Melting and crystal structure. The onset of ‘rotation’ on melting

Extensive randomization of molecular orientations or ‘rotation’ is generally thought to occur on melting of certain fairly simple substances. The onset of ‘rotation’ on melting of more complex molecules is reviewed. Special attention has been directed to the behaviour of ‘stiff’ molecules such as benzene derivatives and fused aromatic rings, for which intra­molecular crumpling in the melt is likely to be unimportant. The fractional increase in volume ∆Vf/Vs on melting has been evaluated for a range of compounds from existing density measurements. ∆Vf/Vs has also been determined experimentally for the molecules 2, 6-dimethyl naphthalene, 2, 3-dimethyl naphthalene, acenaphthene, anthracene, phenanthrene and chrysene. Using bond-distances and Van der Waals repulsion envelopes established by X-ray studies, the increase in volume required to permit ‘rotation’ about different mole­cular axes in the melt has been calculated. Comparison with the volume increase on melting observed experimentally shows that in many melts containing ‘stiff molecules’ there can be no ‘rotation’ of the molecules. In the present experiments no anomalies occur in such melts even 50° above the melting-point, but it is not yet possible to say whether ‘rotational’ transitions would be observed at higher temperatures in the liquids, similar to the well- known transitions in crystals. On the other hand, ‘rotation’ about at least one of the mole­cular-axes probably does accompany melting for some of the molecules studied, including benzene, bromobenzene, p-xylene, acenaphthene, naphthalene and 2, 6- but not 2, 3-dimethyl naphthalene.