The Spin Chemistry and Magnetic Resonance of H2@C60. From the Pauli Principle to Trapping a Long Lived Nuclear Excited Spin State inside a Buckyball

• Published in: Accounts of chemical research 2010-02, Vol.43 (2), p.335-345
• Main Authors: Turro, Nicholas J, Chen, Judy Y.-C, Sartori, Elena, Ruzzi, Marco, Marti, Angel, Lawler, Ronald, Jockusch, Steffen, López-Gejo, Juan, Komatsu, Koichi, Murata, Yasujiro
• Format: Article
• Language: English
• Subjects: Catalysis; Fullerenes - chemistry; Hydrogen - chemistry; Magnetic Resonance Spectroscopy; Oxygen - chemistry
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/ar900223d

One of the early triumphs of quantum mechanics was Heisenberg’s prediction, based on the Pauli principle and wave function symmetry arguments, that the simplest molecule, H2, should exist as two distinct speciesallotropes of elemental hydrogen. One allotrope, termed para-H2 (pH2), was predicted to be a lower energy species that could be visualized as rotating like a sphere and possessing antiparallel (↑↓) nuclear spins; the other allotrope, termed ortho-H2 (oH2), was predicted to be a higher energy state that could be visualized as rotating like a cartwheel and possessing parallel (↑↑) nuclear spins. This remarkable prediction was confirmed by the early 1930s, and pH2 and oH2 were not only separated and characterized but were also found to be stable almost indefinitely in the absence of paramagnetic “spin catalysts”, such as molecular oxygen, or traces of paramagnetic impurities, such as metal ions. The two allotropes of elemental hydrogen, pH2 and oH2, may be quantitatively incarcerated in C60 to form endofullerene guest@host complexes, symbolized as pH2@C60 and oH2@C60, respectively. How does the subtle difference in nuclear spin manifest itself when hydrogen allotropes are incarcerated in a buckyball? Can the incarcerated “guests” communicate with the outside world and vice versa? Can a paramagnetic spin catalyst in the outside world cause the interconversion of the allotropes and thereby effect a chemical transformation inside a buckyball? How close are the measurable properties of H2@C60 to those computed for the “quantum particle in a spherical box”? Are there any potential practical applications of this fascinating marriage of the simplest molecule, H2, with one of the most beautiful of all molecules, C60? How can one address such questions theoretically and experimentally? A goal of our studies is to produce an understanding of how the H2 guest molecules incarcerated in the host C60 can “communicate” with the chemical world surrounding it. This world includes both the “walls” of the incarcerating host (the carbon atom “bricks” that compose the wall) and the “outside” world beyond the atoms of the host walls, namely, the solvent molecules and selected paramagnetic molecules added to the solvent that will have special spin interactions with the H2 inside the complex. In this Account, we describe the temperature dependence of the equilibrium of the interconversion of oH2@C60 and pH2@C60 and show how elemental dioxygen, O2, a ground-state triplet, is