Photochemical studies as a function of solvent viscosity. A new photochemical pathway in the reaction of (η5-C5H4Me)2Mo2(CO)6 with CCl4

Braden, Dale A.; Parrack, Eileen E.; Tyler, David R.

doi:10.1039/b202112a

Cited by 7 publications

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References 30 publications

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“…(The solvent viscosity was modified by adding varying amounts of squalane, a viscogen, to the hexanes/CCl 4 solvent The quantum yields as a function of viscosity were then fit to eq 12, where c is a fitting parameter that contains k cP , φ pair is the quantum yield for formation of the radical cage pair, and φ x is an offset parameter that is necessary to account for a minor electron-transfer reaction that occurs in these systems . The value for F cP was then obtained by inserting the value for φ pair into the expression Φ obs = φ pair [1 − F cP ]…”

Section: Resultsmentioning

confidence: 99%

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

2007

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This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination efficiencies of photochemically generated radical cage pairs (denoted as FcP) were measured in the presence and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect. The first method was femtosecond pump-probe transient absorption spectroscopy, which directly measured FcP from reaction kinetics, and the second method (referred to herein as the "steady-state" method) obtained FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (lambda = 515 nm for the pump probe method and lambda = 546 nm for the steady-state method) of Cp'2Mo2(CO)6 (Cp' = eta(5)-C5H4CH3). In addition, radical cage pairs generated from Cp'2Fe2(CO)4 and Cp*2TiCl2 (Cp* = eta(5)-C5(CH3)5) were studied by the steady-state method. The pump-probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.

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Section: Resultsmentioning

confidence: 99%

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

2007

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Microviscosity and wavelength effects on radical cage pair recombination

Harris

Oelkers

Tyler

2007

Journal of Organometallic Chemistry

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Radical Cage Effects in the Photochemical Degradation of Polymers: Effect of Radical Size and Mass on the Cage Recombination Efficiency of Radical Cage Pairs Generated Photochemically from the (CpCH₂CH₂N(CH₃)C(O)(CH₂)_nCH₃)₂Mo₂(CO)₆ (n = 3, 8, 18) Complexes

2003

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This study explored the effect of radical size, chain length, and mass on the cage recombination efficiency of photochemically generated radical cage pairs. Radical cage pairs containing long-chain radicals of the type [(CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(CO)(3)Mo*, *Mo(CO)(3)(CpCH(2)CH(2)(CH(3))NC(O)(CH(2))(n)CH(3))] were generated in hexanes/squalane solution by photolysis (lambda = 546 nm) of the Mo-Mo bonds in (CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(2)Mo(2)(CO)(6) (n = 3, 8, 18). The cage recombination efficiencies (denoted as F(cP), where F(cP) = k(cP)/(k(cP) + k(dP)), k(dP) is the diffusion rate constant, and k(cP) is the radical recombination rate constant) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl(4) (a metal-radical trap) as a function of solvent system viscosity. The results show that F(cP) increases as the length of the chain on a radical center increases. This finding likely provides at least one of the reasons why the quantum yields for photolytic polymer degradation (and long-chain molecules, in general) decrease as the polymer chains get longer. In quantitative terms, plots of k(dP)/k(cP) were linearly proportional to mass(1/2)/radius(2), in agreement with the prediction of Noyes' cage effect theory. The "radius" of a long-chain radical, such as those studied herein, is rather vague, and for that reason a less ambiguous structural parameter was sought to replace the r(2) term in the Noyes expression. Plots of k(dP)/k(cP) vs mass(1/2)/surface area suggest that surface area can be used in place of the radius(2) term in the Noyes expression. The significance of being able to use a particle's surface area in the Noyes expression is that the expression becomes useful for nonspherical particles. The new expression allows the approximate prediction of F(cP) values for radicals of different sizes and masses.

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Photochemical studies as a function of solvent viscosity. A new photochemical pathway in the reaction of (η5-C5H4Me)2Mo2(CO)6 with CCl4

Cited by 7 publications

References 30 publications

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

Microviscosity and wavelength effects on radical cage pair recombination

Radical Cage Effects in the Photochemical Degradation of Polymers: Effect of Radical Size and Mass on the Cage Recombination Efficiency of Radical Cage Pairs Generated Photochemically from the (CpCH₂CH₂N(CH₃)C(O)(CH₂)_nCH₃)₂Mo₂(CO)₆ (n = 3, 8, 18) Complexes

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Photochemical studies as a function of solvent viscosity. A new photochemical pathway in the reaction of (η5-C5H4Me)2Mo2(CO)6 with CCl4

Cited by 7 publications

References 30 publications

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

Microviscosity and wavelength effects on radical cage pair recombination

Radical Cage Effects in the Photochemical Degradation of Polymers: Effect of Radical Size and Mass on the Cage Recombination Efficiency of Radical Cage Pairs Generated Photochemically from the (CpCH2CH2N(CH3)C(O)(CH2)nCH3)2Mo2(CO)6 (n = 3, 8, 18) Complexes

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Radical Cage Effects in the Photochemical Degradation of Polymers: Effect of Radical Size and Mass on the Cage Recombination Efficiency of Radical Cage Pairs Generated Photochemically from the (CpCH₂CH₂N(CH₃)C(O)(CH₂)_nCH₃)₂Mo₂(CO)₆ (n = 3, 8, 18) Complexes