Radical Cations of Branched Alkanes in Solutions: Time-Resolved Magnetic Field Effect and Quantum Chemical Studies

Potashov, Pavel A.; Borovkov, V. I.; Shchegoleva, L. N.; Gritsan, Nina P.; Bagryansky, V. A.; Molin, Yuriy N.

doi:10.1021/jp2113208

Cited by 14 publications

(15 citation statements)

References 22 publications

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“…The time-resolved experiments were performed in isooctane. The primary RCs of this alkane exhibit a significant EPR spectrum width, Δ H pp ≈ 4.2 mT . The addition of HMDS has no effect on excess electron migration and scavenging, and, by our estimates, the EPR spectrum width of HMDS RC is about 0.5–0.6 mT .…”

Section: Methodsmentioning

confidence: 59%

“…The primary RCs of this alkane exhibit a significant EPR spectrum width, ΔH pp ≈ 4.2 mT. 23 The addition of HMDS has no effect on excess electron migration and scavenging, and, by our estimates, the EPR spectrum width of HMDS RC is about 0.5−0.6 mT. 24 Therefore, to reveal the quantum beat pattern determined by HFCs in 3 −• and 3 2 −• , HMDS was added to the solution to provide fast capture of solvent RCs and to reduce the effect of large HFCs in the RC of isooctane on the TR MFE curve.…”

Section: Introductionmentioning

confidence: 93%

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Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family

et al. 2019

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While there is a body of experimental data concerning dimers formed by an aromatic molecule and its radical cation, information on the corresponding dimer radical anions (DRAs) is scarce. In this work, evidence for the formation of the DRAs of decafluorobiphenyl and 4-aminononafluorobiphenyl has been obtained by the optically detected electron paramagnetic resonance and the time-resolved magnetic field effect techniques. Theoretical investigation (DFT B3LYP-D3/6-31+G*) of these DRAs and the DRAs of octafluoronaphtalene and 1,2,4,5-tetrafluorobenzene previously detected by Werst has been undertaken to gain greater insight into the structure of the polyfluoroarene DRAs. Without substituents different from a fluorine atom, an extra electron is evenly delocalized over two fragments; the bonding interaction is π stacking. On the potential energy surfaces (PES), there are two minima of nearly equal energy corresponding to the structures of perfect and parallel displaced sandwiches. Such a PES structure is due to a conical intersection between two electronic states of different symmetry. The DRA of 4-aminononafluorobiphenyl is an ion-molecular associate stabilized by electrostatic interactions involving NH 2 groups. The complex cyclic structure of the PES of this DRA suits the successive electron transfers between the dimer fragments. The calculated hyperfine coupling constants averaged over the PES minima agree well with the experimental ones.

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Section: Methodsmentioning

confidence: 59%

Section: Introductionmentioning

confidence: 93%

Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family

et al. 2019

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“…This basis set led to a good agreement between the calculated and experimental hyperfine coupling (HFC) constants with protons for the RCs of a series of branched alkanes in our previous studies. 11,12 To compare the calculation results with the experiment, the values of inhomogeneous width (ΔH pp ) of the EPR spectrum for the corresponding RCs were estimated. As the calculations showed, the HFC was mostly due to an isotropic interaction with β-protons of methyl groups.…”

Section: Introductionmentioning

confidence: 99%

Primary Radical Cations in Irradiated Poly(isobutylene)

et al. 2020

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Using the method of time-resolved magnetic field effect in radiation-induced fluorescence, primary radical cations (RCs) in irradiated poly(isobutylene) (PIB) have been detected for the first time. A comparison of experimental results with the data of quantum chemical calculations suggests that the initial geometry of the ionized fragment of the PIB molecule is close to the geometry of the neutral polymer in the trans–gauche–trans–gauche conformation. The spin density of the RC in this geometry is delocalized over more than 10 polymer units, and the width of the RC’s EPR spectrum is about ΔH pp ≈ 1.3 mT. At a temperature of 273 K and lower, the lifetime of the primary RCs with the delocalized spin density exceeds 10 ns. The structural relaxation of the RCs results in the spin density localization on a single C–C bond, which is extended to nearly 0.2 nm, and in the increase in the EPR spectrum width to ΔH pp ≈ 2.4 mT. It looks likely that this intramolecular structural relaxation is coupled strongly with those types of molecular motions that determine the process of dielectric β-relaxation in the polymer.

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“…(1), but by its integral convolution with the fluorescence probability of excited molecules, which changes exponentially with time, and the Gaussian instrumental function (see, e.g., [4]). Radical ions studied in this work have more than two different HFC constants.…”

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confidence: 99%

“…However, at sufficiently high frequencies of ρ ss (t) oscillations caused by large HFC constants, the fluorescence kinetics must be described, not by Eq. (1), but by its integral convolution with the fluorescence probability of excited molecules, which changes exponentially with time, and the Gaussian instrumental function (see, e.g., [4]). Radical ions studied in this work have more than two different HFC constants.…”

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Time-resolved magnetic field effect as a method of studying radical pairs with rapid evolution of spin state

et al. 2015

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103As a rule, organic radical ions in solutions have rather short lifetimes, which makes them unsuitable for study by ESR, and at lifetimes shorter than 10 ns, even by a more sensitive method of optically detected ESR (OD ESR) of spin correlated radical ion pairs [1,2]. At the same time, the method of time resolved mag netic field effect (TR MFE) in recombination fluores cence of such pairs [2, 3] enables the detection of even shorter lived particles. This possibility is determined, first of all, by the time resolution of a setup, which we have managed to improve to ≈1 ns. An additional nec essary condition for identification of such short lived radicals is the existence of hyperfine coupling (HFC) constants in them sufficiently large (≈10 mT) to induce singlet-triplet transitions in the nanosecond time range. It is worth noting that the class of com pounds of interest includes only those with rather short fluorescence times.In this work, the potentialities of the TR MFE method have been demonstrated by the examples of radical anions of two fluoro substituted aromatic compounds-1,2,3,5 tetrafluorobenzene (TFB) and pentafluoroaniline (PFA)-generated by X ray irradi ation of solutions. This method is based on analysis of specific features of the ratio between the kinetic curves of recombination fluorescence in a magnetic field (I B (t) and in the absence of a field (I 0 (t)). In low polar ity solutions, the fluorescence intensity decay caused by recombination of radical ion pairs, in the limit of infinitely short fluorescence time and narrow instru mental function, is given bywhere F(t) is the pair recombination rate, θ is the frac tion of pairs generated in the singlet correlated state, and ρ ss (t) is the population of the singlet stage of such pairs [3]. In this approximation, the ratio(2) referred to as time resolved magnetic field effect, is independent of recombination kinetics F(t). However, at sufficiently high frequencies of ρ ss (t) oscillations caused by large HFC constants, the fluorescence kinetics must be described, not by Eq.(1), but by its integral convolution with the fluorescence probability of excited molecules, which changes exponentially with time, and the Gaussian instrumental function (see, e.g., [4]). Radical ions studied in this work have more than two different HFC constants. Therefore, in simulation of TR MFE curves, the singlet state population ρ ss (t) in strong and zero fields was calculated numerically. The spin relaxation times of the radical ions in the pair were considered in the framework of approximations described elsewhere [3,5].Studies of radical anions by the TR MFE method were carried out in 1-10 mM liquid solutions of TFB and PFA in 2,2,4 trimethylpentane (isooctane). In isooctane, the mobility of excess electrons is very high, and radical anions of electron acceptors form in the subnanosecond time domain. To reduce the contribu tion of the isooctane radical cation (the ESR spectrum width, ≈2 mT) to the spin evolution of the radical pair, a positive charge acceptor-hexameth...

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Radical Cations of Branched Alkanes in Solutions: Time-Resolved Magnetic Field Effect and Quantum Chemical Studies

Cited by 14 publications

References 22 publications

Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family

Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family

Primary Radical Cations in Irradiated Poly(isobutylene)

Time-resolved magnetic field effect as a method of studying radical pairs with rapid evolution of spin state

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