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

Abstract

The bimolecular contribution, R 1 (M−1 s−1), to the T 1 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 R 1 values in the presence of the various M(acac) x complexes [M = Fe(III), Cr(III), Cu(II), Co(III)] scale with μeff 2 of the metal complex. The R 1 values were significantly smaller for the trapped species than the free H2O. Surprisingly, R 1 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.

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]. Some of the limitations of solution NMR can be overcome to a considerable extent when spectra are obtained from solid samples at low temperatures.…”

Infrared spectroscopy of small-molecule endofullerenes

“…Three spectroscopic techniques NMR, INS and infrared (IR), have been used to study endohedral hydrogen. NMR studies cover spin lattice relaxation rates of H 2 @C 60 in organic solvents [22,23], and in the presence of paramagnetic relaxants [24,25]. NMR was used to follow the ortho-para conversion in H 2 @C 60 in the presence of molecular oxygen at 77 K [26] or upon photoexcitation of a C 70 triplet state [27].…”

Gas storage of simple molecules in boron oxide nanocapsules

“…Encapsulation of atoms and molecules by fullerenes has been a subject of many experimental and theoretical works . Although a number of atomic endohedral complexes of C 60 are known, but encapsulation of molecules has been difficult.…”

“…For example, a trace amount of N 2 @C 60 , N 2 @C 70 , CO@C 60 , and H 2 O@C 60 have been prepared by heating fullerenes under high pressures of the corresponding gases or ion implantation . Also, among selected small molecules (H 2 , N 2 , H 2 O, CO, NH 3 , and CH 4 ) trapped in the chemically opened C 60 and C 70 , only the hydrogen and water containing endohedral fullerenes could be chemically closed. Two review articles have been published on this topic …”

“…They found that the complexation could take place for one H 2 molecule for C 60 and two H 2 molecules for C 70 . This result was in agreement with the experimental synthesis of H 2 @C 60 , H 2 @C 70 , and 2H 2 @C 70 species via molecular surgery methods . Lee and McKee studied the interactions of several H 2 molecules [(H 2 ) n , n = 1–5] inside C 60 , C 70 , and C 82 .…”