Nuclear Magnetic Resonance Studies of Multi-site Chemical Exchange. II. Hindered Rotation in N,N-Dimethyl Carbamyl Fluoride

Abstract

The matrix formulation of the Bloch equations including chemical exchange reported earlier is extended to include indirect spin–spin coupling in first-order spectra. The ABX (JAB = 0) spin system is treated in detail and particular attention is paid to the determining effect on the nuclear magnetic resonance (n.m.r.) lineshapes of the relative signs of the coupling constants JAX and JBX. The hindered rotation for N,N-dimethyl carbamyl fluoride in CCl4 as solvent has been studied using a complete 1H n.m.r. lineshape analysis and the activation parameters obtained are: ΔG≠ = 18.1 ± 0.6 kcal mol−1, ΔH≠ = 17.7 ± 0.6 kcal mol−1, and ΔS≠ = −1.4 ± 2.1 cal deg−1 mol−1 at 25 °C.The complete lineshape fits give very precise values of the relative shifts (16.5 mol% in CCl4) of the methyl groups and of the coupling constants JAX (0.30 ± 0.05 Hz) and JBX (0.80 ± 0.05 Hz) at all temperatures. A 40% change in JBX (1.10 ± 0.05 Hz) is observed in neat DMCF from a lineshape fit at −15 °C. The origin of changes in chemical shifts with temperature and JBX with solvent is discussed.