Proton NMR Spectra of m- and p-Substituted Thioformanilides and the Substituent Effects on the Chemical Shifts

Abstract

The prbton NMR spectra of twelve m- and P-substituted thioformanilides in dimethyl sulfoxide were measured. The assignment to the rotational isomers (1) and (2) was med on the basis of the Hammett-type dependence of the iH chemical shifts of the thioamide group In the trans orm(1), the chemical shifts are correlated with the Yukawa-Tsuno s equation with the common r-value for both electron-donating conjugative and electronwithdrawing conjugative substituents (Eq. (3) and Fig.4). This result means that the aryl ripg exists in the same plane as that of the thioamide group (Fig.3), and that the aryl and the thioamide groups constitute an expanded conjugated system over the whole molecule.
In the cis-form (2), the chemical shifts of the thioformyl protons are correlated with the ae-val, ues (Eq. (5)). This corresponds with the model that the aryl and the thioamide groutps are obliged to be twis ted owing to the steric repulsion between the hydrogen atom of the aryl ring and the sulphur atom of the thioamide group, as is shown in Fig.3.
Thioformanilides provide us an example, in which substiuent effects are determined by a common Yukawa-Tsuno s equation for both electron-donating conjugative and electron-withdrowing conjugative substituents,