The structure of PX3 (X = Cl, Br, I) molecular liquids from X-ray diffraction, molecular dynamics simulations, and reverse Monte Carlo modeling

Abstract

Synchrotron X-ray diffraction measurements have been conducted on liquid phosphorus trichloride, tribromide and triiodide. Molecular Dynamics simulations for these molecular liquids were performed with a dual purpose: (1) to establish whether existing intermolecular potential functions can provide a picture that is consistent with diffraction data; (2) to generate reliable starting configurations for subsequent Reverse Monte Carlo modelling. Structural models (i.e., sets of coordinates of thousands of atoms) that were fully consistent with experimental diffraction information, within errors, have been prepared by means of the Reverse Monte Carlo method.Comparison with reference systems, generated by hard sphere-like Monte Carlo simulations, was also carried out to demonstrate the extent to which simple space filling effects determine the structure of the liquids (and thus, also estimating the information content of measured data). Total scattering structure factors, partial radial distribution functions and orientational correlations as a function of distances between the molecular centres have been calculated from the models. In general, more or less antiparallel arrangements of the primary molecular axes that are found to be the most favourable orientation of two neighbouring molecules. In liquid PBr 3 electrostatic interactions seem to play a more important role in determining intermolecular correlations than in the other two liquids; molecular arrangements in both PCl 3 and PI 3 are largely driven by steric effects.

Structure of Neat Liquids Consisting of (Perfect and Nearly) Tetrahedral Molecules

“…In the previous sections, computational methods have been introduced whose results (including RDF, PRDFs, or total scattering function) depend on the applied set of interparticle potentials. In this subsection, a simulation method is introduced that is biased by the agreement with the measured diffraction data and that has been quite extensively applied for liquids consisting of tetrahedral molecules over the past nearly 20 years. , …”

“…It was found that both the neutron and the X-ray diffraction data were reproduced at a nearly quantitative level by the MD simulation that applied the OPLS all-atom force field, see Figure 2 of ref . (Note that this is not true for the hard-sphere reference system, for which significant deviations were detected from both sets of experimental data, see again Figure 2 of ref .) The RMC calculation that followed the MD simulation acted as a refinement to the MD configuration and was able to achieve a perfect fit to both X-ray and neutron diffraction data.…”

“…Phosphorus tribromide is a (not too dangerous, in contrast with the trichloride) liquid under ambient conditions, and perhaps this is the reason why several diffraction measurements could be performed conveniently on this system that has become a prototype of tetrahedral XY 3 materials. Starting from the late 1980s, two neutron , and one synchrotron X-ray diffraction experiments have been carried out. Van Tricht et al determined the neutron-weighted TSSF up to about 13 Å –1 using monochromatic neutrons from a reactor source.…”

“…The only X-ray diffraction experiment was carried out in conjunction with an extensive computer modeling effort in which molecular dynamics simulation and reverse Monte Carlo modeling were both applied, and hard-sphere reference systems were also prepared by Monte Carlo simulation. It was found that both the neutron and the X-ray diffraction data were reproduced at a nearly quantitative level by the MD simulation that applied the OPLS all-atom force field, see Figure 2 of ref . (Note that this is not true for the hard-sphere reference system, for which significant deviations were detected from both sets of experimental data, see again Figure 2 of ref .)…”

Surface modification of alumina-coated silica nanoparticles in aqueous sols with phosphonic acids and impact on nanoparticle interactions

“…For 3D-visualization of particle configurations compatible with our experimentally observed scattered intensities, a reverse Monte Carlo (RMC) algorithm was applied to I(q). [39][40][41][42] It is based on the experimental size distribution of spheres, and calculation of I(q) of homogeneous interacting polydisperse spheres with an excluded volume. 43 The basic design of the algorithm is outlined in ref.…”