Phenol vs water molecule interacting with various molecules: Sigma-type, pi-type, and chi-type hydrogen bonds, interaction energies, and their energy components.

The nature of interactions of phenol with various molecules (Y = HF, HCl, H2O, H2S, NH3, PH3, MeOH, MeSH) is investigated using ab initio calculations. The optimized geometrical parameters and spectra for the global energy minima of the complexes match the available experimental data. The contribution of attractive (electrostatic, inductive, dispersive) and repulsive (exchange) components to the binding energy is analyzed. HF favors sigma(O)-type H-bonding, while H2O, NH3, and MeOH favor sigma(H)-type H-bonding, where sigma(O)-/sigma(H)-type is the case when a H-bond forms between the phenolic O/H atom and its interacting molecule. On the other hand, HCl, H2S, and PH3 favor pi-type H-bonding, which are slightly favored over sigma(O)-, sigma(H)-, sigma(H)-type bonding, respectively. MeSH favors chi(H)-type bonding, which has characteristics of both pi and sigma(H). The origin of these conformational preferences depending on the type of molecules is elucidated. Finally, phenol-Y complexes are compared with water-Y complexes. In the water-Y complexes where sigma(O)/sigma(H)-type involves the H-bond by the water O/H atom, HF and HCl favor sigma(O)-type, H2O involves both sigma(O)-/sigma(H)-type, and H2S, NH3, PH3, MeOH, and MeSH favor all-type bonding. Except for HF, seven other species have larger binding energies with a phenol molecule than a water molecule.