The ultraviolet spectra of nitrones, the configurations of which were assumed from their dipole moments and chemical reactivities, and their related compounds have been measured in various solvents. The following results have been obtained: (1) Nitrones show a characteristic strong π-π* band in the 280∼370 mμ region as well as one or two additional bands with a relatively low intensity in the 200∼260 mμ region. The molecular extinction coefficient (ε) of the former is about 20000. The decrease in intensity of the π-π* band is observed in the spectra of the sterically-hindered compounds. (2) These bands always show a blue shifs with the increasing polarity of the solvent. The general rule of solvent effect on heterocyclic N-oxides applies also to nitrones, the conjugated N-oxide band of which has the same property as that of heterocyclic N-oxides. (3) It has been confirmed by ultraviolet and infrared spectra that the formation of a hydrogen bond such as N→O…H (solvent) plays an important role in this blue shift phenomenon.
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Kubota et al. (1963) studied this question.