Tarasov Valery F, White Ryan C, Forbes Malcolm D E
Semenov Institute of Chemical Physics RAS, Moscow 199999, Russia.
Spectrochim Acta A Mol Biomol Spectrosc. 2006 Mar 13;63(4):776-83. doi: 10.1016/j.saa.2005.10.005. Epub 2006 Feb 2.
Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles are presented and simulated using the microreactor model. They are discussed in terms of the two-site model with a particular emphasis on longitudinal relaxation mechanisms. The geminate radical pair is created by photo-oxidation of dyglicine by the excited triplet states of an anthraquinone salt. The strong chemically induced electron spin polarization observed is due to three mechanisms: TM, RPM, and SCRPM. Relative contributions from these mechanisms depend on the water pool volume and the time of observation. There are three types of longitudinal relaxation in radical pairs. The first is relaxation of the RPM induced longitudinal magnetization in spin correlated radical pairs. The second is the longitudinal relaxation in radical pairs which are not correlated (with a zero value of the double quantum coherence). In such pairs, the generation of longitudinal magnetization due to RPM is impossible, and the spin-selective recombination of the pairs is ineffective. Under all experimental conditions, the first type of relaxation is slower than the second type. For both, the physical mechanism leading to relaxation is modulation of the Heisenberg electron spin exchange interaction. This is an internal relaxation process. The third relaxation type occurs in radical pairs due to ordinary longitudinal relaxation in non-interacting radicals. Normally, relaxation of the third type is the slowest of the three. This explains time and micelle size dependence of the relative contribution of RPM into TREPR spectra. It seems reasonable to suggest that the creation of the initial spin state populations is partially adiabatic.
本文展示并利用微反应器模型模拟了在AOT反胶束中产生的相关自由基对的时间分辨电子顺磁共振谱（X波段）。根据双位点模型对其进行了讨论，特别强调了纵向弛豫机制。双自由基对是由蒽醌盐的激发三重态对二甘氨酸进行光氧化而产生的。观察到的强烈化学诱导电子自旋极化归因于三种机制：TM、RPM和SCRPM。这些机制的相对贡献取决于水池体积和观察时间。自由基对中存在三种类型的纵向弛豫。第一种是自旋相关自由基对中RPM诱导的纵向磁化强度的弛豫。第二种是不相关自由基对（双量子相干值为零）的纵向弛豫。在这种自由基对中，由于RPM产生纵向磁化强度是不可能的，并且自由基对的自旋选择性重组是无效的。在所有实验条件下，第一种弛豫类型比第二种弛豫类型慢。对于这两种情况，导致弛豫的物理机制是海森堡电子自旋交换相互作用的调制。这是一个内部弛豫过程。第三种弛豫类型发生在自由基对中，是由于非相互作用自由基的普通纵向弛豫。通常，第三种弛豫类型是三种弛豫类型中最慢的。这解释了RPM对时间分辨电子顺磁共振谱相对贡献的时间和胶束尺寸依赖性。认为初始自旋态布居的产生部分是绝热的，这似乎是合理的。
