Pressure- and temperature-dependent ¹H NMR studies of N-methylpiperidine ring inversion in the gas phase

Gas-phase proton NMR spectra of N-methylpiperidine (MPD) obtained at temperatures between 277 and 303 K display exchange-broadened line shapes that are both temperature and pressure dependent. Gas-phase unimolecular activation parameters for the chair to chair ring inversion process are ΔH^‡ = 50.6 (0.8) kJ mol^-1, ΔS^‡ = 1 (4) J K^-1 mol^-1, and ΔG^‡₂₉₈ = 50.2 (0.4) kJ mol^-1. The gas-phase free energy of activation at 298 K is 0.8 kJ mol^-1 lower than that obtained in a previous liquid study.¹ This phase dependence is consistent with an activation volume (ΔV^‡) of ca. 7 cm³ mol^-1. Rate constants obtained at 300.9 K for pressures between 5.4 and 2548.3 Torr can be adequately modeled using RRKM kinetic theory, indicating statistical or nearly statistical internal vibrational redistribution in MPD at an internal energy of 51.6 (3.3) kJ mol^-1 and a state density of ∼24 000 states/cm^-1.