Effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol: An ab initio molecular orbital study

The effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol has been studied by SCF ab initio MO calculations at the HF/6-31G * level. The systems investigated are eight phenol-water complexes and the dimer and trimer of phenol. Optimized geometries show that hydrogen bond formation causes a consistent pattern of changes in the structure of the molecule. When phenol acts as a proton donor, the expected increase of r (O-H) is accompanied by a slight decrease of r(C-O) and of the internal ring angles at the ipso and para positions, and by an increase of r(Cipso©ortho). These changes suggest that the relative contribution of polar canonical forms to the electronic structure of the molecule increases upon hydrogen bond formation, since this enhances the strength of the interaction. The opposite changes occur when phenol acts as a proton acceptor, except for r(O-H), which is the same as in the free molecule. If phenol acts as a proton donor and as a proton acceptor, the two hydrogen bonds become stronger due to a synergic effect. In this case, however, the structural deformation of the molecule is less pronounced than in the previous cases, due to the opposite effect of the two hydrogen bonds. The available experimental evidence on gas-crystal structural differences for phenol is critically reviewed, also in the light of the present results on gas-phase complexes. © 1995 Plenum Publishing Corporation.