Time-domain EPR studies of slow molecular motion

One example is a study of the molecular dynamics of an elongated nitroxide in frozen toluene matrices by two-dimensional electron spin echo spectroscopy (ESE) carried out by Poluektov and Doubinski at 95 GHz. The spectra were 

Another example is a recent study of confinement effects in ionomers carried out at W-band. Electronic Tx and T2 relaxation as a function of temperature was measured at W-band for spin probes localized at the interface between the ionic clusters and the polymer. Excellent angular selection W-band EPR which resolves x-, y-, and z-orientations of the nitroxides in the magnetic field allowed authors to probe electronic relaxation along those orientations while the sample remained macroscopically disordered. Based on these measurements of the electronic relaxation as a function of the nitroxide orientation, it was concluded that reorientation of these spin probes has clear uniaxial features. Moreover, evidence was presented that the dynamic constraints on the poly(isoprene) chains in the diblock copolymer propagate over the whole chain consisting of approximately 170 monomer units. 

Clearly, HF EPR and especially time-domain techniques have enormous potential in studies of slow molecular motion in complex and heterogeneous systems. It is my belief that ongoing development of more sophisticated time-domain HF EPR instrumentation and experimental and theoretical methods will lead to a new understanding of molecular motion in condensed matter and will open new applications for the method. 

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