Effect of different cations on the N—CO rotational barrier of N,N-dimethylacetamide. Variable temperature proton magnetic resonance study

Abstract

The rates of rotation around the N—CO bond of N,N-dimethylacetamide in the presence of several metal perchlorates in propylene carbonate have been measured as a function of temperature and electrolyte to amide ratio.

The data for systems involving lithium perchlorate showed considerable concentration dependence which was attributable to incomplete complexation of the amide to the cation.

Arrhenius plots gave pre-exponential factors in the range 10^12·6–10^13·5, in agreement with the value of 10^12·9 expected for a unimolecular process at the temperatures studied.

The activation energies for N,N-dimethylacetamide complexed to the various cations ranged from 69.5 to 88.3 kJ mol^–1. A linear correlation was found between the activation energies of the complexed amide and the intensity of the electrical field at the surface of the complexed cation for six of the eight systems studied, which supports the idea of an electrostatic stabilization of negative charge at the carbonyl oxygen by most cations.

Silver complexes are unique among those studied in that the activation energy is less than that for the uncomplexed amide.