C-H HYPERCONJUGATION IN ALPHA-CHLOROCARBOCATIONS

The lowering of CBH stretching frequencies in carbocations 1a-d and 2a -c induced by hyperconjugation was tested as a possible probe for esti mating the electron donating ability of a-substituents. Conclusions ar e based on the results of high level quantum chemical calculations con firmed with experimental FT-IR spectra. Because the decrease in the CB H stretching frequency is comparable in Ib and in Ic, and in 2b and 2c respectively, it follows that a-substitution by a methyl group or by chlorine stabilizes a carbocation with almost the same effectiveness.