Photostability versus photodegradation in the excited-state intramolecular proton transfer of nitro enamines: Competing reaction paths and conical intersections

The phototautomerization mechanism of a model nitro enamine (NEA) chromophore (incorporated in the structure of a highly photolabile pesticide, tetrahydro-2-(nitromethylene)-2H-1,3-thiazine) has been studied using complete active space self-consistent field reaction path computations. The optically accessible (1)pi pi* excited state of NEA involves separation of charge and correlates diabatically with the ground state of the tautomerized acinitro imine (ANI) form. For optimum photostabilization, the (1)pi pi* state of NEA should be S-1: in this case, the tautomer would be efficiently formed via a diabatic intramolecular proton-transfer pathway passing through an S-1/S-0 conical intersection, followed by a facile thermal back proton-transfer reaction. However, in NEA itself the lowest excited states correspond to nitro group (1)n pi* states, and there are additional surface crossings that provide a mechanism for populating the (1)n pi* manifold. The above results indicate that the high photolability observed for the pesticide [Kleier, D.; Holden, I.; Casida, J. E.; Ruzo, L. O. J. Agric. Food Chem. 1985, 33, 998-1000] has to be ascribed to photochemistry originating on the (1)n pi* manifold of states, populated indirectly from the (1)pi pi* state.