PRESSURE-DEPENDENT AND TEMPERATURE-DEPENDENT H-1-NMR STUDIES OF N-METHYLMORPHOLINE RING INVERSION

Gas-phase 1H NMR spectra of N-methylmorpholine (MM) display exchange-broadened line shapes that are both temperature and pressure dependent. Infinite pressure, unimolecular gas-phase inversion rate constants are lower and the free energy of activation is higher than observed in liquid-phase samples. This phase dependence is consistent with an activation volume, Δ*, of -9 cm3 mol-1 and a transition state smaller than the equilibrium chair conformation. Gas-phase unimolecular activation parameters for the chair to twist-boat inversion are Eact, = 52.5 (0.4) kj mol-1, A = 1.8 (0.3) × 1013 s-1, ΔG* = 49.9 (0.4) kJ mol-1, ΔH‡ = 50.3 (0.4) kJ mol-1, and ΔS‡ = 1.3 (2.6) J mol-1 K-1. Activation parameters for a liquid sample containing 1.00 mol % MM in CS2 are ΔG‡ = 46.3 (0.4) kJ mol-1, ΔH‡ = 47.3 (0.8) kJ mol-1, and ΔS‡ = 3.3 (3.3) J mol-1 K-1. The pressure dependence of the inversion rate constants agrees with RRKM calculations, indicating stochastic intramolecular vibrational redistribution (IVR) in MM at internal energies of ca. 50.2 kJ mol-1 and 9800 vibrational states/cm-1. © 1990 American Chemical Society.