Nitroxide free radicals orientation dependence

Most commonly used probe is nitroxide radical. A significant amount of work on this topic is available in the literature. Nitroxide free radical (probe) produces a three-line hyperfine structure whose properties such as peak shape and splitting depend upon the environment of the radical. The shape of the ESR signal depends also on the orientation of the magnetic field relative to the axis of the radical. 

Electron spin resonance (ESR) spectroscopy has been widely used to obtain information about the molecular dynamics of polymers. The method requires the introduction of a stable free-radical reporter group, such as a nitroxide, into the system. Nitroxide spin labels can be covalently attached to the polymer of interest, and can therefore serve as probes of the local backbone dynamics of the polymer, providing information on the local orientation, stracture, dynamics, and enviromnent. " A commonly used nitroxide is shown in Eig. 1. Depending on the ESR frequency, motion on time scales between 10 and 10 ° s may be investigated by this method, making it ideal to study the dynamics of macromolecules and macromolecular structures or assemblies. 

In real media, nitroxides change their orientations with respect to the external magnetic field due to the Brownian thermal rotational mobility usually characterized by the rotational correlation time, The anisotropic hyperfine interaction between the unpaired electron and nitrogen nucleus is modified by these changes with a frequency dependent on Xj. In this way, frequency-dependent perturbations are generated, which modify the energy levels and transition probabilities in the system. As a result, the line shape of ESR spectra of nitroxides (and of other free radicals in which anisotropic magnetic interactions occur) depends on the correlation time x. ... 

The line shape of the ESR spectrum can be calculated using the spin Hamiltonian H (f) for the nitroxide radical. The time dependence of the spin Hamiltonian describes the orientation-dependent part which contains terms characterizing the anisotropy of the A- and g-tensors, and terms characterizing the rotational reorientation of the free radical as a classic stochastic process using the Wigner rotation matrix elements. A comprehensive theory of ESR spectra of nitroxides has been described step-by-step in contributions to two monographs and papers cited therein. Computer programs suitable for calculation of theoretical line shapes of ESR spectra measured in continuous wave (CW) and pulse experiments have become indispensable tools for practical applications. The ESR spectra of nitroxides are also described in chapters 1 and 3 in this volume. 

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