The correlation of molecular orientation in liquid water by neutron and X-ray scattering

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Abstract
The neutron diffraction structure factor for heavy water at 22°c has been measured for momentum transfers, ħQ, up to a Q of 13 å -1. The results have been interpreted by abstracting the molecular structure effects as far as possible and using models for the correlation of orientation of pairs of molecules ranging from completely uncorrelated orientation to that for adjacent molecules in ice I. Neither these nor any of the popular models for the structure of water, nor a non-hydrogen bonded model, fit the neutron diffraction data over the whole range of Q. Nevertheless, information about the molecular-centres structure factor is obtained, especially for Q up to about 3 å -1. The method of analysis is generalized and applied to x-ray diffraction data and in particular to x-ray data for liquid water. It is shown that this method of analysis has some advantages over the conventional one. The effect of correlation of molecular orientation is less than that of molecular structure on x-ray scattering by water and rather more extensive information is obtained for the molecular-centres structure factor than from neutron data—if the conventional assumption of atomic electron clouds is accepted. A comparison of the x-ray and neutron scattering data for liquid heavy water shows that the two sets of data are reasonably consistent for any of the usual models for water, except for the region of Q between 3 and 5 å -1. The molecular-centres structure factor of water has a weak double humped first maximum with peaks at 2·0 å -1 and 2·9å -1, probably followed by a damped oscillation. This contrasts with atomic liquids but is similar to that found in liquid carbon tetrachloride. The structure of liquid water is not, therefore, necessarily unique or unusual.