Weak hydrogen bonds: insights from vibrational spectroscopic studies

Abstract The review presents a critical analysis of the data obtained from vibrational spectroscopic studies on a narrow selection of weak hydrogen-bonded binary molecular complexes for measurements performed under isolated conditions, addressing the nature, properties, physical origins of the binding forces, and the role of such hydrogen bonds in dynamics of vibrational relaxations. In the recent history of studies of chemical bonding, hydrogen bond certainly occupies the centre stage. Although the bottom line of our knowledge for structure of hydrogen bonded systems is based on crystallographic data, it is well recognised that the constrained environment of a molecular crystal seriously perturbs the shallow interaction potentials of hydrogen bonds, and particularly their weaker variants. Binary complexes of different categories of molecular prototypes are the most convenient systems to look into the attributes and role played by the weak hydrogen bonds in promoting a chemically significant event. A variety of weak hydrogen bonded binary complexes, having mostly two types of binding motifs, CH··O and OH⋯π, have been considered for this review. The vital molecular parameter that has been primarily considered in the present analysis is the hydrogen bond induced spectral shift (ΔνΧ-Η) of the stretching vibrational fundamental of the donor group (X–H), for measurements performed in inert gas matrixes and also in the gas phase. The changes in infrared spectral band shapes of νX–H transitions have been considered to suggest the influence of the hydrogen bond in vibrational dynamics of the excited X–H stretching mode. Attempts are made to correlate the observed spectral shifts in homologous series of complexes for a particular binding motif with different energetic and electronic structure parameters, and those correlations have been used to get insights into the underlying molecular interactions and origin of vibrational spectral shifts. The other vital parameters of binary molecular complexes are the low-frequency intermolecular vibrations, which appear typically in terahertz range of the electromagnetic spectrum. A brief analysis of the available data for weak hydrogen bonded complexes, obtained by employing LIF spectroscopic method, is presented, and information obtained from complementary spectroscopic methods, like far-infrared absorption, are discussed. The spectral data presented are mostly from the published work of the authors.