Are Quantum Chemistry Semiempirical Methods Effective to Predict Solid State Structure and Adsorption in Metal Organic Frameworks?

Abstract

Semiempirical quantum chemistry methods have been used mainly to treat organic and biological compounds with hundreds of atoms and problems involving larger systems. However, their use in the description of solids is still quite restricted. Our aim was to show that semiempirical methods can be used to study MOF (Metal–Organic Framework) systems. The present study analyzed the predictive power of AM1, PM3, PM6, and PM7 methods with respect to the calculation of up to 72 crystal structures of MOFs, and evaluated how the use of the algorithm MOZYME impacted the predictions. Our results showed that PM6 and PM7 methods yielded an accurate description of the geometric arrangement of these MOFs, also observing that MOZYME does not compromise the accuracy of these methods and, for larger systems (above 700 atoms), the computation time is reduced to about 50%. Supported by these results, we chose to evaluate whether the semiempirical methods can be applied to investigate gas adsorption, using a system theoretically and experimentally well investigated: Mg-MOF-74/CO2. PM6 obtained a description for the geometry of host–guest interaction and adsorption enthalpy in agreement with traditional DFT, while PM7 is in agreement with experimental results and DFT estimates with the use of dispersion corrections.