Infrared absorption spectra of aldonitrones. I. Infrared spectra of benzaldehyde N-methyl and N-phenyl nitrones.

Abstract

The infrared spectra of benzaldehyde N-methyl and N-phenyl nitrones and the corresponding imines, as well as those of the related monosubstituted benzenes, were determined and the assignment was made for most of the vibrations in the 1700∼650 cm-1 region and the characteristic frequencies for aldonitrones were discussed. The N-O stretching frequencies were assigned to the strong absorption at 1172 cm-1 in the N-methyl and the strong doublet at 1088 and 1071 cm-1 (1082 and 1067 cm-1 as solid) in the N-phenyl nitrones, respectively. From these positions, it was concluded that these aldonitrones constitute a transitory group located between heteroaromatic and aliphatic amine N-oxides with respect to the nature of the N-oxide function, the N-methyl nitrone being closer to the former and the N-phenyl nitrone to the latter. The C=N stretching frequencies showed a large shift to a lower frequency in the nitrones as compared with those in the corresponding imines. Study of the solvent effect showed that the N-O and the C=N frequencies shifted markedly to a lower and a higher frequency, respectively, in a solvent capable of hydrogen bond formation. These results were discussed in terms of resonance contribution in the electronic state of the nitrones. Photochemical reaction of these nitrones was investigated spectrophotometrically and their conversion products on irradiation of ultraviolet light were identified and discussed.