Paramagnetic relaxation of nuclear singlet states

Nuclear singlet states often display lifetimes that are much longer than conventional nuclear spin relaxation times. Here we investigate the effect of dissolved paramagnetic species on the singlet relaxation of proton pairs in solution. We find a linear correlation between the singlet relaxation rate constant T-S(-1) and the longitudinal relaxation rate constant T-1(-1). The slope of the correlation depends on the nature of the paramagnetic relaxation agent, but typically, singlet states are between two to three times less sensitive to paramagnetic relaxation than conventional nuclear magnetization. These observations may be interpreted using a model of partially-correlated local fields acting on the nuclear sites. We explore the effect on singlet relaxation of adding a metal-ion-chelating agent to the solution. We also investigate the effect of ascorbate, which reacts with dissolved oxygen.