Triplet-state radical pairs

Fig. 14.13 The oscillating magnetic field effect (OMFE) in the triplet state radical pair reaction.

Triplet-State Radical Pairs from the Photoreduction of Benzophenone by Hydrogen Donors... 

In other studies from the group of Jansen and co-workers, " the encapsulation of different radicals was studied, that is 0.3 6 pyrrolidinyloxy triplet radical moieties, " and 7,7,8,8-tetracianoquinodimethane (TCNQ), " in a box of polypropy-lenepolyamine (fifth generation) dendrimer. Intermolecular ferromagnetic alignment of the encapsulated triplet radicals to triplet state radical pairs was detected when more than one radical was trapped into the dendritic box. Similarly,... 

Since Ag is positive and is negative, Q is larger for the p state than for the a state. Radical pairs in the p nuclear spin state will experience a faster intersystem crossing rate than those in the a state with the result that more RPs in the p nuclear spin state will become triplets. The end result is that the scavenging product, which is fonned primarily from triplet RPs, will have an excess of spins in the p state while the recombination product, which is fonned from singlet RPs, will have an excess of a nuclear spin states. 

Heavy atom effects on the decay dynamics of triplet state ion pairs have been studied in chloranil-naphthalene 1 2 termolecular systems. The triplet exciplexes and radical pairs involved in geminate recombination which occurs with the methylene blue triplet and p-iodoaniline have been examined for spin-orbit coupling induced magnetic field effects . 

Sometimes the mechanism of damage is such that these pairs dominate the spectra, in which case the basic pattern will be that of a triplet-state radical. 

Fig. 7 Triplet and radical-pair TREPR spectra of flavoproteins. (a) TREPR data set of the photoexcited triplet state of FMN in frozen aqueous solution measured 1 ps after pulsed laser excitation at 80 K [93].

Torres, M., Safarik, I., Murai, H., and Strausz, O.P, Detection of Keto-, Thioxo- and Iminocarbenes and the Quintet State Triplet-Triplet Ketocarbene Radical Pairs, Rev. Chem. Intermed., 7,243,1986. 

Fig. 8.2 Spin wavefunction in an external field of 0.33 T, which at the moment of crossrecombination collapses onto a triplet state for pairs (1,4). a Nuclear configuration 14-1 4-2 +3 +4> and b I 4-1 —2 4-3 4-4>. Number in subscript represents the radical where the magnetic nucleus resides. Black and red line represents (respectively) the singlet and triplet probability simulated using i/rtot throughout green and blue dots represent the same using the decomposition method. After 0.1 p.s, the spin wavefunction for pair 2, 3 is shown...

An atom or a molecule with the total spin of the electrons S = 1 is said to be in a triplet state. The multiplicity of such a state is (2.S +1)=3. Triplet systems occur in both excited and ground state molecules, in some compounds containing transition metal ions, in radical pair systems, and in some defects in solids. 

CIDNP involves the observation of diamagnetic products fonned from chemical reactions which have radical intemiediates. We first define the geminate radical pair (RP) as the two molecules which are bom in a radical reaction with a well defined phase relation (singlet or triplet) between their spins. Because the spin physics of the radical pair are a fiindamental part of any description of the origins of CIDNP, it is instmctive to begin with a discussion of the radical-pair spin Hamiltonian. The Hamiltonian can be used in conjunction with an appropriate basis set to obtain the energetics and populations of the RP spin states. A suitable Hamiltonian for a radical pair consisting of radicals 1 and 2 is shown in equation (B1.16.1) below [12]. 

An individual radical from the RP may encounter a radical from a different RP to fomi what are known as random RPs or F pairs. F pairs which happen to be in the singlet state have a high probability of recombining, so the remaining F pairs will be in the triplet state. Consequently, the initial condition for F pairs is the triplet state in nearly all cases. 

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. 

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