EPR line shifts and line shape changes due to spin exchange of nitroxide free radicals in liquids: 6. Separating line broadening due to spin exchange and dipolar interactions.
EPR line shifts and line shape changes due to spin exchange of nitroxide free radicals in liquids: 6. Separating line broadening due to spin exchange and dipolar interactions.
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DOI：
10.1021/jp8093947
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发表时间：
2009-04-30
期刊：
影响因子：
--
通讯作者：
Peric M
中科院分区：
文献类型：
--
作者：
Bales BL;Meyer M;Smith S;Peric M
本文研究了全氘代2，2，6，6-四甲基-4-氧代哌啶-1-氧（PDT）在水和70wt%甘油水溶液中的EPR谱随浓度c和温度T的变化。在所有温度下，在两种溶剂中，在三条超精细线上平均的本征线宽的增加，Δ Btot Δ，与c呈线性变化，截距为零;因此， 独立于C。自旋交换诱导的色散（可从中计算自旋交换频率ωe）随Δ Btot Δ线性增加，在高温下通过水和70%甘油中的原点;然而，在低温下，偶极相互作用使光谱变宽，由于偶极相互作用对色散的贡献，直到Δ Btot Δ> 1 G才出现线性。由于自旋交换的加宽常数，，发现从线性区域的斜率，允许偶极常数的计算。因此，浓度展宽分离成自旋交换和偶极贡献的影响，而不必呼吁一些假设的温度依赖性的两个相互作用。在两种溶剂中，由自旋交换引起的分数增宽Ω（T）在高温下接近1，在273 K下在70%甘油中降至零。Ω（T）是PDT的逆旋转相关时间的连续函数，但作为T/η的函数是不连续的，其中η是剪切粘度。Ω（T）= 0.5，其中自旋交换和偶极相互作用对线宽的贡献相等，发生在70%甘油中T/n = 20 ± 1 K/cP处。水动力预测 通过Stokes-Einstein（SE）方程计算的结果在70%甘油中非常精确，与一系列烷烃的结果相当。在水中， 与T /η呈线性，零截距符合SE要求;然而，斜率小0.73倍。 仅在η/T非常小的情况下，SE可以合理地预测，并且非常快地遵循近似对数依赖性，而不是线性预测。值 接近高于η/T = 0.20 cP/K的平台，约为固态极限的一半。由于缺乏对重新相遇之间的时间的了解，由于自旋交换引起的谱线位移对于推导Ω（T）的值还没有用;然而，它们可以用于验证由谱线加宽和自旋交换引起的色散确定的值。在70%甘油中低温下的一些效应表明，偶极相互作用的影响是不充分的广泛接受的理论描述。
EPR spectra of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (PDT) are studied as functions of molar concentration, c, and temperature, T, in water and 70 wt% glycerol in water. The increase of the intrinsic linewidth averaged over the three hyperfine lines, 〈Btot〉, varies linearly with c with zero intercept in both solvents at all temperatures; therefore is independent of c. The spin exchange induced dispersion, from which the spin exchange frequency, ωe, may be computed, increases linearly with 〈Btot〉, passing through the origin in water and in 70 % glycerol at high temperatures; however, at low temperatures, where dipolar interactions broaden the spectra, linearity does not prevail until 〈Btot〉 > 1 G due to a contribution of dipolar interactions to the dispersion. The broadening constant due to spin exchange, , is found from the slope of the linear region, permitting a computation of the dipolar constant, . Thus, the separation of concentration broadening into spin exchange and dipolar contributions is effected without having to appeal to some supposed temperature dependence of the two interactions. The fractional broadening by spin exchange, Ω(T), is near unity at high temperatures in both solvents, decreasing to zero in 70 % glycerol at 273 K. Ω(T) is a continuous function of the inverse rotational correlation time of PDT, but is discontinuous as a function of T/η where η is the shear viscosity. Ω(T) = 0.5, where spin exchange and dipolar interactions contribute equally to the linewidth occurs at T/η = 20 ± 1 K/cP in 70 % glycerol. Hydrodynamic predictions of via the Stokes-Einstein (SE) equation are remarkably accurate in 70 % glycerol comparable with the results in a series of alkanes. In water, is linear with T /η with zero intercept as required by the SE; however, with slope a factor of 0.73 smaller. is reasonably predicted by the SE only at very small values of η/T very quickly following an approximately logarithmic dependence rather that the linear prediction. Values of approach a plateau above η/T = 0.20 cP/K that is about one-half the solid state limit. Line shifts due to spin exchange are not yet useful to deduce values of Ω(T) due to a lack of knowledge of the time between re-encounters; however, they may be used to verify the values determined from line broadening and spin exchange induced dispersion. Some effects at low temperatures in 70% glycerol suggest that the effects of dipolar interaction are inadequately described by the widely accepted theory.
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影响因子：
3.3
作者：
Bales, BL;Peric, M
通讯作者：
Peric, M
影响因子：
--
作者：
CURRIN, JD
通讯作者：
CURRIN, JD
影响因子：
2.2
作者：
SALIKHOV, KM
通讯作者：
SALIKHOV, KM
影响因子：
4.4
作者：
Nevzorov, AA;Freed, JH
通讯作者：
Freed, JH
影响因子：
2.9
作者：
Bales, BL;Peric, M
通讯作者：
Peric, M