A CASPT2//CASSCF study of the quartet excited state tilde{a}^{4}A^{primeprime} of the HCNN radical

Complete active space self-consistent field and second-order multiconfigurational perturbation theory methods have been performed to investigate the quartet excited state ${tilde{a}}^{4}{A^{primeprime}}$ potential energy surface of HCNN radical. Two located minima with respective cis and trans structures could easily dissociate to CH $({tilde{a}}^{4}Sigma^{-})$ and $N_{2} ({tilde{X}}^{1}Sigma_{rm g}^{+})$ products with similar barrier of about 16.0 kcal/mol. In addition, four minimum energy crossing points on a surface of intersection between ${tilde{a}}^{4}A^{primeprime}$ and X ( $X={tilde{X}}^{2}A^{primeprime}$ and ${tilde{A}}^{2}A^{prime}$ ) states are located near to the minima. However, the intersystem crossing ${tilde{a}}^{4}A^{primeprime} rightarrow X$ is weak due to the vanishingly small spin–orbit interactions. It further indicates that the direct dissociation on the ${tilde{a}}^{4}{A^{primeprime}}$ state is more favored. This information combined with the comparison with isoelectronic HCCO provides an indirect support to the recent experimental proposal of photodissociation mechanism of HCNN.