Mechanisms for N3 formation in radiated solid nitrogen: Computational predictions including excited electronic states

A computational investigation of the azide radical formation from two N2 molecules was carried out employing the CASSCF and MRCI electronic structure methods. Potential energy curves were built considering various electronic excited states along proposed reaction coordinates. Reaction paths with C2v and Cs symmetries were considered and energy barriers were estimated, as well as intersections between key electronic states were found. A possible reaction mechanism is suggested involving a nitrogen atom ion to produce linear N3. The distinction between N3 formed by UV radiated ices and other sources of energy is presented and rationalized. Among other results, we predict that the N3 radical can react barrierlessly with N(2D) atoms, yielding N2 + N2 products. CASSCF/MRCI approaches have been employed to rationalize a possible reaction mechanism for azide radical formation from UV photolysis of solid N2 and results were compared to available experimental energy threshold for the process. The outcomes of the performed calculations indicate that the reaction involves two electronically excited N2 molecules and is only possible when very high photon flux is used.