The ring inversion of gaseous N-methylpiperazine. Pressure and temperature dependent ¹H NMR studies

Pressure dependent gas phase ring inversion rate constants of N-methylpiperazine (MPZ) obtained from line-shape analysis of exchange broadened ¹H NMR spectra can be adequately modeled using RRKM theory indicating that intramolecular vibrational redistribution in critically energiezed MPZ molecules is statistical or nearly so and that transition state theory is a useful approximation at high pressures for providing a framework for comparisons of rate data obtained for this and other similar molecules. Temperature dependent gas phase ¹H NMR spectra of MPZ are consistent with the following activation parameters for the chair ⇌ chair ring inversion process: ΔG^‡ = 51.5(1.2) kJ mol^-1, ΔH^‡ = 51.9(1.7) kJ mol^-1, ΔS^‡ = 2.1(1.7) J mol^-1 K^-1, E_act = 55.7(1.7) kJ mol^-1 and A_∞ = 2.2(1.3) × 10¹³ s^-1. ΔH^‡ and E_act for MPZ are slightly higher than values obtained for gaseous N-methylpiperidine, indicating that replacement of the CH₂ group at position four of the ring with an NH group increases the barrier to ring inversion. ΔG^‡₂₄₇ in the gas phase is 3.3 kJ mol^-1 higher than in a 20% solution of MPZ in CH₂Cl₂, and 2.5 kJ mol^-1 higher than in a 20% solution of MPZ in CH₃OH, indicating that interactions with polar solvent molecules facilitate the ring inversion process.