Magnetic resonance studies of triphenylcarbonium ions—II : Carbon-13 magnetic resonance studies of charge delocalization in para-substituted triphenylcarbonium ions

Abstract

Carbon-13 NMR shifts of para-substituted triphenylcarbinols, ( p-XC 6H 4) 3COH and the corresponding carbonium ions, ( p-XC 6H 4) 3C + where X = H, NO 2 F, Cl, Me, OMe and NMe 2) have been measured and interpreted in terms of charge densities, via the empirical relation between C-13 shifts and charge densities. The general features of the charge distributions given by the C-13 shifts are in accord with a simple resonance picture, with relatively large positive charges at positions alpha and meta to the substituent. The amount of positive charge delocalized from the central carbon is proportional to the substituent parameter, σ +. The validity of an additivity relation for the C-13 shifts of the carbonium ion species indicates that the charge distribution in the substituted ion is, approximately, the superposition of the distributions in the unsubstituted ion and the corresponding monosubstituted benzene. However, the substituents OMe and NMe 2 bear a relatively large portion of the charge delocalized from the central carbon. The charge densities derived from C-13 shifts for triphenylcarbonium ion correlate well with those calculated by the CNDO method, even though the agreement is not quantitative.