Radical pair mechanism polarization

Further evidence consistent with the polar radical pair mechanism was provided by a crossover experiment (Scheme 6.26). A 1 1 mixture of labeled 8Z /8 and unlabeled 8Z/8E was heated in xylene at 125 °C for 2h and at 135 °C for 4h to afford hydroxypyrimidinones 3 and 3. Analysis of the products by high resolution mass spectrometry showed no crossover between the labeled and unlabeled fragments. This result reinforces the computational results discussed previously wherein PRP recombines to give product within the solvent cage (Scheme 6.24). 

Any CIDNP-based assignment of the sign and relative magnitude of hfcs is valid only if the radical pair mechanism (RPM) is operative they become invalid if an alternative process is the source of the observed effects. The triplet-Overhauser mechanism (TOM) is based on electron nuclear cross-relaxation. For effects induced via the TOM, the signal directions depend on the mechanism of cross-relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. 

Fig. 1.14. Comparison between integrated continuous light-induced (upper trace) and time-resolved pulsed laser-induced (lower trace) EPR spectra from 45A Ti02 (0.3M) modified with dopamine (0.08 M).The lower trace was obtained with a 550 nm laser (laser intensity 10 mJ per pulse, 10 ns pulse duration, 20 scans), 1 ps after the laser pulse. Both spectra were recorded at 8 K. Right section shows how triplet radical pair mechanism of CIDEP in addition to fast exchange can contribute to the observed polarized spectrum.

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

It is now well established that both CIDEP and CIDNP have their origins in the formation and removal reactions of free radicals. As a result of this, it is now possible to gain information, not normally obtained from magnetic resonance studies, for those photochemical reactions which show CIDEP and CIDNP. An example of this is those photochemical reactions in which the primary radicals react immediately to regenerate the starting compounds. The regenerated compounds may show CIDNP, and this is often the only evidence that this reaction has occurred. In the radical-pair mechanism, spin polarization is caused by the spin-selective reaction. While it is generally not possible to monitor by esr the selective reactivity of the radical pairs as a function of their nuclear spin states, CIDNP has proved to be a valuable tool to probe the small difference in reactivity of the nuclear spin states of the radical pairs. 

The utility of CIDEP in photochemistry was greatly enhanced when it was realized (131) that the radical-pair mechanism is not the exclusive spin polarization mechanism. Initial triplet spin polarization produced by the different intersystem-crossing rates to the excited triplet sublevels can be "transferred" to radicals formed by the photochemical reaction of the polarized triplet. 

The current theories of chemically induced magnetic polarization can therefore be summarized into the two basically different mechanisms the photoexcited triplet mechanism (PTM) responsible for the initial electron polarization and the observed Overhauser effect in nuclear polarization, and the radical-pair mechanism which, to date, accounts for almost the remaining bulk of the known polarization systems. We proceed to describe the simple physical models of these two mechanisms by beginning with the more sophisticated radical-pair theory. 

On the other hand, the overwhelming success of the radical-pair mechanism is well supported by many experimental investigations (2,18,49,106,119). In a systematic study of the photoreduction of quinones by phenols, Adeleke and Wan (2) have confirmed the effect of the sign of the Ag on the polarization of the semiquinone and the phenoxy radicals as dictated by eq. 10, and Elliot and Wan (49) have obtained relative individual enhancement factors of the esr lines of the durosemiquinone radical formed in the photoreduction of the parent quinone by isopropanol which agree very well with the values predicted by eq. 16. 

Based upon the current theories of CIDEP and CIDNP, we propose that in many photochemical systems the primary photochemical reaction of the excited triplet state contributes to magnetic polarization via the triplet mechanism. The secondary reaction of the polarized primary radicals may transfer their initial polarization to the "secondary radicals" provided that the radical reactions can compete with the radical spin-lattice relaxation process (59,97). On the other hand, secondary reactions of the primary radical pair or the uncorrelated F pair contribute to polarization by the radical-pair mechanism. A general scheme showing the possible and simultaneous operations of both the... 

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

CIDNP studies have proven to be a valuable tool in investigating the mechanisms of decarbonylation and disproportionation reactions in micelles27 29). Since the mechamisms involve the formation of triplet radical pairs, nuclear polarization of the protons near the radical centers occurs and results in the observation of emission or enhanced absorption in the NMR spectra of products of the radical pairs. For example, the photolysis of di-t-butyl ketone (11) in HDTCI yields both decarbonylation and disproportionation products (Scheme VII)27,29). The CIDNP spectra (Fig. 12) taken at various concentrations of copper chloride (free radical scavenger) illustrates that the intramicellar product is isobutylene (72), while 2,2,4,4-tetramethylbutane (13) and 2-methyl-propane (14) are the extramicellar products. 

The changes of polarizations with the addition of various sensitizers may be regarded as strong evidence for the radical pair mechanisms of CIDNP and provide a nice example of the dependence of the effects on the precursor states. Two other important features of CIDNP also become apparent from the results of the studies outlined above. 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

In 1963, Fessenden and Schuler [1] found during irradiation of liquid methane (CFLt and CD4) at 98 K with 2.8 MeV electron that the low-field line for both hydrogen and deuterium atoms appeared inverted (emissive signals) and that the central deuterium atom line was very weak. Although the cause of such anomalous ESR spectra was not clear at that time, similar anomalous signals have been observed in many reactions and have been called " Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP should be due to non-equilibrium electron spin state population in radicals and could also be explained later by the radical pair mechanism as CIDNP. 

In this section, we will see how CIDEP can be generated from the radical pair mechanism. The spin polarization (P) of ESR transition is represented as shown in Fig. 5-1. Here, P is given as follows ... 

The transients formed from phenol (irradiation at 266 nm in ethanol) have been identified as solvated electrons, phenoxyi radicals (an absorption around 400 nm) and the triplet state of phenol (450 nm)". The formation of phenoxyi radicals and hydrated electrons display a low-frequency/high-field absorption and a high-frequency (low-field) emission polarization pattern generated by a radical pair mechanism. Phenoxyi radicals have also been observed following electron transfer from phenols (as solutes) to molecular radical cations of some non-polar solvents (cyclohexane, n-dodecane, 1,2-dichloroethane, n-butyl chloride). This study used pulsed radiolysis and the formation of the phenoxyi radicals is thought to involve Scheme 1. 

At temperatures sufficiently low that the cytochromes are unable to transfer electrons rapidly to the photooxidized P840, the oxidized primary donor recombines with the reduced electron acceptor P to produce the spin-polarized triplet state through the radical-pair mechanism ... 

The RC of green filamentous bacteria contain a membrane-bound cytochrome c554, which with a redox potential is -hO.26 V can reduce P865 in 10 /js at room temperature. When electron transfer is interrupted either on the donor or the acceptor side, P865 and 1 recombine to form the spin-polarized triplet state through the radical-pair mechanism. The decay time of P865 at room temperature is 90 ps. 

When electron transfer to the secondary acceptor is disrupted, the separated charges recombine in a few nanoseconds, via the radical pair mechanism, to form the spin-polarized triplet state of the primary donor, P. As shown in Fig. 11, the decay time of P865 in the green filamentous bacterium Cf. aurantiacus is 6 //s at ambient temperature. At 1.2 K it is 75 /js. Reaction centers of Cf. aurantiacus contain two menaquinone molecules, MQa and MQg, which behave the same way as a pair of analogous quinones in purple bacteria and photosystem II. Under non-physiological conditions, MQa recombines with P865 in 60 ms and MQb in 1 s. 

From the start, these phenomena were recognized as spin polarizations (deviations of the populations of the nuclear spin states from the Boltzmann distribution) caused by radical reactions. As the first attempts to understand their generation erroneously focussed on Overhauser effects, they were christened "chemically induced d)mamic nuclear polarizations". Although only partially correct, that name has stuck, possibly because its acronym CIDNP (usually pronounced "kidnap") evokes the picture of radical scavenging. However, only 2 years later the now universally accepted quite different explanation, the hitherto unknown radical-pair mechanism, was found, again by two groups independently." ... 

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