On the Conformation of a Sterically Congested Amide Group in the Crystalline State and in Solution

The conformation in the crystalline state and in solution of the sterically congested tetramethylpiperidinederived amide group of the symmetric diamide I formed from 2,2,6,6-tetramethylpiperidine ( b ) and 1,2,5-thiadiazole-3,4-dicarbonyl dichloride ( c ), and of the mixed diamide II derived from b , c , and piperidine ( a ) has been investigated. In crystals, as observed with II , this group is strongly bent out-of-plane at both the N-atom and the carbonyl C-atom, and there is also a sizable twisting around the amide bond. Furthermore, the amide bond is abnormally long (1.37 Å). In CD₂Cl₂ (or CDCl₃) solution, the group is apparently planar in its ground-state conformation, but the energy barrier to rotation around the amide bond is low. This conclusion is based on low-temperature ¹H-NMR measurements on I , II , and on the symmetric diamide III derived from a and c .