Rodgers Christopher T, Norman Stuart A, Henbest Kevin B, Timmel Christiane R, Hore P J
Contribution from the Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom.
J Am Chem Soc. 2007 May 30;129(21):6746-55. doi: 10.1021/ja068209l. Epub 2007 May 1.
Measurements are reported of the effects of 0-23 mT applied magnetic fields on the spin-selective recombination of Py*- and DMA*+ radicals formed in the photochemical reaction of pyrene and N,N-dimethylaniline. Singlet <--> triplet interconversion in [Py*- DMA*+] radical pairs is probed by investigating combinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2 values for the four isotopomeric radical pairs agree with predictions based on the Weller equation using known hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with the ratio of effective hyperfine couplings, aDMA/aPy. An efficient method is introduced for calculating the spin evolution of [Py*- DMA*+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highly ill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairs and have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. This interval reflects the time scale of re-encounters that are crucial for generating the magnetic field effect. Computer simulations of generalized radical pairs containing six spin-1/2 nuclei show that Weller's equation holds approximately only when the radical pair recombination rate is comparable to the two effective hyperfine couplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effective hyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentially any radical pair should show a significant LFE.
报道了0 - 23 mT外加磁场对芘与N，N - 二甲基苯胺光化学反应中形成的Py和DMA+自由基自旋选择性复合的影响。通过研究完全质子化和完全氘代反应伙伴的组合，探测了[Py* - DMA*+]自由基对中的单重态<-->三重态相互转换。定性地说，四个同位素异构自由基对的实验B1/2值与基于韦勒方程并使用已知超精细耦合常数的预测结果一致。“低场效应”（LFE）的幅度与有效超精细耦合比aDMA/aPy密切相关。引入了一种有效方法来计算总共包含18个自旋1/2和自旋1磁核的[Py* - DMA*+]自由基对的自旋演化。通过蒂霍诺夫和最大熵正则化方法实现了对磁场效应的定量分析，以获得自由基重新相遇概率分布f(t)——一个高度不适定和欠定的问题。四个同位素异构自由基对的所得f(t)函数非常相似，并且在孪自由基对产生后2到10 ns之间具有显著幅度。这个时间间隔反映了对产生磁场效应至关重要的重新相遇的时间尺度。包含六个自旋1/2核的广义自由基对的计算机模拟表明，只有当自由基对复合速率与两个有效超精细耦合相当时，韦勒方程才大致成立，并且显著的LFE需要两个有效超精细耦合相差超过5倍的条件，但这并非保证条件。相比之下，对于非常缓慢的复合，基本上任何自由基对都应该表现出显著的LFE。
