Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211-7600, USA.
J Chem Phys. 2012 Jan 21;136(3):034501. doi: 10.1063/1.3676727.
Molecular dynamics simulations of supported shock waves (shock pressure P(s) ∼ 15 GPa) propagating along the [110], [011], [101], and [111] directions in crystalline nitromethane initially at T = 200 K were performed using the nonreactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. These simulations, combined with those from a preceding study of shocks propagating along [100], [010], and [001] directions in nitromethane for similar conditions of temperature and shock pressure [L. He, T. D. Sewell, and D. L. Thompson, J. Chem. Phys. 134, 124506 (2011)], have been used to study the post-shock relaxation phenomena. Shocks along [010] and [101] lead to a crystal-crystal structure transformation. Shocks propagating along [011], [110], [111], [100], and [001] exhibit plane-specific disordering, which was characterized by calculating as functions of time the 1D mean square displacement (MSD), 2D radial distribution function (RDF), and 2D orientation order parameter P(2)(θ) in orthogonal planes mutually perpendicular to the shock plane; and by calculating as functions of distance behind the shock front the Cartesian components of intermolecular, intramolecular, and total kinetic energies. The 2D RDF results show that the structural disordering for shocks along [100], [110], and [111] is strongly plane-specific; whereas for shocks along [001] and [011], the loss of crystal structural order is almost equivalent in the orthogonal planes perpendicular to the shock plane. Based on the entire set of simulations, there is a trend for the most extensive disordering to occur in the (010) and (110) planes, less extensive disordering to occur in the (100) plane, and essentially no disordering to occur in the (001) plane. The 2D P(2)(θ) and 1D MSD profiles show, respectively, that the orientational and translational disordering is plane-specific, which results in the plane-specific structural disordering observed in the 2D RDF. By contrast, the kinetic energy partitioning and redistribution do not exhibit plane specificity, as shown by the similarity of spatial profiles of the Cartesian components of the intermolecular, intramolecular, and total kinetic energies in orthogonal planes perpendicular to the shock plane.
采用非反应性 Sorescu-Rice-Thompson 力场[D.C. Sorescu、B.M. Rice 和 D.L. Thompson，J. Phys. Chem. B 104, 8406（2000）]对沿[110]、[011]、[101]和[111]方向传播的晶体硝甲烷中传播的支撑激波（激波压力 P(s)∼15 GPa）进行了分子动力学模拟，初始温度为 T=200 K。这些模拟与之前在硝甲烷中沿[100]、[010]和[001]方向传播的激波的模拟相结合，研究了激波传播后的弛豫现象。沿[010]和[101]方向传播的激波导致晶体-晶体结构的转变。沿[011]、[110]、[111]、[100]和[001]方向传播的激波表现出特定于平面的无序化，通过计算时间的一维均方位移（MSD）、正交平面中相互垂直于激波平面的二维径向分布函数（RDF）和二维取向有序参数 P(2)(θ)来对其进行了表征，并且通过计算冲击前缘后面的距离来计算笛卡尔分子间、分子内和总动能的分量。二维 RDF 结果表明，沿[100]、[110]和[111]方向传播的激波的结构无序化具有很强的平面特异性，而沿[001]和[011]方向传播的激波，在垂直于激波平面的正交平面中，晶体结构有序度的损失几乎是等效的。基于整个模拟集，最广泛的无序化趋势发生在(010)和(110)平面，在(100)平面发生较少的无序化，在(001)平面基本没有无序化。二维 P(2)(θ)和一维 MSD 分布分别表明，取向和平移无序化具有平面特异性，这导致在二维 RDF 中观察到的平面特异性结构无序化。相比之下，动能分配和再分配没有表现出平面特异性，这可以通过垂直于激波平面的正交平面中分子间、分子内和总动能笛卡尔分量的空间分布的相似性来证明。
