The optimization of the transition state for the reaction CN + H2O ?OH + HCN has been performed by using the energy gradient approach at UHF SCF/3-21G level. The calculated activation barrier of the reaction is 32.6 kJ/mol (exptl. value 31.2 ?0.6 kJ/mol). The Intrinsic Reaction Coordinate (IRC) for the reaction is traced by using Morokuma's magnitude method. Along the IRC, the variations for geometries of the reactants, the coupling constants between the IRC and other normal vibrations, and the changes of the vibrational frequencies are evaluated. By combining the vibrational frequencies with the coupling constants, more dynamic information about the reaction has been provided. The theor. rate constants in the conventional transition state theory, variational transition state theory and tunneling correction approach are calculated and compared with the experimental rate constants over the temperature of 500 ~ 1100 K.

Key words: WATER, ACTIVATION ENERGY, REACTION RATE CONSTANT, TRANSITION STATE THEORY, HYDROGEN CYANIDE, GEOMETRICAL ISOMERISM, IRC