Characterization of two types of intermolecular interactions on halogen monoxide monohydrates.

Monohydrates of halogen monoxides ClO.H2O and BrO.H2O have been studied by means of DFT (B3LYP) and ab initio (MP2) correlated calculations with aug-cc-pVnZ basis sets ranging from triple- up to quintuple-zeta. These complexes might be formed in the troposphere and stratosphere and participate in chemical reactions involved in ozone depletion. Two stable structures are found that differ in the intermolecular interaction which takes place, namely: conventional XO...HOH hydrogen bond and OX...OH2 halogen bond. We demonstrate that both types of interactions participate in the formation of these complexes yet all the computational methods tested predict a slightly greater stability for the latter OX...O link. Both intermolecular interactions are characterized upon analyzing electron density distribution, charge transfer effects, and electron localization domains. These analyses reveal the central role played by electron redistribution. Because of this, the greater spatial extent of the electron density in Cl or Br as compared to H could be the main cause to yield a slightly greater stability for the O-X...O halogen bond with respect to the O...H-O hydrogen bond. Copyright 2009 Wiley Periodicals, Inc.