| Abstract
| - The phototautomerization mechanism of a model nitro enamine (NEA) chromophore (incorporatedin the structure of a highly photolabile pesticide, tetrahydro-2-(nitromethylene)-2H-1,3-thiazine) hasbeen studied using complete active space self-consistent field reaction path computations. Theoptically accessible 1ππ* excited state of NEA involves separation of charge and correlates diabaticallywith the ground state of the tautomerized acinitro imine (ANI) form. For optimum photostabilization, the1ππ* state of NEA should be S1: in this case, the tautomer would be efficiently formed via a diabaticintramolecular proton-transfer pathway passing through an S1/S0 conical intersection, followed by afacile thermal back proton-transfer reaction. However, in NEA itself the lowest excited states correspondto nitro group 1nπ* states, and there are additional surface crossings that provide a mechanism forpopulating the 1nπ* 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 beascribed to photochemistry originating on the 1nπ* manifold of states, populated indirectly from the 1ππ*state.
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