Isomorphism of 2-methylnaphthalene and 2-halonaphthalenes as a revealer ofa special interaction between methyl and halogen

The systems 2-methylnaphthalene+2-chloronaphthalene and 2-methylnaphthalene +2-bromonaphthalene belong to the exceptional group of binary systems where the formation of mixed crystals goes together with a solid-liquid phase di agram with a maximum. For these systems a thermodynamic analysis is present ed which is based on new phase diagram and thermochemical data. The excess Gibbs energies, excess enthalpies, and excess entropies of the mixed crysta lline state all are negative. These properties correspond to a net attracti on between methyl and substituted halogen. Additional evidence of such an a ttraction is given by the outcome of a statistical search on intermolecular contacts, in the crystalline state of pure substances, among methyl+haloge n, halogen+halogen, and methyl+methyl. (C) 1999 American Institute of Physi cs. [S0021-9606(99)52109-0].