Radical pair mechanism development

On June 11, 1965, the author (H. Hayashi) and Dr. K. Itoh visited Dr. Y. Kurita at his office in The Basic Research Laboratory of Toyo Rayon Company, Ltd. and saw his beautiful ESR spectra of radical pairs ( J and K ) in single crystals of dimethylglyoxime irradiated by X-rays at 77 K [2]. Here, the radical pairs J and K are symmetric and asymmetric pairs, respectively, as shown in Fig. 4-2. The typical ESR spectra observed for the radical pairs J and K are shown in Fig. 4-3. The author noticed from Fig. 4-3(b) that the central three lines of the nine hyperfme (HF) lines due to two nitrogen atoms of K were not equally spaced [3], but that there is no anomaly in the HF lines of J as shown in Fig. 4-3(a). We found that the anomalous HF lines of K could be explained by the mixing of the singlet and triplet states of a radical pair in the complete Spin Hamiltonian of the pair developed by Dr. Itoh [3]. This theory has been called "the radical pair mechanism". 

The reencounter mechanism, usually just called radical pair mechanism, involves two steps, i.e. the radicals must separate and reencounter later. When the radicals are separated, the exchange integral becomes negligible (3 Q )- From Eq. (11-40), an off-diagonal element of the density matrix is developed... 

As stated above, CIDNP denotes the transient occurrence of anomalous line intensities in NMR spectra recorded during chemical reactions or shortly after their completion. The phenomenon was first observed in 1967 by Bargon, Fischer and Johnsen [35a] in thermal decompositions of peroxides and azo compounds, and, independently, by Ward and Lawler [35b] in the reactions of alkyl lithium with alkyl halides. It was immediately realized that the line anomalities are caused by populations of the nuclear spin states in the reaction products that deviate from the Boltzmann populations. After initial attempts of interpreting CIDNP by electron-nuclear cross-relaxation, the radical pair mechanism was developed in 1969 by Kaptein and Oosterhoff [36a], and independently by Closs [36b],... 

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. 

On the other hand, if the correlated radical pair mechanism is operative, a pair of partially overlapping anti-phase doublets is expected.[36-38] The polarization pattern observed, E A E, is similar to that observed for P700 - A in Photosystem I of green plants, [24] and P865 -Q in bacterial reaction centers. [25] Recently, Stehlik et al. [39] have developed a simple theoretical model that can be used to simulate these spectra. This model focuses on the influence of/, D, and g-anisotropy on the EPR spectra of radical pairs. The latter two quantities are particularly useful in determining the distance between the radicals and their mutual orientation. The... 

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

CIDEP Initial Polarization. The establishment and the development of the photoexcited triplet mechanism in CIDEP of transient radicals in solution had been rather controversial, if not as turbulent and exciting as the photoexcitation process itself. The early objections centered around two very important questions. The first one concerns the uncertainty of whether the spin polarization in the molecular frame can be effectively transferred to the laboratory frame for triplet systems in liquid solution. The second related question involves the fact that the polarized triplet molecules are rotating rapidly with respect to the laboratory axes and the triplet spin lattice relaxation time T x (normally between 10 and lO-- - s) would be too short for the polarization to be retained in the radical pair. The earlier photoexcited triplet mechanism developed by Wong et al. (136,137) is based on a "static model" with the excited triplet molecules being randomly oriented. Such a static model cannot deal satisfac-... 

In principle, initial polarization (triplet mechanism) develops immediately following excitation reaction, whereas radical-pair polarization evolves within a time period slightly behind the development of the initial polarization. Time-resolved experiments, therefore, can separate to certain extent the simultaneous contributions of the two different mechanisms. The key problem for the experimentalist who is doing quantitative measurements of the two different types of polarization boils down to how to beat spin relaxation. 

However, in certain cases under photolytic conditions, spectra of the corresponding arylmercury radical cations 6 developed, whereas no mercuration occurred in dark [81] signifying collapse of the ArH +,Hg(TFA)2 radical ion pair 4, provides an alternative path way to Wheeland complex 2 and hence to ArHg(TFA) + 6. Arene radical cations can also be generated from arene and thallium(III) tris-(trifluoroacetate) in trifluoroacetic acid [82], but with a different mechanism proposed by Eberson et al. [83]. Oxidation of anthracene showed 9-trifluoroacetoxy and 9,10-bis(trifluoroacetoxy)anthracene [84, 85], benzo[a]pyrene, 7-methylbenzo-[a]pyrene and 12-methylbenzo[a]pyrene yielded radical cations of 7- and/or 12-trifluoroacetates [86], triptycene (9,10-dihydro-9,10-[l,2]benzanthracene) showed... 

P+B"H is the first radical pair formed. It develops with a rate constant k and decays with the rate k2 in the subsequent electron transfer (ET) from P+B H to P+BH". In the unistep mechanism the radical ion pair P BH" is formed directly from P BH with the rate k according to... 

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