Isomerization of OH-Isoprene Adducts and Hydroxyalkoxy Isoprene Radicals

This paper employs quantum chemical methods to investigate gaps in our understanding of the fates of radical intermediates in the OH-initiated degradation of isoprene. We employ two density functional theory (DFT) approaches:  the well-known B3LYP functional and the recently constructed MPW1K functional. The Complete Basis Set method CBS-QB3 is used selectively to verify certain DFT results. The paper focuses on the configuration of the isoprene-OH adducts with the hydroxyl radical bound to carbons 1 or 4 of isoprene and the fate of the δ-hydroxyalkoxy radicals produced from these adducts. The chemically activated isoprene-OH adducts undergo prompt E/Z isomerization in competition with quenching. This reaction allows formation of the δ-hydroxyalkoxy radicals possessing the (Z) configuration, enabling a fast 1,5 H-shift reaction to dominate the fate of these radicals. The (E) isomer of the δ-hydroxyalkoxy radical that cannot undergo a 1,5 H-shift is predicted to react exclusively with O2. The (E) isomer of the δ-hydroxyalkoxy radical appears likely to undergo a 1,5 H-shift reaction, but that conclusion depends more sensitively than the other conclusions on the assumed rate of the O2 reaction. The effect of tunneling, which has been ignored in most previous calculations of the rate constants of 1,5 H-shift reactions, is estimated using an asymmetric Eckart potential

Reactions of the Alkoxy Radicals Formed Following OH-Addition to α-Pinene and β-Pinene. C−C Bond Scission Reactions

The atmospheric degradation pathways of the atmospherically important terpenes α-pinene and β-pinene are studied using density functional theory. We employ the correlation functional of Lee, Yang, and Parr and the three-parameter HF exchange functional of Becke (B3LYP) together with the 6-31G(d) basis set. The C−C bond scission reactions of the β-hydroxyalkoxy radicals that are formed after OH addition to α-pinene and β-pinene are investigated. Both of the alkoxy radicals formed from the α-pinene−OH adduct possess a single favored C−C scission pathway with an extremely low barrier (∼3 kcal/mol) leading to the formation of pinonaldehyde. Neither of these pathways produces formaldehyde, and preliminary computational results offer some support for suggestions that 1,5 or 1,6 H-shift (isomerization) reactions of alkoxy radicals contribute to formaldehyde production. In the case of the alkoxy radical formed following OH addition to the methylene group of β-pinene, there exists two C−C scission reactions with nearly identical barrier heights (∼7.5 kcal/mol); one leads to known products (nopinone and formaldehyde) but the ultimate products of the competing reaction are unknown. The single C−C scission pathway of the other alkoxy radical from β-pinene possesses a very low (∼4 kcal/mol) barrier. The kinetically favored C−C scission reactions of all four alkoxy radicals appear to be far faster than expected rates of reaction with O2. The rearrangement of the α-pinene−OH adduct, a key step in the proposed mechanism of formation of acetone from α-pinene, is determined to possess a barrier of 11.6 kcal/mol. This value is consistent with another computational result and is broadly consistent with the modest acetone yields observed in product yield studies

Intramolecular Hydrogen Bonding and Double H-Atom Transfer in Peroxy and Alkoxy Radicals from Isoprene

Quantum mechanical calculations were used to determine the structure and stability of second-generation peroxy and alkoxy radicals formed in the atmospheric degradation of isoprene (2-methyl-1,3-butadiene). Certain of these radicals exhibit a novel hydrogen bonding motif, consisting of two intramolecular hydrogen bonds. The hydrogen bonds are donated in series, with an enol group donating a hydrogen bond to a −CH2OH group, which donates in turn to the oxygen radical center. This hydrogen bonding motif opens a new reaction pathway:  the simultaneous transfer of two H-atoms across the hydrogen bonds with a barrier of only ∼5 kcal/mol in the alkoxy radicals, but ∼20 kcal/mol in the peroxy radicals. Rate constants for these reactions were calculated, and the effects of tunneling on the rate constant were examined. All species and reactions were analyzed at the B3LYP/6-311G(2df,2p) level of theory; the transition states for the double H-atom transfer reactions were also studied using the MPW1K functional and the CBS-QB3 method. Similar chemistry is possible for alkoxy and peroxy radicals derived from other volatile organic compounds of atmospheric interest

Prompt Chemistry of Alkenoxy Radical Products of the Double H-Atom Transfer of Alkoxy Radicals from Isoprene

Quantum mechanical calculations have previously shown that certain second-generation alkoxy radicals formed in the atmospheric degradation of isoprene (2-methyl-1,3-butadiene) will undergo a novel reaction pathway:  the simultaneous jumping of two H-atoms across the hydrogen bonds with a barrier of only ∼5 kcal/mol. The alkenoxy radical products of these double H-atom transfers are formed with ∼20−25 kcal/mol of energy, and may promptly decompose in competition with quenching to thermal energies. The fate of the energized alkenoxy radicals was determined under atmospheric and common laboratory conditions with use of stochastic Master equation analyses with relative energies determined at the B3LYP/6-311G(2df,2p) level of theory. The analyses accounted for the differing stabilities of, and interconversions between, conformers of the alkenoxy radicals with different numbers and arrangements of hydrogen bonds. Atmospheric implications of the double H-atom transfer are discussed

Effects of Olefin Group and Its Position on the Kinetics for Intramolecular H-Shift and HO₂ Elimination of Alkenyl Peroxy Radicals

Two classes of unimolecular reactions of peroxy radicals are key to autoignition, namely, intramolecular H-atom shift (which promotes autoignition) and concerted HO2 elimination (which inhibits autoignition). Olefin groups are prominent functional groups in biodiesel fuels. This paper explores the effects of the presence of an olefin group and its position on the kinetics of unimolecular reactions of peroxy radicals. CBS-QB3 calculations were carried out for 10 selected alkyl- and alkenylperoxy radicals. Transition-state theory was used to determine the temperature dependence of the high-pressure limiting rate constants, and Rice−Ramsperger−Kassel−Marcus/master equation simulations were performed to determine the pressure dependence of selected rate constants. Tunneling effects were computed using the asymmetric Eckart potential. The contribution of internal rotors to partition functions were included by using the hindered-rotor treatment

Quality Structures, Vibrational Frequencies, and Thermochemistry of the Products of Reaction of BrHg^• with NO₂, HO₂, ClO, BrO, and IO

Quantum chemical calculations have been carried out to investigate the structures, vibrational frequencies, and thermochemistry of the products of BrHg^• reactions with atmospherically abundant radicals Y^• (Y = NO₂, HO₂, ClO, BrO, or IO). The coupled cluster method with single and double excitations (CCSD), combined with relativistic effective core potentials, is used to determine the equilibrium geometries and harmonic vibrational frequencies of BrHgY species. The BrHg–Y bond energies are refined using CCSD with a noniterative estimate of the triple excitations (CCSD­(T)) combined with core–valence correlation consistent basis sets. We also assess the performances of various DFT methods for calculating molecular structures and vibrational frequencies of BrHgY species. We attempted to estimate spin–orbit coupling effects on bond energies computed by comparing results from standard and two-component spin–orbit density functional theory (DFT) but obtained unphysical results. The results of the present work will provide guidance for future studies of the halogen-initiated chemistry of mercury

Quantum Chemistry, Reaction Kinetics, and Tunneling Effects in the Reaction of Methoxy Radicals with O₂

The reaction of the methoxy radical with O₂ is the prototype for the reaction of a range of larger alkoxy radicals with O₂ in the lower atmosphere. This reaction presents major challenges to quantum chemistry, with CCSD­(T) overpredicting the barrier height by about 7 kcal/mol in the complete basis set limit. CCSD­(T) calculations also indicate that the CH₃OOO^• analog of the HOOO^• radical is energetically unstable with respect to CH₃O^• + O₂, a finding that seems unlikely. The previous successful prediction of the barrier height using CCSD­(T)/cc-pVTZ energies at CASSCF/6-311G­(d,p) geometries is shown to rely on the use of a metastable Hartree–Fock reference wave function. The performance of several density functionals is explored and B3LYP is selected to examine the role of tunneling, including the competition between small curvature tunneling (SCT) and large curvature tunneling (LCT). SCT is found to be sufficient to describe tunneling, in contrast to the typical findings for bimolecular hydrogen-abstraction reactions. The previously proposed mechanism of a cyclic transition state yields rate constants for CH₃O^• + O₂ that faithfully reproduces the experimentally derived Arrhenius pre-exponential term. Predictions of the branching ratios for the competing reactions CH₂DO^• + O₂ → CHDO + HO₂ (1a) and CH₂DO^• + O₂→ CH₂O + DO₂ (1b) are also in good agreement with experiment

Quantum Chemical Study of Autoignition of Methyl Butanoate

Methyl butanoate is a widely studied surrogate for fatty acid esters used in biodiesel fuel. Here we report a detailed analysis of the thermodynamics and kinetics of the autoignition chemistry of methyl butanoate. We employ composite CBS-QB3 calculations to construct the potential energy profiles of radicals derived from methyl butanoate. We compare our results with recently published G3MP2B3 results for reactions of peroxy (ROO^•) and hydroperoxy alkyl (^•QOOH) radicals and comment on differences in barrier heights and reaction enthalpies. Our emphasis, however, is on hydroperoxy alkylperoxy (^•OOQOOH) radicals that are critical for autoignition of diesel fuel. We examined four classes of reactions: peroxy radical interconversion of ^•OOQOOH (^•OOQOOH→ HOOQOO^•), H-migration reactions (from carbon to oxygen), HO₂ elimination, and cyclic ether formation with elimination of OH radical. We evaluate the significance of reaction pathways by comparing rate coefficients in the high-pressure limit. Unexpectedly, we find a low activation barriers for 1,8 H-migration of RC­(O)­OCH₂OO^•. We also find peroxy radical interconversion of ^•OOQOOH radicals from methyl butanoate commonly possess the lowest barriers of any unimolecular reaction of these radicals, despite that they proceed via 8-, 10- and 11-member ring transition states. At temperatures relevant to autoignition, these peroxy radical interconversions are dominant or significant reaction pathways. This means that some ^•OOQOOH radicals that were expected to be produced in negligible yields are, instead, major products in the autoignition of methylbutanoate (MB). These reactions have not previously been considered for MB, and will require revision of models of autoignition of methyl butanoate and other esters

Quantum Chemistry Guide to PTRMS Studies of As-Yet Undetected Products of the Bromine-Atom Initiated Oxidation of Gaseous Elemental Mercury

A series of BrHgY compounds (Y = NO₂, ClO, BrO, HOO, etc.) are expected to be formed in the Br-initiated oxidation of Hg(0) to Hg­(II) in the atmosphere. These BrHgY compounds have not yet been reported in any experiment. This article investigates the potential to use proton-transfer reaction mass spectrometry (PTRMS) to detect these atmospherically important species. We show that reaction of the standard PTRMS reagent (H₃O⁺) with BrHgY leads to stable parent (M + 1) ions, BrHgYH⁺, for most of these radicals, Y. Rate constants for the proton transfer reaction H₃O⁺ + BrHgY are computed using average dipole orientation theory. Calculations are also carried out on the commercially available compounds HgCl₂, HgBr₂, and HgI₂ to enable tests of the present work