A CASPT2//CASSCF study of the quartet excited state $$\tilde{a}^{4}A^{\prime\prime}$$ 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 $${\tilde{a}}^{4}{A^{\prime\prime}}$$ 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 $${\tilde{a}}^{4}A^{\prime\prime}$$ and X ( $$X={\tilde{X}}^{2}A^{\prime\prime}$$ and $${\tilde{A}}^{2}A^{\prime}$$ ) states are located near to the minima. However, the intersystem crossing $${\tilde{a}}^{4}A^{\prime\prime} \rightarrow X$$ is weak due to the vanishingly small spin–orbit interactions. It further indicates that the direct dissociation on the $${\tilde{a}}^{4}{A^{\prime\prime}}$$ 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. [ABSTRACT FROM AUTHOR]