A proton magnetic resonance study of hindered internal rotation in some substituted N,N-dimethylamides

Abstract

Several studies of hindered internal rotation about the C-N bond in amides have been made by high-resolution proton magnetic resonance (p.m.r.) spectroscopy. Various methods have been reported for obtaining the activation energies Ea and frequency factors A for internal rotation from the temperature dependence of the p.m.r. spectrum. Recent methods make it possible to reduce the rather large experimental errors associated with these parameters in the earlier studies. We therefore have made a redetermination of the energy barrier and frequency factor for internal rotation about the central C-N bond of N,N-dimethylacetamide (DMA). The same experimental techniques and methods of calculation then were applied for the measurements of the barrier heights for internal rotation in a series of substituted N,N-dimethylamides, none of which had been studied previously. A detailed treatment of the errors in the present method leads to the conclusion that the probable error in Ea has been reduced to less than ±0.8 kcal./mole. The effect of structure on Ea therefore may be discussed with some assurance that differences among different substituted amides are significant.