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With MgH^+ ions as a test case, we investigate to what extent the rotational motion of smaller polar molecular ions sympathetically cooled into Coulomb crystals in linear Paul traps couples to the translational motions of the ion ensemble. By comparing results obtained from rotational state-selective resonance-enhanced photo-dissociation experiments with data from theoretical simulations, we conclude that the effective rotational temperature exceeds the translational temperature (< 100 mK) by more than two orders of magnitude, indicating a very weak coupling. In fact, the experimental results are consistent with the rotational temperature being in thermal equilibrium with the surrounding environment at ~300 K.
We have used different computational methods, including B3LYP, CCSD(T)-F12 and CBS-QB3, to study and compare the addition–elimination reaction of the nitrate radical NO₃ with four sulfur-containing species relevant to atmospheric chemistry: hydrogen sulfide (H₂S), dimethyl sulfide [(CH₃)₂S], dimethyl sulfoxide [(CH3)₂SO] and sulfur dioxide (SO₂). We find that the reaction with (CH₃)₂SO to give NO₂ + (CH₃)₂SO₂ has a very low barrier, and is likely to be the dominant oxidation mechanism for (CH₃)₂SO in the atmosphere. In agreement with previous experimental data and computational results, we find that the reaction with H₂S and SO₂ is very slow, and the reaction with (CH₃)₂S is not competitive with the hydrogen abstraction route. The differences in reaction energetics and rates between the four species are explained in terms of stabilizin...
We have studied the oxidation of SO₂ to SO₃ by four peroxyradicals and two carbonyl oxides (Criegee intermediates) using both density functional theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. All the studied peroxyradicals react very slowly with SO₂ due to energy barriers (activation energies) of around 10 kcal/mol or more. We find that water molecules are not able to catalyze these reactions. The reaction of stabilized Criegee intermediates with SO₂ is predicted to be fast, as the transition states for these oxidation reactions are below the free reactants in energy. The atmospheric relevance of these reactions depends on the lifetimes of the Criegee intermediates, which, at present, is highly uncertain.
We have investigated the reaction of the one-carbon stabilized Criegee intermediate (H_2COO, formaldehyde oxide) with ozone, theoretically, using high level coupled cluster ab initio methods. Key to the reactivity of the Criegee intermediate with ozone is the strongly exothermic formation of an intermediate consisting of five oxygen and one carbon atoms (H_2CO_5) in a six-membered ring structure. This intermediate proceeds via a spin-allowed route over two transition states with low energy barriers to form molecular oxygen and formaldehyde. The reaction may contribute to the loss of these biradicals in the atmosphere.
We present a hypothesis that autoxidation (inter- and intramolecular hydrogen abstraction by peroxy radicals) plays an important role in the oxidation of organic compounds in the atmosphere, particularly organic matter associated with aerosol. In the laboratory, we determine the rate of this process at room temperature for a model system, 3-pentanone. We employ ab initio calculations to investigate H-shifts within a broader group of substituted organic compounds. We show that the rate of abstraction of hydrogen by peroxy radicals is largely determined by the thermochemistry of the nascent alkyl radicals and thus is highly influenced by neighboring substituents. As a result, autoxidation rates increase rapidly as oxygen-containing functional groups (carbonyl, hydroxy, and hydroperoxy) are added to organic compounds. This mechanism is co...
Long recognized as an important process in the degrdn. of unsatd. fats and oils, wine, and components of biol. organisms, and in low temp. combustion processes, the role of autoxidn. in the conversion of atm. chems. is only just beginning to be understood. Here, recent exptl. and theor. work on the rate limiting step of autoxidn., namely, the H-shift isomerization of peroxy radicals (RO_2 → QOOH), for several systems relevant to atm. chem. will be highlighted. Trends in the peroxy radical isomerization rates with functionalization will be discussed. The potential impact of autoxidn. toward the gas- and condensed-phase org. chem. of the atm. will be hypothesized, and directions for future work to test this hypothesis will be suggested.
We investigate the oxidation of methacryloylperoxy nitrate (MPAN) and methacrylicperoxy acid (MPAA) by the hydroxyl radical (OH) theoretically, using both density functional theory [B3LYP] and explicitly correlated coupled cluster theory [CCSD(T)-F12]. These two compounds are produced following the abstraction of a hydrogen atom from methacrolein (MACR) by the OH radical. We use a RRKM master equation analysis to estimate that the oxidation of MPAN leads to formation of hydroxymethyl–methyl-α-lactone (HMML) in high yield. HMML production follows a low potential energy path from both MPAN and MPAA following addition of OH (via elimination of the NO_3 and OH from MPAN and MPAA, respectively). We suggest that the subsequent heterogeneous phase chemistry of HMML may be the route to formation of 2-methylglyceric acid, a common component of ...
Peroxy radicals formed by addition of OH and O_2 to the olefinic carbon atoms in methacrolein react with NO to form methacrolein hydroxy nitrate and hydroxyacetone. We observe that the ratio of these two compounds, however, unexpectedly decreases as the lifetime of the peroxy radical increases. We propose that this results from an isomerization involving the 1,4-H-shift of the aldehydic hydrogen atom to the peroxy group. The inferred rate (0.5 ± 0.3 s^(–1) at T = 296 K) is consistent with estimates obtained from the potential energy surface determined by high level quantum calculations. The product, a hydroxy hydroperoxy carbonyl radical, decomposes rapidly, producing hydroxyacetone and re-forming OH. Simulations using a global chemical transport model suggest that most of the methacrolein hydroxy peroxy radicals formed in the atmosphe...
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