NITROXIDE RADICAL AS A PROBE FOR MOLECULAR-CONFORMATION .1. C-13 NMR-STUDY OF THE INTERACTION OF N,N-DIMETHYLAMIDES WITH NITROXIDE RADICAL

C-13 NMR contact shifts induced by the 2,2,6,6,-tetramethylpiperidine nitroxide radical (TMPN) were measured for N,N-dimethyl-para-substitut ed benzamides and N,N-dimethyl-beta-bromo- and -alpha,beta-methylacryl amides. The shifts were linear functions of radical concentration, c(r ). It was found that N-methyl group anti to the carbonyl group is a be tter acceptor site than the syn-N-methyl group and that there is a rel ationship between dDELTAdelta/dc(r) and the non-planarity of molecules of amides with an s-skew conformation. The electrostatic potential of amide molecules obtained from MNDO calculations was used to predict t he stereospecific interaction with radicals.