Substituent effects on conformational equilibria:: Substituted thiobenzanilides

Infrared absorption spectra of 38 thiobenzanilides in tetrachloromethane solutions were recorded in the nu(N-H) region. From the integrated absorption intensity of the computer-resolved N-H bands, the abundance of the Z and E conformations was calculated. The values of pertinent log K-C depend on substitution in an unexpected way which cannot be described by the Hammett equation or by any other known type of modified constants sigma. The only regularity observed is that the substituent effects from the two benzene rings of thioanilide are additive with a remarkable precision. The nu(N-H) frequencies are not much sensitive to substitution and the dependence is not simple: only nu(N-H) of the E conformer depend on substitution in a regular way.