Recognition of hydrogen isotopomers by an open-cage fullerene.

We present our study on the recognition of hydrogen isotopes by an open-cage fullerene through determination of binding affinity of isotopes H₂/HD/D₂ with the open-cage fullerene and comparison of their relative molecular sizes through kinetic-isotope-release experiments. We took advantage of isotope H₂/D₂ exchange that generated an equilibrium mixture of H₂/HD/D₂ in a stainless steel autoclave to conduct high-pressure hydrogen insertion into an open-cage fullerene. The equilibrium constants of three isotopes with the open-cage fullerene were determined at various pressures and temperatures. Our results show a higher equilibrium constant for HD into open-cage fullerene than the other two isotopomers, which is consistent with its dipolar nature. D₂ molecule generally binds stronger than H₂ because of its heavier mass; however, the affinity for H₂ becomes larger than D₂ at lower temperature, when size effect becomes dominant. We further investigated the kinetics of H₂/HD/D₂ release from open-cage fullerene, proving their relative escaping rates. D₂ was found to be the smallest and H₂ the largest molecule. This notion has not only supported the observed inversion of relative binding affinities between H₂ and D₂, but also demonstrated that comparison of size difference of single molecules through non-convalent kinetic-isotope effect was applicable.