Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211-7600, USA.
J Chem Phys. 2009 Dec 14;131(22):224513. doi: 10.1063/1.3271349.
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.
最近有研究报告称[J. Chem. Phys. 131, 064503 (2009)]，采用分子动力学和非反应势能，对垂直于（100）晶面的冲击加载后硝基甲烷的结构弛豫进行了研究。初始温度分别为 T(0)=50 和 200 K，使用冲击速度 U(p) 范围从 0.5 到 3.0 km s(-1) 对冲击进行了模拟；对于冲击速度为 2.0 km s(-1) 及更高的冲击，明显观察到了熔化现象。在此，我们报告了使用基于 Eckart 框架正则模态分析的方法对这些模拟数据进行分析的结果，该方法允许将动能分配到分子自由度上。通过平均平动和转动动能以及各振动模式加热速率，获得了能量传递的描述。一般来说，在冲击后早期，平动和转动自由度（对应于晶体的声子模式）会出现很大的过热度。最低频率的振动（门控模式）会迅速被激发，也会出现过热度。其余振动模式的激发发生得更慢。对于 T(0)=50 K 和 U(p)=2.0 km s(-1) 的冲击条件，观察到对称 C-H 伸缩模式的早期快速激发，这是由于分子取向与冲击方向的有利排列以及振动和冲击速度之间的频率匹配的综合作用。
