Comparison of kinetic results obtained by NMR line shape and equilibration methods

Abstract

Rates of internal rotation around carbonyl-to-nitrogen bonds are determined for the rotamers IA and IB of N-benzyl-N,2,4,6-tetramethylbenzamide by equilibration of IA and for N,N,2,4,6-pentamethylbenzamide (II) by 1H-NMR line shapes. The activation parameters (Table V) obtained by the two methods agree well within error limits. This represents the first experimental confirmation of the soundness of kinetic measurements by line shapes. The range of rate constants accessible by the two techniques amounts to almost 10 7 sec −1. Further molecular systems are suggested as suitable for such a comparison; the nitrosamines VIIA and VIIB, for example, will allow both methods to be applied to the isomers of the same compound. Steric contributions to the free enthalpies of activation are evaluated for the internal rotation in some 2,4,6-trisubstituted benzamides.