Infrared spectra and structure of carbanions: XVIII. A semiempirical study of carbanions of cyanoacetic acid derivatives

The IR spectra of cyanoacetic acid, ethyl cyanoacetate and cyanoacetamide as well as those of related mono- and, whenever possible, dianions have been studied in dimethyl-sulphoxide (DMSO) and DMSO-d₆.The observed nitrile and carbonyl absorption frequencies correlate linearly with the corresponding Wiberg bond indices given by CNDO/2 calculations with full geometry optimization. These calculations predict carbanionic structures throughout except in the case of the dinegative ion of cyanoacetamide, which could be considered as originating from the aminoacetylenic tautomer of NCCH₂CONH₂. Parallel MINDO/3 calculations, however, predict that the latter dianion is again a carbanion. This result is in reasonable agreement with normal coordinate calculations and the experimental isotopic shifts of vibrational frequencies of the dianion ¹⁵NCCH^?CONH^?.