Rate constants for anilidyl radical cyclization reactions

[reaction: see text] N-Aryl-5,5-diphenyl-4-pentenamidyl radicals (3) were produced by 266 nm laser-flash photolysis of the corresponding N-(phenylthio) derivatives, and the rate constants for the cyclizations of these radicals were measured directly. The 5-exo cyclization reactions were fast (k(c) > 2 x 10(5) s(-1)), and radicals 3 generally behaved as electrophilic reactants with a Hammett correlation of rho = 1.9 for five of the six radicals studied. However, the p-methoxyphenyl-substituted radical 3f cyclized much faster than expected from the Hammett analysis. Variable temperature studies of parent radical 3a (aryl = phenyl) gave an Arrhenius function with log k = 9.2 - 4.4/2.3RT (kcal/mol). The rate constant for the reaction of p-ethylphenyl-substituted anilidyl radical 3b with Bu(3)SnH at 65 degrees C was k(T) = 4 x 10(5) M(-1) s(-1).     

Ketene reactions with the aminoxyl radical tempo: preparative, kinetic, and theoretical studies

Tetramethylpiperidinyloxy (TEMPO, TO*) reacts with ketenes RR(1)C=C=O generated by either Wolff rearrangement or by dehydrochlorination of acyl chlorides to give products resulting from addition of one TEMPO radical to the carbonyl carbon and a second to the resulting radical. Reactions of phenylvinylketenes 4b and 4f, phenylalkynylketene 4c, and the dienylketene AcOCMe=CHCH=CHCMe=C=O (11) occur with allylic or propargylic rearrangement. Even quite reactive ketenes were generated as rather long-lived species by photochemical Wolff rearrangement in isooctane solution, characterized by IR and UV, and used for kinetic studies. The rate constants of TEMPO addition to eight different ketenes have been measured and give a qualitative correlation of log k(2)(TEMPO) = 1.10 log k(H(2)O) -3.79 with the rate constants for hydration of the same ketenes. Calculations at the B3LYP/6-311G//B3LYP/6-311G level are used to elucidate the ring opening of substituted cyclobutenones leading to vinylketenes and of 2,4-cyclohexadienone (17) forming 1,3,5-hexatrien-1-one (18).     

Factors influencing C-ON bond homolysis in alkoxyamines: unexpected behavior of SG1 (N-(2-methyl-2-propyl)- N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl)-based alkoxyamines

Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium constant K (k(d)/k(c)), i.e., the increasing rate constant k(d) of the homolysis of the C-ON bond between the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO derivatives, we observed two relationships for the plot E(a) vs BDE(C-H), one for the nonpolar released alkyl radicals (E(a) (kJ/mol) = -133.0 + 0.72BDE) and the other one for the polar released alkyl radicals (E(a) (kJ/mol) = -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO derivatives), the rate constants k(d) of the C-ON bond homolysis were correlated to the cleavage temperature T(c) (log(k(d)(s(-)(1))) = 1.51 - 0.058T(c)). Such correlations should help to design new alkoxyamines to use as regulators and to improve the tuning of NMP experiments.     

Reactivity of cyano-substituted fluorenes in reaction with the tempo radical

The rate constants for the reaction of the TEMPO radical with fluorene, 2-cyanofluorene, and 9-cyanofluorene were determined by ESR. A comparison was made of the reactivity of the hydrocarbons in reaction with the TEMPO radical and the peroxyl radical. The quantum-chemical characteristics of the radicals and particles taking part in the reactions and also the characteristics of the transition states in the reactions of the TEMPO radical and cumenylperoxyl radical with fluorene were obtained by the PM3 method. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 19–22, January–February, 2006.     

Radical additions of TEMPO to ketenes: correlation of free radical and nucleophilic reactivity

[reaction: see text] Tetramethylpiperidinyloxy (TEMPO, TO*) reacts with a variety of ketenes R1R2C=C=O by rate-limiting attack on carbonyl carbon to give the 1,2-bis(adducts) R1R2C(OT)CO2T. The alpha,beta-unsaturated ketenes (E)-PhCH=CHCH=C=O (8b) and PhC=CCH=C=O (8c) give the 1,4-bis(adducts) PhCH(OT)CH=CHCO2T and PhC(OT)=C=CHCO2T. The ketenes may be generated in situ for these reactions in the presence of TEMPO by either dehydrochlorination of R1R2CHCOCl with Et3N or Wolff rearrangement. Ketenes PhCH=C=O (8a), 8b, and 8c had not previously been observed as long-lived species at room temperature, but when formed by photochemical Wolff rearrangement, these could be characterized in solution by conventional IR spectroscopy and used for kinetic studies for reaction with TEMPO using UV detection. The reactions of six ketenes with TEMPO in hydrocarbon solvents follow second-order kinetics, with a range of 2.5 x 10(5) in the rate constants, which are correlated with unit slope with the corresponding rate constants for hydration.     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

Nitroxyl radical reactions with 4-pentenyl- and cyclopropylketenes: new routes to 5-hexenyl- and cyclopropylmethyl radicals

4-Pentenylketenes 4a and 9 and cyclopropylketenes 3a, 13, 14 (RCH=C=O) are generated by photochemical Wolff rearrangements and observed by IR as relatively long-lived species at room temperature in hydrocarbon solvents. The reactions of these ketenes with the nitroxyl radicals tetramethylpiperidinyloxyl (TEMPO, TO*) and tetramethylisoindoline-2-oxyl (TMIO, IO*) form carboxy substituted 5-hexenyl and cyclopropylmethyl radicals which are either trapped by a second nitroxyl radical or undergo rearrangements followed by trapping. The rate constant of the reaction of 4a with TEMPO was similar to that of n-BuCH=C=O (1b), while 3a was 4.3 times more reactive, indicating cyclopropyl stabilization of the incipient radical.     

An accelerating-reporting group for studies of radical heterolysis reactions and its application in an acid-catalyzed fragmentation reaction of an alpha,beta-dimethoxy radical

The 2,2-diphenylcyclopropyl group was employed to accelerate reactions of alpha-methoxy radicals containing beta-leaving groups, to trap the products of either migration or heterolysis of the leaving group, and to provide a useful chromophore for laser flash photolysis kinetic studies. The reporting group biases reactions in favor of heterolytic fragmentation and most likely intercepts radical cations in ion pairs. The 1-methoxy-1-methyl-2-(diethylphosphatoxy)-2-(2,2-diphenylcyclopropyl)ethyl radical (3a) reacted faster than the kinetic resolution of the instrument (k > 2 x 10(8) s(-1)) in all solvents studied, and the 2-acetoxy analogue (3b) reacted much faster than related radicals that do not contain the cyclopropyl group (e.g., k = 1.1 x 10(6) s(-1) in CH3CN at ambient temperature). The rate constants and Arrhenius parameters for reactions of 3b indicated that the rate-limiting step in the reaction was heterolytic cleavage. The 1,2-dimethoxy-1-methyl-2-(2,2-diphenylcyclopropyl)ethyl radical (26) reacted in a general acid-catalyzed heterolysis reaction, and rate constants for protonation of the beta-methoxy group by a series of carboxylic acids were determined. The results suggest that acid-catalyzed reactions of beta-alkoxy radicals might be employed in synthetic conversions.     

Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation

The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) and the hydroxylamine TEMPO-H by transfer of H(*) (H(+) + e(-)) (acac = 2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k(3H) = 1400 +/- 100 M(-1) s(-1) at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/A(4D)) = -0.5 +/- 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru(II)imH(D) + TEMPO(*) that are greater than those observed: k(3H,calc)/k(3H) = 31 +/- 4 and k(3D,calc)/k(3D) = 140 +/- 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.     

Toluene combustion: reaction paths, thermochemical properties, and kinetic analysis for the methylphenyl radical + O2 reaction

Aromatic compounds such as toluene and xylene are major components of many fuels. Accurate kinetic mechanisms for the combustion of toluene are, however, incomplete, as they do not accurately model experimental results such as strain rates and ignition times and consistently underpredict conversion. Current kinetic mechanisms for toluene combustion neglect the reactions of the methylphenyl radicals, and we believe that this is responsible, in part, for the shortcomings of these models. We also demonstrate how methylphenyl radical formation is important in the combustion and pyrolysis of other alkyl-substituted aromatic compounds such as xylene and trimethylbenzene. We have studied the oxidation reactions of the methylphenyl radicals with O2 using computational ab initio and density functional theory methods. A detailed reaction submechanism is presented for the 2-methylphenyl radical + O2 system, with 16 intermediates and products. For each species, enthalpies of formation are calculated using the computational methods G3 and G3B3, with isodesmic work reactions used to minimize computational errors. Transition states are calculated at the G3B3 level, yielding high-pressure limit elementary rate constants as a function of temperature. For the barrierless methylphenyl + O2 and methylphenoxy + O association reactions, rate constants are determined from variational transition state theory. Multichannel, multifrequency quantum Rice-Ramsperger-Kassel (qRRK) theory, with master equation analysis for falloff, provides rate constants as a function of temperature and pressure from 800 to 2400 K and 1 x 10(-4) to 1 x 10(3) atm. Analysis of our results shows that the dominant pathways for reaction of the three isomeric methylphenyl radicals is formation of methyloxepinoxy radicals and subsequent ring opening to methyl-dioxo-hexadienyl radicals. The next most important reaction pathway involves formation of methylphenoxy radicals + O in a chain branching process. At lower temperatures, the formation of stabilized methylphenylperoxy radicals becomes significant. A further important reaction channel is available only to the 2-methylphenyl isomer, where 6-methylene-2,4-cyclohexadiene-1-one (ortho-quinone methide, o-QM) is produced via an intramolecular hydrogen transfer from the methyl group to the peroxy radical in 2-methylphenylperoxy, with subsequent loss of OH. The decomposition of o-QM to benzene + CO reveals a potentially important new pathway for the conversion of toluene to benzene during combustion. A number of the important products of toluene combustion proposed in this study are known to be precursors of polyaromatic hydrocarbons that are involved in soot formation. Reactions leading to the important unsaturated oxygenated intermediates identified in this study, and the further reactions of these intermediates, are not included in current aromatic oxidation mechanisms.     

Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique

The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, beta-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for beta-scission of tert-alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)()), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO(2), and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 +/- 0. 14)Sigmasigma - (0.31 +/- 0.04)SigmaE(s)(c), was obtained].     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

Kinetic results implicating a polar radical reaction pathway in the rearrangement catalyzed by alpha-methyleneglutarate mutase

alpha-Methyleneglutarate mutase (MGM) catalyzes the rearrangement of 2-methyleneglutarate to 3-methylitaconate (2-methylene-3-methylsuccinate). A putative mechanism for the MGM-catalyzed reaction involves 3-exo cyclization of the 2-methyleneglutaric acid-4-yl radical to a cyclopropylcarbinyl radical intermediate that ring opens to the 3-hydroxycarbonyl-2-methylenebutanoic acid-4-yl radical (3-methylitaconic acid radical). Model reactions for this mechanism were studied by laser flash photolysis kinetic methods. alpha-Ester radicals were produced by 266 nm photolysis of alpha-phenylselenyl ester derivatives. Rate constants for cyclizations of the (Z)-1-ethoxycarbonyl-4-(2,2-diphenylcyclopropyl)-3-buten-1-yl radical ((Z)-8a) and (E)- and (Z)-1,3-di(ethoxycarbonyl)-4-(2,2-diphenylcyclopropyl)-3-buten-1-yl radicals ((E)- and (Z)-8b) were determined. The ester group in (Z)-8a accelerates the 3-exo cyclization in comparison to the parent radical lacking an ester group by a factor of 3, an effect ascribed to a polarized transition state. The ester groups at C3 in radicals 8b slow the 3-exo cyclization reaction by a factor of 50. The rate constant for cyclization of the 2-methyleneglutaric acid-4-yl radical is estimated to be k approximately 2000 s(-1) at ambient temperature. When coupled with the estimated partitioning of the intermediate cyclopropylcarbinyl radical, the overall rate constant for the conversion is estimated to be k approximately equal to 1 x 10(-3) s(-1), which is much too small for any radical reaction and several orders of magnitude too small for kinetic competence for the MGM-catalyzed process. The possibility that the radical reaction in nature involves an unusual mechanism in which polar effects are important is discussed.     

Diffusion controlled hydrogen atom abstraction from tertiary amines by the benzyloxyl radical. The importance of C-H/N hydrogen bonding

The rate constants for H-atom abstraction (k(H)) from 1,4-cyclohexadiene (CHD), triethylamine (TEA), triisobutylamine (TIBA), and DABCO by the cumyloxyl (CumO(?)) and benzyloxyl (BnO(?)) radicals were measured. Comparable k(H) values for the two radicals were obtained in their reactions with CHD and TIBA whereas large increases in k(H) for TEA and DABCO were found on going from CumO(?) to BnO(?). These differences are attributed to the rate-determining formation of BnO(?) C-H/amine N lone-pair H-bonded complexes.     

Experimental and computational studies of the phenyl radical reaction with propyne.

The kinetics for the gas-phase reaction of phenyl radical with propyne has been measured by cavity ring-down spectrometry (CRDS), and the mechanism and initial product branching have been elucidated with the help of quantum chemical calculations. Absolute rate constants measured by the CRDS technique can be expressed by the following Arrhenius equation: (k/cm(3) mol(-1) s(-1)): k(propyne)(T=301-428 K)=(3.68+/-0.92) x 10(11)exp[-(1685+/-80)/T]. The experiment is unable to distinguish between the possible reactive channels, but theory indicates that phenyl radicals preferably add to the unsaturated terminal carbon atom in propyne under our experimental conditions. Theoretical kinetic calculations, employing high-level G2M(RCC, RMP2) and G3 energetic and IRCMax(RCCSD(T)//B3LYP-DFT) molecular parameters, reproduce the total experimental rate constants within a factor of three. Calculated total and branching rate constants are provided for high-T kinetic modeling. Addition reactions of phenyl to C3H4 are estimated to be less important molecular-growth pathways in high-T conditions (T>1000 K) in comparison to the C6H5 + C2H2 reaction.     

Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis

The intramolecular reaction of cysteine thiyl radicals with peptide and protein alphaC-H bonds represents a potential mechanism for irreversible protein oxidation. Here, we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides. For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3, Cys thiyl radicals abstract hydrogen atoms from Gly with k(f) = (1.0-1.1 x 10(5) s(-1), generating carbon-centered radicals, while the reverse reaction proceeds with k(r) = (8.0-8.9) x 10(5) s(-1). The forward reaction shows a normal kinetic isotope effect of k(H)/k(D) = 6.9, while the reverse reaction shows a significantly higher normal kinetic isotope effect of 17.6, suggesting a contribution of tunneling. For N-Ac-CysAla2AspAla3, cysteine thiyl radicals abstract hydrogen atoms from Ala with k(f) = (0.9-1.0) x 10(4) s(-1), while the reverse reaction proceeds with k(r) = 1.0 x 10(5) s(-1). The order of reactivity, Gly > Ala, is in accord with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that k(f) < k(r) suggests some secondary structure of the model peptides, which prevents the adoption of extended conformations, for which calculations of homolytic bond dissociation energies would have predicted k(f) > k(r). Despite k(f) < k(r), model calculations show that intramolecular hydrogen abstraction by Cys thiyl radicals can lead to significant oxidation of other amino acids in the presence of physiologic oxygen concentrations.     

Kinetics and products of the reactions of selected diols with the OH radical

Using a relative rate method, rate constants have been measured at 296 ± 2 K for the gas‐phase reactions of OH radicals with 1,2‐butanediol, 2,3‐butanediol, 1,3‐butanediol, and 2‐methyl‐2,4‐pentanediol, with rate constants (in units of 10^?12 cm³ molecule^?1 s^?1) of 27.0 ± 5.6, 23.6 ± 6.3, 33.2 ± 6.8, and 27.7 ± 6.1, respectively, where the error limits include the estimated overall uncertainty of ±20% in the rate constant for the reference compound. Gas chromatographic analyses showed the formation of 1‐hydroxy‐2‐butanone from 1,2‐butanediol, 3‐hydroxy‐2‐butanone from 2,3‐butanediol, 1‐hydroxy‐3‐butanone from 1,3‐butanediol, and 4‐hydroxy‐4‐methyl‐2‐pentanone from 2‐methyl‐2,4‐pentanediol, with formation yields of 0.66 ± 0.11, 0.89 ± 0.09, 0.50 ± 0.09, and 0.47 ± 0.09, respectively, where the indicated errors are the estimated overall uncertainties. Pathways for the formation of these products are presented, together with a comparison of the measured and estimated rate constants and product yields. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 310–316, 2001     

Rate constants for the reactions of a series of alkylperoxy radicals with NO

The rate constants for the gas-phase reactions of isopropyl- and tert-butylperoxy radicals with nitric oxide (NO) have been studied at 298 +/- 2 K and a total pressure of 3-4 Torr (He buffer) using a laser flash photolysis technique coupled with a time-resolved negative-ionization mass spectrometry. The alkyl peroxy radicals were generated by the reaction of alkyl radicals with excess O(2), where alkyl radicals were prepared by laser photolysis of several precursor molecules. The rate constants were determined to be k(i-C(3)H(7)O(2) + NO) = (8.0 +/- 1.5) x 10(-12) and k(t-C(4)H(9)O(2) + NO) = (8.6 +/- 1.4) x 10(-12) cm(3) molecule(-1) s(-1). The results in combination with our previous studies are discussed in terms of the systematic reactivity of alkyl peroxy radicals toward NO.     

Peroxyl radical clocks

A series of peroxyl radical clocks has been developed and calibrated based on the competition between the unimolecular beta-fragmentation (k(beta)) of a peroxyl radical and its bimolecular reaction with a hydrogen atom donor (k(H)). These clocks are based on either methyl linoleate or allylbenzene and were calibrated directly with alpha-tocopherol or methyl linoleate, which have well-established rate constants for reaction with peroxyl radicals (k(H-tocopherol) = 3.5 x 10(6) M(-1) s(-1), k(H-linoleate) = 62 M(-1) s(-1)). This peroxyl radical clock methodology has been successfully applied to determine inhibition and propagation rate constants ranging from 10(0) to 10(7) M(-1) s(-1).     

Aminoalkyl radicals: direct observation and reactivity toward oxygen, 2,2,6,6-tetramethylpiperidine-N-oxyl, and methyl acrylate

The reactivity of 11 aminoalkyl radicals toward different additives [oxygen, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and methyl acrylate (MA)] has been investigated through laser flash photolysis and quantum mechanical calculations. The transient absorption spectra of the radicals were recorded: good agreement was found with the spectra calculated by using quantum mechanical calculations. All the interaction rate constants were measured. A large range of values are obtained: (0.04-3) x 10(9) M(-1) s(-1) for O2, (0.002-5) x 10(8) M(-1) s(-1) for TEMPO, and (<0.004-2) x 10(7) M(-1) s(-1) for MA. Generation of the decarboxylated aminoalkyl radical derived from N-phenylglycine was unambiguously demonstrated. It was clearly found that both the addition to oxygen and the recombination with TEMPO were strongly governed by the reaction exothermicity. On the other hand, both polar and enthalpy factors have a large influence on the rate constants of the addition reaction to the acrylate unit, which were ranging over at least 4 orders of magnitude. This paper provides a set of new data to characterize the structure/reactivity relationships of aminoalkyl radicals.