Vibrational spectroscopic evidence of unconventional hydrogen bonds

In this article, infrared (IR) spectroscopic evidence for two classes of unconventional hydrogen bonds is presented. The first one involves drastic change in the hydrogen bonding properties in cationic states relative to neutral species. The phenol-benzene cluster shows an extremely high enhancement of π-hydrogen bonding interaction upon ionization. On the other hand, the benzene-water cluster shows a dramatic change from the π-hydrogen bonded to C-H···O type hydrogen bonded structure following ionization. In the case of alkyl phenols, the O-H stretching vibration of o-cis-isomer is shifted to a lower frequency upon ionization, which reflects the formation of an unconventional hydrogen bond of the type O-H···C, wherein the alkyl carbon acts as a proton acceptor. The second is novel hydrogen bonding between borane-amines and acidic protons. The vibrational spectra of O-H and N-H stretching vibrations in phenol and 2-pyridone (2PY), respectively, clearly indicate that these compounds act as proton donors to borane-amines. The experimental observations along with the density functional theoretical (DFT) calculation clearly establishes the formation of "dihydrogen bonding" between the O-H/N-H and B-H groups. Further, the two interacting hydrogens in a dihydrogen bond get eliminated as molecular hydrogen in the cluster cation leading to dehydrogenation reaction.