Solvent cage effect in the photolysis of azomethane in aqueous alcohols and other media: a semiempirical correlation with macroscopic solvent parameters

NODELMAN, N.; Martin, J. C.

doi:10.1021/ja00437a031

Cited by 21 publications

(15 citation statements)

References 12 publications

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“…Secondary cage effects are those in which the geminate radicals recombine to reform the parent molecule after primary cage escape has occurred but before a random distribution of the radicals in solution is achieved. 41,42 This paper describes a method to obtain the primary and § An alternative method for obtaining f pair is to make the standard assumption 38 that the rate constant for geminate recombination, k c , is independent of solvent viscosity (h), and therefore the only viscosity dependence arises from the k d process. (In fact, recent experimental observations confirm that k c is independent of solvent viscosity.…”

Section: Methodsmentioning

confidence: 99%

Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

Oelkers

Tyler

2008

Photochem Photobiol Sci

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A method is reported for measuring the recombination efficiency of secondary geminate radical cage pairs. The procedure involves measuring the recombination efficiency for primary geminate recombination (F(c1)) using pump-probe laser methods and measuring the "apparent" (or net) recombination efficiency (F(cP)) for all geminate pairs (primary and secondary) using steady-state irradiation methods. A mathematical relationship between F(cP), F(c1), and F(c2) (where F(c2) is the recombination efficiency for secondary geminate recombination) is derived and demonstrated using the photolysis reactions of the [(CpR)Mo(CO)(3)](2) molecules, where CpR = eta(5)-C(5)H(4)CH(3) and eta(5)-C(5)H(4)(CH(2))(2)C(O)NCH(3)(CH(2))(n)CH(3) (n = 3, 8, 13, 18). As an example of the results obtained using the new method, it was found that F(c1) = 0.43 and F(c2) = 0.68 for the molecule with CpR = eta(5)-C(5)H(4)CH(2)CH(2)N(CH(3))C(O)(CH(2))(18)CH(3). The value of F(c2) decreased as the side-chain on the Cp ring got shorter; F(c2) is equal to 0.0 for the molecules with n = 3 and for CpR = eta(5)-C(5)H(4)CH(3). It is hypothesized that a longer side-chain prevents facile diffusion of the radicals out of the secondary cage, whereas the smaller side-chains permit more facile diffusion apart of the radicals. A general conclusion is that the reactions of large radicals in particular may be especially impacted by secondary geminate cage recombination.

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Section: Methodsmentioning

confidence: 99%

Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

Oelkers

Tyler

2008

Photochem Photobiol Sci

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“…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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“…Various models have been proposed that attempt to quantify F cP and the cage effect in terms of solvent parameters other than bulk viscosity. For example, models involving internal pressure, cohesive energy density, and solvent density have all been proposed. − However, all of these models inadequately rely on bulk solvent parameters. Furthermore, several of these parameters are exceedingly complex and not conveniently measured.…”

mentioning

confidence: 99%

Radical Cage Effects: Comparison of Solvent Bulk Viscosity and Microviscosity in Predicting the Recombination Efficiencies of Radical Cage Pairs

2016

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This study reports the results of experiments that probed how solvents affect the recombination efficiency (FcP) of geminate radical cage pairs. The macroviscosity of solvents has traditionally been used to make quantitative predictions about FcP, but experiments reported here show that FcP varies dramatically for solvent systems with identical macroviscosities. Experiments show that FcP correlates with the solvent microviscosity: five different solvent systems (consisting of a solvent and a structurally similar viscogen) were examined, and FcP was the same for all five solvent systems at any particular microviscosity. The translational diffusion coefficient of the radicals (measured by DOSY) in the solvent system was used to define the microviscosity of the solvent system.

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Solvent cage effect in the photolysis of azomethane in aqueous alcohols and other media: a semiempirical correlation with macroscopic solvent parameters

Cited by 21 publications

References 12 publications

Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

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

Radical Cage Effects: Comparison of Solvent Bulk Viscosity and Microviscosity in Predicting the Recombination Efficiencies of Radical Cage Pairs

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