RADICAL-ANION AND MEISENHEIMER COMPLEXES ARISING FROM INTERACTION OF THE HYDROXYL ION OR ALIPHATIC-AMINES WITH PHOTOEXCITED 4-NITROVERATROLE STUDIED BY ELECTRON-SPIN-RESONANCE AND TIME-RESOLVED RESONANCE RAMAN-SCATTERING

An electron spin resonance study of the reaction of aliphatic amines and OH- with 4-nitroveratrole (NV) in its lowest electronic triplet state is presented. It is concluded that both nucleophiles transfer an electron to triplet-state NV in the first chemical step of the reaction. This is then followed by rapid intersystem crossing. It seems that the intersystem crossing in the case of amines results from hyperfine coupling between electron and nuclear spins in the two resulting radical ions. In the case of OH- the intersystem crossing is much faster than in the former case, very likely owing to the large spin-orbit coupling in the resulting OH radical. In the second step of the reaction, a singlet-state Meisenheimer complex is formed only via reaction of the geminate radical pairs produced in the electron transfer step. In the case of OH- no radical anions of NV could be detected, most likely because of a very fast and efficient second step. A time-resolved resonance Raman study is presented, which is devoted to the characterization of Meisenheimer complexes formed via interaction of excited NV and the nucleophiles. The Raman spectra of triplet-state NV and of a Meisenheimer complex of NV and piperidine are presented, together with an assignment of the observed lines.