Molecular reorientation in plastic crystals: Infrared and Raman band shape analysis of neopentane

Abstract

An infrared and Raman band shape analysis of the broadened 924 cm−1 fundamental for neopentane in its liquid and plastic crystal phases is presented. Correlation functions and times for molecular reorientation derived from both the infrared and Raman data show the liquidlike behavior of the plastic phase of neopentane, with the molecules rotating ``freely'' through ∼ 10° (175 °K) to ∼ 30° (300 °K) around an inertial axis with the corresponding reduced intermolecular torques (<L2>1/2/kT) decreasing from 8.2 to 3.8. Futhermore, the linewidth and correlation time results show no indication of a change in rotational behavior in passing through the plastic crystal-liquid phase transition. Theoretical fits of our experimental infrared and Raman correlation function with Gordon's M and J diffusion models, as extended by McClung for spherical molecules, show that the experimental results lie between the functions predicted by these two models. The time between rotational ``collisions'' (angular momentum correlation time) varies continuously from 0.4 × 10−12 sec for the room-temperature liquid to 0.2 × 10−12 sec at the lowest temperature in the plastic phase. Activation energies for molecular reorientation of 4.1 and 3.6 kJ/mol are obtained, respectively, from the experimental half-widths and from the angular momentum correlation times, in good agreement with previous NMR and neutron scattering results. The results prove that neopentane melts in two stages: near 140 °K, the rotational degrees of freedom of the (rigid) molecule are liberated, whereas near 253 °K the translational degrees of freedom are liberated without observable change of the characteristics of the rotational motion.