Carbon-13 N.M.R. studies of carbocations. II. An investigation of the correlation between carbon chemical shifts and σ⁺ constants for 1-phenylethyl cations

Abstract

Eighteen 1-methyl-1-phenylethyl (cumyl) (1) and 1-phenylethyl (styryl) (2) carbocations have been prepared in known concentration in FSO₃H/SbF₅ solution from their corresponding carbinols and their ¹H and ¹³C spectra have been measured. The correlations of both the methyl ¹H shifts [δH(Me)] and methyl ¹³C shifts [δC(Me)] with the cationic carbon shifts [δ(C⁺)] have been determined. In the case of δC(Me) curves are obtained indicating non-linearity of chemical shifts with charge density. Correlations of δ(C⁺) with Brown's σ⁺ constants are only fair, the deviation from linearity being interpreted in terms of non-linear dependence of either δ(C⁺) or σ⁺, or both, on charge density at this level of charge. The use of an enhanced substituent constant (σ⁺⁺) and/or the dual substituent parameter equation improves the correlation. Non-linearity in the plot of δ(C 1) against σ⁺ reported for 2-arylnorborn-5-en-2-yl(4) and 2-arylnorbornan-2-yl cations (5) has been shown to occur also in the case of δC(Me) of the cumyl cations (1). The implications of this finding with respect to the structures of (4) and (5) are discussed.