Quantum beats in recombination of spin-correlated radical ion pairs with equivalent protons

Bagryansky, V. A.; Усов, О. А.; Borovkov, V. I.; Kobzeva, T. V.; Molin, Yu. N.

doi:10.1016/s0301-0104(00)00078-1

Cited by 41 publications

(54 citation statements)

References 21 publications

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“…The top two curves in this Figure refer to the fluorescence decay of 30 mM pTP in n-octane (in the absence of 2-Me 8 ) at 233 and 313 K. Figure 7 (circles) illustrates the time-resolved magnetic field (TRMF) effect observed at 293 K for the above solution of 2-Me 8 as the ratio I B (t)/I 0 (t) of the fluorescence decay in a 0.1 T magnetic field I B and in the absence of a magnetic field (I 0 ). [18][19][20][21][22] Similar results were also obtained at other temperatures. The effect of the external magnetic field proves that the fluorescence arises from the recombination of radical ion pairs.…”

Section: Magnetic Field Effect On the Decay Of The Recombination Fluosupporting

confidence: 71%

“…20,24 The solutions were degassed by repeated freeze-pump-thaw cycles. The temperature of the solution during measurements was stabilized within (0.5 K, except at >270 K where the accuracy was about (1 K.…”

Section: Methodsmentioning

confidence: 99%

“…The total width of the ESR spectrum is determined by the time at which I B (t)/I 0 (t) reaches a maximum, i.e., it is well-defined from the simulation. Other parameters for the simulation were the time constants for phase and spin-lattice relaxation, 20,21 which can be considered as the inverse overall-relaxation rates for the radical pair involved, and the fraction θ of singlet spin-correlated radical pairs, which is determined by the probability of geminate recombination in the radiation track. The parameter θ governs the amplitude of the TRMF effect.…”

Section: Magnetic Field Effect On the Decay Of The Recombination Fluomentioning

confidence: 99%

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Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

et al. 2005

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The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me 8 ) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me 8 ), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me 8 ), 2 •+ and 2-Me 8 •+ are stable to opening of the second ring, because in this case the resulting species would be a non-Kekulé hydrocarbon with a quartet ground state. Similarly to 1, octamethyl substitution in 2 promotes the tendency to rearrangement. Thus, ESR and ENDOR studies indicate that the primary radical cation 2 •+ , which is formed upon γ-irradiation of 2 in a CFCl 3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me 8 is treated in the same way, only the rearranged radical cation 2a-Me 8 •+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a •+ and 2-Me 8 •+ , is revealed by other methods. According to UV and IR studies, X irradiation of 2 in an Ar matrix leads directly to the ring-opened radical cation 2a •+ . Moreover, magnetic field effects on the decay of fluorescence, which appears upon recombination of the radical anion of p-terphenyl with a radical cation generated from 2-Me 8 in liquid octane, strongly suggest that 2-Me 8 •+ (and not 2a-Me 8 •+ ) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me 8 •+ f 2a-Me 8 •+ is estimated. The radical cations 2a •+ and 2a-Me 8 •+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene π-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the π-system.

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Section: Magnetic Field Effect On the Decay Of The Recombination Fluosupporting

confidence: 71%

Section: Methodsmentioning

confidence: 99%

Section: Magnetic Field Effect On the Decay Of The Recombination Fluomentioning

confidence: 99%

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Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

et al. 2005

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“…To account for recombination into singlet state, the time-dependent state of the pair is projected back onto its singlet state, and interference terms of the form Bcos(Aco~t) appear in Pss(t, H). If a time-resolved experiment is performed, these terms would produce the well-known quantum beats in recombination fluorescence [8]. However, in a stationary experiment such as MARY, Pss(t, H) is averaged over lifetime distribution f(t) Eq.…”

Section: Physical Background Of Mary Spectroscopymentioning

confidence: 99%

Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

2005

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The paper describes the underlying principles and discusses the most important advantages and limitations of the experimental technique of magnetically aŸ reaction yield spectroscopy as developed in the authors' laboratory and guides the reader step by step through a typical experimenta[ sequence using as example the problem of short-lived radical cations of a series of methyl-substituted benzenes in X-irradiated nonpolar solutions. For two of the eight target substances -benzene itself and mesithylene -the paper reports the first unequivocal observation of their radical cations in liquid alkane so[ution at room temperature and provides a lower estimate of about 10 ns for their relaxation times in low magnetic field.

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“…Experimentally, information on spin dynamics is obtained by studying the TRMFE curves plotted as the ratio of the recombination fluorescence intensities measured in the presence and absence of a magnetic field [4][5][6]. Such an approach allows us to eliminate the factor from I ( t ) equal to the radical pair recombination rate, which decreases rapidly with time and is not known with an adequate accuracy.…”

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confidence: 99%

Manifestation of Inhomogeneous Broadening in the Spin Dynamics of Spin-Correlated Radical Pairs

Bagryansky

2005

Dokl Phys Chem

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The time-resolved magnetic field effect (TRMFE) method in recombination fluorescence is one of the powerful tools for detecting short-lived radical ions in liquid solutions [1][2][3]. In such experiments, a short pulse of ionizing radiation gives rise to singlet-correlated radical ion pairs in an irradiated solution. Under the action of an external magnetic field, hyperfine coupling (HFC), and paramagnetic relaxation, these radical ion pairs are converted into the triplet state. The change in the population of the singlet state is easily monitored by measuring the fluorescence intensity of excited molecules that are formed upon recombination of singlet pairs.The TRMFE method provides information on the HFC constants, g factors, and paramagnetic relaxation times of radical ions and, sometimes, makes it possible to measure the kinetics of their chemical transformations (see reviews [2,3]). It is worth noting that this method is applicable for studying radical ions whose lifetimes are as short as a few nanoseconds, which makes them undetectable for ESR and even for optically detected ESR.Experimentally, information on spin dynamics is obtained by studying the TRMFE curves plotted as the ratio of the recombination fluorescence intensities measured in the presence and absence of a magnetic field [4][5][6]. Such an approach allows us to eliminate the factor from I ( t ) equal to the radical pair recombination rate, which decreases rapidly with time and is not known with an adequate accuracy. Inasmuch as not all recombining pairs in tracks arise in the spin-correlated state, this ratio takes the form (1)where θ is the fraction of spin-correlated pairs and ( t ) and ( t ) are the probabilities of the pair being found in the singlet state, with the proviso that it was initially prepared in the singlet state, calculated for a zero and a nonzero magnetic field, respectively. In the TRMFE method, the spectral parameters of radical ions are extracted by simulation of the experimental curve using relationship (1), in which the singlet populations are obtained by solving the problem of evolution of the spin state of a radical ion pair in a static external magnetic field and in a zero magnetic field.Only isotropic HFC is significant for radical ions in solutions, and it is believed that the interaction between the spins of radical-ion electrons can be ignored. An exact solution of this problem in an arbitrary magnetic field is known only for HFC with a group of equivalent nuclei in each radical [7][8][9]. This severely limits the application of the TRMFE method and compels one to search for approximate approaches. In particular, for a field that is much stronger than HFC, the solution in the strong-field approximation is known for HFC of an arbitrary type [7,8]. For the spin evolution in a zero field, an exact solution exists for the case of no more than two groups of equivalent nuclei in each of the radicals [10] and a solution in the semiclassical approximation [11] is known for an unresolved ESR spectrum, i.e., in the limit o...

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Quantum beats in recombination of spin-correlated radical ion pairs with equivalent protons

Cited by 41 publications

References 21 publications

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

Manifestation of Inhomogeneous Broadening in the Spin Dynamics of Spin-Correlated Radical Pairs

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