Spin relaxation of a short-lived radical in zero magnetic field

A short-lived radical containing only one I = 1/2 nucleus, the muoniated 1,2-dicarboxyvinyl radical dianion, was produced in an aqueous solution by the reaction of muonium with the dicarboxyacetylene dianion. The identity of the radical was confirmed by measuring the muon hyperfine coupling constant (hfcc) by transverse field muon spin rotation spectroscopy and comparing this value with the hfcc obtained from DFT calculations. The muon spin relaxation rate of this radical was measured as a function of temperature in zero magnetic field by the zero field muon spin relaxation technique. The results have been interpreted using the theoretical model of Fedin et al. (J. Chem. Phys., 2003, 118, 192). The muon spin polarization decreases exponentially with time after muon implantation and the temperature dependence of the spin relaxation rate indicates that the dominant relaxation mechanism is the modulation of the anisotropic hyperfine interaction due to molecular rotation. The effective radius of the radical in solution was determined to be 1.12 ± 0.04 nm from the dependence of the muon spin relaxation rate on the temperature and viscosity of the solution, and is approximately 3.6 times larger than the value obtained from DFT calculations.