Electronic structure and reactivity of the CNO/NCO/CON isomers

Abstract

Three-dimensional potential energy surfaces (PESs) have been calculated for the lowest electronic states of NCO, CNO and CON isomers, using internally contracted Multi Reference Configuration Interaction (MRCI) and Coupled-Clusters RCCSD(T) ab initio methods. For the low lying doublet and quartet excited states of the three isomers, the N–CO, O–CN, C–NO and C–ON collinear dissociation paths were mapped by the Complete Active Space SCF (CASSCF) approach and the energy variations with the bending coordinate have been explored. Several regions of conical intersections have been located and the spin–orbit interactions between states of different spin symmetry have been evaluated in the region of intersections of these states. The analysis of the PESs allows one to identify the main interactions governing the reactivity of the lowest electronic states. The NCO and CNO isomers have stable X2Π electronic ground states, for CON the X2Π ground state is separated from the dissociative [CO + N] asymptote by a barrier of 0.11 eV and crosses the dissociative 4Σ - state close to its minimum. At their equilibrium ground state geometries the spin–orbit interactions A SO between the two electronic components of the X2Π states were calculated to be -95.6, -109.6 and -57.1 cm−1 for NCO, CNO and CON, respectively. The predissociation of the vibrational levels of the A2Σ+ and B2Π states of NCO has been explained.