Neutron-diffraction study of liquid hydrogen iodide

Neutron-diffraction experiments on liquid DI, HI, and on an equimolar mixture of HI and DI are presented. All these three samples were in the same thermodynamic state corresponding to the orthobaric liquid at T=253 K. The three partial structure factors ${\mathit{S}}_{\mathrm{II}}$(Q), ${\mathit{S}}_{\mathrm{HI}}$(Q), ${\mathit{S}}_{\mathrm{HH}}$(Q) are derived exploiting the standard isotopic substitution procedure. The corresponding pair correlation functions ${\mathit{g}}_{\mathrm{II}}$(r), ${\mathit{g}}_{\mathrm{HI}}$(r), and ${\mathit{g}}_{\mathrm{HH}}$(r) are evaluated and compared with those given by a model that neglects all orientational correlations. Our data indicate that ${\mathit{g}}_{\mathrm{II}}$(r) (which is essentially the center-center correlation function) is well reproduced by the pair distribution function of a monatomic Lennard-Jones fluid and that ${\mathit{g}}_{\mathrm{HI}}$(r) (which should be sensitive to the correlations between molecular and intermolecular axes) is very similar to the one derived neglecting orientational correlations. On the contrary, orientational correlations between the molecular axes are clearly present in the ${\mathit{g}}_{\mathrm{HH}}$(r), which deviates significantly from the uncorrelated model results. These facts are consistent with the idea that H bonding is not present in liquid HI and indicate also that the only relevant terms of the anisotropic intermolecular potential are those due to the electric multipolar interactions.