Paramagnet Enhanced Nuclear Spin Relaxation in H 2

Relaxation rates of endo-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 endo-H2 in similar cages. T1 relaxation enhancement of the endo-H2O and H2 induced by either intraor 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. In a recent paper, we reported the synthesis and nuclear spin relaxation of H2O in a H2O@Open-C60 paramagnetic nitroxide derivativewith aneight-membered ring opening in the C60 cage. Such enhancement of relaxation by paramagnetic catalysts plays an important role in developing methods for enriching spin isomers of small molecules encapsulated in fullerene cages for possible applications in magnetic resonance imaging or other practical purposes. Due to the availability of multiple double bonds on the C60 cage and the unsymmetrical features of the product molecule, a variety of isomers were obtained and differentiated by proton NMR spectroscopy of the endo-H2O. We measured and briefly discussed the relaxation times (T1) of some of the H2O@Open-C60 nitroxide isomers. The results were compared with those of structurally similar nitroxide isomers of a H2@Open-C60 compound. 1 Similar methodology was employed to attach a nitroxide to the parent H2O@C60 cage. 4 This made it possible to compare the relaxation properties of H2O and H2 inside the same cage with and without an attached paramagnet. The relative sensitivities of the two endomolecules to enhancement of the relaxation rate, 1/T1, i.e. relaxivity, R1 (M s ), by bimolecular encounterswith a nitroxide paramagnet were also determined. We present here an extension of these measurements to include the relaxivity of two Open-C60 endo fullerenes containing H2O or H2 induced by a nitroxide. We also Columbia University. Brown University. Kyoto University. Deceased November 24, 2012. (1) Li, Y.; Lei, X.; Lawler, R. G.; Murata, Y.; Komatsu, K.; Turro, N. J. Org. Lett. 2012, 14, 3822–3825. (2) Turro, N. J.; Chen, J. Y.-C.; Sartori, E.; Ruzzi, M.; Marti, A.; Lawler, R. G.; Jockusch, S.; Lopez-Gejo, J.; Komatsu, K.; Murata, Y. Acc. Chem. Res. 2010, 43, 335–345. (3) Datta, A.; Raymond, K. N. Acc. Chem. Res. 2009, 42, 938–947. (4) Li, Y.; Chen, J. Y.-C.; Lei, X.; Lawler, R. G.; Murata, Y.; Komatsu, K.; Turro, N. J. J. Phys. Chem. Lett. 2012, 3, 1165–1168.