Role of steric and electrostatic effects in the short-range order of quasitetrahedral molecular liquids

Abstract

The study of how both steric and electrostatic interactions affect the structure of liquids formed by quasitetrahedral molecules has been undertaken in this work. We have studied trichlorobromomethane (CBrCl3) and dibromodichloromethane (CBr2Cl2), both displaying a dipole along their C3v and C2v molecular symmetry axes, respectively. The short-range order of the liquid state has been determined using neutron diffraction experiments that were modeled through the reverse Monte Carlo (RMC) technique. To study changes in steric effects due to the distortion of the tetrahedral symmetry, we have compared our results with a previous RMC modeling of carbon tetrachloride (CCl4). The subtle effects of the dipole in the structure of the liquid have been determined using a set of molecular dynamics simulations with and without atomic partial charges, being the force field validated via comparison with the diffraction data. In a first approximation, neither steric nor electrostatic interactions are able to modify the molecular ordering of a fully tetrahedral liquid such as CCl4. A more detailed analysis indicates that, although the interaction between dipoles does not have appreciable effects when aligned along the C3v molecular axes, as for the CBrCl3, it enhances the antiparallel orientation of dipoles when it is oriented along the C2v axes, as in the case of CBr2Cl2.Postprint (published version

Structure determination of liquid carbon tetrabromide via a combination of x-ray and neutron diffraction data and Reverse Monte Carlo modelling

“…Generally speaking, distance-dependent probabilities of each class follow the regular characteristics shown for perfect tetrahedral [12,17] and for CBr 4−x Cl x type molecules [18,32,19] (neglecting the differences between Cl and Br atoms). The first maxima for the classes follow each other from the closely contacting 3:3 pairs at the intermolecular CC cutoff distance through 2:3 (5.4 Å), 2:2 (6 Å), 1:3 (6.2 Å) and 2:1 (7.2 Å) to the 1:1 (about 7.4-7.7 Å) correlations.…”

“…Nevertheless, the majority of orientational correlations could be captured within errors by using one single experimental result. That is, recent RMC studies on the CBr 4−x Cl x family of molecular liquids [32,19] were most certainly able to capture most structural details of the real systems.…”

Reverse Monte Carlo investigations concerning recent isotopic substitution neutron diffraction data on liquid water

“…From the particle configurations, further structural characteristics (coordination numbers, nearest neighbor distances, bond angle distributions, etc...) may be calculated. Over the past nearly three decades, RMC has been successfully applied to a wide variety of systems, from simple liquids (see, e.g., the review of McGreevy [30]), through metallic [35] and covalent [36] glasses, simple [37,38] and hydrogen-bonded [19] molecular liquids, as well as for characterizing disordered crystalline structures [39,40].…”

Differences in first neighbor orientation behind the anomalies in the low and high density trans-1,2-dichloroethene liquid

“…24 Figure 5 shows the angular position (θ CM and ϕ CM ) of the center of mass of the first neighbor with respect to the reference molecule (for more details and examples on bivariate analysis see Refs. [25][26][27][28], where no qualitative difference has been observed between both liquids, since their first neighbor is equally located around θ CM ≈ 90 • and ϕ CM ≈ 90 • . Figure 6 shows two examples of molecular pairs in such angular position.…”