Influence of the alkyl-chain on the symmetry and structural changes of Zn4O(RCO2)6 complexes in film phase -: An FT-IR study

The Zn4O(RCO2)(6) complexes of a selection of straight- and branched-chain carboxylic acids have been prepared in CCl4 and characterized by their C-13 NMR, H-1 NMR and mid-FT-IR spectra. Their structural stability was studied by FT-IR-spectroscopy over a 4-day long period of time in CCl4 solution and as cast films. The observed characteristic spectroscopic features have been discussed. The cast film samples showed extremely diverse behaviour from instant decomposition to the perfect conservation of the original T-d symmetry, even though water was not excluded from the system. An explanation, supported by ab initio quantum chemical calculations on the conformation of the free carboxylate ions with various alkyl-chains, has been given for the diverse behaviour of the samples. Formation of a hydrocarbon moiety, with somewhat extended anti-periplanar (all-trans) conformation, perpendicular to the carboxylate plane was the essential factor in the preservation of the structure and the quaternary alpha-carbon was an additional requirement to sustain its symmetry. Two attempts at preparing the corresponding Zn4O(RCO2)(6) complexes from water have been successful in the cases when the R-groups fulfilled the requirements in geometry stated above. (C) 2006 Elsevier B.V. All rights reserved.