Infrared spectra and structure of isomeric (cyanophenyl)acetonitriles and their carbanions: an ab initio force field treatment

Abstract

The structures of o-, m- and p-(cyanophenyl)acetonitrile molecules and their carbanions were studied on the basis of infrared spectroscopic data and ab initio force field calculations. The assignment was given for the 3100-1100 cm −1 bands of the substances studied. The scaled theoretical infrared band frequencies agree well with those measured experimentally. An excellent linear correlation ( R = 0.999) was found between the theoretical and experimental v CN frequencies of both molecules and carbanions. The calculations predict well the strong increase in intensity (1.5- to 70-fold) of the v CN, v 8 and v 19 bands, which accompanies the conversion of the isomeric (cyanophenyl)acetonitrile molecules into the corresponding carbanions. The structures of the lithium, sodium and potassium derivatives of the nitriles studied in dimethyl sulphoxide are close to those of the kinetically free carbanions. The carbanionic centres are practically planar; the cyano groups carry considerable negative charges, but their influences on the carbanionic centres are mainly inductive. The carbanionic charges are delocalized over the phenylene rings (0.35-0.40 e −), methide (0.22-0.29 e −), α-cyano (0.24-0.27 e −) and ring-cyano (0.08-0.14 e −) groups.