Small Crown-Ether Complexes as Molecular Models for Dihydrogen Adsorption in Undercoordinated Extraframework Cations in Zeolites

1:1
metal complexes of small crown ethers are structurally similar
to extraframework sites in metal-exchanged zeolites. Using ab initio calculations, we show that adsorbed molecular
hydrogen follows the same trends in adsorption energies and vibrational
frequencies at both types of metal sites. Unlike zeolites, crown ethers
can be characterized in the gas phase, which opens new possibilities
for understanding the bonding of dihydrogen at undercoordinated metal
sites to help guide the rational design of porous materials for hydrogen
isotope separation. Because more strongly binding adsorbates affect
the geometry of the hosts, the similarity of crown ethers and zeolites
with regard to the vibrational spectra of the adsorbed molecule seems
to be limited to H2.