Structures, Vibrational Frequencies, and Bond Energies of the BrHgOX and BrHgXO Species Formed in Atmospheric Mercury Depletion Events

Photochemistry during the polar spring leads to atmospheric mercury depletion events (AMDEs): Hg(0), which typically lives for months in the atmosphere, and can experience losses of more than 90% in less than a day. These dramatic losses are known to be initiated largely by Br + Hg + M → BrHg• + M, but the fate of BrHg• is a matter of guesswork. It is believed that BrHg• largely reacts with halogen oxides XO (X = Cl, Br, and I) to form BrHgOX compounds, but these species have never been studied experimentally. Here, we use quantum chemistry to characterize the structures, vibrational frequencies, and thermodynamics of these BrHgOX species and their BrHgXO isomers. The BrHgXO isomers have never previously been studied in experiments or computations. We find the BrHgOX species are 24–28 kcal/mol more stable than their BrHgXO isomers. When formed during polar AMDEs, BrHgBrO and BrHgIO appear sufficiently stable in that they will not dissociate before undergoing deposition, but BrHgClO is probably not that stable

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

Cis–Trans Isomerization of Chemically Activated 1-Methylallyl Radical and Fate of the Resulting 2-Buten-1-peroxy Radical

The cis–trans isomerization of chemically activated 1-methylallyl is investigated using RRKM/Master Equation methods for a range of pressures and temperatures. This system is a prototype for a large range of allylic radicals formed from highly exothermic (∼35 kcal/mol) OH + alkene reactions. Energies, vibrational frequencies, anharmonic constants, and the torsional potential of the methyl group are computed with density functional theory for both isomers and the transition state connecting them. Chemically activated radicals are found to undergo rapid cis–trans isomerization leading to stabilization of significant amounts of both isomers. In addition, the thermal rate constant for trans → cis isomerization of 1-methylallyl is computed to be high enough to dominate reaction with O₂ in 10 atm of air at 700 K, so models of the chemistry of the (more abundant and more commonly studied) <i>trans</i>-alkenes may need to be modified to include the cis isomers of the corresponding allylic radicals. Addition of molecular oxygen to 1-methylallyl radical can form 2-butene-1-peroxy radical (CH₃CHCHCH₂OO^•), and quantum chemistry is used to thoroughly explore the possible unimolecular reactions of the cis and trans isomers of this radical. The cis isomer of the 2-butene-1-peroxy radical has the lowest barrier (via 1,6 H-shift) to further reaction, but this barrier appears to be too high to compete with loss of O₂

Temperature-Dependent Branching Ratios of Deuterated Methoxy Radicals (CH₂DO•) Reacting With O₂

The methoxy radical is an intermediate in the atmospheric oxidation of methane, and the branching ratio (<i>k</i>_1a/<i>k</i>_1b) (CH₂DO• + O₂ → CHDO + HO₂ (1a) and CH₂DO• + O₂ → CH₂O + DO₂ (1b)) strongly influences the HD/H₂ ratio in the atmosphere, which is widely used to investigate the global cycling of molecular hydrogen. By using the FT-IR smog chamber technique, we measured the yields of CH₂O and CHDO from the reaction at 250–333 K. Kinetic modeling was used to confirm the suppression of secondary chemistry. The resulting branching ratios are well fit by an Arrhenius expression: ln­(<i>k</i>_1a/<i>k</i>_1b) = (416 ± 152)/<i>T</i> + (0.52 ± 0.53), which agrees with the room-temperature results reported in the only previous study. The present results will be used to test our theoretical understanding of the role of tunneling in the methoxy + O₂ reaction, which is the prototype for the entire class of alkoxy + O₂ reactions

Rate Constants and Kinetic Isotope Effects for Methoxy Radical Reacting with NO₂ and O₂

Relative rate studies were carried out to determine the temperature dependent rate constant ratio <i>k</i>₁/<i>k</i>_2a: CH₃O· + O₂ → HCHO + HO₂· and CH₃O· + NO₂ (+M) → CH₃ONO₂ (+M) over the temperature range 250–333 K in an environmental chamber at 700 Torr using Fourier transform infrared detection. Absolute rate constants <i>k</i>₂ were determined using laser flash photolysis/laser-induced fluorescence under the same conditions. The analogous experiments were carried out for the reactions of the perdeuterated methoxy radical (CD₃O·). Absolute rate constants <i>k</i>₂ were in excellent agreement with the recommendations of the JPL Data Evaluation panel. The combined data (i.e., <i>k</i>₁/<i>k</i>₂ and <i>k</i>₂) allow the determination of <i>k</i>₁ as 1.3_–0.5^+0.9 × 10^–14 exp[−(663 ± 144)/<i>T</i>] cm³ s^–1, corresponding to 1.4 × 10^–15 cm³ s^–1 at 298 K. The rate constant at 298 K is in excellent agreement with previous work, but the observed temperature dependence is less than was previously reported. The deuterium isotope effect, <i>k</i>_H/<i>k</i>_D, can be expressed in the Arrhenius form as <i>k</i>₁/<i>k</i>₃ = (1.7_–0.4^+0.5) exp((306 ± 70)/<i>T</i>). The deuterium isotope effect does not appear to be greatly influenced by tunneling, which is consistent with a previous theoretical work by Hu and Dibble. (Hu, H.; Dibble, T. S., <i>J. Phys. Chem. A</i> <b>2013</b>, <i>117</i>, 14230–14242.