Structure and Dynamics of Benzene in One-Dimensional Channels

Molecular dynamics investigation of benzene in one-dimensional channel systems AlPO4-5, VPI-5, and carbon nanotube is reported. The results suggest that, in all the three host systems, the plane of benzene is almost perpendicular to the channel axis when the molecule is near the center of the channel and the plane of benzene is parallel to the channel axis when the molecule is near the wall of the channel. The density distribution of benzene as a function of channel length, z, and the radial distance, r, from the channel axis is also different in the three host structures. Anisotropy in translational diffusion coefficient, calculated in body-fixed frame of benzene, suggests that benzene prefers to move with its plane parallel to the direction of motion in AlPO4-5 and VPI-5 whereas in carbon nanotube the motion occurs predominantly with the plane of the benzene perpendicular to the direction of motion. Anisotropy associated with the rotational motion is seen to alter significantly in confinement as compared to liquid benzene. In AlPO4-5, the rotational anisotropy is reversed as compared to liquid benzene thereby suggesting that anisotropy arising out of molecular geometry can be reduced. Reorientational correlation times for C 6 and C 2 axes of benzene are reported. Apart from the inertial decay of reorientational correlation function due to free rotation, two other distinct regimes of decay are observed in narrower channels (AlPO4-5 and carbon nanotube): (i) an initial fast decay (0.5−2 ps) and (ii) a slower decay (>2 ps) of reorientational correlation function where C 6 decays slower than C 2 similar to what is observed in liquid benzene. In the initial fast decay, it is seen that the decay for C 6 is faster than C 2 which is in contrast to what is observed in liquid benzene or for benzene confined in VPI-5.