Rotational Motion of Pentane in the Flat γ Cages of Zeolite KFI

Abstract

The orientation distribution and confined rotational motion of n -pentane in the oblate γ cages of zeolite KFI is computed by use of MD simulation. Pentane is preferably oriented with its end-to-end vector in the x and y direction perpendicular to the C 4 symmetry axis of the cage. For comparison with a previous NMR study (Zorine et al .; J. Phys. Chem. B 2004, 108, 5600), the orientational autocorrelation function (OACF) of the C 1 C 2 C 3 bi-sector is obtained from the simulations between 125 and 450 K, and decomposed into three exponential components. The slowest component follows the Arrhenius law with an activation energy of 13 kJ mol -1 and corresponds well to the effective rotation correlation times obtained from NMR relaxation above 200 K. MD trajectory analysis suggests that this component reflects gauche−trans conformation changes within the cage. According to our present findings, rotation of trans−trans pentane about the cage symmetry axis, which was previously proposed as the source for NMR relaxation, is actually too fast. Based on the OACF analysis, the non-Arrhenius behavior observed in the mentioned NMR study is now explained by a simultaneous increase of the correlation time and relative amplitude of the slowest OACF component versus inverse temperature.