Hindered internal rotation in [15N]trifluoroacetamide

Abstract

The ¹H n.m.r. spectrum of [¹⁵N]trifluoroacetamide has been measured in dioxan and methyl propyl ketone from 15 to 80 °C. From the temperature dependence of the n.m.r. spectra the values for the energy barrier E_a to internal rotation about the central C–N bond were determined by means of total line-shape analysis. It has been found that the barrier was almost independent of the nature of the solvents used: E_a= 76·5 ± 2·9 in dioxan and 77·8 ± 2·5 kJ mol^–1 in methyl propyl ketone. The other thermodynamic activation parameters ΔG^‡(298 K), ΔH^‡ and ΔS^‡ were calculated to be 74·0, 74·5, and 2·1 in dioxan and 73·6 kJ mol^–1, 74·5 kJ mol^–1, and 3·3 J mol^–1 K^–1 in methyl propyl ketone. The results obtained are compared with those for related compounds.