Ab initio studies of neutral and anionic p-benzoquinone-water clusters

Since the energy conversion reactions in most biological systems involve the interactions of various negatively charged quinones with water molecules, we have investigated the equilibrium structures, binding energies, and vibrational frequencies of the water monomer/dimer complexes of p-benzoquinones (Q) and its corresponding negatively charged anionic species (Q(-) and Q(2-)). The calculations, which include the evaluation of charges and aromaticity, indicate the existence of double hydrogen bonds in the water monomer and water dimer complexes of neutral p-benzoquinone. When two water molecules are complexing with p-benzoquinone, a conformer exhibiting an H-bond between two water molecules (W(2)Q) is energetically more favored than the conformer WQW in which there is no direct interaction between the water molecules. However, an increase in the negative charge of p-benzoquinone leads to the energetic stabilization of the WQ(2-)W conformer over the Q(2-)W(2) conformer. The charge transfer Q to water molecules is found to be significant in the case of Q(2-), up to -0.23 a.u. in the presence of two water molecules. An increase in the aromaticity of the p-benzoquinone molecule upon its interaction with the water molecules is noted from an analysis of the Nucleus Independent Chemical Shift (NICS) values. (C) 2003 American Institute of Physics.