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

Abstract

Temperature dependent gas phase 1H NMR spectra of N-ethylmorpholine are consistent with the following activation parameters for the ring inversion process: Δ G Δ = 47.7(0.3) kJ mol −1, Δ H Δ = 47.1(2.3) kJ mol −1, Δ S 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) × 10 13s −1. Rate constants in the gas phase and in CDCl 3 solution are very similar while those obtained in CS 2 solutions are almost twice as large at corresponding temperatures. Rate constant obtained for a 1% solution of N-methylmorpholine in CDCl 3, obtained in the present study and those previously obtained in the gas phase and CS 2 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.