Experimental and ab initio MO studies on the IR spectra and structure of cyanoacetic acid, its oxyanion and dianion

The structures of cyanoacetic acid NC-CH2-CO2H , its oxy anion NC-CH2-CO2- and dianion NC-(C) over bar H-CO2- were studied by means of both IR spectra and ab initio HF and MP2 calculations. Both spectral and structural changes, accompanying the conversion of cyanoacetic acid into the oxyanion, are localized mainly within the carboxylato group; the oxyanion --> dianion changes spread over the whole anionic species. The theory predicts qualitatively well the drastic frequency decrease (146 cm(-1)) and the intensity increase (34 fold) of the C = N stretching band, the carboxy --> carboxylato IF: changes etc. resulting from the conversions studied. The greater part (0.70 e(-)) of the first (oxyanionic) electric charge remains localized within the carboxylato group; the second (carbanionic) charge is delocalized over the methide (0.43 e(-)), carboxylato (0.29 e(-)) and cyano (0.28 e(-)) groups. The possible conformers of the cyanoacetic acid molecule are also discussed. The presence of solvent-separated ion pairs in the DMSO solutions of the alkali-metal mono-and di-derivatives of cyanoacetic acid is assumed. (C) 1998 Elsevier Science B.V.