation of the A-H ... B hydrogen bond. Ab initio calculations carr ied out with an appropriate wavefunction model and basis set, and using the harmonic approximation, can reasonably reproduce the shift of the A-H stre tching band upon hydrogen bonding, if the equilibrium structure exists in a relatively deep potential well on the surface, so that both the v = 0 and the v = 1 vibrational states of the proton-stretching mode are confined wit hin this well. However, if the equilibrium structure is found in a region o f the surface which is broad and relatively flat, or if a second region of the surface can be accessed in either the v = 0 or the v = 1 vibrational st ate of the proton-stretching mode, then the harmonic approximation fails to describe the anharmonicity inherent in the surface. For such complexes, ex perimental gas-phase structures and experimental IR spectra obtained in low -temperature rare-gas matrices may give conflicting descriptions of the hyd rogen bond, and discrepancies will exist between experimental and computed harmonic IR spectra. Anharmonic frequencies for both fundamental and combin ation bands are needed to understand and reproduce qualitatively the most i mportant features of the experimental spectra. In this article, an overview of the calculation of anharmonic frequencies is presented, and results of one- and two-dimensional anharmonic treatments of vibration are reported fo r a variety of hydrogen-bonded complexes. Computed frequencies are compared with experimental gas-phase frequencies when these ape available, and with experimental frequencies obtained in low-temperature rare-gas matrices.