Abstract
PIXEL has been used to perform calculations of adsorbate-adsorbent interaction energies between a range of metal-organic frameworks (MOFs) and simple guest molecules. Interactions have been calculated for adsorption between MOF-5 and Ar, H2, and N2; Zn2(BDC)2(TED) (BDC = 1,4-benzenedicarboxylic acid, TED = triethylenediamine) and H2; and HKUST-1 and CO2. The locations of the adsorption sites and the calculated energies, which show differences in the Coulombic or dispersion characteristic of the interaction, compare favourably to experimental data and literature energy values calculated using density functional theory.
Highlights
Metal–organic frameworks (MOFs) are made up of metalbased nodes connected by organic bridging ligands to form 3-dimensional porous networks
Monte Carlo methods were used by Dubbeldam et al.[37] to probe the energies of the adsorption sites of argon in MOF-5 found in Rowsell's X-ray diffraction study.[17]
Site III is filled preferentially over site II, the binding energies for these sites are CrystEngComm persion term, and the ordering of the different adsorption sites correlates with the number of short contacts that can be made between the adsorbing argon atoms and the atoms of the framework
Summary
Metal–organic frameworks (MOFs) are made up of metalbased nodes connected by organic bridging ligands to form 3-dimensional porous networks. Density functional theory (DFT) has the advantage that it does not require system-specific parameterisation, but it is more computationally demanding than force-field methods. It has the well-known disadvantage that it fails to capture dispersion, an important term for framework–guest interaction energies. Van der Waals functionals have been developed, treatment of dispersion is most commonly accomplished by an extra semi-empirical term. Perturbation methods, such as MP2, can account for dispersion energies, but they are computationally expensive. The PIXEL method is a semi-empirical technique for evaluating intermolecular interactions based on integrations over calculated electron densities of molecules. PIXEL method is a potentially valuable addition to established techniques applied to metal-containing systems
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