Rotational Motion of n-Pentane in H-ZK5

gamma and (b) alpha cage in zeolite ZK5. 2. Methodology The structure model of the zeolite H-ZK5 was obtained by an optimization using the semi-empirical method MNDO, since no experimental structure data were available. Moreover, the influence of the electric field of the proton has been examined. Two clusters were selected from the optimized zeolite structure, the neutral (Si2O7H6) and the acid (SiAlO7H7) cluster. Several configurations of each cluster and a methane molecule were generated. Then, quantum calculations at HF and B3LYP level with the 6-31G(d,p) basis set were performed to estimate the interaction between the methane molecule and the cluster for each configuration. The potential energy curves of both models have been compared. The united atoms approximation is used in which the CH3 and CH2 groups are treated as spherical force centers. They interacted with the corresponding force centers in other pentane molecules and with the zeolite lattice by Lennard-Jones potentials [1]. The potential for the torsional elasticity has been taken from [2]. The loading of n-pentane in H-ZK-5 was equivalent to an experimental [3] density of 1 molecule in each cage. The lattice including 8 unit cells was kept rigid during the simulation. The average temperature was adjusted to 298 K. The rotation of n-pentane in both types of cages of ZK5 has been investigated in terms of the auto-correlation function (ACF) of the quantity 3cosθ-1 where θ is the angle between a fixed axis of the coordinate system and an intramolecular vector. In addition, the preferred orientation and the position of the n-pentane molecule especially in the gamma cage have been examined.