Heat capacities and thermodynamic functions of the ZIF organic linkers imidazole, 2-methylimidazole, and 2-ethylimidazole

Highlights: • Heat capacity data and thermodynamic properties for imidazole, 2-methylimidazole, and 2-ethylimidazole are presented. • The heat capacity data show a transition in the 2-ethylimidazole material near room temperature. • The heat capacity data suggests lattice dynamics plays a key role in the ZIF-8 material. -- Abstract: Metal organic frameworks (MOFs) are a novel class of materials that consist of a lattice of metal centers linked by organic molecules. Previously, heat capacities and thermodynamic functions have been reported for a series of crystalline polymorphs of Zn(EtIm)₂, a zeolitic imidazolate framework family of MOFs that have potential applications in CO₂ sequestration. In those studies, an anomalous trend in the heat capacity and thermodynamic functions was observed that was not predicted. To further investigate these materials and their thermodynamic data, the low-temperature heat capacities of imidazole, 2-methylimidazole, and 2-ethylimidazole, organic linkers that are present in Zn(EtIm)₂ and similar materials, were measured by a Quantum Design Physical Property Measurement System (PPMS) from 1.8–300 K. The data collected was fit to theoretical functions below 10 K, orthogonal polynomials from 5–60 K, and a sum of Debye and Einstein functions above 50 K. These functions were used to generate C_p,m°, Δ₀^TS_m°, Δ₀^TH_m°, and Φ_m° values at smoothed temperatures from 0–300 K. This investigation revealed the presence of a second-order phase transition in 2-ethylimidazole that may yield insight into the anomalous heat capacity behavior present in Zn(EtIm)₂.