Rate constants for self‐ and cross terminations of transient radicals in solution by effect modulated electron spin resonance spectroscopy

Rate constants for the bimolecular self-reaction of isopropylol radicals [(CH&COH] in various solvents are determined as functions of temperature by kinetic electron spin resonance. For hydrocarbon solvents they are well described by theoretical equations for reactions controlled by translational diffusion if diffusion coefficients of 2-propanol, a constant reaction distance, and a spin statistical factor of ' /4 are applied. Deviations from 2kt -D a t high diffusion constants agree with trends expected from recent theoretical models. For hydrogenbonding solvents large negative deviations are observed. They are attributed to steric constraints and slower rotational diffusion of radical-solvent aggregates. The disproportionation-to-combination ratio of isopropylol increases with solvent viscosity. As previously for tert-butyl, this is explained by anisotropic reorientation during encounters. Further, rate data are given for the decarbonylation of the 2-hydroxy-2-methylpropanoyl radical and for several hydrogen abstraction reactions of isopropylol.

Section: Self-diffusion Coefficients From [3334]supporting

confidence: 93%

Effects of diffusion on the self‐termination kinetics of isopropylol radicals in solution

Lehni

Fischer

1983

“…Their molar yields were 27.6, 48.9, 3.3, 14.5, 2.3, and 3.2%, respectively, for T = 237 K and 38.0, 52.0, 8.3, 0.7, <0.3, and 1.1% for T = 298 K. Apart from t-butanol the formation of the products is easily explained by Type I cleavage of the C--0-bond of the excited ester, followed by decarboxylation of the t-butoxicarbonyl radical and radical terminations. 2(CH3),C --+ (CH3)3CH + CH2C(CH& (18) 2(CH3)3C -(CH3)3CC(CH3)3 (19)…”

Section: Resultsmentioning

confidence: 99%

“…Self-and cross-termination rate constants often obey the cross-termination Ansatz (24) k: = 2(k: * k8)"z at least approximately [5,18]. We have tested this relation for t-butyl and t-butoxicarbonyl radicals by time-resolved experiments employing photolysis of t-butylpivalate in butene-l-oxide at the low temperature of 209 K. Under these conditions decarboxylation is unsignificant and good fits of Eqs.…”

Section: Resultsmentioning

confidence: 99%

Rate constants for the decarboxylation of the t‐butoxicarbonyl radical and concurring radical terminations directly obtained by kinetic ESR

Rüegge

Fischer

1986

Rate constants of various simultaneous reactions of t-butoxicarbonyl and t-butyl radicals generated by photolysis of t-butylpivalate in n-heptane are directly determined by kinetic electron spin resonance. The temperature dependence of the decarboxylation reaction t-BuOCO + t-Bu + CO, obeys log(k/s-') = 13.8-49.0/8 where 8 = 2.303 RT/ kJ . mol-'. The self-and cross-termination of the radicals are diffusion limited.

“…In the absence of steric hindrance, the bimolecular self-termination of small alkyl radicals occurs close to the diffusion-controlled limit with rate constants on the order of 10s-lO1oM-l-s-l. The presence of radical stabilizing substituents, such as vinyl [6,7], phenyl [8], or heteroatom-bearing groups [3,9,10] at the radical center does not lower these values to a measurable extent. A common radical-stabilizing substituent is the cyano group, for which the thermodynamical stabilization energy is known to be 20-30 kJ/mol (see [11,12] and references cited therein).…”

Section: Introductionmentioning

confidence: 97%

Rate constants for the bimolecular self‐reaction of cyano‐substituted alkyl radicals in solution

Korth¹,

Lommes²,

Sicking³

et al. 1983

Cyano-substituted methyl radicals (cyanomethyl-and 2-cyano-2-propyl radicals) and synand anti-1-cyano-ally1 radicals were generated, and their recombination kinetics in solution were investigated between -50 and +5OoC by time-resolved electron-spin-resonance spectroscopy. The comparison of the activation energies for recombination with the activation energies of the solution viscosities proves that the dimerizations of the radicals are diffusion controlled with rate constants on the order of 108-109M-'-s-'. In the case of cyanomethyl radicals an additional pseudo-first-order process, hydrogen abstraction, was detected and analyzed kinetically. Product analyses support the kinetic measurements.