Rate coefficient isoprene

Other research in tire area of addition reactions onto unsaturated carbon-carbon bonds has included measurement of the rate coefficients for tire addition of NO3 to chloro- and trichloro-ethene,147 relative rate measurement for N03 addition to isoprene,148 TF-/(SR-measured muonium addition to vinyl aromatics12 and EPR studied addition of radical (65) to alkenes.150 In this latter study a linear dependence of the rate constant of addition with tire donor/acceptor properties of the alkene partner was highlighted. 

For example, the six first-order rate coefficients listed in Appendix 2 can be divided naturally into 3 groups according to the initial concentration of isoprene. 

To prove these assertions, the rate of disappearance of isoprene had to be shown to be first order with respect to isoprene at a given active centre concentration. For this purpose, the concentration of isoprene was measured dilatometrically at a series of times following its addition to a solution of active centres a series of such experiments were carried out, some to test the reproducibility of the data and some at different initial concentrations of isoprene. These data were analysed on the basis of eqn. (36), the rate coefficients being obtained from the particular form of the general rate expression... 

The calculations set out in this table show that the six rate coefficients listed in Table 3 determined at three different initial concentrations of isoprene constitute an homogeneous set... 

An objective of LACTOZ has been to provide accurate rate coefficient data for OH attack on the more complex, higher molecular weight VOCs. As a result of the studies there is now a much improved data set for aromatic compounds, alkenes, including isoprene and other biogenic hydrocarbons. Kinetic data have also been obtained for some oxygenated compounds and nitrates for the first time. 

Isoprene and the terpenes may be attacked by OH, NO3 and O3. The rate coefficients at room temperature for reaction with isoprene are of the order of 10 , 10 and 10 cm molecule" s , respectively. In the case of attack by OH, and in the presence of oxygen, a peroxy-hydroxy radical is formed up to six isomers may be produced. Product studies indicate that the initial addition is to one of the terminal carbon atoms. The peroxy-hydroxy radical may then either react with NO or with other RO2 radicals (including HO2) to form a variety of products, as indicated in Fig. 11. Under conditions where the peroxy-hydroxy radical reacts exclusively with NO, approximately 50 % of the carbon balance is accounted for by three main products methacrolein, methyl vinyl ketone and formaldehyde. Other carbonyl products and hydroxy-nitrates are thought to make up the carbon balance although there is presently no clear indication of the exact identity of these compounds or their yields. 

The products of the reaction with OH, especially in the absence of NOx, and of the reaction with O3 have been investigated. In addition rate coefficients for the reactions with OH and O3 were measured for some recently identified products of the isoprene oxidation. The concentrations of the reactants were in the low ppm range (usually 1-10 ppm). OH radicals were generated either by continuous UV photolysis of H2O2 (254 nm) with or without NO present, or by photolysis of CH3ONO. 

Rate coefficients for the reactions of OH and O3 with 2- and 3-methyl-3-butene-1,2-diol and l,2-epoxy-3-methyl-3-butene, the latter having been reported very recently as product from isoprene + O3 [11], were determined by use of a relative rate technique. In the O3 experiments sufficient CO was added to scavenge > 90 % of OH radicals eventually being formed. For the O3 reaction of the diols absolute measurements were carried out in addition by monitoring the decay of O3 under pseudo-first order conditions. Absolute and relative rate data were in good agreement. The average values at 295 2 K are ... 

Product distributions resulting from the OH radical induced oxidation of the following hydrocarbons have been determined 2-methyl-propane, 2,3-dimethyl-butane, 2-methyl-butane, n-pentane, cyclohexane, methySubstituted 1-butenes, isoprene, toluene. Whenever possible, branching ratios for the self-reactions of alkylperoxy radicals and decomposition rate coefficients for alkoxyl radicals were derived. 

Percent product distribution Methylvinylketone (MVK) 49.0 1.1, metha-croleine (MAC) 44.3 1.6, 3-methyl-fiiran (MFU) 6.7 0.6 for isoprene mole fractions 400 ppm. The ratio of MVK to MAC under these conditions was 1.1 0.6. In the presence of NO the ratio is close to 1.4 [4]. Computer simulations based on our results for methylsubstituted 1-butenes showed that the observed [MVK]/[MAC] ratios are obtained with essentially equal probabilities for OH attack at the two double bonds of isoprene. This result contradicts predictions based on OH reaction rate coefficients for various methylsubstituted butenes [5]. Similar to results obtained for the 1-butenes, diols and hydroxy carbonyl compounds were found in small yields among the products. These are not included in the above product distribution because they are difficult to quantify. ... 

There are some requirements for monomers used for the emulsion polymerization. The primary requirement for monomers is that they must have a limited water solubility and be soluble in the formed polymer. However, the solubility in water should not be too high, otherwise this monomer would tend to polymerize in the water phase. In the mechanism for emulsion polymerization one of the driving forces is the absorption of monomer into the polymeric particles if the monomer and polymer are not mutually soluble then this process will not be efficient. Many different vinyl monomers are currently used in practical emulsion polymerization, including acrylates, methacrylates, St, AN (in copolymers), VAc, isoprene, and 1,3-butadiene. In addition, the monomers would not react with water, surfactants and other additives. Table 11.6 presents the propagation rate coefficients of various monomers examined in emulsion polymerization. 

NO [NO]+)fcH02 [H02]+ ro2 [RO2] (where k, is the rate coefficient for the reaction of ISOPO2 isomers with species X). In comparison, the proposed ISOPO2 isomerisation processes to produce the HPALD and HO2 were expected to occur with it > 1 s for the ISOPO2 isomer produced by 1-OH addition to isoprene and it > 8 s for the ISOPO2 isomer produced following 4-OH addition. 

Examples of the use of activation volume for the reactions in supercritical fluids is the unimolecular decomposition of a-chlorobenzyl methyl ether in 1,1-difluoroethane Jc = 113.4°C), studied by Johnston and coworkers [54] and the Diels-Alder reaction of isoprene and maleic anhydride in supercritical carbon dioxide [56], described in section 3.1.2.1. In the latter study the large variation in the rate coefficient at 35°C and near-critical pressures was quantified as AV = - 1.39 X lO cm moF in the highly compressible region, as compared with - 38.4 cm mol at 200 bar. Paulaitis and Alexander interpreted their results as a solvent effect stemming from an induced quadrupole moment in the carbon dioxide molecule. 

The atmospheric lifetime of peroxymethacrilic nitric anhydride with respect to the removal by reaction with OH radicals is calculated to be around 4 h using [OH] = 2.5 x 10 molecule cm and (OH + MPAN) = 2.9 x 10 cm molecule" s Orlando et al. (2002) have suggested that the reaction with OH could be more rapid than its thermal decomposition in specific conditions. The rate coefficient for the thermal decomposition of MPAN is given in table VIII-M-4. The shorter lifetime of MPAN compared to PAN or PPN will restrict its presence to regions near its source (i.e., in isoprene rich regions) hence its capacity to transport reactive nitrogen will be limited compared to that of PAN and PPN (Orlando et al., 2002). 

The Mainz isoprene mechanism (Poschl et al., 2000) was based on and developed by intercomparison with the MCM, but it is highly condensed with only 16 species and 44 reactions. It was designed to provide a compact representation of the key isoprene chemistry in global models which incorporate explicit O3, NO , , HO , , CO, and CH4 chemistry. Table X-B-3 shows the MIM species included and the compounds they represent. Note that some important species formed in isoprene oxidation, such as CH2O and CH3OOH, are not included as separate species, because they are already included in the CH4 chemistry they appear in the 44 reaction mechanism of MIM as products only. Considerable simplification is introduced by including only MACR its rate coefficients with OH and O3 are taken as the arithmetic mean of those for methacrolein and methylvinylketone. 

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