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-nitropyridine N-oxide (POM) obtained by time-resolved resonance Raman spectroscopy (TR3) are reported. NPO and NPO-d4 keep the C2v symmetry in their T1 state but the structure of the cycle is modified towards a pronounced quinonoid conformation. An important charge transfer from the N-oxide to the NO2 group is observed. The TR3 activity of T1-POM vibrations is very similar to T1-NPO, the T1 --> T(n) transitions of these molecules are of the same nature. Ab initio SCF calculations have been performed and the results obtained for both geometry and electron deformation density are in good agreement with spectroscopic data,