Simulations of condensed phase photochemistry: Cage effect and internal conversion in azoalkanes and nitrosamines

This is a comparative theoretical study of cage effect in the photolysis of two compounds, azomethane and dimethylnitrosamine. Both compounds belong t o classes, azoalkanes and nitrosamines, which undergo an almost complete in hibition of photodissociation in condensed phase. We have set up a semiquan titative simulation of the primary photochemical step, based on Langevin dy namics and surface hopping. This required the ab initio determination of qu asi-diabatic states and potential energy surfaces (PES). In the PES of both compounds one can identify a region of strong nonadiabatic coupling, marke d by the presence of conical intersections, which determines molecular geom etry and time of the internal conversion (IC) from S-1 to S-0. Azomethane u ndergoes a very fast IC when the torsion angle around the N=N bond is about 90 degrees, followed by vibrational energy loss to the solvent: the outcom e is isomerization rather than dissociation. On the contrary, the dissociat ion of nitrosamine takes place essentially in the S-1 potential surface: in this case, the mechanism of cage effect can be described as fast recombina tion of the fragments, favored by strong nonadiabatic coupling with S-0 at medium-long internuclear distances.