Showing papers in "International Journal of Chemical Kinetics in 1990"

A product study of the gas-phase reaction of Isoprene with the OH radical in the presence of NOx

Abstract: The gas-phase reaction of isoprene with the OH radical, in the resence of NOx, was investigated at 298 ± 2 K and atmospheric pressure of air by long path length FT-IR spectroscopy. The primary products identified and their formation yields were: methacrolein, 0.21 ± 0.05; methyl vinyl ketone, 0.29 ± 0.07; and HCHO, with the observed yield being consistent with the sum of the methacrolein and methyl vinyl ketone yields. Combined with the previously reported yield of 0.044 ± 0.006 for 3-methylfuran, these products accounted for 55 ± 9% of the isoprene which reacted. Under conditions where the dark reaction of isoprene with NO2is not significant, the balance of the isoprene consumed could possibly be accounted for by the “organic nitrates” and “other carbonyl compounds” formed in estimated overall yields of ca. 12% and ca. 25%, respectively.

Rate constants for the gas-phase reactions of O3 with a series of monoterpenes and related compounds at 296 ± 2 K

Abstract: Rate constants for the gas-phase reactions of O 3 with a series of monoterpenes and related compounds have been determined at 296±2 K and 740 torr total pressure of air or O 2 using a combination of absolute and relative rate techniques. Good agreement between the absolute and relative rate data was observed, and the rate constants obtained (in units of 10 −17 cm 3 molecule −1 s −1 ) were: α-pinene, 8.7; β-pinene, 1,5; △ 3 -carene, 3,8; 2-carene, 24; sabinene, 8.8; d-limonene, 21; γ-terpinene, 14; terpinolene, 140: α-phellandrene, 190; α-terpinene, 870; myrcene, 49; trans-ocimene, 56; p-cymene, C=C< bond(s)

A pyrolysis mechanism for ammonia

Abstract: The mechanism of NH 3 pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time-history measurements of the species NH and NH 2 were made using narrow-linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH 2 in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH 2 profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: NH 2 +H→NH+H 2 with a rate coefficient of 4.0×10 13 exp(−3650/RT) cm 3 mol −1 s −1 , NH 2 +NH→N 2 H 2 +H with a rate coefficient of 1.5×10 15 T −0.5 cm 3 mol −1 s −1 and NH 2 +NH 2 →NH+NH 3 with a rate coefficient of 5.0×10 13 exp(−10000/RT) cm 3 mol −1 s −1 . The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction NH 3 +M→NH 2 +H+M were reanalyzed in light of recent data for the rate of NH 3 +H→NH 2 1+H 2 , and an improved rate coefficient of 2.2×10 16 exp(−93470/RT) cm 3 mol −1 s −1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ±15%

Absolute and relative rate constants for the reactions of hydroxyl radicals and chlorine atoms with a series of aliphatic alcohols and ethers at 298 K

Abstract: Rate constants for the gas-phase reactions of hydroxyl radicals and chlorine atoms with aliphatic alcohols and ethers have been determined at 298 ± 2 K and at a total pressure of 1 atmosphere. The OH radical rate data were obtained using both the absolute technique of pulse radiolysis combined with kinetic UV spectroscopy and a conventional photolytic relative rate method. The Cl atom rate constants were measured using only the relative rate method. Values of the rate constants in units of 10−12 cm3 molecule−1 s−1 are: The above relative rate constants are based on the values of (OH + c-C6H12) = 7.49 × 10−12 cm3 molecule−1 s−1 and (Cl + c-C6H12) = 311 × 10−12 cm3 molecule−1 s−1. Attempts to corre late the trends in the rate constant data in terms of the bond dissociation energies and inductive effects are discussed.

Kinetics and nitro-products of the gas-phase OH and NO3 radical-initiated reactions of naphthalene-d8, Fluoranthene-d10, and pyrene

Abstract: The kinetics and nitroarene product yields of the gas-phase reactions of naphthalene-d 8 , fluoranthene-d 10 , and pyrene with OH radicals in the presence of NO x and in N 2 O 5 -No 3 -NO 2 -air mixtures have been investigated at 296±2 K and atmospheric pressure of air. Using a relative rate method, naphthalene-d 8 was shown to react in N 2 O 5 −NO 2 3−NO 2 −air mixtures a factor of 1.22±0.10 times faster than than did naphthalene, with the 1- and 2-nitronaphthalene-d 7 product yields being similar to those of 1- and 2-nitronaphthalene from naphthalene. From the measured PAH concentrations and the nitroarene product yields, formation yields of 2-, 7-, and 8-nitrofluoranthene-d 9 and 2- and 4-nitropyrene of 0.03, 0.01, 0.003, 0.005, and 0.0006, respectively, were determined from the OH radical-initiated reactions. Effective rate constants for the reactions of fluoranthene-d 10 and pyrene with N 2 O 5 in N 2 O 5 −NO 3 −NO 2 −air mixtures of ca. 1.8×10 −17 cm 3 molecule −1 s −1 and ca. 5.6×10 −17 cm 3 molecule −1 s -1 , respectively, were derived. Formation yields of 2-nitrofluororanthene-d 9 and 4-nitropyrene of ca. 0.24 and ca. 0.0006, respectively, were estimated for these reaction systems. 2-nitropyrene was also observed to be formed in these N 2 O 5 −NO 3 −NO 2 reactions, but was found to be a function of the NO 2 concentration and, therefore, would be a negligible product under ambient NO 2 concentrations. These product and kinetic data are consistent with ambient air measurements of the nitroarene concentrations

A product study of the gas-phase reaction of methacrolein with the OH radical in the presence of NOx

Abstract: The gas-phase reaction of methacrolein with the OH radical, in the presence of NOx, was investigated at 298 ± 2 K and atmospheric pressure of air. Hydroxyacetone, methylglyoxal, a peroxyacyl nitrate identified as CH2 C(CH3)C(O)OONO2 (peroxymethacryloyl nitrate), formaldehyde, CO, and CO2 were observed to be the major products. The product yield data for these compounds show that OH radical addition to the >C C< bond accounts for ca. 50% of the overall reaction, with the remaining ca. 50% proceeding via H—atom abstraction from the CHO group. The data suggest that the alkoxy radical formed following the addition of OH to the terminal carbon atom, decomposes primarily to give the formyl radical plus hydroxyacetone. A lower limit ratio of 5: 1 has been estimated for OH radical addition to the terminal carbon atom of the double bond relative to addition on the inner carbon atom.

The atmospheric chemistry of Oxygenated fuel additives: t‐Butyl alcohol, dimethyl ether, and methylt‐butyl ether

Abstract: The mechanisms for the Cl-initiated and OH-initiated atmospheric oxidation of t-butyl alcohol (TBA), methyl t-butyl ether (MTBE), and dimethyl ether (DME) have been determined. For TBA the only products observed are equimolar amounts of H 2 CO and acetone, and its atmospheric oxidation can be represented by (7), (CH 3 ) 3 COH+NP+OH→H 2 CO+((CH 3 ) 2 CO+HO 2 +NO 2 (7). The mechanism for the atmospheric oxidation of DME is also straight forward, with the only observable product being methyl formate, (CH 3 )OCH 3 +NO+OH→HCOOCH 3 +NO 2 +HO 2 (8). The mechanism for the atmospheric oxidation of MTBE is more complex, with observable products being t-butyl formate (TBF) and H 2 CO. Evidence is presented also for the formation of 2-methoxy-2-methyl propanal (MMP), which is highly reactive and presumably oxidized to products. The atmospheric oxidation of MTBE can be represented by (9) and (10), CH 3 OC(CH 3 ) 3 +NO+OH→0.6HCOOC((CH 3 ) 3 +0.4CH 3 OC(CH 3 ) 2 CHO+HO 2 +NO 2 (9); CH 3 OC(CH 3 ) 2 CHO+OH+2NO→CO 2 +H 2 CO+(CH 3 ) 2 HO 2 +2NO 2 (10). In terms of atmospheric reactivity, DME, TBA and MTBE all compare favorably with methanol. In terms of rate of reaction in the atmosphere, DME, MTBE, and TBA are 1.4, 0.40 and 0.28 times as reactive as CH 3 OH towards OH on a per carbon basis. With regard to chemistry, atmospheric oxidation of CH 3 OH yields highly reactive H 2 CO as the sole carbon-containing products. In contrast, only 25% of the carbon in TBA is converted to H 2 CO, with the balance yielding unreactive acetone. For DME, all the carbon is converted to methyl formate which is unreactive. Finally, for MTBE, 60% is converted to unreactive TBF while the remaining 40% produces highly reactive MMP. Final assessment of the impact of these materials on the atmopheric reactivity of vehicle emissions requires the determination of their emissions rates under realistic operating conditions

The mechanism of the homogeneous pyrolysis of acetylene

Abstract: The experimental literature on the pyrolysis of C 2 H 2 is reviewed and summarized. These observations divide naturally into three temperature regimes: (i) T 1800 K, where a fragment chain carried by C 2 H and H drives a polymerization to polyacetylenes. A combined molecular/fragment chain mechanism is developed and applied to the modeling of three shock-tube studies which span the two higher-temperature regimes. The core of the mechanism is a set of 5 mainly molecular reactions whose rates are derived from an application of unimolecular rate theory and detailed balance to recent measurements on the decomposition of vinylacetylene. The combination of this core with consequent fragment radical reactions provides a satisfactory description of most features of the chosen experiments including several-isotope exchange, hydrogenation ot C 2 H 4 , and C 6 H 2 formation-which are almost entirely the result of fragment radical chain reaction. Possible detailed paths for the molecular dimerization are suggested and some possible molecular paths for the Berthelot synthesis of benzene are also proposed

High temperature reaction rate coefficients derived from N-atom ARAS measurements and excimer photolysis of NO

Abstract: Mixtures of NO and NO/H 2 in Ar were shock-heated and photolyzed with an ArF excimer laser. Measurements in these experiments of N-atom profiles using atomic resonance absorption spectrophotometry (ARAS) permitted the determination of two rate coefficients. The rate coefficient for the reaction (R1) N+NO→O+N 2 was found to be 4.29×10 13 exp(−787/T) cm 3 mol −1 sec −1 (±20% at 1400 K to ±10% at 3500 K). This is the first direct high temperature measurement of this rate coefficient in the exothermic direction. The rate coefficient for the reaction (R2) N+H 2 →H+NH was found to be 1.60×10 14 exp(−12650/T)(±35% from 1950 to 2850 K). To our knowledge, this is the first direct measurment of this rate coefficient. A study of the N-atom ARAS absorption behavior revealed a noticeable pressure dependence, as well as a weak temperature dependence, in the Beer-Lambert law absoprtion coefficient. Proper consideration of these effects is important when the N-atom ARAS diagnostic is used for absolute concentration measurements

Ethylene pyrolysis and oxidation: A kinetic modeling study

Abstract: Ethylene oxidation and pyrolysis was modeled using a comprehensive kinetic reaction mechanism. This mechanism is an updated version of one developed earlier. It includes the most recent findings concerning the kinetics of the reactions involved in the oxidation of ethylene. The proposed mechanism was tested against ethylene oxidation experimental data (molecular species concentration profiles) obtained in jet stirred reactors (1–10 atm, 880–1253 K), ignition delay times measured in shock tubes (0.2–12 atm, 1058–2200 K) and ethylene pyrolysis data in shock tube (2–6 atm, 1700–2200 K). The general prediction of concentration profiles of minor species formed during ethylene oxidation is improved in the present model by using more accurate kinetic data for several reactions (principally: HO2 + HO2 H2O2 + O2, C2H4 + OH C2H3 + H2O, C2H2 + OH Products, C2H3 C2H2 + H).

Toward a comprehensive mechanism for methanol pyrolysis

Abstract: A single kinetic mechanism for methanol pyrolysis is tested against multiple sets of experimental data for the first time. Data are considered from static, flow, and shock tube reactors, covering temperatures of 973 to 2000 K and pressures of 0.3 to 1 atmosphere. The model results are highly sensitive to the rates of unimolecular fuel decomposition and of various chain termination reactions that remove CH2OH and H radicals, as well as to experimental temperature uncertainties. The secondary fuel decomposition reaction CH3OH = CH2OH + H, which has previously been included only in mechanisms for high temperature conditions, is found to have a significant effect at low temperatures as well, through radical recombination. The reaction CH3O + C = CH3 + CO2, rather than CH3OH + H = CH3 + H2O, is found to be the dominant source of CH3 at low temperatures. The reverse of CH3 + OH = CH2OH + H is important to CH3 production at high temperatures.

Kinetics of iron-catalyzed decomposition of hydrogen peroxide in alkaline solution

Abstract: The decomposition of alkaline hydrogen peroxide solutions at 20°C has been studied in the presence of both supported iron catalysts and in systems with iron initially in solution. Studies with an iron-alumina supported catalyst showed the decomposition reaction was first order with respect to total peroxide concentration, while studies with alkaline Fe3+ produced more complex behavior. This has been attributed to the presence of at least two distinct catalytically active iron species. The first species is highly active and gives rise to high initial rates of reaction. A decrease in concentration of this species is observed together with an increase in concentration of a second, less active, iron species. The catalytic behavior of this “aged” iron species was found to be very similar to that of the supported iron catalyst.

Kinetics of the reactions of O(3P) and Cl(2P) with HBr and Br2

Abstract: A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of reactions (1)–(4) as a function of temperature. In all cases, the concentration of the excess reagent, i.e., HBr or Br2, was measured in situ in the slow flow system by UV-visible photometry. Heterogeneous dark reactions between XBr (X = H or Br) and the photolytic precursors for Cl(2P) and O(3P) (Cl2 and O3, respectively) were avoided by injecting minimal amounts of precursor into the reaction mixture immediately upstream from the reaction zone. The following Arrhenius expressions summarize our results (errors are 2σ and represent precision only, units are cm3 molecule−1 s−1): 1 = (1.76 ± 0.80) × 10−11 exp[(40 ± 100)/T]; 2 = (2.40 ± 1.25) × 10−10 exp[−(144 ± 176)/T]; 3 = (5.11 ± 2.82) × 10−12 exp[−(1450 ± 160)/T]; 4 = (2.25 ± 0.56) × 10−11 exp[−(400 ± 80)/T]. The consistency (or lack thereof) of our results with those reported in previous kinetics and dynamics studies of reactions (1)–(4) is discussed.

A shock tube study of the CH2O + NO2 reaction at high temperatures

Abstract: The reaction of CH 2 O with NO 2 has been studied with a shock tube equipped with two stabilized cw CO lasers. The production of CO, NO, and H 2 O has been monitored with the CO lasers in the temperature range of 1140-1650 K using three different Ar-diluted CH 2 O-NO 2 mixtures. Kinetic modeling and sensitivity analysis of the observed CO, NO, and H 2 O production profiles over the entire range of reaction conditions employed indicate that the bimolecular metathetical reaction, NO 2 +CH 2 O→HONO+CHO (1) affects most strongly the yields of these products. Combination of the kinetically modeled values of k 1 with those obtained recently from a low temperature pyrolytic study, ref. [8], leads to k 1 =8.02×10 2 T 2.77 e −6910/T cm 3 /mol sec for the broad temperature range of 300-2000 K

Kinetics of alkaline hydrolysis of organic esters and amides in neutrally‐buffered solution

Abstract: Reaction rate constants for the hydrolysis of organic esters and amides were determined at temperatures of 100–240°C in aqueous solutions buffered at pH values between 5.5 and 7.3. Experiments are modeled assuming alkaline hydrolysis with a thermodynamic solution model included to account for the temperature dependence of hydroxide ion concentration. In most cases, the ester hydrolysis second order rate constants agree well with published values from experiments in strongly basic solutions at pH values from 11 to 14 and temperatures from 25–80°C, despite the large extrapolations required to compare the data sets. The amide hydrolysis rate constants are about one order of magnitude higher than the extrapolated results from other investigators, but the reaction rate increased proportionally with hydroxide ion concentration, suggesting that an alkaline hydrolysis mechanism is also appropriate. These data establish the validity of the alkaline hydrolysis mechanism and can be used to predict hydrolysis reaction rates in neutrally-buffered solutions such as many groundwater and geothermal fluids.

Kinetic studies of the deactivation of O2(1Σg+) and O(1D)

Abstract: The kinetics of the deactivation of O 2 ( 1 Σ g + ) is studied in real time. O 2 ( 1 Σ g + ) is generated in this system by the O( 1 D)+O 2 reaction following O 3 laser flash photolysis in the presence of excess O 2 , and it is monitored by its characteristic emission band at 762 nm. Quenching rate constants were obtained for O 2 , O 3 , N 2 , CO 2 , H 2 O, CF 4 and the rare gases. Since O( 1 D) is the precursor for the formation of O 2 ( 1 Σ g + ), the addition of an O( 1 D) quencher effectively lowers the initial concentration of O 2 ( 1 Σ g + ). By measuring the initial intensity of the 762 nm fluorescence signal, the relative quenching efficiencies were determined for O( 1 D) quenching by N 2 , CO 2 , Xe, and Kr with respect to O 2 ; the results are in good agreement with literature values

Background reactivity in smog chambers

Abstract: Data from several smog chamber reaction vessels have been analyzed in an attempt to elucidate the chemical species which are responsible for chamber specific background phenomena, and the nature of the processes which determine the heterogeneous interactions of those species. There is good evidence for the emission of a compound which yields both NOx, and free radicals (probably HONO) and emissions of reactive organics (e.g. HCHO) may also be deduced. Total integrated chamber emission of these compounds may be as high as 20 to 60 ppb during a typical smog chamber experiment. In addition to the direct emission of these contaminants, the surface reaction of NO2 and H2O to HONO is examined. In some cases this reaction may have as great an effect on a smog chamber experiment as the emission of trace contaminants. Overall chamber perturbations to gas phase chemistry have been estimated for several experiments and were found to be less than 20 percent in the majority of cases, although higher perturbations were found in experiments involving compounds of low reactivity such as butane.

Kinetics and mechanism for the oxidation of 1,1,1‐trichloroethane

Abstract: The reaction mechanisms for oxidation of CH3CCl2 and CCl3CH2 radicals, formed in the atmospheric degradation of CH3CCl3 have been elucidated. The primary oxidation products from these radicals are CH3CClO and CCl3CHO, respectively. Absolute rate constants for the reaction of hydroxyl radicals with CH3CCl3 have been measured in 1 atm of Argon at 359, 376, and 402 K using pulse radiolysis combined with UV kinetic spectroscopy giving (OH + CH3CCl3) = (5.4 ± 3) 10−12 exp(−3570 ± 890/RT) cm3 molecule−1 s−1. A value of this rate constant of 1.3 × 10−14 cm3 molecule−1 s−1 at 298 K was calculated using this Arrhenius expression. A relative rate technique was utilized to provide rate data for the OH + CH3 CCl3 reaction as well as the reaction of OH with the primary oxidation products. Values of the relative rate constants at 298 K are: (OH + CH3CCl3) = (1.09 ± 0.35) × 10−14, (OH + CH3CClO) = (0.91 ± 0.32) × 10−14, (OH + CCl3CHO) = (178 ± 31) × 10−14, (OH + CCl2O) < 0.1 × 10−14; all in units of cm3 molecule−1 s−1. The effect of chlorine substitution on the reactivity of organic compounds towards OH radicals is discussed.

Identification of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) pyrolysis products by simultaneous thermogravimetric modulated beam mass spectrometry and time-of-flight velocity-spectra measurements

Abstract: The pyrolysis and products formed during the isothermal decomposition of HMX at 211 o C are H 2 O, HCN, CO, CH 2 O, NO, N 2 O, methylformamide, C 2 H 6 N 2 O, octahydro-1-nitroso-3,5,7-trinito-1,3,5,7-tetrazocine, and a nonvolatile residue. The temporal bahaviors of these products during the decomposition are presented. The method for usinf time-of-flight (TOF) velocity spectra to assist mass-spectrometry measurements in identifying the different gaseous products formed from the pyrolysis of material by determining the approximate molecular weights of the different gaseous products contributing to the different m/z values in the mass spectrum of the mixture is described. The ion fragmentation of HMX as a function of electron energy shows complete fragmentation of the HMX molecular ion for electron energies ≥ 12.4 eV. No fragments from the pyrolysis of HMX other than those mentioned above are observed

High temperature pyrolysis of methane in shock waves. Rates for dissociative recombination reactions of methyl radicals and for propyne formation reaction

Abstract: The pyrolysis of 2% CH4 and 5% CH4 diluted with Ar was studied using both a single–pulse and time–resolved spectroscopic methods over the temperature range 1400–2200 K and pressure range 2.3–3.7 atm. The rate constant expressions for dissociative recombination reactions of methyl radicals, CH3 + CH3 C2H5 + H and CH3 + CH3 C2H4 + H2, and for C3H4 formation reaction were investigated. The simulation results required considerably lower value than that reported for CH3 + CH3 C2H4 + H2. Propyne formation was interpreted well by reaction C2H2 + CH3 P-C3H4 + H with = 6.2 × 1012 exp(−17 kcal/RT) cm3 mol−1 s−1.

Thermal decomposition kinetics of polysilanes: Disilane, trisilane, and tetrasilane

Abstract: The decomposition kinetics of disilane with added butadiene, trisilane both neat and with added butadiene, trimethylsilane or H 2 , and normal and iso-tetrasilane both neat and in the presence of added butadiene are reported; Arrhenius parameters of the primary dissociation reactions are determined : A-factors suggest that polysilane decompositions (1) have similar intrinsic activation entropies (△S∼6.2±5 e.u.) and (2) have activation energies which increase with increasing reaction endothermicities. Relative trapping efficiencies of SiH 4 , Si 2 H 6 , Si 3 H 8 , C 4 H 6 , Me 3 SiH, and h 2 toward SiH 2 and SiH 3 SiH are also determined. Other results include the heat of formation of silylsilylene △H f ° (Sih 3 SiH)=75.3 Kcal/mol, and the activation energy for 1,1-H 2 elimination from disilane (E H2 =57.8 kcal/mol)

Reactions of naphthalene in N2O5 ? NO3 ? NO2 ? air mixtures

Abstract: The reactions of naphthalene in N2O5NO3NO2N2O2 reactant mixtures have been investigated over the temperature range 272–297 K at ca. 745 torr total pressure and at 272 K and ca. 65 torr total pressure using long pathlength Fourier transform infrared absorption spectroscopy. 2,3-Dimethyl-2-butene was added to the reactant mixtures at 272 K to rapidly scavenge the NO3 radicals both initially present in the added N2O5 and formed from the thermal decomposition of N2O5 during the reactions. The data obtained in the presence and absence of added 2,3-dimethyl-2-butene showed that napthalene undergoes initial reaction with the NO3 radical to form an NO3-naphthalene adduct, which either rapidly decomposes back to the reactants (at a rate of ca. 5 × 105 s−1 at 298 K) or reacts exclusively with NO2 to form products. When NO3 radicals, N2O5 and NO2 are in equilibrium, this overall process is kinetically equivalent to reaction of naphthalene with N2O5, and previous kinetic and product studies have indeed assumed the reactions of naphthalene and alkyl-substituted naphthalenes in N2O5NO3NO2air mixtures to be with N2O5, and not with NO3 radicals.

The rate constant for the intramolecular isomerization of pentyl radicals

Abstract: The thermal decomposition of n-pentane has been investigated in the temperature range 737 to 923 K. Making various assumptions, the detailed distribution of major products (methane, ethane, ethene, propene, and 1-butene) is used to evaluate the rate constant for the unimolecular isomerization CH 3 CH 2 CH 2 CH 2 CH 2 →CH 3 CHCH 2 CH 2 CH 3 which proceeds via a five-membered, cyclic transition state. Two alternative sets of assumptions are used. Common to both of them are assumptions concerning the thermochemistry and rate constants for decomposition of the C 5 H 11 radicals. Method 1 assumes that all secondary C−H bonds are equally reactive towards hydrogen abstraction in which case, in addition to the value of k 10 , the ratio of the rate constants for abstraction from primary and secondary C−H bonds is evaluated. Values about a factor of two higher than published values for similar molecules are obtained. The alternative, method 2, assumes that the ration of abstraction from the 1- and 2- positions of n-pentane is the same as that published for n-butane, in which case, in addition to the value of k 10 , the ration of the rates of abstraction from the 3− and 2− positions of n-pentane is obtained. The value obtained is 0.401 which is to be compared with the statistically expected (and assumed in method 1) 0.5. Detailed discussions of the values of k 10 obtained leads to the conclusion that method 1 leads to the best value log(k 10 /s −1 )=11.96±0.77−(23.4±2.0)/θ where θ=2.303RT in kcal/mol and error limits are two standard deviations. Combination of this value with values recalculated from published lower temperature data gives log(k 10 /s −1 )=11.08−20.04/θ which, it is concluded, is the best value in the range 438 to 923 K

Rate constants for the gas‐phase reactions of the OH radical with the cresols and dimethylphenols at 296 ± 2K

Abstract: Using a relative rate technique, rate constants for the gas-phase reactions of the OH radical with the three cresols and the six dimethylphenols have been determined at 296±2 k and atmospheric pressure. The rate constants for the cresols, which range from 4.3×10 ―11 cm 3 molecule ―1 s ―1 to 6.8×10 ―11 cm 3 molecule ―1 s ―1 , are in good agreement with previous literature data. The rate constants for 2,3-, 2,4-, 2,5-, 2,6-, and 3,4-dimethylphenol are all in the range (6.6-8.1)×10 ―11 cm 3 molecule ―1 s ―1 , with the rate constant for 3,5-diethylphenol being somezhat higher at 1.13×10 ―1 cm 3 molecule ―1 s ―1 . Rate constants estimated from the number, identity, and positions of the substituent groups around the aromatic ring agree to within a factor of approximately 2 with these experimentally determined rate constants

UV-Absorption spectrum of the methylperoxy radical and the kinetics of its disproportionation reaction at 300 K

Abstract: Molecular modulation spectroscopy combined with ultraviolet spectroscopic techniques have been used to observe the behavior of the CH 3 O 2 radicals generated in the gas phase by near-ultraviolet modulated photolysis of flowing Cl 2 -CH 4 -O 2 mixture. The kinetics of the disproportionation reaction (1) (1(a)) CH 3 O 2 +CH 3 O 2 →CH 3 O+CH 3 O+O 2 (1(b)) →CH 2 O+CH 3 OH+O 2 (1(c)) →CH 3 OOCH 3 +O 2 and the absorption cross-sections of CH 3 O 2 were measured by computer fitting of the modulated absorption traces obtained in the wavelength range 220 to 270 nm at 300 K and 240 torr. The rate constant for the elementary self-reaction k 1 =k 1(a) +k 1(b) +k 1(c) was determined to be (3.61±0.55)×10 −13 cm 3 molecules −1 s −1 . The parameter k obs /σ (where k obs is the observed apparent second-order rate constant) was measured from the decay curves in the dark phase of the modulated photolysis period in the wavelength range 230-260 nm, and had a value 1.16×10 5 cm s −1 at 250 nm. At 250 nm the absorption cross-section was determined as σ(CH 3 O 2 )=4.14×10 −18 cm 2 molecule −1 , leading to a value of k obs =(4.8±0.5)×10 −13 cm 3 molecule −1 s −1 . In addition, the absorption spectrum of CH 3 O 2 was measured in the range 210-295 nm using diode array spectroscopy. A detailed review of all previous studies concerning the kinetics and spectrum of the CH 3 O 2 radical is presented, and a recommended spectrum, representing an average from selected recent studies, is proposed

Initiation in hydrocarbon autoxidation at elevated temperatures

Abstract: Kinetic and mechanistic investigations of liquid phase autoxidation of hexadecane at 120 to 190°C have shown that, in the early stages of oxidation, the initiation process involves homolytic decomposition of hydroperoxides. The values of a composite first order rate constant for this decomposition, k 1 , have been determined in stirred flow reactor experiments from the rate of formation of termination products and in batch reactor studies using inhibitor methods. Arrhenius parameters derived from these studies (log (A/s −1 )=8.5 ane E a =26 kcal mol −1 ) allow calculation of k 1 as a function of temperature. This permits determination of the rate of initiation as a function of hydroperoxide concentration and facilitates determination of absolute rate constants for other oxidation reactions occurring in this system

A kinetic study of the autocatalytic permanganate oxidation of formic acid

Abstract: The kinetics of the permanganate oxidation of formic acid in aqueous perchloric acid has been studied. The results indicate that this reaction is autocatalyzed by both manganese(II) ion (formed as a reaction product) and colloidal manganese dioxide (formed as an intermediate). The apparent rate constants corresponding to the noncatalytic and autocatalytic reaction pathways are given, respectively, by the following equations The activation energies associated with the true rate constants, , , , , , and are 37.2, 62.5, 70.9, 52.5, 40.8, and 59.9 kJ mol−1, respectively. The percentage of the total reaction corresponding to each pathway is given for typical experimental conditions. Mechanisms in agreement with the kinetic data are proposed for the six different reaction pathways observed.

Influence of the side chain on the stability of Schiff‐bases formed between pyridoxal 5′‐phosphate and amino acids

Abstract: The stability of some Schiff-bases formed between PLP and different amino acids has been investigated in a wide range of pH. The kinetic constants of formation of these compounds and their hydrolysis rate constants have been determined. Results show that the α-position of the carboxyl group of amino acid plays an important role on the mechanism of water attack upon the C=N―bond. the absence of ionic groups in the surrounding of that bond must be an important factor of stability. Bulky hydrophobic substituents in the amino acid, near the amine part, protect the imine bond against hydrolysis

Rate constants for the reactions of OH radicals and Cl atoms with diethyl sulfide, Di‐n‐propyl sulfide, and Di‐n‐butyl sulfide

Abstract: Rate constants for the reactions of OH radicals and Cl atoms with diethyl sulfide (DES), di-n-propyl sulfide (DPS), and di-n-butyl sulfide (DBS) have been determined at 295 ± 3 K and a total pressure of 1 atm. Hydroxyl radical rate data was obtained using the absolute technique of pulse radiolysis combined with kinetic spectroscopy. The chlorine atom rate constants were measured using a conventional photolytic relative rate method. The rate constant for the reaction of Cl atoms with dimethyl sulfide (DMS) was also determined. The following rate constants were obtained : k(OH+DES)=(11.6±2)×10 −12 cm 3 molecule −1 s −1 ; k(OH+DPS)=(21.5±3)×10 −12 cm 3 molecule −1 s −1 ; k(OH+DBS)=(37.4±5)×10 −12 cm 3 molecule −1 s −1 ; k(Cl+DMS)=(32.2±3)×10 −11 cm 3 molecule −1 s −1 ; k(Cl+DES)=(44.1±4)×10 −11 cm 3 molecule −1 s −1 ; k(Cl+DPS)=(51.8±4)×10 −11 cm 3 molecule −1 s −1 ; k(Cl+DBS)=(64.6±2)×10 −11 cm 3 molecule −1 s −1

Kinetics of the reactions of Cl(2PJ) and Br(2P3/2) with O3

Abstract: A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the important stratospheric reactions Cl(2PJ) + O3 ClO + O2 and Br(2P3/2) + O3 BrO + O2 as a function of temperature. The temperature dependence observed for the Cl(2PJ) + O3 reaction is nonArrhenius, but can be adequately described by the following two Arrhenius expressions (units are cm3 molecule−1 s−1, errors are 2σ and represent precision only): 1(T) = (1.19 ± 0.21) × 10−11 exp [(−33 ± 37)/T] for T = 189–269K and 1(T) = (2.49 ± 0.38) × 10−11 exp[(−233 ± 46)/T] for T = 269–385 K. At temperatures below 230 K, the rate coefficients determined in this study are faster than any reported previously. Incorporation of our values for 1(T) into stratospheric models would increase calculated ClO levels and decrease calculated HCl levels; hence the calculated efficiency of ClOx catalyzed ozone destruction would increase. The temperature dependence observed for the (2P3/2) + O3 reaction is adequately described by the following Arrhenius expression (units are cm3 molecule−1 s−1, errors are 2σ and represent precision only): 2(T) = (1.50 ± 0.16) × 10−1 exp[(−775 ± 30)/T] for T = 195–392 K. While not in quantitative agreement with Arrhenius parameters reported in most previous studies, our results almost exactly reproduce the average of all earlier studies and, therefore, will not affect the choice of 2(T) for use in modeling stratospheric BrOx chemistry.