Nitroxide Paramagnet-Induced Para−Ortho Conversion and Nuclear Spin Relaxation of H2 in Organic Solvents

• Published in: Journal of the American Chemical Society 2008-09, Vol.130 (38), p.12752-12756
• Main Authors: Sartori, Elena, Ruzzi, Marco, Lawler, Ronald G, Turro, Nicholas J
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja8037195

The kinetics of para−ortho conversion and nuclear spin relaxation of H2 in chloroform-d 1 were investigated in the presence of nitroxides as paramagnetic catalysts. The back conversion from para-hydrogen (p-H2) to ortho-hydrogen (o-H2) was followed by NMR by recording the increase in the intensity of the signal of o-H2 at regular intervals of time. The nitroxides proved to be hundreds of times more effective at inducing relaxation among the spin levels of o-H2 than they are in bringing about transitions between p-H2 and the levels of o-H2. The value of the encounter distance d between H2 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.