Structural effects in solvolytic reactions; carbon-13 NMR studies of carbocations^†: Effect of increasing electron demand on the carbon-13 NMR shifts in substituted tert-cumyl and 1-aryl-1-cyclopentyl carbocations—correlation of the data by a new set of substituent constants, σ^C+,

Abstract

The cationic carbon substituent chemical shifts (ΔδC⁺) for nine representative meta-substituted tert-cumyl carbocations are correlated satisfactorily by the σ_m⁺ substituent constants (slope ρ+ = -18.18, correlation coefficient r = 0.990). However, the substituent chemical shifts (ΔδC⁺) for the corresponding para derivatives are not correlated by the σ_p⁺ substituent constants. The possibility of developing a set of substituent constants capable of correlating such ¹³C NMR shifts was examined. The slope of the line defined by the meta substituents (ρ+ = -18.18) was utilized to calculate σ^C+ constants for both meta and para substituents. The utility of these constants was then tested by their ability to correlate the ¹³C NMR shifts in the cations for a different system, the 1-aryl-1-cyclopentyl cations. Indeed, these σ^C+ values correlate very well with the ΔδC⁺ values, yielding ρ^C+ = -16.84, r = 0.999.

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