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A real space ab initio molecular dynamics method reinforced by the double grid technique was applied to the simulations of the reactions of OH radical/OH anion with formaldehyde to examine their reactivities and also to test the efficiency of the method. The simulations revealed that the H-abstraction reaction by the OH radical takes place without the potential energy barriers within the level of generalized gradient approximation.

A novel quantum chemical approach recently developed has been applied to an ionic dissociation of a water molecule (2H2O→H3O++OH-) in ambient and supercritcal water. The method is based on the quantum mechanical/molecular mechanical (QM/MM) simulations combined with the theory of energy representation (QM/MM-ER), where the energy distribution function of MM solvent molecules around a QM solute serves as a fundamental variable to determine the hydration free energy of the solute according to the rigorous framework of the theory of energy representation. The density dependence of the dissociation free energy in the supercritical water has been investigated for the density range from 0.1 to 0.6 g/cm3 with the temperature fixed at a constant. It has been found that the product ionic species significantly stabilizes in the high density region as compared with the low density. Consequently, the dissociation free energy decreases monotonically as the density increases. The decomposition of the hydration free energy has revealed that the entropic term (-TΔS) strongly depends on the density of the solution and dominates the behavior of the dissociation free energy with respect to the variation of the density. The increase in the entropic term in the low density region can be attributed to the decrease in the translational degrees of freedom brought about by the aggregation of solvent water molecules around the ionic solute. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The quantum mechanical/molecular mechanical (QM/MM) method is combined with the theory of energy representation (ER) to study the acidity of acetic acid in aqueous solution. The analysis has shown that the QM/MM-ER method provides an adequate description for the solvation free energies related to the acidity/basicity calculations of organic molecules.

A hybrid real space quantum mechanical/molecular mechanical (RS-QM/MM) method has been applied to an ionic S(N)2 reaction (OH- + CH3Cl --> CH3OH + Cl-) in water solution to investigate dynamic solvation effects of the supercritical water (SCW) on the reaction. It has been demonstrated that the approaching process of OH- to methyl group is prevented by water molecules in the ambient water (AW), while the reaction takes place easily in the gas phase. Almost the same solvation effect on the dynamics of OH- is observed in the SCW, though the bulk density of water is substantially reduced compared with that of the AW. It has been shown that the solvation of the SCW around the OH anion is locally identical to that of the AW due to the strong ion-dipole interactions between OH- and water molecules. At the transition state, the QM/MM simulations have revealed that the excess electron is quite flexible, and the charge volume, as well as the fractional charges on atoms, vary seriously depending on the instantaneous solvent configurations. However, it has been found that the solvation energy in the SCW can be qualitatively related to the HOMO volume of the system by Born's equation., (Copyright 2002 Wiley Periodicals, Inc.)