Solid state 13 C NMR and infrared studies on the molecular packing of anhydrous Zinc(II) n-alkanoates Richard A. Taylor * ,1 , Henry A. Ellis, Paul T. Maragh Department of Chemistry, University of the West Indies, Mona Campus, Kingston 7, Jamaica article info Article history: Received 18 November 2008 Received in revised form 16 December 2008 Accepted 19 December 2008 Available online 29 December 2008 Keywords: Zinc(II) n-alkanoates Infrared spectroscopy Solid state 13 C NMR spectroscopy Methyl group Packing efficiency Odd–even effect Powder X-ray diffraction abstract The molecular packing of the hydrocarbon chains of a homologous series of zinc(II) n-alkanoates from chain length, n c = 4–20 inclusive, have been studied using infrared (IR) and solid state 13 C NMR spectros- copy. 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, m a (CH 3 ) and NMR chemical shift (d) values for the methyl carbon of the hydrocarbon chain. The data strongly suggest that CH 3 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. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Long chain carboxylates of polyvalent metal ions are substances of considerable importance and are used commercially in many applications [1]. Many metal carboxylates exhibit one or more thermotropic liquid crystalline phases that are currently of increas- ing interest for their potential applications in liquid crystal dis- plays (LCD’s). Thus, in many of these applications, the thermal behaviour on heating the room temperature solid to the isotropic liquid is of some importance. Several studies have shown that knowledge of the crystalline structure is a pre-requisite to under- standing phase behaviour; in many cases the phase behaviour was seen to be chain length dependent [1]. However, the structures of the long chain length compounds are not clearly understood be- cause they do not readily form crystals suitable for single crystal X-ray analysis. Usually, the crystals are thin needles that are very fragile. Consequently, for the zinc(II) carboxylates (ZnC n ), the few structures that have been reported are those of the short chain compounds [2–9]. The compounds were reported to be isostructur- al, in that, each zinc ion was tetrahedrally coordinated to oxygen atoms from four different alkanoate groups and each group forms a Z,E-type bidentate bridge with two tetrahedral zinc ions resulting in a syn–anti arrangement. In previous publications [10,11], room temperature molecular and lattice structures were proposed for the zinc(II) series from the butanoate to the eicosanoate, inclusive. However, powder X- ray diffraction data suggested that the packing arrangement of the hydrocarbon chains was different in short and long chain com- pounds; a double bilayer and an interdigitated bilayer arrangement for the short and long chain homologues, respectively. Indeed, there is overwhelming evidence of the bilayer arrangement for the short chain homologues from the butanoate to octanoate, inclusive [2– 11] but no specific evidence or explanation for the interdigitation has been presented. Additionally, densities and melting points for the long chain homologues of the series showed an odd–even effect with increasing chain length [11]. To explain the data, geometric models based on the work of Boese and coworkers [12] were pre- sented to illustrate the different packing arrangements between odd and even long chain homologues; however, there was no spec- troscopic evidence to validate the proposed models. In this publication, infrared and solid state 13 C NMR spectro- scopic data are used to investigate more closely the hydrocarbon packing arrangement in anhydrous zinc(II) n-alkanoates, from the butanoate to eicosanoate, inclusive. This is to provide definitive evidence for the molecular factors influencing the packing arrange- ments particularly in relation to the number of carbon atoms in the 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.12.042 * Corresponding author. Tel.: +1 868 662 6013x2272; fax: +1 868 645 3771. E-mail addresses: richard.taylor@sta.uwi.edu, richard.taylor@uwimona.edu.jm (R.A. Taylor). 1 Address: Department of Chemistry, Faculty of Science and Agriculture, University of the West Indies, St. Augustine Campus, St. Augustine, Trinidad and Tobago. Journal of Molecular Structure 921 (2009) 118–125 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc