ИСТИНА

In the investigations of molecular dynamics and structure of liquids, glasses, polymers, colloids, and liquid crystals, the use of the spin probe technique is widely applied. It is generally agreed that for reliable interpretation of electron paramagnetic resonance (EPR) spectra in order to obtain information about rotation mobility and orientation order, the quantitative simulation of experimental spectra with the use of least-squares optimization should be performed. Among numerous spin probes used, biradical molecules show particular promise. Due to the large anisotropic electron-electron dipolar interaction, these probes are expected to have profound sensitivity towards orientation order and rotation movements of molecules in the investigated media. However, the problem of quantitative simulation of EPR spectra of biradicals in the slow-motional regime and in the ordered samples has not been solved until now. In the present work the numerical simulation of EPR spectra of nitroxide biradical both in isotropic and aligned media was performed. The models suitable for description of the spectra of the probes, both in the rigid limit and in presence of rotation motions, were developed and successfully applied to model systems. The examples of spectra simulation are illustrated in Fig. 1 for the slow-motional regime (Fig. 1a) and in the rigid limit (Fig. 1b). The simulation of EPR spectra allows obtaining the following information about molecular structure and dynamics: the values of orientation order parameters, the type of rotation mobility and its quantitative characteristics, the sign and value of the spin exchange constant of the biradical. Model systems used in this work include solutions of nitroxide biradical in a viscous solvent (squalane) in the range of temperatures 100¬–370K and in the aligned liquid crystal n-octylcyanobiphenyl (8CB, 100–298.5K). Unexpectedly, it was found that in 8CB the main orientation axis of biradical molecule is perpendicular to the longest molecular axis. The authors acknowledge the financial support from RFBR (grant Nos. 16-33-60139 mol-dk and 14-03-00323a).