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

Abstract

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: Δ G ‡ = 51.5(1.2) kJ mol −1, Δ H ‡ = 51.9(1.7) kJ mol −1, Δ S ‡ = 2.1(1.7) J mol −1 K −1, E act = 55.7(1.7) kJ mol −1 and A ∞ = 2.2(1.3) × 10 13 s −1. Δ H ‡ and E act for MPZ are slightly higher than values obtained for gaseous N-methylpiperidine, indicating that replacement of the CH 2 group at position four of the ring with an NH group increases the barrier to ring inversion. Δ G ‡ 247 in the gas phase is 3.3 kJ mol −1 higher than in a 20% solution of MPZ in CH 2Cl 2, and 2.5 kJ mol −1 higher than in a 20% solution of MPZ in CH 3OH, indicating that interactions with polar solvent molecules facilitate the ring inversion process.