Spin polarization radical-pair mechanism

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. 

A subset of electron-hole radical pairs exhibits features of Spin Correlated Radical Pair (CRRP) electron spin polarization mechanism [101] which can be observed at somewhat longer times via light/field modulated (LFM) EPR measurements. This technique is only sensitive to the light dependent part of the EPR spectrum on the time scale of the light modulation frequency (millisecond regime, insert Fig. 1.15). Using LFM EPR it was observed that both the transitions of the holes localized on the surface modifier and electrons localized on the Ti02... 

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. 

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... 

Extensive studies of the sensitizer dependence and the solvent dependence of the polarization patterns led to the identihcation of two parallel pathways of that deprotonation. One is a proton transfer within the spin-correlated radical pairs, with the radical anion A acting as the base. The other is a deprotonation of free radicals, in which case the proton is taken up by surplus starting amine DH. Furthermore, evidence was obtained from these experiments that even in those situations where the polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6 these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by deprotonation of the radical cation by either of the described two routes. The whole mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited for an analysis is the product V. 

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 ... 

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." ... 

The question of whether there are other mechanisms leading to CIDNP besides the radical pair mechanism is of central importance because chemical conclusions that are drawn from CIDNP results on the basis of the latter mechanism might of course be entirely wrong with another mechanism being the source of the polarizations. There has been some evidence [67-72] that cross-relaxation in radicals, by which electron spin polarization (C1DEP) is converted into CIDNP, could provide such a mechanism. Depending on whether cross-relaxation occurs by flip-flop transitions (Am = 0) or by double spin flips (Am = 2), opposite or equal phases of CIDEP and CIDNP would result. Since the origin of the electron spin polarizations is usually the triplet mechanism, this cross-relaxational mechanism is sometimes referred to as the triplet mechanism of CIDNP. 

The most fascinating development in this field of CIDNP within the last years has been the observation, by Zysmilich and McDermott [146], of nuclear spin polarized (solid state) 15NNMR spectra from photosynthetic reaction centers in which the forward electron transfer from the primary charge-separated state to the accepting quinone was blocked. The all-emissive polarizations were proposed to be due to a radical pair mechanism, though many of the details are still not very clear. The reaction scheme is virtually identical to that of Chart VIII (Section V.A.2), the donor D being the special pair and the acceptor A the pheophytin. As in that example, the polarizations from the triplet exit channel are hidden in the triplet product 3D for the lifetime of the latter. This feature, in combination with the fact that nuclear spin relaxation in the molecular triplet localized on the special pair is relatively fast, serves to avoid the cancellation of CIDNP that would occur otherwise because the products from both exit channels are identical. 

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

We then proceed to give a critical discussion of the special case of cyclic reactions, which are of paramount importance in biochemical applications. Due to the basic spin-sorting nature of the radical pair mechanism (partial) cancellation of polarization may occur in cyclic reactions where no net chemical change occurs. This obscures information and makes quantitative evaluation of CIDNP intensities difficult. 

Recalling the spin-sorting nature of the radical pair mechanism, we can anticipate that in the absence of nuclear spin relaxation, random recombination will eventually lead to exact cancellation of the + polarization when the + polarization in A is transferred to A (making the usual assumption that chemical reaction preserves nuclear spin orientation). In such a situation, polarization in A could only be observed in a time-resolved experiment before all the radicals had recombined. Relaxation of the nuclei A , however, allows some of the escape polarization to "leak away preventing complete cancellation (II). Thus, unless the radical lifetime is very much smaller than the nuclear... 

The photoionization of the amino acid tyrosine in alkaline solution was studied by CW TR EPR. The photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive CIDEP spectrum produced by the radical-pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction J is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of the photoionization of deprotonated tyrosine is only seen upon the addition of the electron acceptor 2-bromo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic add). This channel is also found to be monophotonic. 

The EPR spectrum of TAPD -ZP-NQ in Figure 5 can be attributed to 2 radicals, TAPD with a broad linewidth at lower g-factor and NQ with a narrow linewidth at higher g-factor. Polarization is observed on a millisecond time scale because the spin-lattice relaxation times of the radicals are long at 5 K. Spin polarization of TAPD -ZP-NQ can result from two mechanisms. The first mechanism is the usual radical pair mechanism, RPM, of CIDEP . S-To mixing in TAPD-ZP -NQ is followed by polarization transfer to a non-interacting radical pair TAPD -ZP-NQ, i.e. J = 0. The second mechanism assumes that TAPD -ZP-NQ is itself an interacting spin correiated... 

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