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

Simple rules for resolved level-crossing spectra in magnetic field effects on reaction yields

In this work we derive conditions under which a level-crossing line in a magnetic field effect curve for a recombining radical pair will be equivalent to the electron spin resonance (ESR) spectrum and discuss three simple rules for qualitative prediction of the level-crossing spectra.

Spin-Correlated Radical Ion Pairs Generated in Liquid Haloalkanes Using High-Energy Radiation

The probability of formation of spin-correlated secondary radical ion pairs (RIPs) in diluted solutions of charge acceptors in irradiated haloalkanes is believed to be extremely low due to the dissociative attachment of excess electrons to solvent molecules. Contrary to this, it has been found that spin-correlated RIPs can be formed upon irradiation in some liquid chloroalkanes with yield sufficient to observe the recombination fluorescence of the RIP's partners. This allowed the study of primary radical cations (RCs) as well as radical ionic states of molecules dissolved in haloalkanes using the method of time-resolved magnetic field effect (TR MFE) in radiation-induced fluorescence. With this method, the magnetic resonance characteristics of the solvent RCs in a series of liquid haloalkanes were examined for the first time. For the 1,2-dichloroethane RC, the rate of scavenging by solute molecules and the dominant mechanisms of paramagnetic relaxation were determined. Polysulfone and poly(ethyl methacrylate) were used to demonstrate that due to their high dissolving ability, chloroalkanes can be exploited as solvents to study the magnetic resonance characteristics of radical ionic states of polymeric molecules in solutions with the TR MFE method.

Solvent Radical Anions in Irradiated Aliphatic Ketones and Esters as Observed Using Time-Resolved Magnetic Field Effects in the Recombination Fluorescence

It has been found that addition of alcohols (~0.1 M) to some liquid ketones and esters results in well-pronounced oscillations in the decay of the delayed fluorescence intensity from irradiated solutions. The analysis of the time-resolved magnetic field effects (TR MFEs) in the recombination fluorescence has shown that these oscillations are a manifestation of singlet-triplet transitions in spin-correlated radical ion pairs (RIPs) created by irradiation. Comparison with literature data indicates that the transitions are due to hyperfine couplings (HFCs) in the solvent radical anion (RA), stabilized due to the presence of alcohol molecules. In acetone, this stabilization effect has been observed for methanol, ethanol, 2- propanol, and, to a smaller extent, for tert-butanol. Similar effects have also been observed in diethyl ketone, ethyl acetate, and methyl propionate but not in methyl tert-butyl ketone and ethyl trimethylacetate. The results obtained indicate that the interaction between the radical anions (RAs) of carbonyl compounds and alcohol molecules is of importance in pulse radiolysis studies of organic liquids and their mixtures.

Estimation of the Fraction of Spin-Correlated Radical Ion Pairs in Irradiated Alkanes using Magnetosensitive Recombination Luminescence from Exciplexes Generated upon Recombination of a Probe Pair

We suggest a convenient probe exciplex system for studies in radiation spin chemistry based on a novel acceptor-substituted diphenylacetylene, 1-(phenylethynyl)-4-(trifluoromethyl)benzene that has a very short fluorescence lifetime (<200 ps) and low quantum yield (0.01) of intrinsic emission, provides efficient electron capture in alkanes and efficient exciplex formation upon recombination in pair with DMA radical cation, while exhibiting a shifted to red exciplex emission band as compared to the parent system DMA – diphenylacetylene. After chemical, luminescent, radiation and spin-chemical characterization of the new system we used the magnitude of magnetic field effect in its exciplex emission band for experimental estimation of the fraction of spin-correlated radical ion pairs under X-irradiation with upper energy cutoff 40 keV in a set of 11 alkanes. For linear and branched alkanes magnetic field effects and the corresponding fractions are approximately 19–20% and 0.28, while for cyclic alkanes they are lower at 16–17% and 0.22, respectively.

Real-time observation of the spin-state mixing process of a micellized radical pair in weak magnetic fields by nanosecond fast field switching

The singlet-triplet spin-state mixing process of a singlet-born radical pair confined in a sodium dodecyl sulfate (SDS) micelle was studied by observing the nanosecond switched external magnetic field (SEMF) effect on the transient absorption signals. A long-lived singlet radical pair is generated by the photoinduced bond cleavage reaction of tetraphenylhydrazine in an SDS micelle. Application of off-on type SEMF results in the increase of the free radical yield contrary to the decrease produced by an applied static magnetic field. The S-T mixing process in low magnetic field was observed by means of a delay-shift SEMF experiment. Observed incoherent mixing processes are explained in terms of the interplay between coherent hyperfine interaction and fast dephasing processes caused by the fluctuation of electron-spin interactions. Singlet-triplet and triplet-triplet dephasing rate constants are determined independently to be 2 x 10(8) and 0.2 x 10(8) s(-)1, respectively, by a simulation based on a modified single-site Liouville equation. This is the first direct observation of the incoherent spin-state mixing process at magnetic fields comparable to the hyperfine interactions of the radical pair.

Radical Cations of Branched Alkanes As Observed in Irradiated Solutions by the Method of Time-Resolved Magnetic Field Effect

Spin dynamics in radical ion pairs formed under ionizing irradiation of n-hexane solutions of two branched alkanes 2,3-dimethylbutane and 2,2,4-trimethylpentane has been studied by the method of time-resolved magnetic field effect in recombination fluorescence. Experimental curves of the magnetic field effect are satisfactorily described by assuming that the spin dynamics is determined by the hyperfine interactions in the radical cation (RC) of branched alkane under study with hyperfine coupling (HFC) constants averaged by internal rotations of RC fragments. The HFC constants determined from the magnetic field effect curves are close to those estimated within DFT B3LYP approach. Analysis of the results indicates that at room temperature the lifetimes of the RC of the studied branched alkanes amount to, at least, tens of nanoseconds.

Longitudinal Electron Spin Relaxation Induced by Degenerate Electron Exchange as Studied by Time-Resolved Magnetic Field Effects

T(1) paramagnetic relaxation of radical ions induced by degenerate electron exchange (DEE) reactions is studied theoretically and experimentally. Our theoretical analysis shows that T(1) relaxation time is well described by the Redfield theory at arbitrary values of the characteristic DEE time tau. Longitudinal relaxation of norbornane (NB) radical cation is studied by means of the time-resolved magnetic field effects (TR-MFE) technique; the rate constant of DEE involving NB(*+) radical cation and NB neutral molecule is obtained. Advantages of the TR-MFE technique and its potential for measuring the short DEE times are discussed in detail.

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me8), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me8), 2*+ and 2-Me8*+ 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 gamma-irradiation of 2 in a CFCl3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me8 is treated in the same way, only the rearranged radical cation 2a-Me8*+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a*+ and 2-Me8*+, 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-Me8 in liquid octane, strongly suggest that 2-Me8*+ (and not 2a-Me8*+) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me8*+ --> 2a-Me8*+ is estimated. The radical cations 2a*+ and 2a-Me8*+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene pi-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the pi-system.

Spin evolution of radical pair with radical containing two groups of equivalent magnetic nuclei

Analytical solution is obtained for time-resolved magnetic field effects (TR-MFE) on recombination fluorescence of radical-ion pair (RIP) containing radical ion with two groups of magnetically equivalent nuclei. The present theoretical approach is applied to three experimental systems: RIPs containing radical cations of 2,3-dimethylbutane, 2,2,6,6-tetramethylpiperidine, or diisopropylamine and radical anion of p-terphenyl-d14 in nonpolar alkane solutions. Good agreement between theory and experiment is found for all the three systems, hyperfine coupling constants of radical cations are obtained by fitting the experimental TR-MFE traces. The potential of the TR-MFE technique for studying radical ions with nonequivalent nuclei is discussed in detail. The wide applicability of the theoretical model and the experimental technique make them useful for studying short-lived radical species that are often beyond the reach of the conventional electron paramagnetic resonance spectroscopy.