Density functional theory of chemical reactivity indices in some ion-molecule reaction systems

Three isoelectronic reactions, proton transfer (PT), hydrogen abstraction (HA), and electron transfer (ET), of NH3+ with NH3, H2O, and HF have been studied using ab initio molecular orbital calculations. For the reaction of NH3+ + H2O, the energy of the transition state (TS) is higher than that of the reactants. This is consistent with the experimental observation that the rate constant is less than the average dipole orientation (ADO) rate constant. It seems reasonable that the reaction rate for the reaction NH3+ + H2O would hardly depend on the nu(2) mode of NH3+ at least for low-lying excited states (E(int) less than or equal to 0.714 eV) of the nu(2) mode, because the nu(2) mode contributes mainly to the normal mode orthogonal to the reaction coordinate at the TS. This is consistent with experimental observation. A similar prediction can be made for the NH3+ + HF reaction. The electron-transfer processes for the HA reactions have been examined in terms of the intrinsic reaction coordinate (IRC). The order of reactivity with NH3+ is NH3 > H2O > HF. It is found that the degree of the electron transfer and the reactivity are correlated with the absolute hardness (eta) of NH3, H2O, and HF. This is in accord with the softness as the chemical reactivity index in the density functional theory. (C) 1996 John Wiley & Sons, Inc.