Room temperature structures and odd–even behaviour of a homologous series of anhydrous lithium n-alkanoates

The molecular structures of a homologous series of lithium n-alkanoates have been determined at room temperature using infrared spectroscopy, polarizing light microscopy and X-ray powder diffraction in conjunction with density and melting point measurements. For all the compounds investigated, asymmetric ionic metal–carboxylate coordination is proposed, with the molecules located within a triclinic crystal system with P  1¯ space group. The molecules are nearly all of similar structure and are arranged within lamellar layers with four molecules per unit cell. The hydrocarbon chains, in nearly all trans conformation, are arranged tail-to-tail and tilted at an average angle of 55ο to the planes containing lithium ions. The unit cell parameters such as sides: b and c increase linearly with increasing chain length whilst side a shows a linear decrease. Furthermore, the measured densities and melting points show odd–even behaviour, suggesting differences in molecular packing between odd and even chain length homologues. Geometric models are proposed to explain molecular orientation within a lamella and odd–even behaviour, involving the influence of terminal groups on the packing geometry of hydrocarbon chains within the lattice.