John E. Bennett

Kinetic electron paramagnetic resonance study of the reactions of t-butylperoxyl radicals in aqueous solution

The kinetics of reactions of t-butylperoxyl radicals in aqueous solution have been measured using electron paramagnetic resonance, ultraviolet absorption spectroscopy and gas chromatography. The rate constants for the overall self-reaction, the separate terminating and non-terminating reactions are very similar to those observed in non-polar solvents and the gas phase. The t-butoxyl radicals, formed by the non-terminating reaction, can either undergo scission, which leads to methylperoxyl radicals, or react with further t-butyl hydroperoxide to regenerate t-butylperoxyl radicals. The cross-termination reaction between methylperoxyl and t-butylperoxyl radicals is an important route in the overall termination sequence. The propagation reaction occurs significantly only at high concentrations of t-butyl hydroperoxide, ([ButOOH] > 0.3 mol dm–3) and its rate constant is much lower than that in non-polar solutions.

ESR spectrum of the α-styryl radical

Abstract The ESR spectrum shows that the structure of the α-styryl radical, C 6 H 5 CH 2 , is not that of the 1-phenylvinyl radical but that of the α-methylenebenzyl radical in which the unpaired electron is delocalized into the π-system of the aromatic ring.

ESR spectra of the trimethyl—tin and trimethyl—lead radicals

Abstract The ESR spectra of (CH 3 ) 3 Sn · and (CH 3 ) 3 Pb · have been observed. The g -tensors are anisotropic with g zz g fs g yy ≈ g xx , and the hyperfine couplings to 117 Sn, 119 Sn and 207 Pb nuclei are extremely large and anisotropic. The results show that there is appreciable spin density in the valence s-orbitals of both radicals, and that the radicals are not planar, in contrast to the carbon analogue, (CH 3 ) 3 C·.

Termination rate constants of t-butyl radicals, in solution: interference by cis-azoisobutane

The rate constant for the mutual termination reaction of t-butyl radicals in solution has been measured by kinetic ESR spectroscopy and is (11.1 ± 3.4) × 109 M−1 sec−1 over the temperature range from 220 to 330°K. Azoisobutane was shown to be unsuitable as a photo-initiator in the experiments because the formation of the thermally unstable cis-isomer during photolysis led to erroneous values of the rate constant.

Reactions of alkylperoxyl radicals in solution. Part 1.—A kinetic study of self-reactions of 2-propylperoxyl radicals between 135 and 300 K

Absolute rate constants for the self-reaction of 2-propylperoxyl radicals in solution have been obtained over a wide temperature range and in four solvents. The kinetic behaviour is in agreement with that reported for other secondary peroxyl radicals in solution, but is not entirely consistent with the corresponding data for the 2-propylperoxyl radical in the gas phase. The thermodynamics of the equilibrium between the radicals and their dimer have been investigated to interpret their behaviour below 200 K.

Electron spin resonance spectra of secondary alkylperoxy radicals

A series of secondary and two primary alkylperoxy radicals have been prepared photochemically in the liquid phase and their electron spin resonance (e.s.r.) spectra observed. Under suitable conditions of temperature and dissolved oxygen concentration the e.s.r. spectra of all the secondary alkylperoxy radicals consisted of well-resolved doublets with hyperfine coupling constants lying in the range 2–8 G and g-factors close to 2.0154. The hyperfine coupling constants varied considerably with the nature of the alkyl substituent. This dependence is attributed to restricted rotation about the C—O bond which leads to a preferential orientation between the alkyl and peroxy groups.

Studies of radical–molecule reactions using a rotating cryostat. Reactions of hydrogen atoms with organic substrates at 77 K

The rotating cryostat has been used in conjunction with electron spin resonance (e.s.r.) spectroscopy to study directly the reactions of hydrogen and deuterium atoms with a range of organic substrates in the solid state at 77 K. In every case the primary radicals formed by the reaction were trapped and could be observed by e.s.r. For ethylene, allene, ketene and acetaldehyde, free radicals formed by a secondary reaction were also observed.No primary abstraction reactions were observed for any of the alkenes studied, but hydrogen abstraction occurred almost exclusively for acetaldehyde and allyl alcohol, and both abstraction and addition reactions were observed for substituted benzenes.The relative amounts of addition to the two ends of the double bond in a series of asymmetric alkenes were measured and are in qualitative agreement with predictions based on quantum mechanical calculations, though for pent-2-ene the experimental selectivity was much greater than expected. A small secondary inverse deuterium isotope effect was observed for the addition of hydrogen atoms to 1,1-dideuteroethylene.For the substituted benzenes preferential addition to the ortho-position of the ring occurred, with much less addition to the para-position and virtually none at the meta-position.

Mechanisms of peroxide decomposition. EPR studies of the one-electron oxidation of the peroxymonosulphate anion (HOOSO3–) and the reactions of SO5˙–

Continuous-flow EPR studies of the reaction between CeIV and the peroxymonosulphate anion HOOSO3– at low pH enable the isotropic EPR spectrum of SO5˙– to be characterized, and provide kinetic information on the formation and reaction of this radical. Alkylperoxyl radicals (RO2˙) detected when the reaction is carried out in the presence of alkenes (e.g. methyl methacrylate) are shown to arise from reactions of SO4˙–(derived from self-reaction of SO5˙–) with the alkene, and subsequent addition of oxygen (also formed from SO5˙–).

A kinetic–electron spin resonance study of the self-reactions of some aromatic and aliphatic sulphonyl radicals and aromatic sulphinyl radicals

A variety of aromatic and aliphatic sulphonyl radicals (RSO2˙) have been found to undergo diffusion-controlled self-reaction in solution (with 2kt typically ca. 109 dm3 mol–1 s–1); reaction proceeds somewhat slower for some sterically hindered arenesulphonyl radicals (ArSO2˙) and for some arenesulphinyl analogues (ArSO˙). The observation of curved Arrhenius plots for some ArSO2˙ radicals, as well as the detection of ArSO˙ from sulphonyl precursors under steady-state conditions, is attributed to the thermal decomposition of an unstable intermediate formed by dimerisation (sulphur-to-oxygen) of ArSO2˙. A thermodynamic and kinetic analysis is presented.

A kinetic study of the self-reaction of prop-2-ylperoxyl radicals in solution using ultraviolet absorption spectroscopy

Overall rate constants for the self-reaction of prop-2-ylperoxyl radicals in the liquid phase (cyclohexane, decane and dodecane) have been measured over the temperature range 293–373 K using ultraviolet absorption spectroscopy to monitor the concentration of alkylperoxyl radicals. The Arrhenius constants for the self-reaction are Et= 20 ± 3 kJ mol–1 and 2At=(5 ± 2.5)× 109 dm3 mol–1 s–1. Above 363 K there is evidence that reaction with the solvent becomes increasingly important.