The structure of fluid trifluoromethane and methylfluoride

Abstract

We present hard X-ray and neutron diffraction measurements on the polar fluorocarbons HCF 3 and H 3 CF under supercritical conditions and for a range of molecular densities spanning about a factor of ten. The Levesque-Weiss-Reatto inversion scheme has been used to deduce the site-site potentials underlying the measured partial pair distribution functions. The orientational correlations between adjacent fluorocarbon molecules -which are characterized by quite large dipole moments but no tendency to form hydrogen bonds -are small compared to a highly polar system like fluid hydrogen chloride. In fact, the orientational correlations in HCF 3 and H 3 CF are found to be nearly as small as those of fluid CF 4 , a fluorocarbon with no dipole moment.PACS: 61.20.Qg, 61.20.Ja, 61.25.Em IntroductionThe understanding of dielectric properties resulting from orientational correlations [1,2], as well as the determination of orientational correlations from diffraction experiments [3,4], are long standing problems in the physics of molecular fluids. The simple fluorocarbons HCF 3 and H 3 CF are very interesting model substances in this context, as they posses rather large dipole moments (1.65 · 3.336 · 10 −30 Cm), in the case of trifluoromethane -the same as that of the water molecule) but no tendency to form hydrogen bonds [5]. The investigation of the structure of the simple fluorocarbons H 3 CF and HCF 3 thus enables the study of the structural effect of the molecular dipole alone.Although the properties of these fluorocarbons are interesting, only very limited structural information is available so far. HCF 3 is discussed as replacement for chlorinated hydrocarbons as refrigerant since it it has no ozone damaging effect, it has a shorter atmospheric lifetime and, hence, a lower global warming potential and it presents no toxological risk [6,7]. HCF 3 is discussed for extraction applications [8] and it has been shown, that the enatioselectivity of asymmetric catalysis can be controlled by the density of the fluoroform solvent [9]. The crystal structure of HCF 3 has been determined by a neutron powder diffraction experiment [10] and the molecular geometry by a gas phase electron diffraction

On the Structure of [K(crypt‐222)]+

“…Let us now see whether or not the new structures refined in the correct space group can shed light on the nature of the CF 3 unit. The structures of CHF 3 in the gas, [21] solid, [22] and liquid [23] phases are in good agreement with the DFT data [15] (Table 1). For CF À 3 , only gas-phase DFT-computed geometries are available, [15] [24] which differ noticeably from those of CHF 3 .…”

Reverse Monte Carlo modelling

“…A number of methods have been developed which produce structural models from experimental data, but via the refinement of an interatomic potential (e.g. [65][66][67][68][69][70]). Probably the most extensively used of these is the empirical potential structure-refinement (EPSR) method due to Soper [68].…”

The structure of liquid carbon dioxide and carbon disulfide

“…The molecular density of both systems was held within 0.3% of 0.009 93 Å −3 , occurring at atmospheric pressure for CS 2 and at ϳ66 bars for CO 2 . The CO 2 was enclosed in an aluminum pressure cell described earlier 34 that allows investigation of samples with x rays and neutrons in the same sample environment in order to minimize systematic errors. This capability is especially useful in this case as it is necessary to reproduce the same thermodynamic state in both neutron and the x-ray experiments as precisely as possible, especially since CO 2 is fairly close to its critical point under the experimental conditions, and therefore small changes in pressure and temperature lead to fairly large changes in the density.…”