Experimental and ab initio MO studies on the IR spectra and structure of cyanoacetamide, its carbanion and dianion

The structures of cyanoacetamide NC-CH2-CONH2, its carbanion NC-(C) over bar H-CONH2 and dianion NC-(C) over bar CH-CONH2 and of their perdeutero and N-15 labelled analogues have been studied by means of both IR spectra and ab initio HF 6-31G and 6-31 + G(d) calculations. Both the spectral and structural changes, accompanying the conversion of cyanoacetamide into the carbanion, spread over the whole molecule; the carbanion --> dianion changes are localized mainly within the carboxamido group. The theory predicts qualitatively well the strong frequency decreases (165 and 247 cm(-1)) and the strong intensity increases (29 fold and 1.7 fold) of the C=N and C=O stretching bands respectively, resulting from the conversions studied. The first (carbanionic) electric charge is delocalized over the methide (0.36 e(-)), carboxamide (0.33 e(-)) and cyano (0.31 e(-)) groups the greater part (0.74 e(-)) of the second (nitranionic) charge remains localized within the carboxamido group. The possible conformers of the species studied are also discussed. The anionic species exist as ion pairs in dimethyl sulfoxide solutions. (C) 1998 Elsevier Science B.V. All rights reserved.