Chain Length and Thermal Sensitivity of the Infrared Spectra of a Homologous Series of Anhydrous Silver(I) n-Alkanoates

Abstract

The thermal and chain length sensitivity of the infrared spectra of some solid state anhydrous silver(I) salts (n-octanoate to n-eicosanoate, inclusive) are discussed. At ambient temperature, the tilted alkyl chains, anchored to the metal planes via chelating bidentate coordination to the silver ions, are crystallized in the fully extended all-trans conformation. Interestingly, though all compounds are crystallized in a monoclinic crystal system, their lateral chain packing, van der Waals effects, and hence vibrational features are chain length-dependent. This is a direct result of electrostatic effects of the COO group in addition to vibrational coupling between CH2, CH3, and COO modes, an effect which varies significantly with chain length. Variable temperature infrared measurements indicate significant irreversible changes in the metal-carboxyl coordination sphere, most likely due to bond fission. For long chain adducts (nc>12), thermally induced crystal system switching, monoclinic to triclinic, indicates greater thermal sensitivity of their alkyl chains. During heating, the regions of the hydrocarbon chains, furthest from the COO, become increasingly molten and mobile as the stepwise melt advances towards the solid COO moieties. This solid-liquid melting behaviour is responsible for mesophase formation in metal carboxylate systems.