Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product (original) (raw)
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The Journal of Physical Chemistry, 1994
The kinetics and mechanism of the reactions C6H5CH202 + C6HsCH202-2C6H5CH20 + 0 2 (3a), C&-CHzO2 + C~H S C H Z O~-C&sCHO + C&CH2OH + 0 2 (3b), and CaHsCH202 + HOz-C&CHzOOH + 0 2 (4) have been investigated using two complementary techniques: flash photolysis/UV absorption for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses and branching ratio determinations. The reaction of chlorine atoms with toluene was found to yield benzyl radicals exclusively and was used to generate benzylperoxy radicals in excess oxygen. During this study, relative reaction rate constants of chlorine atoms with compounds related to those involved in the reaction mechanism have been measured at room temperature: k(Cl+toluene) = (6.1 f 0.2) X lo-", k(Cl+benzaldehyde) = (9.6 f 0.4) X 10-11, k(Cl+benzyl chloride) = (9.7 f 0.6) X 10-l2, k(Cl+benzyl alcohol) = (9.3 f 0.5) X 10-11, k(Cl+benzene) < 5 X 10-l6, all in units of cm3 molecule-1 s-l. The products identified following the self-reaction 3 were benzaldehyde, benzyl alcohol, and benzyl hydroperoxide. The latter is the product of the reaction of C6H5-CHzO2 with HO2. The yield of products allowed us to determine the branching ratio ar = ksa/k3 = 0.4. The UV absorption spectrum of the benzylperoxy radical was determined from 220 to 300 nm. It was similar to those of alkylperoxy radicals, with a maximum cross section at 245 nm of 6.8 X 10-18 cm2 molecule.-' Kinetic data were obtained from the detailed simulation of experimental decay tracea recorded at 250 nm over the temperature range 273-450 K. The resulting rate expressions are k3 = (2.75 f 0.15) X 10-14 exp[(1680 f 140)K/T] cm3 molecule-l s4 and k4 = (3.75 f 0.32) X IO-" exp[(980 f 230)K/T) cm3 molecule-' s-1 (errors = la). The UV absorption traces in the flash-photolysis kinetic study were well accounted for by the identified products in the FTIR study, thus providing good confidence in the results. However, about 20% of the products have remained unidentified. Some uncertainties persist in the reaction mechanism leading us to assign a fairly large uncertainty of about 50% to the rate constants k3 and k4 over the whole temperature range. This work shows that the aromatic substituent does not provide any specificity in the reactivity of peroxy radicals and confirms that large radicals tend to react faster with H 0 2 than generally assumed in current atmospheric models.
Journal of Physical Chemistry A, 2005
The laser-induced fluorescence (LIF) excitation spectra of the 4-methylcyclohexoxy and d11-cyclohexoxy radicals have been measured for the first time. LIF intensity was used as a probe in direct kinetic studies of the reaction of O 2 with trans-4-methylcyclohexoxy and d11-cyclohexoxy radicals from 228 to 301 K. Measured rate constants near room temperature are uniformly higher than the Arrhenius fit to the lower-temperature data, which can be explained by the regeneration of cyclic alkoxy radicals from the product of their-scission and the effect of O 2 concentration on the extent of regeneration. The Arrhenius expressions obtained over more limited ranges were k O 2) (1.4-1 +8) × 10-13 exp[(-810 (400)/T] cm 3 molecule-1 s-1 for trans-4methylcyclohexoxy (228-292 K) and k O 2) (3.7-1 +4) × 10-14 exp)[(-760 (400) /T] cm 3 molecule-1 s-1 for d11-cyclohexoxy (228-267 K) independent of pressure in the range 50-90 Torr. The room-temperature rate constant for the reaction of trans-4-methylcyclohexoxy radical with O 2 (obtained from the Arrhenius fit) is consistent with the commonly recommended value, but the observed activation energy is ∼3 times larger than the recommended value of 0.4 kcal/mol and half the value previously found for the reaction of normal cyclohexoxy radical with O 2 .
Reactions of the HO 2 Radical with CH 3 CHO and CH 3 C(O)O 2 in the Gas Phase
The Journal of Physical Chemistry A, 2001
Flash photolysis UV absorption techniques were used to study the HO 2 + CH 3 C(O)O 2 reaction. It was found that the reaction HO 2 + CH 3 CHO h CH 3 CH(OH)O 2 (2, -2) can interfere with the kinetic measurements. Thus, the kinetics and thermochemistry of this reaction were investigated. The UV spectrum of the CH 3 CH-(OH)O 2 radical was determined, and the rate constant for the association reaction was measured to be k 2 ) 4.4 × 10 -14 cm 3 molecule -1 s -1 at 298 K, with an uncertainty factor of about 2. The reaction was found to be equilibrated near room temperature, and the equilibrium constant was determined between 298 and 373 K:
Chemical Physics Letters, 1994
Alkyl and alkyl peroxy radicals from t-butyl alcohol (TBA), HOC(CH,),CH; and HOC(CH9)&H20i have been studied in the gas phase at 298 K. Two techniques were used: pulse radiolysis UV absorption to measure the spectra and kinetics, and long path-length Fourier transform infrared spectroscopy (FTIR) to identify and quantify the reaction products. Absorption cross sections were quantified over the wavelength range 220-320 nm. At 240 nm, ~~~~~"~,~~=(2.4fO.3)~ lo-" and ~~HOC(CH1j2CH202= (3.4f0.5) x 1O-18 cm2 molecule-' have been obtained. Observed rate constants for the self-reaction of HOC(CH3)2CH;
The Journal of Physical Chemistry A, 2004
The UV absorption spectrum along with the self-reaction and oxidation reaction kinetics of the hydroxycyclohexadienyl radical, C 6 H 6 OH (which results from OH addition to benzene), were studied using excimer laser photolysis coupled to transient UV absorption. The radicals were generated by photolysis of N 2 O/H 2 O/ C 6 H 6 /He mixtures at 193 nm in a series of chemical reactions initiated by O( 1 D). The radical has continuous absorption in the range 260-340 nm with a maximum absorption cross-section of (8.1 ( 1.4) × 10 -18 cm 2 molecule -1 at 280 nm. Reaction of the radical with molecular oxygen, C 6 H 6 OH + O 2 h C 6 H 6 (OH)OO (1), and self-reaction C 6 H 6 OH + C 6 H 6 OH f products (2), were studied over the 252-285 K temperature range at 1.01 ( 0.02 bar (He). The radical temporal profiles were recorded via transient absorption at 315 nm. In reaction 1, two-time-domain "equilibration" kinetics were recorded in the temperature range 252-273 K. The rate constant of the addition reaction is k 1 ) (1.4 ( 0.8) × 10 -12 exp(-18.6 ( 1.7 kJ mol -1 /RT) cm 3 molecule -1 s -1 . The standard enthalpy of reaction 1 was determined from the measured equilibrium constants using the third law method: ∆H°2 98 ) -43.6 ( 2.0 kJ mol -1 . The measured rate constant of self-reaction 2 is k 2 ) (6 ( 3) × 10 -11 exp(-2.00 ( 1.6 kJ mol -1 /RT) cm 3 molecule -1 s -1 .
Kinetics and mechanism of hydroxyl radical reaction with methyl hydroperoxide
The Journal of Physical Chemistry, 1989
The reaction of hydroxyl radical with methyl hydroperoxide, CH300H, was investigated in the temperature range 203-423 K by pulsed photolytic generation of OH and detection by laser-induced fluorescence. The rate coefficient for the overall reaction, OH + C H 3 0 0 Hproducts ( k , ) was measured by using 180H and OD in place of OH. The rate coefficient for the CH3O2 production channel OH + CH3OOH -CH3O2 + H 2 0 (k,,) was obtained by using OH. The channel that yields CHzOOH, OH + C H 3 0 0 H -+ C H 2 0 0 H + H 2 0 (klb), is not observed when monitoring OH since C H 2 0 0 H rapidly falls apart to give back OH (and CH20) but is observed when studying the 180H or OD reaction with CH300H. By monitoring OH production in OD + C H 3 0 0 H reaction at 249 K, the two-channel mechanism was confirmed, and the values for k , and k , , were also determined. Both reaction 1 and channel la show negative activation energies, with k , = (2.93 & 0.30) X exp((220 & 21)/7') cm3 molecule-' s-I, where the indicated error is la, including estimated systematic errors and uA = AulnA. The rate coefficient for the reaction of OD with CH300D is at least a factor of 2 smaller than that for reaction la. The thermal decomposition lifetime for C H 2 0 0 H to give OH + CH20 is deduced to be shorter than 20 ws at 205 K. The mechanism of reaction 1 and the implications of our kinetic and mechanistic results to Earth's atmospheric chemistry are discussed. The measured value of kl and the branching ratio, kla/klb, at 298 K are compared with previous indirect measurements of Niki et al. [ J . Phys. Chem. 1983, 87, 21901. exp((l90 f 14)/7') cm3 molecule-' s-l (average of I80H and OD studies) and k,, = (1.78 & 0.25) X Phys. Lett. 1987, 139, 513. 7837 (9) Moortgat, G. K.; Burrows, J. P.; Schneider, W.; Tyndall, G. S.; Cox, R. A.
Thermochemistry, Reaction Paths, and Kinetics on the Secondary Isooctane Radical Reaction with 3 O2
International Journal of Chemical Kinetics, 2013
Ab initio and density functional calculations are performed to determine thermochemical and kinetic parameters in analysis of the 2 hydroperoxy-ethyl radical association with O 2. The system serves as an initial model for O 2 association with higher molecular weight alkyl-hydroperoxide radicals and is an important component in the well-studied ethyl radical plus O 2 reaction system. The CBS-Q//B3LYP/6-31G(d,p) and G3(MP2) composite methods are utilized to calculate energies. The well depth is determined as 35 kcal/mol and transition state results show two low energy paths (barriers below the entrance channel) for reaction to new products: (i) a HO 2 molecular elimination and (ii) a hydrogen shift path. Intramolecular hydrogen transfer (five-member ring) leads to 2 hydroperoxide acetadehyde + OH, where the barrier is ca. 7 kcal/mol lower than previously estimated. The HOOCH 2 CH(dO) formed here is chemically activated and a significant fraction dissociates to OH + formyl-methoxy radical, before stabilization. The barrier for hydrogen transfer is several kcal/mole lower than the corresponding reaction in a conventional hydrocarbon for this five-member ring transition state because the weak C-H bond on the hydroperoxide carbon. The second path is unimolecular HO 2 elimination leading to a vinyl hydroperoxide + HO 2. The vinyl hydroperoxide has a weak (22.5 kcal/mol) CH 2 dCHO-OH bond and rapidly dissociates to formyl methyl plus OH radicals; a second low energy chain branching path in low-temperature HC oxidation. Kinetic analysis with falloff on chemical activation and unimolecular dissociation, illustrate that both low energy paths are competing. Results also show significant formation of a diradical, • OCH 2 CH 2 OO • + OH, an additional new path to chain branching, which results from the chemical activation reaction. The HO 2 molecular elimination plus vinyl hydroperoxide dominates the H transfer by a factor of 1.8 at low temperatures, a result of its small entropy advantage. At high temperatures, dissociation to the higher energy, but loose transition state, hydroperoxide ethyl radical + O 2 (back to reactants) is the dominant path.
The Journal of Physical Chemistry A, 2000
The hydrogen abstraction reactions from CH 3 CHO by OH and OD radicals were studied in a fast-flow reactor by observation of the infrared chemiluminescence from H 2 O and HOD. The fraction of available energy released as the vibrational energy of water was 〈f v 〉 ) 0.52 with ∼30% released as bending excitation and ∼65% released as stretching excitation of the newly formed OH bond. The pattern of energy disposal closely resembles that for OH + H 2 CO or (CH 3 ) 2 S reactions, but differs from abstraction from secondary C-H bonds of hydrocarbons. Secondary reactions of CH 3 CO radical with NO 2 , OH, and H also were observed by infrared emission of products formed from the unimolecular decomposition of CH 3 C(O)ONO, CH 3 C(O)OH, and CH 3 C(O)H intermediate adducts. The CO 2 vibrational distributions from the decomposition of CH 3 C-(O)OH, CH 3 C(O)ONO, and HC(O)ONO are compared.