Shock-induced melting of (100)-oriented nitromethane: Energy partitioning and vibrational mode heating.

A study of the structural relaxation of nitromethane subsequent to shock loading normal to the (100) crystal plane performed using molecular dynamics and a nonreactive potential was reported recently [J. Chem. Phys. 131, 064503 (2009)]. Starting from initial temperatures of T(0)=50 and 200 K, shocks were simulated using impact velocities U(p) ranging from 0.5 to 3.0 km s(-1); clear evidence of melting was obtained for shocks initiated with impacts of 2.0 km s(-1) and higher. Here, we report the results of analyses of those simulation data using a method based on the Eckart frame normal-mode analysis that allows partitioning of the kinetic energy among the molecular degrees of freedom. A description of the energy transfer is obtained in terms of average translational and rotational kinetic energies in addition to the rates of individual vibrational mode heating. Generally, at early times postshock a large superheating of the translational and rotational degrees of freedom (corresponding to phonon modes of the crystal) is observed. The lowest frequency vibrations (gateway modes) are rapidly excited and also exhibit superheating. Excitation of the remaining vibrational modes occurs more slowly. A rapid, early excitation of the symmetric C-H stretch mode was observed for the shock conditions T(0)=50 K and U(p)=2.0 km s(-1) due to a combination of favorable alignment of molecular orientation with the shock direction and frequency matching between the vibration and shock velocity.