AB-INITIO STUDY OF THE POTENTIAL-ENERGY SURFACES FOR THE REACTION N(S-4(U))+CH(X(2)PI(R))-]CN(X(2)SIGMA+,A(2)PI(I))+H(S-2(G))

The three lowest triplet potential energy surfaces of HCN have been investigated with the complete active space multiconfigurational self consistent field (CAS/MCSCF) method. Two surfaces are issued from the ground state reactants, while the third one is issued from the first excited state of the reactants, and is coupled to the others by a conical intersection. Of the two surfaces issued from the ground state reactants, one possesses a very small potential barrier in the entrance channel, while the other surface has none, thus making the reaction possible at very low temperature. For each surface, two potential wells have been found, corresponding to bent structures HCN and HNC. These potential wells are separated by high isomerisation saddle points. The approach of H toward CN shows a potential barrier for each of the three surfaces. Some parts of the conical intersection have been calculated and found energetically lower than the ground state reactants. Thus, transitions between surfaces should play an important role in the reaction dynamics.