M. Tafazzoli et al., THE RING INVERSION OF GASEOUS N-METHYLPIPERAZINE - PRESSURE AND TEMPERATURE-DEPENDENT H-1-NMR STUDIES, Journal of molecular structure, 317(1-2), 1994, pp. 137-145
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.