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

Abstract

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 T2HE (HE = Heisenberg exchange) were measured by electron spin echo. The spin magnetization recovery time constant from inversion recovery experiments, which we denote as T1e*, includes contributions from T1e and T1HE. In the absence of oxygen, values of T1e* 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 T1e and T1HE. In air-saturated solutions of CP T1e* decreases with concentration to about 200 ns at 1 mM, and then remains independent of further concentration increases. T1e and T1N 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 T1e, T1HE, T1N, T2e, and T2HE for both radicals in this fast tumbling regime.