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

Abstract

O ne 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, H 2 , should exist as two distinct speciessallotropes of elemental hydrogen. One allotrope, termed para-H 2 (pH 2 ), was predicted to be a lower energy species that could be visualized as rotating like a sphere and possessing antiparallel (vV) nuclear spins; the other allotrope, termed ortho-H 2 (oH 2 ), was predicted to be a higher energy state that could be visualized as rotating like a cartwheel and possessing parallel (vv) nuclear spins. This remarkable prediction was confirmed by the early 1930s, and pH 2 and oH 2 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, pH 2 and oH 2 , may be quantitatively incarcerated in C 60 to form endofullerene guest@host complexes, symbolized as pH 2 @C 60 and oH 2 @C 60 , 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 H 2 @C 60 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, H 2 , with one of the most beautiful of all molecules, C 60 ? How can one address such questions theoretically and experimentally?A goal of our studies is to produce an understanding of how the H 2 guest molecules incarcerated in the host C 60 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 H 2 inside the complex. In this Account, we describe the temperature dependence of the equilibrium of the interconversion of oH 2 @C 60 and pH 2 @C 60 and show how elemental dioxygen, O 2 , a ground-state triplet, is an excellent paramagnetic spin catalyst for this interconversion. We then describe an exploration of the spin spectroscopy and spin chemistry of H 2 @C 60 . We find that H 2 @C 60 and its isotopic analogs, HD@C 60 and D 2 @C 60 , provide a rich and fascinating platform on which to investigate spin spectroscopy and spin chemistry. Finally, we consider the potential extension of spin chemistry to another molecule with spin isomers, H 2 O, and the potential applications of the use of pH 2 @C 60 as a source of latent massive nu...

Comparison of Nuclear Spin Relaxation of H2O@C60 and H2@C60 and Their Nitroxide Derivatives

“…The latter has proved to be essential in developing methods to manipulate the ratio of ortho and para nuclear spin isomers of H 2 within the cage and study the mechanism of the conversion. 5 Similar studies for the conversion of ortho and para H 2 O@C 60 are in progress. Table 1 summarizes the results of the following types of measurements of T 1 , reported as the inverse rate, 1/T 1 , for H 2 O and H 2 : (1) The intrinsic T 1 for the small molecules in three organic solvents with different viscosities and polarities; (2) the T 1 values for the molecules in the covalently bonded nitroxides, 1 and 2, and the corresponding diamagnetic hydroxylamines, 1D and 2D (Chart 1); and (3) the bimolecular relaxivity, R 1 , obtained from measurements of T 1 as a function of the added nitroxide TEMPO.…”

Nuclear spin isomers of guest molecules in H 2 @C 60 , H 2 O@C 60 and other endofullerenes

“…Based on a variety of spectroscopic studies, the emerging answer is a qualified 'yes' [14,[32][33][34], and references therein] although some coupling of rotational and internal translation modes of the endo H 2 has been predicted theoretically [35] and detected spectroscopically [32]. (It has not, however, been possible to resolve by NMR the spin-rotation fine structure in H 2 @C 60 that is resolved in gas phase molecular beam measurements [36].)…”

“…Enrichment becomes undetectable in liquid solution, however, because of bimolecular quenching of the triplet. a Values for H 2 are estimated from k po measured in toluene-d 8 [34].…”

Interaction potential and infrared absorption of endohedral H2 in C60

“…For H 2 @C 60 it is feasible to reach the thermal equilibrium at liquid nitrogen temperature (77 K), n o /n p = 1, using liquid oxygen as a spin catalyst for orthopara conversion. 19,20 The IR spectrum of H 2 @C 60 at 6 K, presented and analyzed in Ref. 6, consists of sharp lines positioned between 4000 and 5000 cm −1 .…”