NEUTRON-DIFFRACTION FROM LIQUID-HYDROGEN BROMIDE - STUDY OF THE ORIENTATIONAL CORRELATIONS

The three partial structure factors of liquid hydrogen bromide along t he coexistence curve have been measured by neutron diffraction using t he isotopic substitution technique. The structure factors and correspo nding pair correlation functions obtained suggest that it is an excell ent approximation to regard the bromine atom as the center of mass of the hydrogen bromide molecule. The anisotropic terms in the intermolec ular potential manifest themselves primarily by causing a significant degree of correlation between the orientations of neighboring molecule s. In order to gauge the size of these orientational correlations an i mage reconstruction technique is employed. This analysis shows that th ere is a pronounced correlation between the relative orientations of n eighboring molecules, but that there is not a strong directionality in the local coordination of any particular molecule. The results are di scussed in relation to a previous experiment on hydrogen bromide which proposed the formation of hydrogen bonds in this liquid.