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Room-Temperature Electron Spin Relaxation of Triarylmethyl Radicals at the X- and Q-Bands

Kuzhelev, Andrey A., Trukhin, Dmitry V., Krumkacheva, Olesya A., Strizhakov, Rodion K., Rogozhnikova, Olga Yu., Troitskaya, Tatiana I., Fedin, Matvey V., Tormyshev, Victor M., Bagryanskaya, Elena G.
The Journal of physical chemistry 2015 v.119 no.43 pp. 13630-13640
ambient temperature, electron paramagnetic resonance spectroscopy, free radicals, magnetic fields, physical chemistry, secondary amines, viscosity, Finland
Triarylmethyl radicals (trityls, TAMs) represent a relatively new class of spin labels. The long relaxation of trityls at room temperature in liquid solutions makes them a promising alternative for traditional nitroxides. In this work we have synthesized a series of TAMs including perdeuterated Finland trityl (D₃₆ form), mono-, di-, and triester derivatives of Finland-D₃₆ trityl, the deuterated form of OX63, the dodeca-n-butyl homologue of Finland trityl, and triamide derivatives of Finland trityl with primary and secondary amines attached. We have studied room-temperature relaxation properties of these TAMs in liquids using pulsed electron paramagnetic resonance (EPR) at two microwave frequency bands. We have found the clear dependence of phase memory time (Tₘ ∼ T₂) on the magnetic field: room-temperature Tₘ values are ∼1.5–2.5 times smaller at the Q-band (34 GHz, 1.2 T) than at the X-band (9 GHz, 0.3 T). This trend is ascribed to the contribution from g-anisotropy that is negligible at lower magnetic fields but comes into play at the Q-band. In agreement with this, the difference between T₁ and Tₘ becomes more pronounced at the Q-band than at the X-band due to increased contributions from incomplete motional averaging of g-anisotropy. Linear dependence of (1/Tₘ – 1/T₁) on viscosity implies that g-anisotropy is modulated by rotational motion of the trityl radical. On the basis of the analysis of previous data and results of the present work, we conclude that, in the general situation where the spin label is at least partly mobile, the X-band is most suitable for application of trityls for room-temperature pulsed EPR distance measurements.