Local structure and orientational ordering in liquid bromoform

Abstract

The neutron diffraction data of liquid bromoform (CHBr3) at 25°C was analysed using the Empirical Potential Structure Refinement technique in combination with H/D isotopic substitution. Compared to liquid chloroform (CHCl3), CHBr3 displays more spatially defined intermolecular contacts. A preference for polar stacking with collinear alignment of dipole moments is observed for the most closely approaching CHBr3 molecules, although to a lesser extent than in chloroform. Consistent with this and in line with dielectric spectroscopy, the Kirkwood correlation factor from the structural model of CHBr3 is smaller than that of CHCl3. The net antiparallel alignment of dipole moments in CHBr3, as suggested by dielectric spectroscopy, must be due to weak but persistent longrange orientation correlations in CHBr3, which counteract the local polar stacking.

Deconstructing the Local Intermolecular Ordering and Dynamics of Liquid Chloroform and Bromoform

“…The overall similarity of the haloform SDFs, despite chlorine atoms being 0.088 e more negative than bromoform’s bromine atoms, is an indication that local ordering is a strong function of packing effects. Both pairs of SDFs show very good overall agreement with the recent ND-EPSR results of Salzmann and co-workers, , and our analysis of the SDFs in Figure will detail the differences observed.…”

“…Since the two haloforms share C 3 v symmetry, steric character (one hydrogen and three halogens), and an overall nearly tetrahedral geometry, similarity between their corresponding RDFs is expected. These RDFs agree well with neutron diffraction data and previous simulations that employ a polarizable model force field. ,, Figure a shows the RDFs of the central carbon atoms, in which each haloform exhibits local ordering extending to three solvation shells, with peaks at approximately 5, 10, and 15 Å. Due to bromoform’s larger size, the bromoform curve is shifted slightly toward larger values than chloroform, with the first peaks at 5.3 and 5.5 Å.…”

“…This is in contrast to earlier diffraction studies reporting that the dominant short-range structural feature is antiparallel, where fully chlorinated faces of two tetrahedral molecules approach each other. , Additionally, Salzmann and co-workers introduce the definition of a “polar stack”, reminiscent of the “Apollo configuration”, where neighboring CHCl 3 molecules have parallel and colinear dipoles with one molecule’s hydrogen atom positioned in the hollow formed by the neighbor molecule’s three chlorine atoms. , Our molecular dynamics simulations of liquid chloroform agreed with the reported population and population distribution of these polar stacks but also suggested that these structures are a result of steric packing, not electrostatic interactions . Salzmann and co-workers recently followed their CHCl 3 work with a similarly detailed ND-EPSR analysis of liquid CHBr 3 , that builds upon earlier scattering-reverse Monte Carlo studies , by elucidating detailed angle-dependent pair correlations (SDFs) and dipole–dipole orientational correlations . These two experimental studies suggest structural similarity but a greater tendency for antiparallel alignment in CHBr 3 than in CHCl 3 , a phenomena the authors attribute to stronger halogen bonding or weak but persistent long-range orientational correlations.…”

“…Polarizable models provided further accuracy at the cost of additional computational expense and allowed for more precise simulation of chloroform’s behavior in solvent/solute and interfacial systems. − We refer the reader to a recent review by Pusztai and co-workers for a more complete discussion of computational and theoretical approaches . The local intermolecular ordering of these haloforms has been investigated in several studies over the last few decades − and has recently received additional attention with the detailed neutron diffraction-empirical potential structure refinement (ND-EPSR) studies performed by Salzmann and co-workers. , …”

Phase Stability of Chloroform and Dichloromethane at High Pressure

“…Both CHCl 3 and CH 2 Cl 2 are characterized by non-zero dipole moments. Hence the study of the interactions of these molecules in the liquid and solid state leads to a better understanding of the interplay between close packing and polar aggregation [3][4][5][6][7]. Both molecules are also model systems for the study of intermolecular interactions such as halogen-halogen bonding [8][9][10], or hydrogen bonding [11][12][13].…”