A high-level ab initio study of the destruction of methanimine under UV radiation

The photodecomposition of methanimine (CH$_2$NH) in the interstellar medium through several possible pathways is investigated by means of high-level multireference configuration interaction {\it ab initio} calculations. Among them are photodissociation pathways involving hydrogen-atom elimination from both the CH$_2$ and NH groups, and fragmentation into CH$_2$ and NH. Potential-energy curves for the ground and several excited electronic states, as well as nonadiabatic couplings between them, are calculated. Possible dissociation mechanisms are discussed for the different pathways. It is found that the minimum excitation energy required for methanimine dissociation is above $7$ eV. By using a two-dimensional representation of methanimine, the CH$_2$NH $\rightarrow$ CHNH$_2$ isomerization is explored as an additional methanimine decomposition pathway. Hydrogen-atom elimination from the CH$_2$ group is also investigated along the isomerization pathway. The results show that the isomerization proceeds by overcoming a transition state that in the first two excited states would require excitation energies similar to or somewhat lower than the typical minimum energies needed for breaking the molecule through the fragmentation pathwys. Therefore, CH$_2$NH $\rightarrow$ CHNH$_2$ isomerization can effectively contribute to methanimine decomposition, competing efficiently with the photodissociation pathways. The radiation content present in the interstellar medium makes possible the occurrence of all the pathways studied.