THE NATURE OF ROTATIONAL BARRIERS OF THE C-N BOND IN THIOAMIDES AND THE ORIGIN OF THE NONPLANARITY FOR THIOUREA

The most important donor/acceptor interactions about the C(O)-N bond r otation in formamide are the barrier-forming N(1p)/C-O(pi) interactio n and the antibarrier-forming N(1p)/C-O(sigma) interaction. There is a large difference between the rotational barriers of formamide and ur ea. The most significant causes which lead to the low rotational barri er of urea are the reduction of the barrier-forming N(1p)/C-O(pi) int eraction energy and the existence of the antibarrier-forming N'(1p)/C- O(pi) interaction. The nonplanarity of the ground state for urea is c aused by the competition of the two former interactions. They come int o conflict with each other to a greater extent in the planar structure than in the nonplanar structure and this excessive conflict results i n the instability of the planar structure. To relax this strain, the g eometry is transformed into a stable nonplanar one. We found that the same interpretation as for formamide is applied to the thioamide syste m. The amide pi-orbital system was investigated to understand the chan ge in the C-N and C-O(S) bond length during the rotation process.