Temperature Dependence of Electron Spin Relaxation of 2,2-Diphenyl-1-Picrylhydrazyl in Polystyrene

The electron spin relaxation rates for the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) doped into polystyrene were studied by inversion recovery and electron spin echo at X-band and Q-band between 20 and 295 K. At low concentration (340 μM, 0.01 %), spin–lattice relaxation was dominated by the Raman process and a local mode. At high concentration (140 mM, 5 %), relaxation is orders of magnitude faster than at the lower concentration, and 1/T ₁ is approximately linearly dependent on the temperature. Spin lattice relaxation rates are similar at X-band and Q-band. The temperature dependence of spin echo dephasing was faster at about 140 K than at higher or lower temperatures, which is attributed to a wagging motion of the phenyl groups.     

Temperature Dependence of Hyperfine Interaction for 15N Nitroxide in a Glassy Matrix at 10–210 K

Principal ¹⁵N hyperfine interaction (hfi) values in ¹⁵N-substituted nitroxide spin probe 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl dissolved in nematic liquid crystal 4-pentyl-4′-cyanobiphenyl (5CB) were measured in a wide temperature range of 10–210 K, for 5CB frozen to a glassy state. X-band continuous-wave electron paramagnetic resonance (CW EPR) and pulse X- and Q-band ¹⁵N electron-nuclear double resonance (ENDOR) techniques were employed. To avoid microwave saturation at low temperatures in CW EPR studies, a holmium complex Ho(Dbm)₃Bpy (where Dbm is dibenzoylmethane and Bpy is 2,2′-bipyridine) was added. X- and Q-band ¹⁵N-ENDOR data have shown that the nitroxide hfi tensor is axially symmetric. The combination of data from all techniques allowed us to obtain the temperature dependence of isotropic and anisotropic parts of the nitroxide hfi tensor. Above ~100 K, a linear dependence of the anisotropic hfi value was observed, whereas below 30 K it was found to be nearly temperature independent. Such a behavior can be interpreted using the model of restricted orientational motions (librations) of a spin probe in a glassy matrix, with quantum effects occurring at low temperature (“freezing” of the librations). The energy quantum for the libration motion estimated from the temperature dependence of hfi of the spin probe is 84 cm^?1. Low-frequency Raman spectra of 5CB were also obtained, which provided the mean vibrational frequency of 76 cm^?1 for glassy 5CB.     

Electron spin relaxation of triarylmethyl radicals in fluid solution

Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1:1 water:glycerol, and 1:9 water:glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H(2)O solution, T(1) is 17+/-1 micros at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T(1) in 1:1 water:glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H(2)O solution, T(2) is 11+/-1 micros at X-band, increasing to 13+/-1 micros at L-band. For more viscous solvent mixtures, T(2) is much shorter than T(1) and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T(2). In water and 1:1 water:glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1:9 water:glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T(2)-determined contributions to the linewidths at higher frequencies.     

Sensitivity of Saturation Transfer Electron Spin Resonance Extended to Extremely Slow Mobility in Glassy Materials

A novel extension of the saturation transfer (ST) ESR technique that enables the determination of extremely long rotational correlation times of nitroxide spin labels up to values around 10⁴s is proposed. The method is based on the observation that the integral of ST-ESR spectra is sensitive to the spin–lattice relaxation time of the electron of the spin label, which in turn is directly dependent upon the rotational correlation time. The method is applied to the spin label TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) in glycerol. From the known viscosity data and the related rotational correlation times of the TEMPOL spin label in glycerol, the rotational correlation times of unknown samples can be determined. The method is especially applicable to systems with a very high viscosity, such as glassy materials. The method is applied to a 20 wt% glucose–water mixture in the glassy state, giving a value for the highest limiting rotational correlation time of about 10³s at a temperature of 45 K below the glass transition temperature of this system. This is an extension by six decades for the rotational correlation time, as compared to the current application of ST-ESR.     

Frequency dependence of electron spin relaxation for threeS = 1/2 species doped into diamagnetic solid hosts

Electron spin lattice relaxation rates (1/T₁) were measured as a function of temperature at two or three microwave frequencies for threeS = 1/2 species in temperature ranges with different dominant relaxation processes. Between 10 and 50 K the contribution from the direct process to the relaxation rate was substantially greater at 94 than at 9.5 GHz for a vanadyl porphyrin doped into zinc tetratolylporphyrin. For bis(diethyldithiocarbamato)copper(II) doped into the diamagnetic Ni(II) analog the relaxation rate between 25 and 100 K is dominated by the Raman process and exhibits little frequency dependence between 9.2 and 94 GHz. For 4-hydroxy-2,2,6,6-tetramethylpiperidinoloxy (tempol) doped into a diamagnetic host the relaxation rate between about 40 and 100 K is dominated by the Raman process. In this temperature range, relaxation rates at 3.2, 9.2, and 94 GHz exhibit little frequency dependence. Above about 130 K, the relaxation rate for tempol decreases in the order S-band s> X-band > W-band. The relaxation rates in this temperature range fit a model in which 1/T₁ is dominated by a thermally activated process that is assigned as rotation of the methyl groups on the nitroxyl ring.     

Electron spin lattice relaxation rates for S = 12 molecular species in glassy matrices or magnetically dilute solids at temperatures between 10 and 300 K.

The temperature dependence of X-band electron spin-lattice relaxation between about 10 and 300 K in magnetically dilute solids and up to the softening temperature in glassy solvents was analyzed for three organic radicals and 14 S = 12 transition metal complexes. Contributions from the direct, Raman, local vibrational mode, thermally activated, and Orbach processes were considered. For most samples it was necessary to include more than one process to fit the experimental data. Debye temperatures were between 50 and 135 K. For small molecules the Debye temperature required to fit the relaxation data was higher in 1:1 water:glycerol than in organic solvents. For larger molecules the Debye temperature was less dependent upon solvent and more dependent upon the characteristics of the molecule. The coefficients of the Raman process increased with increasing g anisotropy and decreasing rigidity of the molecule. For the transition metal complexes the data are consistent with major contributions from local modes with energies in the range of 185 to 350 K (130 to 240 cm-1). The coefficient for this contribution increases in the order 3d < 4d transition metal. For C-60 anions there is a major contribution from a thermally activated process with an activation energy of about 240 cm-1. For low-spin hemes the dominant contribution at higher temperatures is from a local mode or thermally activated process with a characteristic energy of about 175 cm-1.     

Frequency (250 MHz to 9.2 GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

Electron spin relaxation times for four triarylmethyl (trityl) radicals at room temperature were measured by long-pulse saturation recovery, inversion recovery, and electron spin echo at 250 MHz, 1.5, 3.1, and 9.2 GHz in mixtures of water and glycerol. At 250 MHz T(1) is shorter than at X-band and more strongly dependent on viscosity. The enhanced relaxation at 250 MHz is attributed to modulation of electron-proton dipolar coupling by tumbling of the trityl radicals at rates that are comparable to the reciprocal of the resonance frequency. Deuteration of the solvent was used to distinguish relaxation due to solvent protons from the relaxation due to intra-molecular electron-proton interactions at 250 MHz. For trityl-CD(3), which contains no protons, modulation of dipolar interaction with solvent protons dominates T(1). For proton-containing radicals the relative importance of modulation of intra- and inter-molecular proton interactions varies with solution viscosity. The viscosity and frequency dependence of T(1) was modeled based on dipolar interaction with a defined number of protons at specified distances from the unpaired electron. At each of the frequencies examined T(2) decreases with increasing viscosity consistent with contributions from T(1) and from incomplete motional averaging of anisotropic hyperfine interaction.     

Electron spin relaxation studies of La@C82 in the solid state by EPR

The temperature dependence of EPR spectrum of La@C₈₂ in the powder of empty C₆₀ and C₇₀ mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g _‖=2.0021,g _⊥=1.9970,^La A _⊥=7.8 MHz,^La A _‖～0 MHz and an isotropic¹³C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.     

丙三醇浓溶液玻璃化转变与结构松弛参数DSC分析

利用差示扫描量热仪研究了5种高浓度丙三醇水溶液(60%、70%、80%、90%、100%)的玻璃化转变行为,以考察水分含量和升降温速率对其玻璃化转变行为和结构松弛参数的影响.采用4种线性升降温速率(10、15、20、25K/min)获得玻璃化转变的相关参数.利用GordonTaylor方程对玻璃化转变温度的分析结果表明,水对丙三醇增塑常数的计算结果与升降温速率和玻璃化转变温度的读取方法有很大关系.玻璃化转变过程的比热容变化不仅随水分含量的增加而增加,而且与升降温速率也有一定的依赖关系.结构松弛活化能的计算结果表明,随体系水分含量的增加,体系的结构松弛活化能和动力学脆度都逐渐降低.随水分含量的变化,热力学脆度和动力学脆度表现出相反的变化趋势.     

Non-equilibrium relaxation and tumbling times of polymers in semidilute solution

The end-over-end tumbling dynamics of individual polymers in dilute and semidilute solutions is studied under shear flow by large-scale mesoscale hydrodynamic simulations. End-to-end vector relaxation times are determined along the flow, gradient, and vorticity directions. Along the flow and gradient directions, the correlation functions decay exponentially with sinusoidal modulations at short times. In dilute solution, the decay times of the various directions are very similar. However, in semidilute solutions, the relaxation behaviors are rather different along the various directions, with the longest relaxation time in the vorticity direction and the shortest time in the flow direction. The various relaxation times exhibit a power-law shear-rate dependence with the exponent -?2/3 at high shear rates. Quantitatively, the relaxation times are equal to the tumbling times extracted from cross-correlation functions of fluctuations of radius-of-gyration components along the flow and gradient direction.     

Electron spin relaxation of copper(II) complexes in glassy solution between 10 and 120 K

The temperature dependence, between 10 and 120 K, of electron spin-lattice relaxation at X-band was analyzed for a series of eight pyrrolate-imine complexes and for ten other copper(II) complexes with varying ligands and geometry including copper-containing prion octarepeat domain and S100 type proteins. The geometry of the CuN4 coordination sphere for pyrrolate-imine complexes with R=H, methyl, n-butyl, diphenylmethyl, benzyl, 2-adamantyl, 1-adamantyl, and tert-butyl has been shown to range from planar to pseudo-tetrahedral. The fit to the recovery curves was better for a distribution of values of T1 than for a single time constant. Distributions of relaxation times may be characteristic of Cu(II) in glassy solution. Long-pulse saturation recovery and inversion recovery measurements were performed. The temperature dependence of spin-lattice relaxation rates was analyzed in terms of contributions from the direct process, the Raman process, and local modes. It was necessary to include more than one process to fit the experimental data. There was a small contribution from the direct process at low temperature. The Raman process was the dominant contribution to relaxation between about 20 and 60 K. Debye temperatures were between 80 and 120 K. For samples with similar Debye temperatures the coefficient of the Raman process tended to increase as gz increased, as expected if modulation of spin-orbit coupling is a major factor in relaxation rates. Above about 60 K local modes with energies in the range of 260-360 K (180-250 cm-1) dominated the relaxation. For molecules with similar geometry, relaxation rates were faster for more flexible molecules than for more rigid ones. Relaxation rates for the copper protein samples were similar to rates for small molecules with comparable coordination spheres. At each temperature studied the range of relaxation rates was less than an order of magnitude. The spread was smaller between 20 and 60 K where the Raman process dominates, than at higher temperatures where local modes dominate the relaxation. Spin echo dephasing time constants, Tm, were calculated from two-pulse spin echo decays. Near 10 K Tm was dominated by proton spins in the surroundings. As temperature was increased motion and spin-lattice relaxation made increasing contributions to Tm. Near 100 K spin-lattice relaxation dominated Tm.     

Local and global dynamics in the glass-forming di-isobutyl phthalate as studied by H NMR

Spin-lattice relaxation times T₁ and T_1ρ as well as ¹H NMR spectra have been employed to study the dynamics of the glass-forming di-isobutyl phthalate in the temperature range extending from 100 K, through the glass transition temperature T_g, up to 340 K. Below T_g NMR relaxation is governed by local dynamics and may be attributed to rotation of methyl groups at low temperatures and to motion of isobutyl groups in the intermediate temperature interval. Above T_g the main relaxation mechanism is provided by overall molecular motion. The observed relaxation behavior is explained by motional models assuming asymmetrical distributions of correlation times. The motional parameters obtained from Davidson-Cole distribution, which yields the best fit of the data at all temperatures are given.     

环氧树脂高温分子链松弛与玻璃化转变特性

环氧树脂是电力设备中广泛应用的一种绝缘材料, 其介电性能受到分子链运动特性的影响. 本文制备了直径为50 mm、厚度为1 mm的环氧树脂试样, 采用差示扫描量热仪和宽频介电谱仪测试了环氧树脂的玻璃化转变温度和介电特性. 实验结果表明, 环氧树脂的玻璃化转变温度为105 ℃, 在玻璃化转变温度以上, 高频段出现了由分子链段运动造成的松弛过程, 低频段出现了由载流子在材料中迁移造成的直流电导过程. 发现环氧树脂不同尺寸分子链段的松弛时间不同, 其松弛时间分布较宽, 计算得到了分子链段在不同温度下的松弛时间分布特性. 分子链松弛峰频率和直流电导随温度的变化关系服从Vogel-Tammann-Fulcher公式. 拟合实验结果得到分子链松弛峰频率和直流电导的Vogel温度和强度系数. 由Vogel温度计算得到了与差示扫描量热测试结果一致的玻璃化转变温度, 约为102 ℃. 结果表明玻璃化转变温度以上环氧树脂的自由体积增大, 分子链段有足够的空间来响应外电场从而产生分子链松弛极化, 载流子有足够的能量在材料中迁移形成电导.     

The electronic structure and the ferromagnetic intermolecular interactions in the crystal of TEMPO radicals

Based on the generalized gradient approximation, full potential linearized augmented plane wave (FP-LAPW) calculations have been performed to study the electronic band structure and the intermolecular ferromagnetic (FM) interactions for the two TEMPO radicals 4-Benzylideneamino-2,2,6,6-tetramethylpiperidin-1-oxyl (1) and 4-(2-naphtylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl (2). The total and the partial density of states and the atomic spin magnetic moments are calculated and discussed. The calculation revealed that the two TEMPO radicals have the intermolecular FM interactions, and the spontaneous magnetic moment is 1.0 μ_B per molecule of each crystal, which is in good agreement with the experimental value. It is found that the unpaired electrons in these compounds are localized in a molecular orbital constituted primarily of π* (NO) orbital, and the main contribution of the spin magnetic moment comes from the NO-free radical. The origin of FM is also studied in detail.     

非晶材料玻璃转变过程中记忆效应的热力学

低温下处于非平衡态的非晶材料升温到玻璃转变以上,要先后发生弛豫和回复最终达到平衡过冷液态,其中弛豫过程中释放的能量在回复过程中以等量的方式获取,表现出明显记忆行为.本文基于氧化物、金属与小分子等多种非晶形成体系,全面探讨了在围绕玻璃转变的一个冷却加热循环过程中的焓弛豫特征,建立了弛豫谱,发现弛豫焓在数值上与熔化焓密切相关.基于弛豫焓与非晶材料动力学Fragility之间的关联,展示了非晶体系在动力学极限(m=175)条件下的玻璃转变热力学基本特征,与热力学二级相变进行了对比.研究深化了对非晶弛豫与玻璃转变热力学的理解.     

X-band rapid-scan EPR of nitroxyl radicals

X-band rapid-scan EPR spectra were obtained for dilute aqueous solutions of nitroxyl radicals (15)N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeuteromethyl-pyrrolin-1-(15)N-oxyl-d(12)) and (15)N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl-piperidinyl-(15)N-oxyl-d(16)). Simulations of spectra for (15)N-mHCTPO and (15)N-PDT agreed well with the experimental spectra. As the scan rate is increased in the rapid scan regime, the region in which signal amplitude increases linearly with B(1) extends to higher power and the maximum signal amplitude increases. In the rapid scan regime, the signal-to-noise for rapid-scan spectra was about a factor of 2 higher than for unbroadened CW EPR, even when the rapid scan spectra were obtained in a mode that had only 4% duty cycle for data acquisition. Further improvement in signal-to-noise per unit time is expected for higher duty cycles. Rapid scan spectra have higher bandwidth than CW spectra and therefore require higher detection bandwidths at faster scan rates. However, when the scan rate is increased by increasing the scan frequency, the increase in noise from the detection bandwidth is compensated by the decrease in noise due to increased number of averages per unit time. Because of the higher signal bandwidth, lower resonator Q is needed for rapid scan than for CW, so the rapid scan method is advantageous for lossy samples that inherently lower resonator Q.     

Spin relaxation and ultra-slow Li motion in an aluminosilicate glass ceramic

The ion dynamics in a lithium aluminosilicate glass ceramic was studied using stimulated-echo 7Li-NMR. For temperatures 300 K相似文献   

Importance of volume limitation for tissue redox status measurements using nitroxyl contrast agents: a comparison of X-band EPR bile flow monitoring (BFM) method and 300 MHz in vivo EPR measurement

Methods proposed for in vivo redox status estimation, X-band (9.4 GHz) electron paramagnetic resonance (EPR) bile flow monitoring (BFM) and 300 MHz in vivo EPR measurement, were compared. The spin probe 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (carbamoyl-PROXYL) was utilized for both methods, due to its suitable lipophilicity. EPR signal decay of a nitroxyl spin probe in the bile flow and in the liver region (upper abdomen) of several rat groups with different selenium status were measured by both the BFM and the in vivo EPR method, respectively. The nitroxyl radical clearance measured with in vivo EPR method may be affected not only by the redox status in the liver but also by information from other tissues in the measured region of the rat. On the other hand, the time course of nitroxyl radical level in the bile flow of rats was found to be a reliable index of redox status. Measurement site and/or volume limitation, which was achieved by the BFM method in this paper, is quite important in estimating reasonable EPR signal decay information as an index of tissue/organ redox status.     

Overhauser dynamic nuclear polarization and molecular dynamics simulations using pyrroline and piperidine ring nitroxide radicals

The efficiency of Overhauser dynamic nuclear polarization (DNP) depends on the local dynamics modulating the dipolar coupling between the two interacting spins. By attaching nitroxide based spin labels to molecules and by measuring the ¹H DNP response of solvent water, information about the local hydration dynamics near the spin label can be obtained. However, there are two commonly used types of nitroxide ring structures; a pyrroline based and a piperidine based molecule. It is important to know when comparing different experiments, whether changes in DNP enhancements are due to changes in local hydration dynamics or because of the different spin label structures. In this study we investigate the key parameters affecting DNP signal enhancements for 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl, a 5-membered ring nitroxide radical, and for 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, a 6-membered ring nitroxide radical. Using X-Band DNP, field cycling relaxometry, and molecular dynamics simulations, we conclude that the key parameters affecting the DNP amplitude of the ¹H signal of water to be equal when using either nitroxide. Thus, experiments measuring hydration dynamics using either type of spin labels may be compared.     

Behavior of hafnium fluoride octahedral complex in HF at low temperature studied by TDPAC

The behavior of $\text{HfF}_{6}^{2-}$ octahedral complex in HF has been studied in the temperature range from 300 to 77 K by time differential perturbed angular correlation technique. No time dependent spin relaxation has been observed up to 223 K. At 203 and 193 K, definite nuclear spin relaxations while at 77 K a pure static interaction have been observed. These indicate that at temperatures below 223 K, a different phase of the liquid appears. The rotational correlation times characterizing the tumbling motion of the complex in HF have been determined at 203 and 193 K and found to be 1.6 ns and 2.1 ns, respectively from the measured relaxation constants at these temperatures and the value of quadrupole frequency found at 77 K. The value of rotational correlation time found at 203 K disagrees by an order of magnitude with the earlier measured value which was found to be in the picosecond range.