Ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the peroxy radicals arising from the OH-initiated oxidation of isoprene. Geometry optimizations of the OH-O 2 -isoprene peroxy radicals were performed using density functional theory at the B3LYP/6-31G** level, and individual energies were computed using second-order Møller-Plesset perturbation theory (MP2) and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G* level of theory the zero-point-corrected OH-O 2 -isoprene adduct radical energies are 47-53 kcal mol -1 more stable than the separated OH, O 2 , and isoprene reactants. In addition, we find no evidence for an energetic barrier to O 2 addition and have calculated rate constants for the O 2 addition step using canonical variational transition state theory (CVTST) based on Morse potentials to describe the reaction coordinate. These results provide the isomeric branching between the six isoprene-OH-O 2 adduct radicals.

The kinetics of the isoprene−OH reaction have been studied both experimentally and computationally. Experimental rate constants at pressures in the range 0.5−20 Torr have been determined at 295 K using pulsed photolysis/laser-induced fluorescence detection of the OH radical. A rate constant of (0.99 ± 0.05) × 10-10 molecules-1 cm3 s-1 at 20 Torr in argon was determined, which is consistent with previous results for the high-pressure limiting rate constant. We present the first experimental observation of the falloff region for this reaction and have modeled the pressure dependence of the rates using the Troe formalism. Canonical variational transition state theory calculations were performed on the basis of recent ab initio calculations to determine the relative branching among the four possible isoprene−OH adducts in the high-pressure limit. We find OH addition to the outer carbon positions dominates OH addition to the inner carbon positions. We have employed RRKM/master equation calculations to evaluate the pressure dependence of the overall rate and the rates for the individual isomers in the pressure range 0.25−1000 Torr. The excellent agreement between the calculated and experimental falloff behavior provides an independent test of the ab initio energetics and RRKM/ME treatment. The results shed light on the mechanisms for oxidation of isoprene in the troposphere.

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The photodissociation of CHBr3 at 193 run has been investigated using photo fragment translational spectroscopy with VUV ionization detection. The only primary process observed was the loss of bromine atom. The translational energy distribution for this channel suggests a direct dissociation from an excited electronic state, and the anisotropy parameter, fJ=O.O, is consistent with a transition dipole moment aligned perpendicular to C 3 v axis. The majority of nascent CHBr2 fragments undergo secondary dissociation via two competing channels. The elimination ofHBr and C-Br bond cleavage in CHBr 2 occur with comparable yields. We also provide ab initio calculations on the relevant photochemical species and RRKM estimates of the product branching ratios that are consistent with the experimental observations.

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The kinetics of the decomposition of 4-methyl-1-pentyl radicals have been studied from 927-1068 K at pressures of 1.78-2.44 bar using a single pulse shock tube with product analysis. The reactant radicals were formed from the thermal C-I bond fission of 1-iodo-4-methylpentane, and a radical inhibitor was used to prevent interference from bimolecular reactions. 4-Methyl-1-pentyl radicals undergo competing decomposition and isomerization reactions via beta-bond scission and 1, x-hydrogen migrations (x = 4, 5), respectively, to form short-chain radicals and alkenes. Major alkene products, in decreasing order of concentration, were propene, ethene, isobutene, and 1-pentene. The observed products are used to validate a RRKM/master equation (ME) chemical kinetics model of the pyrolysis. The presence of the branched methyl moiety has a significant impact on the observed reaction rates relative to analogous reaction rates in straight-chain radical systems. Systems that result in the formation of substituted radical or alkene products are found to be faster than reactions that form primary radical and alkene species. Pressure-dependent reaction rate constants from the RRKM/ME analysis are provided for all four H-transfer isomers at 500-1900 K and 0.1-1000 bar pressure for all of the decomposition and isomerization reactions in this system.

A computationally efficient method for calculating C-H and C-X (X ) F, Cl, and Br) bond dissociation energies in haloalkanes has been developed by determining correction factors to MP2/cc-pVtz energies. Corrections for basis set effects were determined by the difference in bond dissociation energies calculated at the MP2/cc-pVtz and MP2/cc-pV5z levels, and correlation effects were corrected by calculating the difference in energies at the MP2/cc-pVtz and CCSD(T)/cc-pVtz levels. Subsequent corrections for the spin-orbit energy of the atomic fragment and zero-point energy were applied to give a final bond dissociation energy. The correction factors were determined using CH 4 , CH 3 F, CH 3 Cl, and CH 3 Br and are found to yield bond dissociation energies in excellent agreement with experimental results. This correction may also be broadly applied to multihalogen compounds, as shown in calculations of the C-H and C-X bond dissociation energies of CH 2 X 2 and CHX 3 (X ) F, Cl, and Br) compounds, which accurately reproduce experimental values.

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