INFRARED-SPECTRA AND STRUCTURE OF PHENYLACETONITRILE AND OF ITS CARBANION - AN AB-INITIO FORCE-FIELD TREATMENT

The structures of phenylacetonitrile and of its carbanion have been studied on the basis of IR spectroscopic data (including literature results) and of ab initio force field calculations. The assignment (D. Croisat et al., J. Org. Chem., 157 (1992) 6435) of the IR bands of phenylacetonitrile, its d(5) analogue, and their carbanions has been confirmed. An excellent linear correlation (R = 0.999) has been found between the theoretical and experimental IR frequencies of the species studied. The calculations predict well the strong increase in intensity (five to 42 fold) of the v(CN), v(S8) and v(19) bands which accompanies the conversion of the phenylacetonitrile molecule to its carbanion. The structures of both sodium and potassium derivatives of phenylacetonitrile in dimethyl sulfoxide are close to that of the kinetically free phenylacetonitrile carbanion. The carbanionic center is practically planar; the cyano group carries a considerable negative charge, but its influence on the carbanionic center is mainly inductive. The carbanionic charge is delocalized over the phenyl ring (0.42 e(-)), methide (0.30 e(-)), and cyano (0.28 e(-)) groups.