Influence of the conjugation effect on the UV-spectra of 4-substituted pyridine N-oxides and their conjugated acids

Abstract

The UV-spectra of three 4-substituted pyridine N-oxides: 4-methoxy pyridine N-oxide, 4-N′,N′-dimethylamine pyridine N-oxide, and 4-nitro pyridine N-oxide, as well as their simple cations in acetonitrile have been compared with the results of CNDO/S-CI calculations, in order to make possible the assignment of bands. For 4-N′,N′-dimethylamine and 4-methoxy derivatives both N-oxide oxygen and substituent protonated forms were considered. It has been found that in all cases except the 4-methoxy derivative conjugation with the substituent influences greatly the transitions of the free and N-oxide oxygen protonated N-oxides. The spectra of the substituent protonated N-oxides resemble that of free pyridine N-oxide which in comparison with the experimental data may indicate the preference of N-oxide oxygen protonation.