Substituent effects on conformational equilibria: Substituted thiobenzanilides

Infrared absorption spectra of 38 thiobenzanilides in tetrachloromethane so lutions 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 su bstitution in an unexpected way which cannot be described by the Hammett eq uation or by any other known type of modified constants sigma. The only reg ularity observed is that the substituent effects from the two benzene rings of thioanilide are additive with a remarkable precision. The nu(N-H) frequ encies are not much sensitive to substitution and the dependence is not sim ple: only nu(N-H) of the E conformer depend on substitution in a regular wa y.