The application of the Hammett equation to ¹³C N.M.R. spectroscopy: correlations in the β-nitrostyrenes

Abstract

A series of 33 β-nitrostyrenes has been prepared and their ¹³C N.M.R. spectra have been determined in dimethyl sulphoxide and in deuterochloroform (where soluble). Hammett correlations between substituent and the chemical shift of both the para ring carbons and the α-carbon of the ethenyl group have been shown to be only approximate. In contrast, an excellent correlation has been observed between the chemical shift of the β-carbon of the ethenyl group and Hammett σ⁺ constants in all cases except those involving para substituents capable of exerting a - R effect. Possible reasons for this are considered. The system appears to be the first reported one where ¹³C measurements for both meta- and para-substituted compounds give rise to correlations in which agreement lies well inside the limits of experimental error, and shows promise as a convenient method for calculating substituent constants and studying their variation with solvent.