Nuclear magnetic resonance study of trifluoroacetamide and N-methyltrifluoroacetamide

Abstract

The high-resolution proton and fluorine resonance spectra of trifluoroacetamide and N-methyltrifluoroacetamide in [²H₆]acetone and tetrahydrofuran, and their temperature-dependence in the range of –80 to +60°C have been measured and analysed. The change of the chemical shifts of amide protons with temperature may be accounted for in terms of the effects of the reaction field and the displacement of a dynamic solvation equilibrium. The coupling constant between the fluorine nuclei and one of the amide protons in trifluoroacetamide was found to be 1·8 Hz from the low-temperature spectrum and the rotational average coupling constant was 0·7 Hz from the higher-temperature spectrum. In N-methyltrifluoroacetamide, the coupling constants J(NH, Me)= 4·8 Hz, J(NH, CF₃)= 1·1 Hz, and J(CF₃, CH₃)= 0·7 Hz were obtained.