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

摘要

One of the early triumphs of quantum mechanicsnwas Heisenberg’s prediction, based on the Paulinprinciple and wave function symmetry arguments, thatnthe simplest molecule, H2, should exist as two distinctnspeciessallotropes of elemental hydrogen. One allo-ntrope, termed para-H2 (pH2), was predicted to be a lowernenergy species that could be visualized as rotating like ansphere and possessing antiparallel (vV) nuclear spins; thenother allotrope, termed ortho-H2 (oH2), was predicted tonbe a higher energy state that could be visualized as rotat-ning like a cartwheel and possessing parallel (vv) nuclearnspins. This remarkable prediction was confirmed by thenearly 1930s, and pH2 and oH2 were not only separatednand characterized but were also found to be stable almost indefinitely in the absence of paramagnetic “spin catalysts”, such as molecu-nlar oxygen, or traces of paramagnetic impurities, such as metal ions.nThe two allotropes of elemental hydrogen, pH2 and oH2, may be quantitatively incarcerated in C60 to form endofullerene guest@hostncomplexes, symbolized as pH2@C60 and oH2@C60, respectively. How does the subtle difference in nuclear spin manifest itself when hydro-ngen allotropes are incarcerated in a buckyball? Can the incarcerated “guests” communicate with the outside world and vice versa? Can anparamagnetic spin catalyst in the outside world cause the interconversion of the allotropes and thereby effect a chemical transformationninside a buckyball? How close are the measurable properties of H2@C60 to those computed for the “quantum particle in a spherical box”?nAre there any potential practical applications of this fascinating marriage of the simplest molecule, H2, with one of the most beautiful ofnall molecules, C60? How can one address such questions theoretically and experimentally?nA goal of our studies is to produce an understanding of how the H2 guest molecules incarcerated in the host C60 can “communicate”nwith the chemical world surrounding it. This world includes both the “walls” of the incarcerating host (the carbon atom “bricks” that com-npose the wall) and the “outside” world beyond the atoms of the host walls, namely, the solvent molecules and selected paramagneticnmolecules added to the solvent that will have special spin interactions with the H2 inside the complex. In this Account, we describe thentemperature dependence of the equilibrium of the interconversion of oH2@C60 and pH2@C60 and show how elemental dioxygen, O2,anground-state triplet, is an excellent paramagnetic spin catalyst for this interconversion. We then describe an exploration of the spin spec-ntroscopy and spin chemistry of H2@C60. We find that H2@C60 and its isotopic analogs, HD@C60 and D2@C60, provide a rich and fasci-nnating platform on which to investigate spin spectroscopy and spin chemistry. Finally, we consider the potential extension of spin chemistrynto another molecule with spin isomers, H2O, and the potential applications of the use of pH2@C60 as a source of latent massive nuclearnpolarization.