On the structure of liquid hydrogen fluoride.

The liquid state is the most complex phase of matter. Densities of liquids are comparable to densities of the solids, implying that the forces between particles in the liquid are of the samemagnitude as those forces present in the solid. However, there is no simplification due to the presence of a lattice and no satisfactory analytic theory of the liquid state exists. However, despite this complexity, the liquid state is an outstandingly important chemical milieu in which many reactions take place. Strongly associated fluids are particularly complex and the structure and properties of these fluids provide an exacting and stringent test of theory. Here, we report the first investigation of the structure of hydrogen fluoride at the level of the distributions of pairwise interatomic distances, the partial pair correlation functions. Liquid HF is an important chemical and it is widely used in the petrochemical industry, as a catalyst for hydrocarbon management, and in the glass and ceramics industries. Academically, its superior properties as a solvent have found application in both organic and inorganic chemistry, and the superacidic properties have been exploited in both disciplines, in the study of reactive intermediates and reaction mechanisms That these highly desirable properties are not more widely applied is mainly due to the exceedingly toxic and corrosive nature of the material, which is severe when anhydrous and only somewhat lessened in solution. Indeed, given the properties of liquid HF, it has been stated that the calculation of its properties is to be preferred over measurement. The true importance of this fluid does not solely rest with its industrial and academic applications; it is the simplest archetype for the strong hydrogen bond, and the molecular simplicity of HF makes it an attractive model for strongly hydrogen-bonded systems. That hydrogen bonding should be so important to understand need not be reiterated, once the importance of this interaction in structural biology, materials science, chemistry and physics is appreciated. This important, directional structural interaction is responsible, inter alia, for protein conformations, the stability of the structure of DNA and the properties of water and other associated fluids. Both the bulk properties 10,11] and microscopic structure of HF have been the focus of intense theoretical investigation; there have been many calculational approaches to the structure and properties of HF using a variety of methods. The overarching feature of these calculations is the complete lack of experimental data with which to compare the results of calculation at the pair correlation function level. The only structural data reported to date are two total structure factor measurements for DFat a variety of thermodynamic state points. Given that the total structure factor is the weighted sum of the partial structure factors, it is unsurprising that there is a variance in the results of the calculated structural models of HF at the pair correlation function level. The hydrogen bond is the dominant feature of the structural chemistry of HF in all phases; the solid is composed of unbranched, zigzag chains while the vapor is composed of cyclic oligomers and clusters. In the liquid, the macroscopic properties are consistent with strong hydrogen bonds, though until this report, there has been no experimental data to confirm this at the pair correlation function level. To determine the atomic structure and therefore the hydrogen-bonded nature of HF, high-energy X-ray and neutron diffraction measurements were performed on samples of HF and DF at 296 2 K and 1.2 0.1 bar. Both types of radiation were used to provide complementary information on the structure. X-rays scatter from electron density, weighting the contribution of each atom to the scattering pattern by Z the atomic number. In contrast, the interaction of neutrons with matter is dependent on the composition of the nucleus and therefore the isotopic nature of each sample defines magnitude of the scattering interaction. Assuming isostructurality between isotopomeric samples, it is possible, by taking linear combinations of diffraction patterns, to solve the structure factor equations and explicitly determine each of the individual structure factors. This technique has been widely applied to diffraction studies of liquids as well as other disordered systems. The pair correlation function is related to the scattered intensity by Fourier transformation as given in Equation (1), where 1 is the atomic number density.