T 1e, T 1N, T 1HE, T 2e, and T 2HE by Pulse EPR at X-Band for Nitroxides at Concentrations Relevant to Solution DNP]]>

摘要

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. T ~(2)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.