NMR shielding of helium-3 in the micropores of zeolites

• Helium adsorbed on a microporous material is investigated by 3He NMR spectroscopy.
• 3He magnetic shielding is directly measured using a standard NMR spectrometer.
• The effect of intermolecular interactions with zeolite is partially covered by BSC.

In the present work direct measurements of 3He magnetic shielding were used to investigate helium-3 gas in the micropores of 5 Å molecular sieve under pressures up to 300 bar. We have observed the distinct effects in helium-3 shielding when helium gas was interacting with zeolite powder, however these effects cannot be accurately determined due to the unknown bulk susceptibility correction (BSC). The real intermolecular effects in helium-3 shielding are much stronger than it is experimentally observed because the diamagnetic BSC partially cover these effects. As shown the intermolecular interactions are better seen when the quantity of helium-3 is considerably limited at lower pressure and it suggests the presence of dominant interactions between helium-3 atoms and metal ions on surface of the zeolite. We confirm that the use of helium-3 as a probe is promising because this gas has many unique physicochemical properties which allow one to study the smallest micropores in zeolites. Our preliminary results also prove that the measurements of 3He magnetic shielding provide more precious data than the appropriate NMR chemical shifts.

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