Local structure and orientational ordering in liquid bromoform

The neutron diffraction data of liquid bromoform (CHBr₃) at 25°C was analysed using the Empirical Potential Structure Refinement technique in combination with H/D isotopic substitution. Compared to liquid chloroform (CHCl₃), CHBr₃ displays more spatially defined intermolecular contacts. A preference for polar stacking with collinear alignment of dipole moments is observed for the most closely approaching CHBr₃ molecules, although to a lesser extent than in chloroform. Consistent with this, and in line with dielectric spectroscopy, the Kirkwood correlation factor from the structural model of CHBr₃ is smaller than that of CHCl₃. The net antiparallel alignment of dipole moments in CHBr₃, as suggested by dielectric spectroscopy, must be due to weak but persistent long-range orientation correlations in CHBr₃, which counteract the local polar stacking.