A Magnetic Switch for Spin-Catalyzed Interconversion of Nuclear Spin Isomers

Abstract

Dihydrogen, H 2 , exists as two allotropes or nuclear spin isomers: a nuclear singlet state with antiparallel (vV) nuclear spins, termed parahydrogen (pH 2 ), and a nuclear triplet state with parallel (vv) nuclear spins, termed ortho-hydrogen (oH 2 ). 1According to the Pauli Principle, singlet pH 2 is required to occupy even rotational states (J ) 0, 2, 4, ...) and triplet oH 2 odd rotational states (J ) 1, 3, 5, ...). Since J ) 0 is the lowest rotational level of H 2 , pH 2 is the ground rotational state and oH 2 a rotationally excited state. The energy gap between the J ) 0 and J ) 1 rotational states is 120 cm -1 (0.343 kcal mol). The equilibrium mixture at any given temperature is termed eH 2 . At room temperature (RT), triplet oH 2 dominates at equilibrium (eH 2 ) 75% oH 2 /25% pH 2 ) because of its higher statistical weight and the small energy gap between the two rotational levels.The two allotropes of hydrogen, oH 2 and pH 2 , may be quantitatively incarcerated into C 60 to form the endofullerene guest@host complexes [2][3][4] oH 2 @C 60 and pH 2 @C 60 , respectively. We have developed 5 the following procedure for producing samples of pH 2 -enriched H 2 @C 60 in which the incarcerated H 2 spin isomers are not in spin equilibrium at RT: (1) adsorbing a sample of eH 2 @C 60 on the external surface of NaY zeolite at RT; (2) cooling the sample to 77 K; (3) immersing the sample in liquid oxygen at 77 K, thereby converting the sample to a 50% oH 2 /50% pH 2 mixture (which is eH 2 at 77 K); (4) removing the spin catalyst (O 2 ) by applying a vacuum; (5) rapidly bringing the sample to RT before the sample can back-convert to equilibrium at RT; and (6) extracting the sample into a solvent at RT. The sample at RT is enriched in pH 2 (nonequilibrium 50% pH 2 rather than the equilibrium 25% pH 2 ) and is termed *pH 2 @C 60 .At RT, the back conversion of *pH 2 @C 60 to eH 2 @C 60 takes days in the absence of an added spin catalyst. The conversion of *pH 2 @C 60 to eH 2 @C 60 (eq 1) is followed conveniently by 1 H NMR spectroscopy using a sample that contains HD@C 60 , which does not have spinrotational isomers, as an internal standard.At RT, nitroxides serve as paramagnetic spin catalysts for the back conversion of *pH 2 @C 60 to eH 2 @C 60 . 5 We reasoned that if a derivative of H 2 @C 60 could be rapidly switched from a diamagnetic nitroxide precursor to a paramagnetic nitroxide, a "magnetic switch" for forward conversion (eH 2 @C 60 f *pH 2 @C 60 ) at 77 K and back conversion (*pH 2 @C 60 f eH 2 @C 60 ) at RT would be available. We describe the design and demonstration of such a magnetic switch based on the reversible nitroxide/hydroxylamine system 7 The red curve corresponds to the 1 H NMR signals after conversion. Clearly, the proportion of oH 2 @1 relative to HD@1 decreased as expected: the signal intensities are consistent with a 50%/50% mixture of oH 2 @1/pH 2 @1, corresponding to *pH 2 @1. At RT, the back conversion rate of *pH 2 @1 to eH 2 @1 without a spin catalyst is similar to that of *pH 2 @C 60 to eH ...

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

“…Methods of removing the auxiliary group, such as flash pyrolysis, to form the parent endofullerene are under development [47,48]; (iii) removal of the triplet state catalyst occurs spontaneously in milliseconds or less after irradiation ceases [49]; and (iv) the radical ion intermediate might similarly be removed almost instantaneously by back electron transfer from a photo-generated radical ion pair, or more slowly by chemical electron transfer.…”

Theory and spectroscopy of an incarcerated quantum rotor: The infrared spectroscopy, inelastic neutron scattering and nuclear magnetic resonance of H2@C60 at cryogenic temperature

“…High resolution solution NMR has proved to be a valuable tool for monitoring the interconversion of the ortho (NMR active) and para (NMR silent) allotropes of H 2 @C 60 and related fullerenes [27,28,43]. However, spatial information may be extracted only indirectly from solution NMR spectra via relaxation time measurements, and even then spatial information is often mixed with dynamical effects which are difficult to evaluate [44][45][46][47].…”

Symmetry-breaking in the H2@C60 endofullerene revealed by inelastic neutron scattering at low temperature

“…Spin conversion in H 2 @C 60 has been observed in the presence of paramagnetic substances. 13,14,29,30 In the absence of spinconversion catalysts, ortho-and para-hydrogen are metastable: no measurable ortho-para conversion in solid pure H 2 @C 60 has been observed in experiments that lasted several days at cryogenic temperatures. 12,[20][21][22][23] In both the para-and ortho-H 2 @C 60 manifold, the energy difference between the ground states and the lowest excited states is in the order of 22 meV, equivalent to 255 K. 21,23,24 Given the typical energy level separations and the ortho-para metastability, only the respective rotational ground states, ortho-H 2 with J = 1, I = 1 and para-H 2 with J = 0, I = 0, are effectively populated at any temperature below 50 K. In the following discussion we neglect the nuclear spin degrees of freedom and the nuclear spin structure of the rotational energy levels, except for Pauli exclusion principle effects.…”