Rate Constants For Addition Of Radicals Research Articles

Pulsed EPR Study of the Initial Steps of Radical-Scavenging Reactions of C70 with Diphenylphosphine Oxide, Hydroxycyclohexyl, and 2-Hydroxypropyl Radicals

The radical scavenging reactions of C70 were investigated using time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) methods. In the diphenylphosphine oxide (DPPO) radical and C70 system, structural isomers of the adduct radicals of C70 appeared in the TR-EPR spectrum with clear hyperfine structures due to the phosphorus atom. Four isomers were identified through the analysis of the hyperfine coupling constants. The assignment of the adduct radicals was confirmed by the semiempirical calculation of the relative addition reaction rate constants that produce each isomer. The radical scavenging rate constants, ksca, of C70 in toluene were determined for DPPO, hydroxylcyclohexyl, and 2-hydroxypropyl radicals through electron spin echo observations using the pulsed-EPR method. The ksca values were nearly 109 mol-1 dm3 s-1 and were almost equal to the diffusion-controlled rate constant in toluene. This proves that C70 acts as an excellent radical scavenger. In addition, the radical addition rate constants, kadd, of C70 for varying carbon atoms in C70 were obtained by considering the peak intensity ratio of the adduct radicals in the TR-EPR spectrum. In this study, we demonstrated that the large number of carbon atoms in pentagons (five-membered rings) is responsible for the high reactivity of fullerenes.

Development and Application of a Peroxyl Radical Clock Approach for Measuring Both Hydrogen-Atom Transfer and Peroxyl Radical Addition Rate Constants.

The rate-determining step in free radical lipid peroxidation is the propagation of the peroxyl radical, where generally two types of reactions occur: (a) hydrogen-atom transfer (HAT) from a donor to the peroxyl radical; (b) peroxyl radical addition (PRA) to a "C═C" double bond. Peroxyl radical clocks have been used to determine the rate constants of HAT reactions (kH), but no radical clock is available to measure the rate constants of PRA reactions (kadd). In this work, we modified the analytical approach on the linoleate-based peroxyl radical clock to enable the simultaneous measurement of both kH and kadd. Compared to the original approach, this new approach involves the use of a strong reducing agent, LiAlH4, to completely reduce both HAT and PRA-derived products and the relative quantitation of total linoleate oxidation products with or without reduction. The new approach was then applied to measuring the kH and kadd values for several series of organic substrates, including para- and meta-substituted styrenes, substituted conjugated dienes, and cyclic alkenes. Furthermore, the kH and kadd values for a variety of biologically important lipids were determined for the first time, including conjugated fatty acids, sterols, coenzyme Q10, and lipophilic vitamins, such as vitamins D3 and A.

ChemInform Abstract: Prediction of Methyl Radical Addition Rate Constants from Molecular Structure.

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Prediction of Methyl Radical Addition Rate Constants from Molecular Structure

Multiple linear regression and computational neural networks (CNNs) are used to develop quantitative structure−property relationships for methyl radical addition rate constants. Structure based descriptors are used to numerically encode substrate information for 191 compounds. Descriptors can be classified as topological, geometric, electronic, or combination. A six-descriptor CNN was developed that produced training set rms error = 0.381 log units and rms error = 0.496 log units for an external prediction set. A seven-descriptor CNN was used to build a model for a subset of 172 of the compounds. Training set rms error was 0.424 log units and prediction set rms error reduced to 0.409 log units. Model predictions were on the order of experimental error.

Radical Addition Rate Constants to Acrylates and Oxygen: α-Hydroxy and α-Amino Radicals Produced by Photolysis of Photoinitiators

Laser flash photolysis of R-hydroxy and R-amino ketones, which are used as photoinitiators in free radical polymerization, lead to the generation of a series of nucleophilic R-hydroxy and R-amino radicals. Absolute addition rate constants of these radicals to n-butylacrylate and oxygen were measured by laser flash photolysis employing an indirect probe technique. Crystal violet and N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9,- 10-perylenedicarboximide were used as selective probe molecules for these nucleophilic initiator radicals to measure the addition rate constants to n-butylacrylate and oxygen, respectively. High acrylate addition rate constants of some initiator radicals were found in acetonitrile solution, e.g., dimethylketyl radical ( kacrylate ) 1.3 10 7 M -1 s -1 ) and 2-morpholino propan-2-yl radical (kacrylate ) 2.9 10 7 M -1 s -1 ).

Effects of ring strain on gas‐phase rate constants. 3. NO3 radical reactions with cycloalkenes

AbstractRate constants for the gas‐phase reactions of NO3 radicals with a series of cycloalkenes have been determined at 298 ± 2 K, using a relative rate technique. Using an equilibrium constant for the NO2 + NO3 ⇄ N2O5 reactions of 3.4 × 10−11 cm3 molecule−1, the following rate constants (in units of 10−13 cm3 molecule−1 s−1) were obtained: cyclopentene, 4.52 ± 0.52; cycloheptene, 4.71 ± 0.56; bicyclo[2.2.1]‐2‐heptene, 2.41 ± 0.28; bicyclo[2.2.2]‐2‐octene, 1.41 ± 0.17; bicyclo[2.2.1]‐2,5‐heptadiene, 9.92 ± 1.13; and 1,3,5‐cycloheptatriene, 12.6 ± 2.9. When combined with previous literature rate constants for cyclohexene and 1,4‐cyclohexadiene, these data show that the rate constants for the nonconjugated cycloalkenes studied depend to a first approximation on the number of double bonds and the degree and configuration of substitution per double bond. No obvious effects of ring strain energy on these NO3 radical addition rate constants were observed. Our previous a priori predictive techniques for the alkenes and cycloalkenes can now be extended to strained cycloalkenes.

Untersuchungen zur Spin‐Trapping‐Kinetik in Systemen von Radikalen unterschiedlicher Reaktivität. Die Photolyse von α‐Phenylbenzoin in Gegenwart von Benzyliden‐tert.‐butylamin‐N‐oxid

Investigations on Spin‐Trapping Kineties in Radical Systems with Different Reactivity. Photolyses of α‐Phenylbenzoine in the Presence of Benzylideneter. ‐butylamine N‐oxideAs a model system to study the spin‐trapping kinetics of systems forming two radicals of different reactivity α‐phenylbenzoine (PhB) was chosen. The photolysis of PhB forms benzoyl (B·) and diphenylketylradikals (K·), respectively, which can be trapped by benzylidene‐tert‐butyl‐amine N‐oxide (PBN), as a spin trap. The rate constants of radical addition to PBN and their reactions with the spin adducts were determined by means of flash photolysis and kinetic calculations of the PBN concentration dependence. For B· and K· addtion rate constants to PBN of k1 = 8,1 × 105 M−1s−1, and k2 = 5,8 · 104 M−1s−1, respectively, were found. The rate constants for the reactions of B· and K· with the spin adducts are in the order of 109…1010 M−1s−1.

Effects of ring strain on gas‐phase rate constants. 2. OH radical reactions with cycloalkenes

AbstractRelative rate constants for the gas‐phase reactions of OH radicals with a series of cycloalkenes have been determined at 298 ± 2 K using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with isoprene of 9.60 × 10−11 cm3 molecule−1 s−1, the rate constants obtained were (X 1011 cm3 molecule−1 s−1): cyclopentene 6.39 ± 0.23, cyclohexene 6.43 ± 0.17, cycloheptene 7.08 ± 0.22, 1,3‐cyclohexadiene 15.6 ± 0.5, 1,4 cyclohexadiene 9.48 ± 0.39, bicyclo[2.2.1]‐2‐heptene 4.68 ± 0.39, bicyclo[2.2.1] 2,5 heptadiene 11.4 ± 1.0, and bicyclo[2.2.2] 2 octene 3.88 ± 0.19. These data show that the rate constants for the nonconjugated cycloalkenes studied depend on the number of double bonds and the degree of substitution per double bond, and indicate that there are no obvious effects of ring strain energy on these OH radical addition rate constants. A predictive technique for the estimation of OH radical rate constants for alkenes and cycloalkenes is presented and discussed.

An electron spin resonance study of radical addition to alkyl isocyanides

Using e.s.r. spectroscopy, a variety of types of free radical (Z·) have been shown to add to the terminal carbon of alkyl isocyanides (RNC) to form imidoyl radical adducts (RNĊZ). At 253 K the rate constants for radical addition to ButNC are in the order k(Ph˙) > k(ButO˙) > k(Me˙), and k(ButO˙)= 6 × 105 l mol–1 s–1. The imidoyl adducts are σ-radicals in which the unpaired electron occupies an orbital orthogonal to the NC π-orbital. The magnitudes of a(13Cα) and a(Nβ) are markedly dependent upon the nature of Z, and a(13Cα) increases with the electronegativity of Z. The radical ButNĊSiEt3 appears to be close to linear at Cα, whilst ButNĊOR is bent. There appear to be two contributions, of opposite sign, to a(Nβ), a positive one resulting from direct overlap of the orbital of the unpaired electron with that of the lone pair on nitrogen, and a negative one resulting from spinpolarisation of the Nβ–Cασ-bonding orbital. The β-scission of some imidoyl radicals has also been studied using e.s.r. spectroscopy. Radicals of the type RNĊXBut(X = O or S) undergo X–C cleavage to form RNCX and But˙, whilst the radicals RNĊY (Y = MeS, Et3Si, or Ph) undergo cleavage to produce R˙ and NCY. At 243 K the rate constants for β-scission were ca. 1.4 × 105(ButNĊSiEt3), 3.1 × 105(ButNĊSBut), and 3.6 × 103 s–1(ButNĊOBut).

Rate constants for radical addition to olefins

1. It has been shown theoretically that the rate constant for a\(\mathop R\limits^ \cdot + {}_ / ^ \setminus C = C\) type reaction is an oscillating function of the heat of reaction. The oscillation frequency is largely determined by the C-C bond vibration frequency. This conclusion has been confirmed by comparison with the results of studies on polymerization reactions. Our results make it possible to use heats of reaction to predict rate constant with considerable accuracy. 2. Alteration of the steric factor increases the reaction rate, the increase being independent of the heat of reaction and the form of the curve showing the logarithm of the rate plotted as a function of the heat of reaction remaining unchanged. The activation energy is a nonlinear function of the heat of reaction.