Microviscosity and wavelength effects on radical cage pair recombination

Harris, John; Oelkers, Alan B.; Tyler, David R.

doi:10.1016/j.jorganchem.2007.03.036

Cited by 6 publications

(7 citation statements)

References 16 publications

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“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19] Studies have also been performed to investigate geminate recombination following excitation of larger neutral radicals. [20][21][22][23][24][25][26][27][28][29][30][31] The measurements on these caged radical pairs produced by photodissociation again provide qualitative agreement with predicted trends, escape rates depend on radical size and solvent fluidity, but quantitative agreement is elusive.…”

Section: Introductionmentioning

confidence: 58%

“…The development of time-resolved spectroscopies with picosecond and femtosecond resolution allowed for much more direct measurement of diffusive cage escape and geminate recombination. Again the iodine molecule became a favored paradigm, with early studies investigating the dissociation, geminate recombination, and vibrational relaxation of iodine as a function of solvent. − Studies have also been performed to investigate geminate recombination following excitation of larger neutral radicals. − The measurements on these caged radical pairs produced by photodissociation again provide qualitative agreement with predicted trends, escape rates depend on radical size and solvent fluidity, but quantitative agreement is elusive.…”

Section: Introductionmentioning

confidence: 87%

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Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

et al. 2009

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Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (<100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.

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Section: Introductionmentioning

confidence: 58%

Section: Introductionmentioning

confidence: 87%

Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

et al. 2009

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“…Overall, a stronger solvent cage would increase F cP . However, numerous studies have shown that the solvent bulk viscosity is inadequate for quantitatively predicting F cP . , In particular, solvents with the same bulk viscosity can have drastically different F cP values. − Other studies have investigated internal solvent pressure, cohesive energy density, and solvent density as a way to predict and interpret F cP . These methods have likewise met with little success. − Furthermore, many of these latter solvent properties are complex and not easily or conveniently measured.…”

Section: Introductionmentioning

confidence: 99%

Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types

2017

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This study reports a method for correlating the radical recombination efficiencies (F cP ) of geminate radical cage pairs to the properties of the solvent. Although bulk viscosity (macroviscosity) is typically used to predict or interpret radical recombination efficiencies, the work reported here shows that microviscosity is a much better parameter. The use of microviscosity is valid over a range of different solvent system types, including nonpolar, aromatic, polar, and hydrogen bonding solvents. In addition, the relationship of F cP to microviscosity holds for solvent systems containing mixtures of these solvent types. The microviscosities of the solvent systems were straightforwardly determined by measuring the diffusion coefficient of an appropriate probe by NMR DOSY spectroscopy. By using solvent mixtures, selective solvation was shown to not affect the correlation between F cP and microviscosity. In addition, neither solvent polarity nor radical rotation affects the correlation between F cP and the microviscosity.

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“…An alternative explanation is that cob(I)alamin always reacts by outer-sphere electron transfer but involves the concept of a solvent cage (Scheme , hypothesis B), wherein the solvent encapsulates the radical pair, which may react with each other prior to diffusing apart to become free radicals − . In this case, transfer of an electron to the substrate results in a caged radical paira substrate-derived vinyl radical and a cob(II)alamin metal-based radical.…”

Section: Resultsmentioning

confidence: 99%

Reconciling Disparate Models of the Involvement of Vinyl Radicals in Cobalamin-Mediated Dechlorination Reactions

Kliegman

McNeill

2009

Environ. Sci. Technol.

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Inner-sphere (nonradical) and outer-sphere (radical-based) mechanisms have been suggested for cobalamin-mediated dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). In this study, the role of free vinyl radicals was investigated using deuterated radical traps: d(8)-isopropanol and d(8)-tetrahydrofuran. For both substrates, addition of trap resulted in production of deuterated dechlorination products, and higher concentrations of trap resulted in increased amounts of deuterated products. However, only a finite proportion of the products were trappable: 8% of the PCE-derived products and 86% of the TCE-derived products result from free radicals. The data show that the reaction does not proceed solely by either an inner-sphere or an outer-sphere mechanism and led to the hypothesis that caged radical intermediates were involved in the mechanism. The untrappable fraction of products are hypothesized to result from in-cage reactions. This hypothesis was investigated using d(5)-glycerol as a radical trap and viscogen. Although increased viscosity resulted in decreased formation of free-radical-derived products, consistent with the cage hypothesis, these data were inconclusive. The role of d(8)-isopropanol in enhancing the production of radicals in this system via an acetone ketyl radical chain mechanism was also investigated, and no evidence for such an effect was found.

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Microviscosity and wavelength effects on radical cage pair recombination

Cited by 6 publications

References 16 publications

Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types

Reconciling Disparate Models of the Involvement of Vinyl Radicals in Cobalamin-Mediated Dechlorination Reactions

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