Magnetic Interaction of Solution-State Paramagnets with Encapsulated H2O and H2

The bimolecular contribution, R1 (M−1 s−1), to the T1 of isolated H2 and H2O protons, as well as free H2O in organic solution, by paramagnetic metal complexes was determined. Isolation was achieved by encapsulation in a fullerene: H2 was trapped in pristine C60 or in C60 with a 13-atom opening, and H2O in an open-C60 with a 19-atom opening. The R1 values in the presence of the various M(acac)x complexes [M = Fe(III), Cr(III), Cu(II), Co(III)] scale with μeff2 of the metal complex. The R1 values were significantly smaller for the trapped species than the free H2O. Surprisingly, R1 was nearly identical for the all three endohedral proton pairs, even in different solvents. This suggests that the magnetic isolation effects of the carbon cage are not significantly affected by the solvent, the completeness of the carbon cage, or separation of the hydrogens by an oxygen atom.