Reconstruction of the orientational pair-correlation function from neutron-diffraction data: The case of liquid hydrogen iodide

An analysis of partial-structure-factor information on molecular liquids is described which uses the spherical harmonic expansion of the site-site structure factors to extract an estimate of the orientational pair-correlation function between molecules. Recently published neutron-diffraction data on the site-site partials in liquid hydrogen iodide at 210 K are used as the input data to this technique. The results, which are presented as a map of the orientational pair-correlation function, g(r,omega1,omega2), show the occurrence of pronounced relative orientational correlations between molecules at this temperature, even though the same orientations are apparently only weakly correlated with the molecular center of mass. The same spherical harmonic expansion procedure can be applied to a number of other molecular liquids where diffraction data are available.