Gas and solution phase NMR studies of N-ethylmorpholine intra- and intermolecular effects on ring inversion rate constants

Temperature dependent gas phase ¹H NMR spectra of N-ethylmorpholine are consistent with the following activation parameters for the ring inversion process: ΔG^Δ = 47.7(0.3) kJ mol^-1, ΔH^Δ = 47.1(2.3) kJ mol^-1, ΔS^Δ = -1.9(4.9) J mol^-1 K^-1, E_act = 49.2(2.3) kJ mol^-1 and A_∞ = 1.2(0.4) × 10¹³s^-1. Rate constants in the gas phase and in CDCl₃ solution are very similar while those obtained in CS₂ solutions are almost twice as large at corresponding temperatures. Rate constant obtained for a 1% solution of N-methylmorpholine in CDCl₃, obtained in the present study and those previously obtained in the gas phase and CS₂ solutions show a similar trend. The observed phase and solvent dependent rate constants do not agree with predictions based on simple steric or dielectric considerations and may indicate that complex static and dynamic solvent effects are important in the liquid solutions.