Gas-Phase 1H NMR Studies of Internal Rotation Barriers and Conformer Stabilities of N-Ethyl, N-Methylthioamides

Abstract

Kinetic parameters and conformational equilibrium constants characterizing the internal rotation in gaseous N-ethyl, N-methylthioformamide (EMSF), N-ethyl, N-methylthioacetamide (EMSA), and N-ethyl, N-methyltrifluorothioacetamide (EMSTFA) are determined from exchange broadened 1H NMR spectra. The ΔG⧧298 values, in kcal mol-1, are 22.6 (0.1), 17.8 (0.1), and 17.2(0.1) for EMSF, EMSA, and EMSTFA, respectively. Conformer Gibbs free energy differences, ΔG° (ΔG° = G° (methyl group syn to thionyl sulfur) − G° (methyl group anti to thionyl sulfur)), in cal mol-1, are −123 (24), 56 (9), and 96 (13) for EMSF, EMSA, and EMSTFA, respectively. These results are compared to solution phase values and to the corresponding oxoamides.