Electron spin–lattice relaxation of nitroxyl radicals in temperature ranges that span glassy solutions to low-viscosity liquids

Abstract

Electron spin-lattice relaxation rates, 1/T 1 , at X-band of nitroxyl radicals (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl, 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-1-oxyl and 3-carbamoyl-2,2,5,5-tetramethylpyrrolin-1-oxyl) in glass-forming solvents (decalin, glycerol, 3-methylpentane, o-terphenyl, 1-propanol, sorbitol, sucrose octaacetate, and 1:1 water:glycerol) at temperatures between 100 K and 300 K were measured by long-pulse saturation recovery to investigate the relaxation processes in slow-to-fast tumbling regimes. A subset of samples was also studied at lower temperatures or at Q-band. Tumbling correlation times were calculated from continuous wave lineshapes. Temperature dependence and isotope substitution ( 2 H and 15 N) were used to distinguish the contributions of various processes. Below about 100 K relaxation is dominated by the Raman process. At higher temperatures, but below the glass transition temperature, a local mode process makes significant contributions. Above the glass transition temperature, increased rates of molecular tumbling modulate nuclear hyperfine and g anisotropy. The contribution from spin rotation is very small. Relaxation rates at X-band and Q-band are similar. The dependence of 1/T 1 on tumbling correlation times fits better with the Cole-Davidson spectral density function than with the Bloembergen-Purcell-Pound model.

A slow relaxing species for molecular spin devices: EPR characterization of static and dynamic magnetic properties of a nitronyl nitroxide radical

“…This is in line with the results on nitroxides in glasses, 69,70 showing a contribution to relaxation due to fast librations. In particular, for T > 80 K the Hahn decay shows an exponential behaviour typical of a dominant relaxation due to the presence of residual motions.…”

First determination of the spin relaxation properties of a nitronyl nitroxide in solution by electron spin echoes at X-band: A comparison with Tempone

“…The strategies to increase the relaxation times for nitroxides are usually based on isotope substitution, i.e. deuteration and/or substitution of 14 N with 15 N [22][23][24][25][26]. Their relaxation rates depend also on the conformational inter-conversions of their rings [27].…”

“…[40][41][42][43][44]). Pulse experiments in solution, either with saturation recovery or with ELDOR methods, have been used for long time for the determination of longitudinal relaxation times T 1 by the groups of Hyde et al [22,24,45,46], Robinson et al [47,48], Eaton et al [23,25,26]. Spin-echo methods, on the other hand, have been mostly used for the determination of transverse relaxation times T 2 (see e.g.…”

Frequency dependence of electron spin–lattice relaxation for semiquinones in alcohol solutions

“…The temperature dependence of relaxation described by Eq. 3 has been observed for molecular species in glassy matrices, including organic radicals and transition metal complexes [17-19]. The magnitude of the contribution from the local mode is larger in soft matrices such as sucrose octaacetate than in harder ones such as sorbitol [18].…”

“…For a molecular species in a glass the local mode is interpreted as an intramolecular vibration that is impacted by interaction with the matrix. For nitroxides in highly viscous media [18, 19] and for trityl radicals in water:glycerol [20] the local mode dominates relaxation in the glassy state below the glass transition temperature. There is no change in slope for a plot of log(1/T 1 ) vs. log(T) in the vinicity of the glass transition temperature, so it is proposed that the local mode relaxation mechanism persists in solution.…”