EPR line shifts and line shape changes due to spin exchange of nitroxide free radicals in liquids
EPR line shifts and line shape changes due to spin exchange of nitroxide free radicals in liquids
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DOI：
10.1021/jp970995g
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发表时间：
1997-10-23
影响因子：
3.3
通讯作者：
Peric, M
中科院分区：
文献类型：
--
作者：
Bales, BL;Peric, M
本文导出了在慢交换极限下进行自旋交换的氮氧自由基的EPR谱线形状的表达式，并通过实验和严格的理论进行了检验。线形是洛伦兹线的吸收和色散的总和，其中色散分量对于外部线具有相反的符号，而对于内部线为零。的吸收和色散的相对振幅示出为自旋交换频率的线性函数。非线性最小二乘拟合的光谱允许分离的吸收和色散分量，确定其相对振幅，以高精度产生的自旋交换频率的值，是从更传统的线加宽的精度可比。这种测量自旋交换频率的新方法是有吸引力的，因为没有偶极相互作用的复杂性，并且不需要在低浓度下进行测量。外层谱线的吸收分量的共振场随着自旋交换频率的平方而向谱线的中心移动，并且由于吸收-色散重叠，所观察到的谱线的位置甚至进一步移动。观察到的位移也是自旋交换频率的二次函数，并涉及未加宽的线宽。这两种位移都可以用来测量自旋交换频率;然而，只有观察到的位移产生的值与加宽和色散振幅技术的精度相当。在67 ℃下，使用过氧胺二磺酸盐在50 mM K2 CO 3中对理论的预测进行实验测试。在此升高的温度下，自由基的浓度以方便的速率降低，使得可以在不干扰样品的情况下研究宽的浓度范围。线性关系良好，相关系数r = 0.99996。从色散分量的振幅导出的自旋交换频率在从谱线展宽导出的自旋交换频率的小于1%的范围内，并且具有相当的精度。自旋交换频率来自移动的线是较低的精度产生值的自旋交换频率约2%，低于那些发现从线加宽。
An expression for the EPR line shape of a nitroxide free radical undergoing spin exchange in the slow exchange limit is derived and tested experimentally and against a rigorous theory. The line shape is the sum of the absorption and dispersion of Lorentzian lines, in which the dispersion component is of opposite signs for the outer lines and zero for the inner. The relative amplitude of the absorption and the dispersion is shown to be a linear function of the spin exchange frequency. Nonlinear least-squares fitting of the spectra allows the separation of the absorption and dispersion components determining their relative amplitudes to high precision yielding values of the spin exchange frequency that are of precision comparable to those derived from the more traditional line broadening. This new way to measure spin exchange frequencies is attractive because there are no complications from dipolar interactions and no measurements at low concentrations are required. The resonance fields of the absorption component of the outer lines shift toward the center of the spectrum as the square of the spin exchange frequency and the observed positions of the lines shift even further due to the absorption-dispersion overlap. The observed shift is also a quadratic function of the spin exchange frequency and involves the unbroadened line width. Both shifts may be used to measure the spin exchange frequency; however, only the observed shift yields values that are of precision comparable with broadening and dispersion-amplitude techniques. The predictions of the theory are tested experimentally using peroxylamine disulfonate in 50 mM K2CO3 at 67 degrees C. At this elevated temperature, the concentration of the radical decreases at a convenient rate making it possible to study a wide concentration range without disturbing the sample. The line broadening was linear with the concentration with a coefficient of correlation r = 0.99996. The spin exchange frequency derived from the amplitude of the dispersion component was within less than 1% of that derived from line broadening and of comparable precision. Spin exchange frequencies derived from the shift of the lines were of lower precision yielding values of the spin exchange frequency about 2% lower than those found from line broadening.