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Predictions of 13C Chemical Shifts in Carbocations. The Use of Scaled Chemical Shifts Calculated Using GIAO DFT Methods.

Accurate prediction of 13C 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/6-311g(d,p)//B3LYP/6-31G(d) level of theory for 16 different carbocations are scaled using a linear correlation equation (delta pred=adelta calc+b). The slope and intercep 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 +/-0.022, 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.

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