Infrared spectra and structure of alkane- and cycloalkanecarbonitriles and of their carbanions: An ab initio force field treatment

The structure of alkane- and cycloalkanecarbonitriles (seven compounds) and of their carbanions has been studied by both infrared spectrometry and ab initio force field calculations. The carbanions (counter ions Li+, Na+ and K+) have been found to exist mainly as ionic aggregates in hexamethylphoshoric triamide solutions. The calculations describe well the marked decrease, by 124-214 cm(-1), in the nitrile band frequencies and also the strong increase, by 1-2 orders, in the nitrile band integrated intensities which accompany the conversion of the parent neutral molecules into carbanions. Cyclopropanecarbonitrile is remarkable as having the highest nitrile band intensity among all the neutral molecules and the lowest one among all the carbanions studied. This result has also been predicted by the calculations, and it can be explained by certain peculiarities in the structure of the particles. The conjugation of the carbanionic charge with the cyano group in the cyclopropanecarbonitrile carbanion is greatly hindered by the considerable deviation (estimated al 56 degrees) of the cyano group from the ring plane. The carbanionic charges of the carbanions studied are delocalized over the cyano groups (0.30-0.41 e(-)), carbanionic centres (0.08-0.29 e(-)) and hydrocarbon moieties (0.34-0.63 e(-)).