Understanding H$_{2}$-H$_{2}$ interactions in Metal Organic Frameworks (MOFs) with unsaturated metal centers*

Unsaturated Metal Organic Frameworks (MOFs) are particularly interesting due to their high H$_{2}$ uptakes with relatively large isosteric heats of adsorption (Q$_{st }>$8 kJ/mol). This work explores H$_{2}$-H$_{2}$ interactions between adsorbed H$_{2}$ at the different sites in MOF-74 (M$_{2}$(dhtp),dhtp=2,5-dihydroxyterephthalate) and combines IR spectroscopy with vdW-DFT calculations. The adsorption sites in MOF-74 are from highest to lowest binding energies the metal, oxygen, benzene and pore-center sites. The frequency of adsorbed H$_{2}$ at the metal site suffers an additional $\sim $-30 cm$^{-1}$ red shift (for Mg and Zn) and $\sim $-84 cm$^{-1}$ (for Co) when the neighboring oxygen site is occupied. The dipole moment of adsorbed H$_{2}$ is also affected. These interactions extend to the benzene sites for MOF-74-Co. A decrease in dipole moment of H$_{2}$ adsorbed at the metal site is observed with the partial occupation of the benzene sites. However, the complete occupation of the benzene sites induces an additional $\sim $-10 cm$^{-1}$ red shift.