Measurement of T1e, T1N, T1HE, T2e, and T2HE by Pulse EPR at X-Band for Nitroxides at Concentrations Relevant to Solution DNP

Аннотация

Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitroxides that are widely used in Overhauser dynamic nuclear polarization (ODNP) experiments. Carboxy proxyl (CP, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) in water and tempol (TP, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) in toluene have been studied with oxygen removed. For comparison, solutions of CP were also studied in air-saturated solution, which is the typical preparation for ODNP experiments. T₂ and T_2HE (HE = Heisenberg exchange) were measured by electron spin echo. The spin magnetization recovery time constant from inversion recovery experiments, which we denote as T_1e*, includes contributions from T_1e and T_1HE. In the absence of oxygen, values of T_1e* for both radicals decrease with increasing concentration up to ~ 1 mM, then increase again towards 10 mM. The concentration dependence results from changes in the relative contributions from T_1e and T_1HE. In air-saturated solutions of CP T_1e* decreases with concentration to about 200 ns at 1 mM, and then remains independent of further concentration increases. T_1e and T_1N were also measured with a digital saturation recovery spectrometer. Using the combined results from spin echo, inversion recovery, and saturation recovery we could extract the values of T_1e, T_1HE, T_1N, T_2e, and T_2HE for both radicals in this fast tumbling regime.