Adsorption of CO2 In a Spin-Crossover Metal Coordination Polymer

We have studied the structure and thermodynamics of CO₂ adsorbed in the metal coordination polymer Fe(pz)Ni(CN)₄ (where pz is pyrazine), which exhibits a hysteretic spin-crossover transition triggered by both loading and temperature changes. We have experimentally measured the IR spectra of the CO₂/Fe(pz)Ni(CN)₄ material as a function of the CO₂ loading and the temperature.  We found that there are distinct frequency shifts in the IR modes of CO₂ adsorbed in the low-spin and high-spin states of the material. We have used both quantum mechanical density functional theory (DFT) and classical statistical mechanical simulations to explore the CO₂-Fe(pz)Ni(CN)₄ interactions at the atomic level. We found that CO₂ molecules form “T” shape structures in Fe(pz)Ni(CN)₄ at high loadings. We have calculated the binding energies for different CO₂ configurations using three different DFT functionals, namely, PBE, DFT-D2, and vdW-DF2, where the latter two functionals include van der Waals interactions. We have also computed binding energies with classical empirical potentials. We computed the vibrational frequencies of CO₂ as a function of loading and at different geometries in Fe(pz)Ni(CN)₄ using the PBE functional. The vibrational frequencies for the low-spin structure are red-shifted when compared with the results for high-spin structure. We found that the calculated vibrational frequencies qualitatively agree with experimental data and explain the origin of the observed frequency shifts.