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 rotation in formamide are the barrier-forming N(lp)/C-O(π)* interaction and the antibarrier-forming N(lp)/C-O(σ)* interaction. There is a large difference between the rotational barriers of formamide and urea. The most significant causes which lead to the low rotational barrier of urea are the reduction of the barrier-forming N(lp)/C-O(π)* interaction energy and the existence of the antibarrier-forming N′(lp)/C-O(π)* interaction. The nonplanarity of the ground state for urea is caused by the competition of the two former interactions. They come into conflict with each other to a greater extent in the planar structure than in the nonplanar structure and this excessive conflict results in the instability of the planar structure. To relax this strain, the geometry is transformed into a stable nonplanar one. We found that the same interpretation as for formamide is applied to the thioamide system. The amide π-orbital system was investigated to understand the change in the C-N and C-O(S) bond length during the rotation process.