THE RING INVERSION OF GASEOUS N-METHYLPIPERAZINE - PRESSURE AND TEMPERATURE-DEPENDENT H-1-NMR STUDIES

Pressure dependent gas phase ring inversion rate constants of N-methyl piperazine (MPZ) obtained from line-shape analysis of exchange broaden ed H-1 NMR spectra can be adequately modeled using RRKM theory indicat ing that intramolecular vibrational redistribution in critically energ ized MPZ molecules is statistical or nearly so and that transition sta te theory is a useful approximation at high pressures for providing a framework for comparisons of rate data obtained for this and other sim ilar molecules. Temperature dependent gas phase H-1 NMR spectra of MPZ are consistent with the following activation parameters for the chair reversible chair ring inversion process: DELTAG(double dagger) = 51.5 (1.2) kJ mol-1, DELTAH(double dagger) = 51.9(1.7) kJ mol 1. DELTAS(dou ble dagger) = 2.1(1.7) J mol-1 K-1, E(act) = 55.7(1.7) kJ mol-1 and A( infinity) = 2.2 (1.3) x 10(13) s-1. DELTAH(double dagger) and E(act) f or MPZ are slightly higher than values obtained for gaseous N-methylpi peridine, indicating that replacement of the CH2 group at position fou r of the ring with an NH group increases the barrier to ring inversion . DELTAG247double dagger 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.