Abstract
We provide a theoretical analysis of spin-selective recombination processes in clusters of n ≥ 3 radicals. Specifically, we discuss how spin correlation can ensue from random encounters of n radicals, i.e., "F-clusters" as a generalization of radical F-pairs, acting as precursors of spin-driven magnetic field effects. Survival probabilities and the spin correlation of the surviving radical population, as well as transients, are evaluated by expanding the spin density operator in an operator basis that is closed under application of the Haberkorn recombination operator and singlet-triplet dephasing. For the primary spin cluster, the steady-state density operator is found to be independent of the details of the recombination network, provided that it is irreducible; pairs of surviving radicals are triplet-polarized independent of whether they are actually reacting with each other. The steady state is independent of the singlet-triplet dephasing, but the kinetics and the population of sister clusters of smaller size can depend on the degree of dephasing. We also analyze reaction-induced singlet-triplet interconversion in radical pairs due to radical scavenging by initially uncorrelated radicals ("chemical Zeno effect"). We generalize previous treatments for radical triads by discussing the effect of spin-selective recombination in the original pair and extending the analysis to four radicals, i.e., radical pairs interacting with two radical scavengers.
Highlights
Spin dynamics can render chemical reactions magnetosensitive.[1–3] The Radical Pair Mechanism (RPM) is such an example.[4–6] Here, magnetosensitivity emerges as the result of the magnetic fielddependent interconversion of electronic singlet and triplet states of a pair of radicals in combination with spin state-discriminating reaction pathways
If a radical pair reacts with a third, initially uncorrelated radical, the reaction induces singlet–triplet conversion in the original pair, even in the absence of coherent interconversion pathways.[24]. This effect, which was suggested by Letuta and Berdinskii and was dubbed the “chemical Zeno effect”, can give rise to large magnetic field effects when combined with the usual hyperfine-driven spin dynamics, as we have shown in the context of the avian compass model,[25,26] and can be used to teleport spin states.[27]
It is surprising that the spin state realized in the surviving spin system in the long-time limit is independent of the reaction topology of the network and the actual values of the rate constants as long as all spins are members of one reaction network
Summary
Spin dynamics can render chemical reactions magnetosensitive.[1–3] The Radical Pair Mechanism (RPM) is such an example.[4–6] Here, magnetosensitivity emerges as the result of the magnetic fielddependent interconversion of electronic singlet and triplet states of a pair of radicals in combination with spin state-discriminating reaction pathways. Scitation.org/journal/jcp rise to non-equilibrium spin configurations when spin-selective recombination pathways exist These F-pair radicals (“F” for freely diffusing) initially encounter one another with a random relative spin orientation, which corresponds to the thermalized state of such systems, expressible as a mixture of 3/4 triplet and 1/4 singlet electron spin pairs.[2,10,11]. If a radical pair reacts with a third, initially uncorrelated radical, the reaction induces singlet–triplet conversion in the original pair, even in the absence of coherent interconversion pathways.[24] This effect, which was suggested by Letuta and Berdinskii and was dubbed the “chemical Zeno effect” ( the chemical anti-Zeno effect might better reflect its properties), can give rise to large magnetic field effects when combined with the usual hyperfine-driven spin dynamics, as we have shown in the context of the avian compass model,[25,26] and can be used to teleport spin states.[27]. The singlet probability conditioned on survival is p(Si,j)/p
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