Tuesday, October 18, 2011: 4:15 PM
205 B (Minneapolis Convention Center)
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.