Triplet radicals

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. 

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

Unrestricted monodeterminantal treatments using different orbitals for different spins for open-shell systems (free radicals, triplet states, etc.) [41,42],... 

Magnetic resonance (ESR, NMR) Chemical structure, tacticity, conformation, polymer mobility (NMR) Radical, triplet state structure and behaviour (ESR)... 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

Selenophenes, dioxirane oxidation, 1157 Selenyl radicals, triplet oxygen domino reactions, 223, 227... 

Thiophene, dioxirane oxidation, 1156 Thiourea, dialkyl peroxide synthesis, 706 1,4-Thioxane, oxidation, 805-6 Thiyl radicals, triplet oxygen domino reactions, 221-3, 224, 225, 226 THPO (1,2,3,4-tetrahydronaphthyl hydroperoxide), 331-2 Thromboxanes, from arachidonic hydroperoxides, 612... 

The results reveal the nature of triplet DPC reaction in a quantitative manner. Thus, triplet DPC reacts with cyclohexane and THE at least 100 times as rapidly as does methyl or benzhydryl radical. Triplet DPC is 30 times less reactive with THE than is the phenyl radical. Thus, DPC has a reactivity toward hydrogen abstraction intermediate between those of methyl and phenyl radicals. 

There has been a great deal of discussion on the question of whether methylene is best described as a radical (triplet) or nonradical (singlet) species and on the related question of whether methylene is linear or bent. The early theoretical work of Lennard-Jones86 indicated a singlet state, and Laidler and Casey82 postulated a bent singlet structure for CH2 with C—H bonds formed from p orbitals of the carbon atom, the... 

A somewhat modified MO LCAO scheme, without restriction on the identity of spin orbitals (p and

unrestricted Hartree-Fock (UHF) method and is usually used to treat open-shell systems (free radicals, triplet states, etc.). Electron correlation is partially taken into account in this method, and therfore it can be expected to be more efficient than the RHF method when applied to calculate potential energy surfaces of chemical rearrangements whose intermediate or final stages may involve the formation of free- or bi-radical structures. The potentialities of the UHF method are now under active study in organic reaction calculations. Also, it is successfully coming into use in chemisorption computations (6). 

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. 

Applicability All chromophores All paramagnetic centers, especially transition metals, radicals, triplets Most nuclei, in biology primarily H, "C and - P... 

This is the main reason why a new type of CIDEP is produced through the radical triplet pair mechanism (RTPM). Many variations of CIDEP due to the RTPM have been observed. For example, net absorptive CIDEP was observed in the T-D quenching for positive J values [10]. Here, the level-crossings between IG3/2) - -D-i/2) Q n)produce such... 

As in the reactions of all coenzyme Bi2-dependent enzymes, the observable free radicals display spin-spin coupling with low-spin Co in cob(II)alamin and are quantitatively characterized as diradicals or radical triplet species. As in DDH, this coupling is weak in EAL, and quantitative analysis indicates a separation of... 

The observation of a kinetically competent cysteinyl-thiyl free radical intermediate implicated Cys408 in a critical mechanistic role. As in other coenzyme Bi2-dependent enzymes, the unpaired electron of the radical was spin coupled to Co of cob(II)alamin. The structure of the thiyl radical was rigorously proven by isotope-edited EPR analysis, and spectral simulations showed that the thiyl radical resided at 5—7 A from Co of cob(II)alamin. The emergence of the radical triplet signal required adenosylcobalamin and dGTP in addition to RTPR. 

The solution behavior of exciplexes is even more complex, and accounts for the formation of radicals, triplet exciplexes and ordinary triplet states (2j0, 22). The general kinetic scheme for the photophysics of exciplexes has been summarized by Mattes and Farid (201 and is diagrammed below. Some of our results may be reconciled with the processes summarized by this scheme ... 

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