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

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 e...