Sulfur-nitrogen bonds, rotation barrier

Compared to the corresponding formamides and acetamides, the non-bonding interaction between the sulfur atom and the hydrogen atom of the methyl group attached to the nitrogen atom is important. Two rotamers of the a,p-un-saturated thioamides 6 and 7 are observed in NMR spectra, and their rotational barriers can be elucidated. The isomers 6, where the thiocarbonyl and benzyl groups are located in a cis position with respect to the C-N single bond. 

In sulfenamides (R S—NR R ), the cation-radicals keep an nnpaired electron occupying a n orbital. This orbital is localized between the sulfur and nitrogen atoms. As a matter of fact, a slight S=N double bond character exists in the neutral sulfenamides. A consequence of this double bond character is an increase in the energy barrier, which restricts rotation around the S—N bond. Restricted rotation about the S—N bond is known in neutral sulfenamides (Kost and Raban 1990). Notably, the energy barrier to this rotation is greater for the derived cation-radicals when compared to the parent compounds (Bassindale and Iley 1990). 

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