title: Nitroxide paramagnet-induced para-ortho conversion and nuclear spin relaxation of H2 in organic solvents.
abstract: 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 para-hydrogen ( 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.
authors: Department of Chemistry, Columbia University, New York, New York 10027, USA.
authors: Elena
authors: E
authors: Sartori
authors: Marco
authors: M
authors: Ruzzi
authors: Ronald G
authors: RG
authors: Lawler
authors: Nicholas J
authors: NJ
authors: Turro