Distance-Dependent Paramagnet-Enhanced Nuclear Spin Relaxation of H₂@C₆₀ Derivatives Covalently Linked to a Nitroxide Radical

A series of H₂@C₆₀ derivatives covalently linked to a nitroxide radical has been synthesized. We report distance-dependent nuclear spin relaxivity of H₂ in these derivatives. The results clearly indicate that the relaxivity of H₂ is distance-dependent and in good agreement with the Solomon−Bloembergen equation, which predicts a 1/<i>r</i>⁶ dependence

Synthesis, Isomer Count, and Nuclear Spin Relaxation of H₂O@Open-C₆₀ Nitroxide Derivatives

H₂O@C₆₀ derivatives covalently linked to a nitroxide radical were synthesized. The ¹H NMR of the guest H₂O revealed the formation of many isomers with broad signals. Reduction to the diamagnetic hydroxylamines sharpened the ¹H NMR signals considerably and allowed for an “isomer count” based on the number of observed distinct signals. For H₂O@K-8, 17 positional isomeric nitroxides are predicted, not including additional numbers of regioisomers; indeed, 17 signals are observed in the ¹H NMR spectrum

Paramagnet Enhanced Nuclear Spin Relaxation in H₂O@Open‑C₆₀ and H₂@Open‑C₆₀

Relaxation rates of <i>endo</i>-H₂O in H₂O@Open-C₆₀ in the presence of a nitroxide radical and of their nitroxide derivatives have been measured and are compared with effects for <i>endo</i>-H₂ in similar cages. <i>T</i>₁ relaxation enhancement of the <i>endo</i>-H₂O and H₂ induced by either intra- or intermolecular interaction is relatively insensitive to the presence of a cage opening. Enhancement of intermolecular relaxation is observed, however, when the cage opening has an OH group

ENDOR Evidence of Electron–H₂ Interaction in a Fulleride Embedding H₂

An endofulleropyrrolidine, with H₂ as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H₂ has been probed by pulsed ENDOR. The experimental hyperfine couplings between the electron and the H₂ guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H₂ guest. The <i>ortho-H</i>_<i>2</i>/<i>para-H</i>_<i>2</i> interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic <i>ortho-H</i>_<i>2</i> to the diamagnetic <i>para-H</i>_<i>2</i> causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride