title: 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 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.
authors.affiliation: Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan. yasujiro@scl.kyoto-u.ac.jp
authors.forename: Yasujiro
authors.initials: Y
authors.lastname: Murata
authors.forename: Shih-Ching
authors.initials: SC
authors.lastname: Chuang
authors.forename: Fumiyuki
authors.initials: F
authors.lastname: Tanabe
authors.forename: Michihisa
authors.initials: M
authors.lastname: Murata
authors.forename: Koichi
authors.initials: K
authors.lastname: Komatsu
keywords: encapsulation; escape; hydrogen; isotopomer; kinetics; open-cage fullerene