Determination of Radical Re-encounter Probability Distributions from Magnetic Field Effects on Reaction Yields
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- 作者：Christopher T. Rodgers ; Stuart A. Norman ; Kevin B. Henbest ; Christiane R. Timmel ; P. J. Hore
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：May 30, 2007
- 年：2007
- 卷：129
- 期：21
- 页码：6746 - 6755
- 全文大小：164K
- 年卷期：v.129,no.21(May 30, 2007)
- ISSN：1520-5126
文摘
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 andN,N-dimethylaniline. Singlet 
triplet interconversion in [Py- DMA+] radical pairs is probed by investigatingcombinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2values for the four isotopomeric radical pairs agree with predictions based on the Weller equation usingknown hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with theratio of effective hyperfine couplings, <aDMA>/<aPy>. An efficient method is introduced for calculating the spinevolution of [Py- DMA+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitativeanalysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highlyill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropyregularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairsand have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. Thisinterval 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 equationholds approximately only when the radical pair recombination rate is comparable to the two effective hyperfinecouplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effectivehyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentiallyany radical pair should show a significant LFE.
- and DMA+ radicals formed in the photochemical reaction of pyrene andN,N-dimethylaniline. Singlet triplet interconversion in [Py- DMA+] radical pairs is probed by investigatingcombinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2values for the four isotopomeric radical pairs agree with predictions based on the Weller equation usingknown hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with theratio of effective hyperfine couplings, <aDMA>/<aPy>. An efficient method is introduced for calculating the spinevolution of [Py- DMA+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitativeanalysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highlyill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropyregularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairsand have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. Thisinterval 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 equationholds approximately only when the radical pair recombination rate is comparable to the two effective hyperfinecouplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effectivehyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentiallyany radical pair should show a significant LFE.