| Abstract
| - A new method for the experimental determination of thecross-relaxation (CR) transition rates in liquid solutionsof paramagnetic compounds has been developed utilizing a pulsed dynamicnuclear polarization (DNP)technique. In contrast to NMR relaxation, which is proportional tothe sum of relaxation rates, the DNPeffect is determined by the ratio of transition rates in thenucleus−electron coupled spin system. By use ofpulsed DNP, the NMR relaxation rates and DNP data can be obtained inthe same experiment. As a result,a set of independent equations for CR and dipole−dipole (DD)transition rates can be derived. The solutionof these equations defines individual cross-relaxation and DDtransition rates, as well as molecular−kineticinformation, and avoids the necessity of performing complicatedvariable frequency and temperaturemeasurements. A pulsed DNP relaxometer operating at a protonfrequency of 0.5 MHz was constructed.The 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPONE) stablefree radical in benzene was chosen forstudy as a system with strong intermolecular dipole−dipoleinteractions. The measurement of individualrates of CR and DD transitions gave us the distance (b =5.0 Å) between spins I and S for the solvatedradical of TEMPONE and also the molecular rotational correlation time(τr = 2.5 ×10-10 s). Another systemstudied was the solvated electron in hexamethylphosphorus amide (HMPA)in which scalar and DD interactionsare present at the same time. The basic characteristics of thisfundamentally important elementary paramagneticcenter have been obtained, such as the lifetime of HMPA molecules inthe solvent cage (τh = 2.9 ×10-9 s)and the intermolecular hyperfine constant value (a = 0.08MHz). Pulsed DNP is shown to be a valuableapproach for the study of very weak hyperfine interactions that are notreadily detected by other traditionalmagnetic resonance methods.
|
