Recognition of hydrogen isotopomers by an open-cage fullerene

Abstract

We present our study on the recognition of hydrogen isotopes by an open-cage fullerene through determination of binding affinity of isotopes H-2/HD/D-2 with the open-cage fullerene and comparison of their relative molecular sizes through kinetic-isotope-release experiments. We took advantage of isotope H-2/D-2 exchange that generated an equilibrium mixture of H-2/HD/D-2 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. D-2 molecule generally binds stronger than H-2 because of its heavier mass; however, the affinity for H-2 becomes larger than D-2 at lower temperature, when size effect becomes dominant. We further investigated the kinetics of H-2/HD/D-2 release from open-cage fullerene, proving their relative escaping rates. D-2 was found to be the smallest and H-2 the largest molecule. This notion has not only supported the observed inversion of relative binding affinities between H-2 and D-2, but also demonstrated that comparison of size difference of single molecules through non-convalent kinetic-isotope effect was applicable.