Br...Br and van der Waals interactions along a homologous series: crystal packing of 1,2-dibromo-4,5-dialkoxybenzenes

Abstract

The crystalline structures of four homologues of the 1,2-dibromo-4,5-dialkoxybenzene series [Br2C6H2(OCnH2n + 1)2forn= 2, 12, 14 and 18] have been solved by means of single-crystal crystallography. Comparison along the series, including the previously reportedn= 10 andn= 16 derivatives, shows a clear metric trend (bandcessentially fixed along the series andagrowing linearly withn), in spite of some subtle differences in space groups and/or packing modes. A uniform packing pattern for the aliphatic chains has been found for then= 12 to 18 homologues, which slightly differs from that of then= 10 derivative. The crystalline structures of all the higher homologues (n= 10–18) seem to arise from van der Waals interchain interactions and, to a lesser extent, type II Br...Br interactions. The dominant role of interchain interactions provides direct structural support for the usual interpretation of melting point trends like that found along this series.Atoms in Molecules(AIM) analysis allows a comparison of the relative magnitude of the interchain and Br...Br interactions, an analysis validated by the measured melting enthalpies.