Organic photochemistry. Part VI. The ultraviolet absorption spectra of N-substituted hydrazones

Abstract

Methylation of the terminal nitrogen atom in the planar hydrazones of aliphatic aldehydes and ketones causes a red shift in the ultraviolet absorption spectrum, readily interpreted by Hückel molecular orbital calculations using the heteroatom model for the methyl group. In ketone NN-dimethylhydrazones, Me₂N·N:NR¹R², which are non-planar because of steric interaction of R¹ and Me, the long-wavelength transition is displaced considerably (∼30 mµ) to the red with respect to the corresponding planar monomethylhydrazones, and shows n→π* characteristics. Calculations show that this surprising effect can be ascribed to the interaction of the unshared pairs of electrons on the adjacent nitrogen atoms of these non-planar hydrazones.