Neuefeind J, Fischer H E, Simonson J M, Idrissi A, Schöps A, Honkimäki V
Spallation Neutron Source, Oak Ridge National Laboratory, 1 Bethel Valley Rd., Oak Ridge, Tennessee 37831, USA.
J Chem Phys. 2009 May 7;130(17):174503. doi: 10.1063/1.3116106.
We present neutron and x-ray scattering data (a 2N+X experiment) of liquid CO(2) and CS(2) at a density of about 10 molecules/nm(3). Because the scattering length contrast of the carbon isotope is very small and, in fact, smaller than anticipated from standard scattering length tables, a direct partial structure factor determination via matrix inversion gives unconvincing results. Instead we search for the best representation of the three independent scattering data sets by a simulation of rigid molecules interacting via a 12-6-1 potential, furthermore restricting the pressure p, the density rho, and the temperature T to the experimental values. We show that a 12-6-1 potential is completely adequate to describe the structure of CO(2); for CS(2) we find that the best 12-6-1 potential still slightly overestimates the height of the sulfur-sulfur pair-distribution function g(SS). Orientational correlations reflect the similarities much more than the differences of the two molecular systems. The distinct differences in the atom-atom pair distribution functions of CO(2) and CS(2) do not mean that their structures are radically different and the comparison with the crystalline structures is somewhat deceptive. A linear transformation, wherein all the parameters describing the interaction and the geometry of CS(2) are changed to those of CO(2), allows us to point out the physical parameters which may be responsible for the differences or similarities in thermodynamic behavior (pressure) and structures (orientations) between the two liquids.
我们展示了密度约为10个分子/纳米³的液态CO₂和CS₂的中子和X射线散射数据（2N + X实验）。由于碳同位素的散射长度对比度非常小，实际上比标准散射长度表预期的还要小，通过矩阵求逆直接确定部分结构因子会得到不可信的结果。相反，我们通过模拟经由12 - 6 - 1势相互作用的刚性分子，来寻找这三个独立散射数据集的最佳表示形式，此外还将压力p、密度ρ和温度T限制在实验值。我们表明，12 - 6 - 1势完全足以描述CO₂的结构；对于CS₂，我们发现最佳的12 - 6 - 1势仍略微高估了硫 - 硫对分布函数g(SS)的高度。取向相关性反映出这两个分子体系的相似性远多于差异性。CO₂和CS₂的原子 - 原子对分布函数的明显差异并不意味着它们的结构截然不同，与晶体结构的比较在某种程度上具有欺骗性。一种线性变换，即将描述CS₂相互作用和几何结构的所有参数都变换为CO₂的参数，使我们能够指出可能导致这两种液体在热力学行为（压力）和结构（取向）上存在差异或相似性的物理参数。
