Experimental and Theoretical Study of Reaction of OH with 1,3-Butadiene

Kinetic and reactivity of gas-phase reaction of acyclic dienes with hydroxyl radical in the 273-318 K temperature range

As dienes contain two C[double bond, length as m-dash]C bonds, theoretically, they are much more chemically reactive with hydroxyl radical (˙OH) than alkenes and alkanes, and the reaction with ˙OH is one of the main atmospheric degradation routes of dienes during the daytime. In our work, rate coefficients of three types of acyclic dienes: conjugated as 3-methyl-1,3-pentadiene (3M13PD), isolated as 1,4-hexadiene (14HD), and cumulated as 1,2-pentadiene (12PD) reaction with ˙OH were measured in the temperature range of 273-318 K and 1 atm using the relative rate method. At 298 ± 3 K, the rate coefficients for those reactions were determined to be k3M13PD+OH = (15.09 ± 0.72) × 10-11, k14HD+OH = (9.13 ± 0.62) × 10-11, k12PD+OH = (3.34 ± 0.40) × 10-11 (as units of cm3 per molecule per s), in the excellent agreement with values of previously reported. The first measured temperature dependence for 3M13PD, 14HD and 12PD reaction with ˙OH can be expressed by the following Arrhenius expressions in units of cm3 per molecule per s: k3M13PD+OH = (8.10 ± 2.23) × 10-11 exp[(173 ± 71)/T]; k14HD+OH = (9.82 ± 5.10) × 10-12 exp[(666 ± 123)/T]; k12PD+OH = (1.13 ± 0.87) × 10-12 exp[(1038 ± 167)/T] (as units of cm3 per molecule per s). The kinetic discussion revealed that the relative position between these two C[double bond, length as m-dash]C could significantly affect the reactivity of acyclic dienes toward ˙OH. A simple structure-activity relationship (SAR) method was proposed to estimate the reaction rate coefficients of acyclic dienes with ˙OH.

Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments

Hydroxyl radicals and olefins are quite important from a combustion and an atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in transportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions ( T = 294-468 K and p ∼ 53 mbar; T = 881-1348 K and p ∼ 1-2.5 bar). We obtained the low- T data in a flow reactor using laser flash photolysis and laser-induced fluorescence (LPFR/LIF), and the high- T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures-revealing that OH-addition channels prevail at low temperatures. Except for the 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behavior as a result of many possible chemical pathways in such a convoluted system.

Atmospheric oxidation of halogenated aromatics: comparative analysis of reaction mechanisms and reaction kinetics

Atmospheric transport is the major route for global distribution of semi-volatile compounds such as halogenated aromatics as well as their major exposure route for humans. Their major atmospheric removal process is oxidation by hydroxyl radicals. There is very little information on the reaction mechanism or reaction-path dynamics of atmospheric degradation of halogenated benzenes. Furthermore, the measured reaction rate constants are missing for the range of environmentally relevant temperatures, i.e. 230-330 K. A series of recent theoretical studies have provided those valuable missing information for fluorobenzene, chlorobenzene, hexafluorobenzene and hexachlorobenzene. Their comparative analysis has provided additional and more general insight into the mechanism of those important tropospheric degradation processes as well as into the mobility, transport and atmospheric fate of halogenated aromatic systems. It was demonstrated for the first time that the addition of hydroxyl radicals to monohalogenated as well as to perhalogenated benzenes proceeds indirectly, via a prereaction complex and its formation and dynamics have been characterized including the respective transition-state. However, in fluorobenzene and chlorobenzene reactions hydroxyl radical hydrogen is pointing approximately to the center of the aromatic ring while in the case of hexafluorobenzene and hexachlorobenzene, unexpectedly, the oxygen is directed towards the center of the aromatic ring. The reliable rate constants are now available for all environmentally relevant temperatures for the tropospheric oxidation of fluorobenzene, chlorobenzene, hexafluorobenzene and hexachlorobenzene while pentachlorophenol, a well-known organic micropollutant, seems to be a major stable product of tropospheric oxidation of hexachlorobenzene. Their calculated tropospheric lifetimes show that fluorobenzene and chlorobenzene are easily removed from the atmosphere and do not have long-range transport potential while hexafluorobenzene seems to be a potential POP chemical and hexachlorobenzene is clearly a typical persistent organic pollutant.

Atmospheric oxidation mechanism of OH-initiated reactions of diethyl ether – the fate of the 1-ethoxy ethoxy radical

The 1-ethoxy ethoxy radical resulting from the secondary peroxy chemistry in the oxidation of diethyl ether (DEE) by hydroxyl radical leads to the formation of ethyl formate in major quantities and ethyl acetate in minor quantities.

Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals with a Series of Methoxylated Aromatic Compounds

Rate coefficients for the reactions of hydroxyl radicals (OH) with a series of oxygenated aromatics (two methoxybenzene and five methoxyphenol isomers) have been obtained using the relative kinetic method in 1080 and 480 L photoreactors at the University of Wuppertal, Germany. The experiments were realized at 295 ± 2 K and 1 bar total pressure of synthetic air using in situ Fourier transform infrared spectroscopy for the chemical analysis. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) were determined: methoxybenzene (anisole), (2.08 ± 0.21) × 10(-11); 1-methoxy-2-methylbenzene, (4.56 ± 0.50) × 10(-11); 2-methoxyphenol (guaiacol), (5.40 ± 0.72) × 10(-11); 3-methoxyphenol, (6.93 ± 0.67) × 10(-11); 4-methoxyphenol, (5.66 ± 0.55) × 10(-11); 2-methoxy-4-methylphenol, (7.51 ± 0.68) × 10(-11); 2,3-dimethoxyphenol, (7.49 ± 0.81) × 10(-11); and 2,6-dimethoxyphenol (syringol), (8.10 ± 0.98) × 10(-11). The rate coefficients for the reactions of OH with 2,3-dimethoxyphenol and 1-methoxy-2-methylbenzene are first time measurements. The rate coefficients determined in this work are compared with previous determinations reported in the literature and also with the values estimated using a structure-activity relationship method. A comparison is performed between the OH rate coefficients obtained for methoxylated aromatics with those of other substituted aromatics in order to understand the influence of the type, number, and position of the different substituents on the reactivity of aromatics toward OH. In addition, a comparison is made between the OH and Cl rate coefficients for the compounds. The principal atmospheric sink of these methoxylated aromatic compounds during daytime is their reaction with OH radicals. The corresponding lifetimes for reaction with OH radicals and Cl atoms are 2-8 and 11-50 h, respectively.

Kinetics study of the reaction of OH radicals with C5-C8 cycloalkanes at 240-340 K using the relative rate/discharge flow/mass spectrometry technique

Rate constants of reactions of hydroxyl radical with cyclopentane (k1), cyclohexane (k2), cycloheptane (k3), and cyclooctane (k4) have been acquired at 240-340 K and a total pressure of about 1 Torr using the technique of relative rate combined with discharge flow and mass spectrometry (RR/DF/MS). At 298 K, the rate constants are determined using two reference compounds, which are averaged to be k1 = (4.81 ± 0.88) × 10(-12), k2 = (6.41 ± 0.85) × 10(-12), k3 = (10.30 ± 1.44) × 10(-12), and k4 = (1.42 ± 0.27) × 10(-11) cm(3) molecule(-1) s(-1). The Arrhenius expressions at 240-340 K for these reactions are determined to be k1(T) = (2.43 ± 0.50) × 10(-11)exp[-(481 ± 58)/T], k2(T) = (3.96 ± 0.60) × 10(-11)exp[-554 ± 42)/T], k3(T) = (5.74 ± 0.66) × 10(-11)exp[-527 ± 33)/T], and k4(T) = (1.12 ± 0.21) × 10(-10)exp[-626 ± 53)/T]. Using the kcycloalkane+OH(277 K) values measured in the present work, the atmospheric lifetime for cyclopentane, cyclohexane, cycloheptane, and cyclooctane is estimated to be about 78, 64, 38, and 29 h, respectively.

Mechanisms and reaction-path dynamics of hydroxyl radical reactions with aromatic hydrocarbons: the case of chlorobenzene

All geometries and energies significant for the first step of tropospheric degradation of chlorobenzene were characterized using the MP2/6-31+G(d,p) and G3 methods. A pre-reaction complex for the addition of OH radical to chlorobenzene was found and the associated transition state was determined for the first time. The reaction path for the association of OH radical and chlorobenzene into the pre-reaction complex was extrapolated from the selected low frequency normal mode of pre-reaction complex. The reaction rate constant for addition of OH radical to chlorobenzene was determined for the temperature range 230-330K, using RRKM theory and G3 energies. The calculated rate constants are in agreement with the experimental results. Regioselectivity was also determined for the title reaction from the ratio of respective reaction rates and our results are in very good agreement with the experimental results, which show the dominance of the ortho and para channels as well as a negligible contribution by the ipso channel.

Experimental and ab initio dynamical investigations of the kinetics and intramolecular energy transfer mechanisms for the OH + 1,3-butadiene reaction between 263 and 423 K at low pressure

The rate constants for the reaction of the OH radical with 1,3-butadiene and its deuterated isotopomer has been measured at 1-6 Torr total pressure over the temperature range of 263-423 K using the discharge flow system coupled with resonance fluorescence/laser-induced fluorescence detection of OH. The measured rate constants for the OH + 1,3-butadiene and OH + 1,3-butadiene- d 6 reactions at room temperature were found to be (6.98 +/- 0.28) x 10 (-11) and (6.94 +/- 0.38) x 10 (-11) cm (3) molecule (-1) s (-1), respectively, in good agreement with previous measurements at higher pressures. An Arrhenius expression for this reaction was determined to be k 1 (II)( T) = (7.23 +/- 1.2) x10 (-11)exp[(664 +/- 49)/ T] cm (3) molecule (-1) s (-1) at 263-423 K. The reaction was found to be independent of pressure between 1 and 6 Torr and over the temperature range of 262- 423 K, in contrast to previous results for the OH + isoprene reaction under similar conditions. To help interpret these results, ab initio molecular dynamics results are presented where the intramolecular energy redistribution is analyzed for the product adducts formed in the OH + isoprene and OH + butadiene reactions.

Kinetics study of OH radical reactions with n-octane, n-nonane, and n-decane at 240-340 K using the relative rate/discharge flow/mass spectrometry technique

The kinetics of the reactions of hydroxyl radical with n-octane (k1), n-nonane (k2), and n-decane (k3) at 240-340 K and a total pressure of approximately 1 Torr has been studied using relative rate combined with discharge flow and mass spectrometer (RR/DF/MS) technique. The rate constant for these reactions was found to be positively dependent on temperature, with an Arrhenius expression of k1 = (2.27 +/- 0.21) x 10(-11)exp[(-296 +/- 27)/T], k2 = (4.35 +/- 0.49) x 10(-11)exp[(-411 +/- 32)/T], and k3 = (2.26 +/- 0.28) x 10(-11)exp[(-160 +/- 36)/T] cm3 molecule(-1) s(-1) (uncertainties taken as 2sigma), respectively. Our results are in good agreement with previous studies at and above room temperature using different techniques. Assuming that the reaction of alkane with hydroxyl radical is the predominant form for loss of these alkanes in the troposphere, the atmospheric lifetime for n-octane, n-nonane, and n-decane is estimated to be about 43, 35, and 28 h, respectively.

On the identity and reactivity patterns of the “second oxidant” of the T252A mutant of cytochrome P450cam in the oxidation of 5-methylenenylcamphor

Density functional calculations show that in the absence of Compound I, the primary oxidant species of P450, the precursor species, Compound 0 (FeOOH), can effect double bond activation of 5-methylenylcamphor (1). The mechanism is initiated by homolytic cleavage of the O-O bond and formation of an OH radical bound to the Compound II species by hydrogen bonding interactions. Subsequently, the so-formed OH radical can either activate the double bond of 1 or attack the meso position of the heme en route to heme degradation. The calculations show that double bond activation is preferred over attack on the heme. Past the double bond activation, the intermediate can either lead to epoxidation or to a glycol formation. The glycol formation is predicted to be preferred, although in the P450(cam) pocket the competition may be closer. Therefore, in the absence of Compound I, Compound 0 will be capable of epoxidizing double bonds. Previous studies [E. Derat, D. Kumar, H. Hirao, S. Shaik, J. Am. Chem. Soc. 128 (2006) 473-484] showed that in the case of a substrate that can undergo only C-H activation, the bound OH prefers heme degradation over hydrogen abstraction. Since the epoxidation barrier for Compound I is much smaller than that of Compound 0 (12.8 vs. 18.9kcal/mol), when Compound I is present in the cycle, Compound 0 will be silent. As such, our mechanism explains lucidly why T252A P450(cam) can epoxidize olefins like 5-methylenylcamphor but is ineffective in camphor hydroxylation [S. Jin, T.M. Makris, T. A. Bryson, S.G. Sligar, J.H. Dawson, J. Am. Chem. Soc. 125 (2003) 3406-3407]. Our calculations show that the glycol formation is a marker reaction of Compound 0 with 5-methylenylcamphor. If this product can be found in T252A P450(cam) or in similar mutants of other P450 isozymes, this will constitute a more definitive proof for the action of Cpd 0 in P450 enzymes.