A complete active space self-consistent field study of the photochemistry of nitrosamine.

Photodissociation mechanisms of nitrosamine (NH2NO) have been studied at the complete active space self-consistent field level of theory in conjunction with atomic-natural-orbital-type basis sets. In addition, the energies of all the critical points and the potential energy curves connecting them have been recomputed with the multiconfigurational second-order perturbation method. Ground state minimum of nitrosamine has a C1 nonplanar structure with the hydrogen atoms of the amino moiety out of the plane defined by the N-N-O bonds. Electronic transitions to the three lowest states are allowed by selection rules: (i) S0-->S3 (7.41 eV) has an oscillator strength of f=0.0006 and it is assigned as an (npO)0-->(piNO*)2 transition, (ii) S0-->S2 (5.86 eV) has an oscillator strength of f=0.14 and it is assigned as an npN-->piNO* transition, and (iii) S0-->S1 (2.98 eV) has an oscillator strength of f=0.002 and it is assigned as an npO-->piNO* transition. It is found that N-N bond cleavage is the most likely process in all the photochemical relevant states, namely, S1 (1 1A"), S2 (2 1A'), and T1 (1 3A"). While S1 and T1 yield exclusively homolytic dissociation: NH2NO-->NH2 (1 2B1)+NO(X 2Pi), on S2 the latter process constitutes the major path, but two additional minor channels are also available: adiabatic homolytic dissociation: NH2NO-->NH2 (1 2A1)+NO(X 2Pi), and adiabatic oxygen extrusion: NH2NO-->NH2N (1 3A1)+O(3P). The excited species NH2 (1 2A1) experiences a subsequent ultrafast decay to the ground state, the final products in all cases the fragments being in their lowest electronic state. We have not found a unimolecular mechanism connecting excited states with the ground state. In addition, homolytic dissociation in the ground state, tautomerizations to NHNOH and NHNHO, and intersystem crossings to T1 are considered. The most favorable process on this state is the isomerization to NHNOH.