Hydrogen bonding between phenol and acetonitrile

The present work aims to resolve theoretically the puzzling effect of downshifting of the nu(OH) stretching mode of phenol, complexed with acetonitrile, as the latter concentration increases, via performing a thorough search of the potential energy surface of the interaction of phenol with acetonitrile at the modest MP2/6-31+G(d,p), MP2/6-31G(d), and B3LYP/6-31+G(d,p) computational levels. We postulate and firmly approve that. besides the conventional a hydrogen-bonded structure of phenol with one acetonitrile molecule, there exists the novel, although less favorable, structure with pi-type hydrogen bonding. Such bonding becomes crucial as the second acetonitrile molecule is added to the 1:1 complex of phenol and acetonitrile, and furthermore, its substantial stabilization in polar solvents provides an explanation of the mentioned controversial issue that has stood unclear during the last four decades.