SPECTROSCOPIC AND THEORETICAL-STUDIES OF 4-NITROPYRIDINE N-OXIDE AND OF ITS RELATED CHARGE-TRANSFER COMPOUNDS IN THEIR EXCITED-STATE T(1)

Lowest triplet state T1 Raman spectra of 4-nitropyridine N-oxide NPO, its deuterated derivative NPO-d4 and its related compound 3-methyl 4-n itropyridine N-oxide (POM) obtained by time-resolved resonance Raman s pectroscopy (TR3) are reported. NPO and NPO-d4 keep the C2v symmetry i n their T1 state but the structure of the cycle is modified towards a pronounced quinonoid conformation. An important charge transfer from t he N-oxide to the NO2 group is observed. The TR3 activity of T1-POM vi brations is very similar to T1-NPO, the T1 --> T(n) transitions of the se molecules are of the same nature. Ab initio SCF calculations have b een performed and the results obtained for both geometry and electron deformation density are in good agreement with spectroscopic data,