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

摘要

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: (CH3)(2)N (1(2)B(1)) and NO (X-2 Pi). However, while on S-1 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.