Abstract by Peter Rosen
Chemistry and Biochemistry
Heat Capacity of Crystalline Forms of the Metal-Organic Framework Zinc 2-methylimidazolate in Vacuum and Under Pressure
Zeolitic imidazolate frameworks (ZIFs) are composed of metal atoms connected with imidazole-like linkers, and these frameworks have potential for applications in molecular sieving, gas sequestration, and catalysis. Of particular interest is the sodalite (SOD) polymorph of the popular zinc 2-methylimidazolate framework, Zn(MeIm)2, which exhibits structural flexibility via a gate opening phenomenon. We have measured the low temperature heat capacities of the sodalite (SOD) and diamondoid (dia) topologies of Zn(MeIm)2 under vacuum, and these results are compared with previous heat capacity measurements on the zinc 2-ethylimidazolate framework, Zn(EtIm)2. Using previous enthalpy measurements, we determined that the Gibbs free energy of the transformation from SOD to dia is −8.5±2 kJ, and this transformation is primarily enthalpically driven. We have also measured the heat capacities of the SOD polymorph under pressurized CO2 atmospheres. These heat capacities had transitions near 20 K and 150 K, providing experimental evidence that CO2 absorption induces structural changes in SOD-Zn(MeIm)2.