Nitroxide paramagnet-induced para-ortho conversion and nuclear spin relaxation of H-2 in organic solvents

The kinetics of para-ortho conversion and nuclear spin relaxation of H-2 in chloroform-d(1), were investigated in the presence of nitroxides as paramagnetic catalysts. The back conversion from parahydrogen (p-H-2) to ortho-hydrogen (o-H-2) was followed by NMR by recording the increase in the intensity of the signal of o-H-2 at regular intervals of time. The nitroxides proved to be hundreds of times more effective at inducing relaxation among the spin levels of o-H-2 than they are in bringing about transitions between p-H-2 and the levels of o-H-2. The value of the encounter distance d between H-2 and the paramagnetic molecule, calculated from the experimental bimolecular conversion rate constant k(0), using the Wigner theory of para-ortho conversion, agrees perfectly with that calculated from the experimental relaxivity R-1 using the force free diffusion theory of spin-lattice relaxation.