The kinetics of the bimolecular self‐reaction of <i>t</i>‐butyl radicals in solution. II. Disproportionation/combination ratios

Schuh, Hans-Herbert; Fischer, Hanns

doi:10.1002/hlca.19780610718

Cited by 53 publications

(24 citation statements)

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“…The observed cage effect on the Disp / Comb selectivity can be explained by considering the spin state of the radical pair, namely, the spin correlation effect, and the collision model originally proposed by Fischer and modified by us (Scheme ) . The spin state of the caged radical pair should be predominantly singlet while that of the encountered diffused pair is triplet.…”

Section: Methodsmentioning

confidence: 96%

“…The observed cage effect on the Disp/Comb selectivity can be explained by considering the spin state of the radical pair, namely,t he spin correlation effect, [24] andt he collisionm odel originally proposed by Fischer and modified by us (Scheme 3). [17,25] The spin state of the caged radicalp air should be predominantly singlet while that of the encountered dif-fused pair is triplet. For the diffused radicals, the triplet pair must undergo intersystem crossing to the singlett hrough a conicalp oint, in which the radical pairm ust take an orthogonal orientation.T he radical pair hast or earrange to thec ollinear orientation before undergoing the termination,a nd this change of radical configurationa ssociates the displacement and reorganization of the surrounding solvents.…”

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confidence: 99%

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The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

Ogihara

Abe

et al. 2019

Chemistry A European J

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The effecto fv iscosity on the diffusion efficiency (F dif )o fa no rganic radical pair in as olvent cage and the termination mechanism, that is,t he selectivity of disproportionation (Disp)a nd combination (Comb)o ft he geminated caged radical pair and the diffused radicals encountered, were investigated quantitatively by following the photolysis of dimethyl 2,2'-azobis(2-methylpropionate) (V-601) in the absence andp resence of PhSD. F dif and Disp/ Comb selectivity outside the cage [Disp (dif) /Comb (dif) ]a re highly sensitivetot he viscosity. In contrast, the Disp/Comb selectivity inside the cage [Disp (cage) /Comb (cage) ]i sr ather insensitive. The differencei nv iscosityd ependence between Disp (cage) /Comb (cage) and Disp (dif) /Comb (dif) is explained by the spin state of the radical pair inside and outside the cage and the spin state dependent configurationalc hangeso f the radical pair upon their collision. Given that the configurational change of the radicals associates the displacement and reorganization of solvents around the radicals, the terminationo utside the cage, which requires larger change than that inside the cage, is highly viscosity dependent. Furthermore,while the bulk viscosity of each solvent shows good correlation with F dif and Disp/Comb selectivity, microviscosity is the better parameterp redicting F dif and Disp (dif) /Comb (dif) selectivity regardless of the solvents.[a] X.

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Section: Methodsmentioning

confidence: 96%

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confidence: 99%

The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

Ogihara

Abe

et al. 2019

Chemistry A European J

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“…They are 1.9 for (CH,),C+COOH, 1.5 for (CH,),C+COOCH3 and 0.3 for (CH,),CH+ COOH (CH,). Comparison with k d / k c z 3 for (CH,),C+ C(CH,), [29] and 0.65 for (CH,),CH+ HC (CH,), [30] reveals a diminution by a factor of 2 which is reasonable on statistical grounds.…”

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confidence: 94%

Norrish Type I Cleavage of Aliphatic Carboxylic Acids and Esters in Solution. A CIDNP.‐study

Kaiser¹,

Fischer²

1979

Helvetica Chimica Acta

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SummaryUV.-irradiation of pivalic and isobutyric acid and their methyl esters as well as of a-hydroxy isobutyric and malonic acid in solution lead to chemically induced dynamic nuclear polarization (CIDNP.) of parent compounds and of various reaction products. C1DNP.-effects and product distributions confirm a-cleavage of the C(u), CO bond to be the major mode of photodecomposition. Scavenger experiments indicate that decomposition of the molecules from a triplet excited state is roughly equally or more probable than from a singlet excited state. Quantum yields of educt disappearance are also given for the methyl esters of pivalic and isobutyric acid.

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“…The geminate reactions of recombination and disproportionation result in the restoration of the initial ketone RÀ COÀR and in the formation of aldehyde RÀCHO and alkene R(À H). The escaped acyl radicals undergo a fast decarbonylation reaction with the rate constant k CO = 2 · 10 7 s À1 for MP (a) [20] and k CO = 8 · 10 6 s À1 for MiB (b) [21]. The consequent bulk reactions of radical termination k t rise to the formation of products RÀR, RH, and R(À H).…”

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confidence: 97%

2,5‐Dihydro‐1H‐imidazole‐Based Nitroxides as Prospective Mediators in Living Radical Polymerization

Zubenko

Kirilyuk

Roshchupkina

et al. 2006

Helvetica Chimica Acta

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Dedicated to the memory of Professor Hanns FischerA number of spectroscopic methods were applied to obtain kinetic parameters of reactions modelling the 2,5-dihydro-1H-imidazole 1-oxide mediated living polymerization of acrylates. The homolysis rate constants of alkoxyamines based on five nitroxides were measured by EPR spectroscopy at different temperatures. The recombination rate constants k c between the corresponding alkyl radicals and the nitroxides were measured by means of laser flash photolysis. The time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) experiments revealed the negligible contribution of disproportionation in the recombination reaction. In addition, the thermodecomposition of alkoxyamines in the NMR probe showed the absence of intramolecular elimination of hydroxylamines from the corresponding alkoxyamines. Analysis of the kinetic parameters showed that the 2,5-dihydro-1H-imidazole 1-oxide type radicals are promising mediators for the living polymerization of acrylates and methacrylates.Introduction. -Nitroxide-mediated polymerization [1] [2] (NMP) is a fine synthetic technique, which allows for the design of new types of homo-and copolymers with welldefined architectures and narrow molecular-mass distributions [3]. The principal mechanism of NMP (Scheme 1) involves the reversible dissociation of a dormant nitroxide end-capped polymer chain with rate constant k d , the propagation of C-centered radicals with rate constant k p , the cross-coupling reaction between the nitroxide and the growing polymer chain with rate constant k c , and simultaneous irreversible self-termination reactions of the C-centered radicals with rate constant k t .

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The kinetics of the bimolecular self‐reaction of t‐butyl radicals in solution. II. Disproportionation/combination ratios

Cited by 53 publications

References 54 publications

The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

Norrish Type I Cleavage of Aliphatic Carboxylic Acids and Esters in Solution. A CIDNP.‐study

2,5‐Dihydro‐1H‐imidazole‐Based Nitroxides as Prospective Mediators in Living Radical Polymerization

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