Neutron diffraction and molecular dynamics study of liquid benzene and its fluorinated derivatives as a function of temperature

The study of the local translational and orientational ordering in benzene (C6H6),1,3,5-trifluorobenzene (C6H3F3) and hexafluorobenzene (C6F6) neat liquids has been performed along their liquid-vapor coexistence curve between the melting point and the boiling point combining neutron diffraction experiments and molecular dynamics simulation. Both experiment and simulation show that the local ordering is only very slightly affected in the temperature range investigated. The analysis of the results show that the orientational local order in liquid benzene is almost isotropic at distances corresponding to the first shell of neighbors. In contrast, for hexafluorobenzene, parallel and perpendicular configurations exhibit maxima of occurrence for distinct r values, parallel configurations being predominant at short distances. For 1,3,5-trifluorobenzene, the orientational order in the first shell is strongly anisotropic, and a stacked configuration involving a pair of molecules is observed at short distances (about 4 Angstrom). This finding together with the value of the coordination number obtained from simulation data clearly shows the existence of dimers (sandwichlike). Finally it is argued that the orientational and translational ordering observed in the hexafluorobenzene is intermediate between those of benzene and those of 1,3,5-trifluorobenzene.