In a few cases, single-bond rotation is so slowed that cis and trans isomers can be isolated even where no double bond exists. One example is Nmethyl-N-benzylthiomesitylide (69 and 70), the isomers of which are stable in the crystalline state but interconvert with a half-life of _25h in CDCl at 50_C. This type of isomerism is rare; it is found chiefly in certain amides and thioamides, because resonance gives the single bond some double-bond character and slows rotation