Solid state 13C NMR and infrared studies on the molecular packing of anhydrous Zinc(II) n-alkanoates

The molecular packing of the hydrocarbon chains of a homologous series of zinc(II) n-alkanoates from chain length, nc = 4–20 inclusive, have been studied using infrared (IR) and solid state 13C NMR spectroscopy. Odd–even variation, in density and melting point are explained on the basis of odd–even variation in values for the IR antisymmetric stretch of the methyl group, νa(CH3) and NMR chemical shift (δ) values for the methyl carbon of the hydrocarbon chain. The data strongly suggest that CH3 groups of the even chain homologues are in close proximity to the zinc ion, pointing into the zinc basal plane of the lattice, as opposed to the odd chain homologues that point along the zinc plane resulting in a less close approach to the zinc ion. Molecular models show that the even chain homologues have a higher packing efficiency, density and melting point than the odd chain homologues.