C-H Hyperconjugation in α-chlorocarbocations

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