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 multicon-figurational 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)_2N (1~2B_1) and NO (X~2Π). 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.