13C Nuclear Magnetic Resonance Studies of the Hindered Internal Rotation and the Molecular Association of N,N-Dimethylacetamide

Abstract

Abstract The concentration and temperature dependences of the 13C chemical shifts of N,N-dimethylacetamide (DMA) in CCl4, H2O, and H2SO4 solutions were measured by means of a 13C FT-NMR spectrometer at 25.15 MHz. From the temperature dependence of the chemical-shift difference between the cis- and trans-NCH3 carbons, the activation energy, Ea, for the hindered internal rotation about the C–N bond was obtained to be 10–17.5 kcal/mol; it was also found to be ca. 10 kcal/mol at an infinite dilution in a CCl4 solution. The chemical shift of each carbon in the DMA molecule was calculated using Pople’s expression under the average excitation-energy approximation by means of the CNDO/2 method. The calculated finding that each carbon in DMA appears in the order of carbonyl, trans-NCH3, cis-NCH3, and acetyl carbons from the low field agrees with the order assigned by McFarlane. Further, as for the molecular association in neat liquid, various models are discussed on the basis of quantum-chemical calculations, and a model is proposed which has antiparallel oriented carbonyl groups in associated states of the molecules of more than two. The observed differences between the chemical shifts of the carbonyl carbons at dilutions in CCl4 and H2O solutions, and between those in CCl4 and H2SO4 solutions, are found to agree well with the results calculated using the model in which the DMA molecules at dilutions in CCl4, H2O, and H2SO4 exist as monomer, hydrogen-bonded DMA with H2O, and protonated DMA respectively.