Dependence of N-nitrosodimethylamine photodecomposition on the irradiation wavelength: excitation to the s(2) state as a doorway to the dimethylamine radical ground-state chemistry.

The photochemistry of N-nitrosodimethylamine after excitation to the S(1) and S(2) states has been studied with the complete active space self-consistent field method (CASSCF) and the second-order multiconfigurational perturbation theory (CASPT2). The calculated vertical transitions agree with the experiment: the S(0) --> S(1) transition occurs at 3.29 eV (f = 0.003 au), the S(0) --> S(2) transition at 5.30 eV (f = 0.17 au) and the first excited triplet state is computed at 2.48 eV. Solvent effects have been reproduced by means of PCM calculations. It is shown that excitation to S(1) and S(2) yields the same photoproducts: (CH(3))(2)N (1(2)B(1)) and NO (X(2)Pi). However, while on S1 the process is adiabatic, the process on S(2) implies an ultrafast decay through a planar S(2)/S(1) conical intersection. Our calculations are consistent with the reversibility of the N-N dissociation after irradiation at 363.5 nm and the observed dimethylamine radical reactions when irradiated at 248 nm, namely, H extrusion, a one-step process (41.3 kcal/mol), and methyl radical extrusion, a two-step process (44.0 kcal/mol and 31.5 kcal/mol). Finally, two more aspects are considered: (i) the topology of the T(1) surface where two minima have been found to correlate with the phosphorescence emission band and (ii) the influence of tautomerizations which is shown to be neglectable.