Reaction series not obeying the Hammett equation:: conformational equilibria of substituted thiobenzanilides

The conformation of the C-N bond of 4,4'-disubstituted thiobenzanilides and the equilibrium Z reversible arrow E were investigated by infrared spectroscopy and by the density functional theory (DFT) at the B3LYP/6-311+G(d,p) level. Together with previous work, 54 derivatives were examined spectroscopically and the conformational equilibrium was determined from the intensities of the nu(N-H) bands. It was confirmed that the substituent effects on log K are not governed by the Hammett equation. This failure is neither due to experimental errors nor to specific shortcomings of the IR method, since the results agree with the DFT calculations and also fulfil the additive relationship. The constant shift of the DFT energies with respect to the experimental Gibbs energies can be explained by entropy factors. The most probable explanation of the failure is as a result of strong resonance effects and, in addition, pi-pi interaction between the two substituted benzene rings in the E conformation.