Reaction control by spin manipulation

Original title: Reaction-control by spin manipulation (RCSM) and product-yield-detected ESR (PYESR):(1-5)

Reaction Control by Spin Manipulation" is briefly explained here on the photoreduction of xanthone (XO) in SDS micellar solution. UV-irradiation of the system yields xanthone excited in the lowest excited triplet state, 3XO*, from 1XO* via the intersystem crossing process. Then, 3XO* abstracts a hydrogen atom from a hydrogen donor, xanthene (XH2) in the present case, and the "geminate radical pair" in the triplet state 3(XH• •XOH) is formed in the cage (SDS micelle).

If one of the electron spins of this radical pair is inverted by the ESR (electron spin resonance) method, the coupling reaction of the component radicals is accelerated to yield the "cage product" XH-XOH. Instead, when a high power resonance rf-field is irradiated, the spin state of radical pair is locked to the triplet state, and the recombination reaction is inhibited.(6) When the yield of one of these compounds is plotted as a function of the magnetic field strength, the ESR spectra of both the radicals are traced in the overlapped form.

The spectrum thus obtained has been named product-yield-detected ESR or PYESR in short.(4,5) The first paper was published in Nature in1986.5) Until this publication no reaction yield had been monitored to obtain the ESR spectrum of the intermediate radical pair, so we named the method PYESR as usual. Spin locking of the radical pair is rather difficult, since it needs a microwave field much larger than the internal magnetic interactions, such as hyperfine coupling. Therefore, it has been most clearly demonstrated with the perdeuteriated systems.(6)

Reaction Control by Spin Manipulation

Upper: The process of photoreduction of xanthone(XO) in the presence of xanthene (XH₂, a hydrogen donor) can be switched at the stage of intermediate radical pair ³(XOH• •XH) by the spin manipulation technique: spin inversion accelerates the coupling reaction of the two radicals and spin locking decelerates it.²⁾

Lower: The effect of spin inversion on the HPLC of product solution. The (XH)₂ peak decreased to about 1/2 by the "spin inversion".³⁾

From the quantum mechanical point of view, it is true that similar experiments had been made by several groups, for example, the Nobosibirsk group (ODESR, optical detection of ESR)(7) and also by the Argonne group (FDMR, fluorescence detected magnetic resonance).8) They produced transient ions by ionizing radiation, and detected their ESR spectrum by detecting the fluorescence, which is emitted upon charge neutralization of the two ions upon returning to the original states.

Most of the scientists in the field of "Spin Chemistry" presume that chemical reactions had been successfully controlled by the technique of “spin manipulation” and the technique had been named as the "reaction-yield-detected magnetic resonance".(9,10) However, the authors of these early reports had only observed a process that is not directly related with a chemical reaction or its products.

A few years before the experiments cited above, Frankevich et al. detected the ESR spectrum of a triplet exciton by its annihilation fluorescence in a crystal.11). They named their experiment RYDMR, reaction yield detected magnetic resonance, where "reaction" was used as an analogy for exciton annihilation. This experiment is a modification of the ODMR (optical detected magnetic resonace) method, which had been demonstrated from the 1960's to observe the ESR of exciton occurred in the solid state.9,10,12) Therefore, their naming of RYDMR is misleading.

The important point of our PYESR experiment is that the chemical bond formation is controlled by the spin operations, and this kind of experiment had not been made before ours.12,13,14) In the full article, the application of "pulse-PYESR" is described. This method is very powerful to study the dynamics of the radical pair in micelle as well as the dynamics of micelle itself.

Referece

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