Modelling X-ray scattering factors from fluids of some fluorinated molecules and related compounds

Abstract

There is current interest, both for basic science and technological applications, in fluorocarbons such as CF₄ and the related molecules. Here we first report a Hartree–Fock calculation of the ground-state electron density in the octahedral molecule SF₆ using the experimentally determined bond length. From this density, the number of electrons lying inside a sphere of radius r, denoted by Q(r), centred on the S nucleus has been extracted. The X-ray scattering factor f(k) is then modelled using Q(r) and compared with f(k) for the isoelectronic united atom ytterbium. The molecule CF₄, together with its isoelectronic partners PF₃, CrF₂ and AsF, is again compared with regard to f(k) with the united atom Mo. Finally, interest in modelling PbF is pointed out, in relation to the quantum-chemical theory already available.

Keywords:

• X-ray scattering
• perfluorinated gases
• CF₄
• SF₆
• isoelectronic series

Acknowledgements

We are indebted to Professor E. Lomba for helpful discussions. JMO acknowledges support from European Union projects FP7-ICT-2009-4-248909-LIMA, FP7-ICT-2009-4-248855-N4E, NMP4–SL–2008–213669–ENSEMBLE and Dirección General de Universidades e Investigación de la Comunidad de Madrid under Grant No. S2009/ESP/1691 and Program MODELICO-CM. DJK thanks the Welch Foundation for financial support (BD-0894). A.T. Balaban thanks the Welch Foundation for financial support (BD-0046). N.H. March is indebted to Professors W.A. Seitz, D.J. Klein and A.T. Balaban for very generous hospitality. N.H. March's continuing affiliation with the University of Antwerp (UA) was made possible by Professors D. Lamoen and C. Van Alsenoy through UA grant BOF-NOI.