Experimental and theoretical study on hydrogen interaction with unsaturated Metal Organic Frameworks*

Infrared absorption spectroscopy (IRAS) is useful to study the interaction of H$_{2}$ molecules inside various materials, since the frequency of its internal stretch mode depends on the adsorption site. Unsaturated Metal Organic Frameworks (MOFs) are particularly interesting due to their high H$_{2}$ uptakes with relatively large isosteric heats of adsorption (Q$_{st }>$8 kJ/mol). Our study focuses on H$_{2}$ in M$_{2}$(dhtp), dhtp=2,5-dihydroxyterephthalate (M= Zn, Ni, Co, Mg) and combines temperature-dependent IRAS measurements and vdW-DF calculations. Results show that the H$_{2}$ stretch frequencies are very sensitive to the chemical environment, with no correlation between binding energies and frequency shifts, as previously observed for saturated MOFs.\footnote{ N. Nijem \textit{et al.} submitted to J.A.C.S 2009} Moreover, the H$_{2}$ stretch vibration closest to the metal site exhibits a strong shift from -30 cm$^{-1}$ to -68 cm$^{-1}$ upon population of neighboring sites (e.g. ``oxygen'' site).