Predictions of 13C chemical shifts in carbocations.: The use of scaled chemical shifts calculated using GIAO DFT methods

Accurate prediction of C-13 chemical shifts (delta(pred)) for the C+ and the alpha- and beta-carbon atoms in alkyl and cycloalkyl carbocations is achieved through scaling. Ab initio calculated chemical shifts (delta(calc)) at the GIAO-B3LYP/6-311 G(d,p)//B3LYP16-31G(d) level of theory for 16 different carbocations are scaled using a linear correlation equation (delta(pred) = a delta(calc) + b). The slope and intercept a and b were determined separately for C+ and the alpha- and beta-carbon positions, and were found to be different for cation structures preferentially stabilized by beta-C-H and beta-C-C hyperconjugation. A very good correlation of all predicted and experimental chemical shifts is obtained (a = 0.999 +/- 0.002, b = 0.231 +/- 0.320). Preliminary results indicate that scaling using linear correlations can also be applied to the C+ carbon chemical shift in benzyl type carbocations.