Paramagnet enhanced nuclear relaxation of H-2 in organic solvents and in H-2@C-60

We have measured the birrolecular contribution (retaxivity) R-1 (M-1 s(-1)) to the spin-lattice relaxation rate for the protons of H-2 and H-2@C-60 dissolved in organic solvents in the presence of paramagnet nitroxide radicals. It is found that the relaxation effect of the paramagnets is enhanced 5-fold in H-2@C-60 compared to H-2 under the same conditions. C-13 relaxivity in C-60 induced by nitroxide has also been measured. The resulting value of R-1 for C-13 is substantially smaller relative to the H-1 relaxation in H-2@C-60 than expected solely on the basis of the smaller magnetic moment of C-13. The observed values of R-1 have been analyzed quantitatively using an outer-sphere model for bimolecular spin relaxation to extract an encounter distance, d, as the dependent variable. The resulting values of d for H-2 and C-13(60) are similar to the sum of the van der Waals radii for the radical and the corresponding molecule. The value of d for H-1(2)@C-60 is substantially smaller than the corresponding van der Waals estimates, corresponding to larger than expected values of R-1. A possible explanation for the enhanced relaxivity is a contribution from hyperfine coupling. Based on the results reported here, it seems that not only is the hydrogen molecule in H-2@C-60 not insulated from magnetic contact with the outside world but also the interaction with paramagnets is even stronger than expected based on distance alone.