A real-space-grid QM/MM study on the ionic/radical association reaction in aqueous phase:: HCHO+OH→HCHO-OH

A real-space-grid quantum mechanical (QM) method based on the density functional theory (DFT) combined with molecular mechanical(MM) approach(QM/MM) have been applied to investigate the solvation effect of the supercritical water (SCW) on the association reaction of OH anion/OH radical to formaldehyde molecule (HCHO). The reactivities of the ionic and radical species in SCW as well as in ambient water (AW) have been discussed. The prior QM/MM simulation for OH- in water solutions has revealed that a simplified point charge model used in the classical simulation is not adequate for the realistic description of anionic systems. In the QM/MM, simulation for the association reaction, the electron cloud of the QM solute (HCHO + OH) has been explicitly considered to realize the diffuse nature of the excess electron on an anionic system, which will cause a significant effect on the energetics along the reaction path. The result shows that the intermediate state of the ionic association reaction is higher in energy than the reactant by similar to 13.6 kcal/mol in the SCW. The same trend has been observed in the ionic reaction in the AW. In the radical processes, on the other hand, there are no potential energy barriers both in the SCW and AW. The radical association will be preferred energetically to the ionic one even in the high temperature and low density region of the water. (C) 2003 Elsevier B.V. All rights reserved.