Infrared absorption intensities of the NO
2 symmetric and antisymmetric stretching vibrations were measured for some aromatic nitrocompounds. It was shown that in the case of ortho alkyl-substituted nitrobenzenes, the intensities of the symmetric stretching vibration decrease in an orderly fashion when the degrees of the twisting of NO
2 group from the plane of benzene ring increase owing to the ortho effect, while the quite opposite behaviour is observed for the antisymmetric stretching. From these results, it was concluded that the factor determining the intensities in question is the conjugation effect between NO
2 group and benzene ring. This was confirmed by the fact that in the case of para substituted nitrobenzenes, completely the same tendencies were observed in relation to the changes in the conjugation effect according to the kinds of substituents. Finally, it was shown that the sum of the intensities of the two vibrations is approximately constant for the compounds treated here, except those containing very strong electron-donating groups such as NH
2 and N (CH
3)
2.
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