Structural Effects in Solvolytic Reactions. 35. Carbon-13 Nuclear Magnetic Resonance Studies of Carbocations. Effect of Increasing Electron Demand on the Carbon-13 Nuclear Magnetic Resonance Shifts in 2-Aryl-2-butyl and 4-Aryl-4-heptyl Carbocations. Correlation of the Data by a New Set of Substituent Constants, σ < sup > C < /sup > < sup > + < /sup >

Abstract

The 13C NMR shifts of a series of meta- and para-substituted 2-phenyl-2-butyl and 4-phenyl-4-heptyl carbocations were measured in SbF5/FSO3H/SO2ClF. The plots of the chemical shifts of the cationic carbon, ∆δC+, in the various substituted derivatives against the values of substituent constants, π and π+, reveal only relatively poor correlations. However, excellent linear correlations are realized in the plots of ∆δc+ against the new σC+ values proposed for these systems. The 2-aryl-2-butyl system yields a pC+ value of -17.78 with a correlation coefficient r = 0.9998. The 4-aryl-4-heptyl system gives a pC+ value of -14.57 with r = 0.999. The nearly perfect linear correlations observed for these systems support the validity and utility of these new σC+ constants. © 1981, American Chemical Society. All rights reserved.