Vacuum ultraviolet absorption spectra of simple amides

Vacuum ultraviolet absorption spectra of six simple amides were measured. It was found that the positions of the firstπ→π ^* transition bands shift appreciably by the substitutions of methyl groups for hydrogen atoms of the NH₂ and(begin{gathered} | hfill H - C = O hfill end{gathered} ) groups. A general tendency is that the substitution in the NH₂ group shifts the band toward longer wavelengths, in the(begin{gathered} | hfill H - C = O hfill end{gathered} ) group however towards shorter wavelengths. This was explained satisfactorily by considering the nature of the band (intramolecular charge-transfer band) and the hyperconjugation effect of the methyl group.