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The ˙OH-initiated oxidation of methyl hydroperoxide was theoretically characterized using high-accuracy composite amHEAT-345(Q) coupled-cluster calculations followed by a two-dimensional E,J resolved master equation analysis.

The reaction of ground state methylidyne (CH) with water vapor (H2O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H–O bond of H2O over a submerged barrier via a well-skipping mechanism yielding solely H and CH2O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC—OH2) and its subsequent isomerization to activated CH2OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH2OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10−10 cm3 s−1 at 50 K, followed by a fairly steep decrease down to 8 × 10−12 cm3 s−1 at 900 K, but increasing again to 5 × 10−11 cm3 s−1 at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10−11 (T/300 K)−1.615 exp(−38.45/T) cm3 s−1 for T = 50–400 K k(T, P = 0) = 1.15 × 10−12 (T/300 K)0.8637 exp(892.6/T) cm3 s−1 for T = 400–1000 K k(T, P = 0) = 4.57 × 10−15 (T/300 K)3.375 exp(3477.4/T) cm3 s−1 for T = 1000–3500 K.

It was recently proposed (Franco et al., Nature 2021) that methanediol (MD, HOCH2OH ) formed by hydration of formaldehyde in liquid cloud droplets is outgassed to a larger extent than previously es...

In air-jet weaving looms, the main nozzle pulls the yarn from the prewinder by means of a high velocity air flow. The flexible yarn is excited by the flow and exhibits high amplitude oscillations. The motion of the yarn is important for the reliability and the attainable speed of the insertion. Fluid-structure interaction simulations calculate the interaction between the air flow and the yarn motion and could provide additional insight into yarn behavior. However, the use of an arbitrary Lagrangian–Eulerian approach for the deforming fluid domain around a flexible yarn typically results in severe mesh degradation, vastly reducing the accuracy of the calculations or limiting the physical time that can be simulated. In this research, the feasibility of using a Chimera technique to simulate the motion of a yarn interacting with the air flow from a main nozzle was investigated. This methodology combines a fixed background grid with a moving component grid deforming along with the yarn. The component grid is, however, not constrained by the boundaries of the flow domain allowing for large deformations with limited mesh degradation. Two separate cases were investigated. In the first case, the yarn was considered to be clamped at the main nozzle inlet. For the second case, the yarn was allowed to move axially as the main nozzle pulled it from a drum storage system.

The reaction of ground state methylidyne (CH) with water vapor (H2O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H-O bond of H2O over a submerged barrier via a well-skipping mechanism yielding solely H and CH2O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC-OH2) and its subsequent isomerization to activated CH2OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH2OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10-10 cm3 s-1 at 50 K, followed by a fairly steep decrease down to 8 × 10-12 cm3 s-1 at 900 K, but increasing again to 5 × 10-11 cm3 s-1 at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10-11 (T/300 K)-1.615 exp(-38.45/T) cm3 s-1 for T = 50-400 K k(T, P = 0) = 1.15 × 10-12 (T/300 K)0.8637 exp(892.6/T) cm3 s-1 for T = 400-1000 K k(T, P = 0) = 4.57 × 10-15 (T/300 K)3.375 exp(3477.4/T) cm3 s-1 for T = 1000-3500 K.

The main nozzle of an air jet loom plays an essential role in the weft insertion process. This role involves sucking the weft yarn from the prewinder and launching it into the reed. Simulating the dynamic behavior of the weft yarn inside the main nozzle involves fluid–structure interaction (FSI). In this work, one-way and two-way FSI simulations of air flow–yarn interaction inside a main nozzle have been performed. A three-dimensional model of the flexible weft yarn, consisting of a chain of line segments, and a two-dimensional axisymmetric model of the supersonic flow have been developed and coupled to perform these simulations. The results of the simulations are compared quantitatively and qualitatively with experimental results. Good agreement has been found between the results of the two-way FSI simulations and the experiment. The coupled fluid and structure models provide an effective numerical tool to optimize the geometry of the main nozzle based on the calculated motion and speed of the weft yarn.

A new chemical mechanism for the oxidation of biogenic volatile organic compounds (BVOCs) is presented and implemented in the Model of Atmospheric composition at Global and Regional scales using Inversion Techniques for Trace gas Emissions (MAGRITTE v1.1). With a total of 105 organic species and over 265 gas-phase reactions, 69 photodissociations, and 7 heterogeneous reactions, the mechanism treats the chemical degradation of isoprene – its main focus – as well as acetaldehyde, acetone, methylbutenol, and the family of monoterpenes. Regarding isoprene, the mechanism incorporates a state-of-the-art representation of its oxidation scheme accounting for all major advances put forward in recent theoretical and laboratory studies. The recycling of OH radicals in isoprene oxidation through the isomerization of Z-δ-hydroxyperoxy radicals is found to enhance OH concentrations by up to 40 % over western Amazonia in the boundary layer and by 10 %–15 % over the southeastern US and Siberia in July. The model and its chemical mechanism are evaluated against the suite of chemical measurements from the SEAC4RS (Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys) airborne campaign, demonstrating a good overall agreement for major isoprene oxidation products, although the aerosol hydrolysis of tertiary and non-tertiary nitrates remain poorly constrained. The comparisons for methylnitrate indicate a very low nitrate yield (<3×10-4) in the CH3O2+NO reaction. The oxidation of isoprene, acetone, and acetaldehyde by OH is shown to be a substantial source of enols and keto-enols, primarily through the photolysis of multifunctional carbonyls generated in their oxidation schemes. Oxidation of those enols by OH radicals constitutes a sizable source of carboxylic acids estimated at 9 Tg (HC(O)OH) yr−1 and 11 Tg(CH3C(O)OH) yr−1 or ∼20 % of their global identified source. The ozonolysis of alkenes is found to be a smaller source of HC(O)OH (6 Tg HC(O)OH yr−1) than previously estimated, due to several factors including the strong deposition sink of hydroxymethyl hydroperoxide (HMHP).

In this research, a fluid-structure interaction (FSI) framework was established to estimate the velocity of a yarn as it is propelled by the main nozzle. To allow the methodology to be used in an optimization context, the computational time was limited as much as possible. The methodology was first validated on polymer coated yarns to avoid any influence of yarn hairiness. Results from the calculations were compared to experiments and adequate agreement was found without tuning. Subsequently, an extension to hairy yarns was made by representing the hairiness as a wall roughness. The roughness height was determined by matching the simulated to the experimental velocity for a single case. The approach was validated by applying the obtained roughness height to different setups and comparing the simulations to the corresponding experiments. Taking into account some limitations, the methodology can be applied for optimization purposes using either smooth or hairy yarns.

A new module for calculating the dry deposition of trace gases is presented and implemented in the Model of Atmospheric composition at Global and Regional scales using Inversion Techniques for Trace gas Emissions (MAGRITTE v1.0). The dry deposition velocities are calculated using Wesely's classical resistance-in-series approach. While relying on analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) for meteorological fields, the aerodynamic resistance calculation module is based on the ECMWF model equations for turbulent transfer within the surface layer. The stomatal resistance for water vapour is calculated using a Jarvis-type parameterization in a multi-layer canopy environment model accounting for the leaf area index (LAI). The gas-phase diffusion coefficients needed to relate the stomatal resistances of different species are calculated from molecular structure. The cuticular, mesophyll and soil resistances depend on the species reactivity and Henry's Law constant (HLC). The HLCs of organic species for which no experimental data is available are estimated using a newly-developed prediction method based on existing methods for vapour pressures (EVAPORATION, Estimation of VApour Pressure of Organics) and infinite dilution activity coefficients (AIOMFAC, Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients). Acknowledging the dominance of stomatal uptake for ozone dry deposition, the stomatal resistance model parameters for 6 of the 7 major plant functional types (PFT) are adjusted based on extensive model comparisons with field measurements of ozone deposition velocity at 24 sites worldwide. The modelled OVOC deposition velocities for 25 different OVOCs are evaluated against field data from a total of 20 studies. The comparison shows the need for a species-dependent adjustment of the canopy resistances in order to match the observed variability among different species. This is realized by multiplying the HLC of each OVOC by a species-dependent parameter f1 adjusted based on the comparisons. The values of f1 span a wide range, from values of the order of unity or less for formaldehyde and several trifunctional compounds, to > 104 for compounds seen to deposit rapidly despite their low water-solubility, like MVK, MACR, CH3CHO and PAN. Despite the acknowledged caveats of the approach, the resulting modelled deposition velocities are consistent with the existing experimental data. The results of global-scale MAGRITTE model simulations demonstrate the importance of OVOC dry deposition on their global abundance. It is found to remove from the atmosphere the equivalent of 27 % of the global NMVOC emissions on a carbon basis, as well as about 8 % of NOx emissions in the form of organic nitrates and PAN-like compounds.

Topics in Catalysis, 50 (1-4), ISSN:1022-5528, ISSN:1572-9028, Recent advances in selective oxidation catalysis

In this work, we theoretically elucidated the mechanism and predicted the major products of the photolysis of alpha-hydroperoxycarbonyls, known to be products of the atmospheric oxidation of biogenic volatile organic compounds (BVOC) and components of secondary organic aerosol (SOA) in rural and remote areas. Using 2-hydroperoxypropanal OCHCH(OOH)CH3 as a model compound, we show that the likely major photolysis mechanism is a fast 1,5 H-shift in the initially excited singlet S1 state followed by spontaneous elimination of singlet oxygen to yield an enol HOCH=CHCH3, while intersystem crossing (ISC) to the triplet T1 state and C–C scission into HCO + HOOCHCH3 followed by expulsion of a hydroxyl radical from the unstable HOOCHCH3 is another product channel. The direct S1 reaction was found to occur at such a high rate that the quantum yield in atmospheric conditions is expected to approach unity. In the atmosphere, the enol should generally react with OH radicals or tautomerize into the more stable carbonyl O=CH–CH2CH3. Vinylalcohol is shown to be a major end product of the photolysis of hydroperoxyacetaldehyde, an isoprene oxidation product. Taking into account also the important enhancement of the absorption cross sections over those of the constituent monofunctional compounds as observed for the analogous beta-ketohydroperoxides, (F. Jorand et al., J. Photochem. Photobiol. A: Chem., 2000, 134, 119–125) the atmospheric photolysis rate of alpha-hydroperoxycarbonyls was estimated to be in the range of (1 to 5) 10-4 s-1, generally faster than the rate of their OH reactions. ispartof: Physical Chemistry Chemical Physics vol:20 issue:10 pages:6970-6979 ispartof: location:England status: published

Unusually rapid (E) ⇌ (Z) isomerization mechanisms are proposed and theoretically quantified for substituted allyloxy radicals, R'RC═CH-CH2O(•), with R and R' alkyl or oxygenated substituents, termed below β,γ-enoxy radicals. These conversions are shown to occur by a sequence of (i) ring closure to nearly isoergic oxiranyl-C(•)RR' radicals, (ii) internal rotation of the oxiranyl-moiety over 180°, and (iii) oxiranyl-ring reopening to yield the (E) ⇌ (Z)-isomerized oxy radicals. The barriers for all three steps were computed at the CCSD(T)/aug-cc-pVTZ//QCISD/6-311(d,p) level of theory to be only ≈5 ± 2 kcal mol(-1), and the rate constants at 298 K for the overall reactions were evaluated using transition-state theory to be in the range of 10(8)-10(9) s(-1). Specifically, and of relevance to the isoprene oxidation mechanism, it is predicted that the (E)-δ-hydroxy-isoprenyloxy radicals resulting from isoprene oxidation at high NO levels should isomerize to their (Z)-analogues at a rate of about 1.5 × 10(9) s(-1), much faster than the competing 1,5-H shift that was proposed earlier as the major fate of these (E)-oxy radicals ( Dibble, T. S. J. Phys. Chem. A 2002, 106, 6643-6650 ). It is concluded that under high-NO conditions the (E)- and (Z)-δ-hydroxy-isoprenylperoxy precursors should yield identical and therefore indistinguishable C5-hydroxycarbonyls as main products.

In this work we identified and theoretically quantified two photolysis mechanisms of HPALDs (hydroperoxy aldehydes) that result from the isomerization of peroxy radicals in the atmospheric oxidation of isoprene at low/moderate NOx. As a first photolysis mechanism, we show that a fraction of the initially excited S1-state HPALDs isomerizes by a near-barrierless 1,5 H-shift at a rate approaching 10^12 s-1 – competing with the equally fast intersystem crossing to the T2 triplet state – forming an unstable biradical that spontaneously expels an OH (hydroxyl) radical. A second mechanism is shown to proceed through the activated T2 triplet biradical – formed from S1 – undergoing a concerted ring-closure and OH-expulsion, yielding an oxiranyl-type co-product radical that quickly ring-opens to enoxy radicals. In both mechanisms, subsequent chemistry of the co-product radicals yields additional first-generation OH. The combined HPALD-photolysis quantum yield by these two mechanisms – which may not be the only photolysis routes – is estimated at 0.55 and the quantum yield of OH generation at 0.9, in fair accordance with experimental data on an HPALD proxy (Wolfe et al., Phys. Chem. Chem. Phys., 2012, 14, 7276–7286). crosscheck: This document is CrossCheck deposited related_data: Supplementary Information identifier: Jozef Peeters (ResearcherID) copyright_licence: The PCCP Owner Societies have an exclusive publication licence for this journal history: Received 13 January 2017; Accepted 3 March 2017; Accepted Manuscript published 3 March 2017; Advance Article published 20 March 2017; Version of Record published 29 March 2017 ispartof: Physical Chemistry Chemical Physics vol:19 issue:13 pages:9096-9106 ispartof: location:England status: published

The Leuven Isoprene Mechanism, proposed earlier to aid in rationalizing the unexpectedly high hydroxyl radical (OH) concentrations in isoprene-rich, low-nitric-oxide (NO) regions (Peeters et al. Phys. Chem. Chem. Phys. 2009, 11, 5935), is presented in an upgraded and extended version, LIM1. The kinetics of the crucial reactions in the proposed isoprene-peroxy radical interconversion and isomerisation pathways are re-evaluated theoretically, based on energy barriers computed at the much higher CCSD(T)/aug-cc-pVTZ//QCISD/6-311G(d,p) level of theory, and using multi-conformer partition functions obtained at the M06-2X/6-311++G(3df,2p) level that, different from the B3LYP level used in our earlier work, accounts for the crucial London dispersion effects in the H-bonded systems involved. The steady-state fraction of the specific Z-δ-OH-peroxy radical isomers/conformers that can isomerise by 1,6-H shift is shown to be largely governed by hydrogen-bond strengths, while their isomerisation itself is found to occur quasi-exclusively by hydrogen atom tunneling. The isomer-specific Z-δ-OH-peroxy 1,6-H shift rate coefficients are predicted to be of the order of 1 s-1 at 298 K, but the experimentally accessible bulk rate coefficients, which have to be clearly distinguished from the former, are two orders of magnitude lower due to the very low Z-δ-OH-peroxy steady-state fractions that are only around or below 0.01 at low to moderate NO and depend on the peroxy lifetime. Two pathways subsequent to the peroxy radical 1,6-H shift are identified, the earlier predicted route yielding the photolabile hydroperoxy-methyl-butenals (HPALDs), and a second, about equally important path, to di-hydroperoxy-carbonyl peroxy radicals (di-HPCARP). Taking this into account, our predicted bulk peroxy isomerisation rate coefficients are about a factor 1.8 higher than the available experimental results for HPALD production (Crounse et al. Phys. Chem. Chem. Phys. 2011, 13, 13607), which is within the respective uncertainty margins. We also show that the experimental temperature dependence of the HPALD production rates as well as the observed kinetic isotope effect for per-deuterated isoprene support quantitatively our theoretical peroxy interconversion rates. Global modeling implementing LIM1 indicates that on average about 28% of the isoprene peroxys react via the 1,6-H shift isomerisation route, representing 100 - 150 Tg carbon per year. The fast photolysis of HPALDs we proposed earlier as primary OH regeneration mechanism (Peeters and Muller, Phys. Chem. Chem. Phys. 2010, 12, 14227) found already experimental confirmation (Wolfe et al. Phys. Chem. Chem. Phys. 2012, 14, 7276); based on further theoretical work in progress, reaction schemes are presented of the oxy co-product radicals from HPALD photolysis and of the di-HPCARP radicals from the second pathway following peroxy isomerisation that are both expected to initiate considerable additional OH recycling. Invited Feature Article, with journal cover ispartof: Journal of Physical Chemistry A vol:118 issue:38 pages:8625-8643 ispartof: location:United States status: published

Photolysis is shown to be a major sink for isoprene-derived carbonyl nitrates, which constitute an important component of the total organic nitrate pool over vegetated areas. Empirical evidence from published laboratory studies on the absorption cross sections and photolysis rates of α-nitrooxy ketones suggests that the presence of the nitrate group (i) greatly enhances the absorption cross sections and (ii) facilitates dissociation to a point that the photolysis quantum yield is close to unity, with O–NO2 dissociation as a likely major channel. On this basis, we provide new recommendations for estimating the cross sections and photolysis rates of carbonyl nitrates. The newly estimated photo rates are validated using a chemical box model against measured temporal profiles of carbonyl nitrates in an isoprene oxidation experiment by Paulot et al. (2009). The comparisons for ethanal nitrate and for the sum of methacrolein- and methyl vinyl ketone nitrates strongly supports our assumptions of large cross-section enhancements and a near-unit quantum yield for these compounds. These findings have significant atmospheric implications: the photorates of key carbonyl nitrates from isoprene are estimated to be typically between ~ 3 and 20 times higher than their sink due to reaction with OH in relevant atmospheric conditions. Moreover, since the reaction is expected to release NO2, photolysis is especially effective in depleting the total organic nitrate pool. ispartof: Atmospheric Chemistry and Physics vol:14 issue:5 pages:2497-2508 status: published

Methyl peroxy, a key radical in tropospheric chemistry, was recently shown to react with the hydroxyl radical at an unexpectedly high rate. Here, the molecular reaction mechanisms are elucidated using high-level quantum chemical methodologies and statistical rate theory. Formation of activated methylhydrotrioxide, followed by dissociation into methoxy and hydroperoxy radicals, is found to be the main reaction pathway, whereas methylhydrotrioxide stabilization and methanol formation (from activated and stabilized methylhydrotrioxide) are viable minor channels. Criegee intermediate formation is found to be negligible. Given the theoretical uncertainties, useful constraints on the yields are provided by atmospheric methanol measurements. Using a global chemistry-transport model, we show that the only explanation for the high observed methanol abundances over remote oceans is the title reaction with an overall methanol yield of ∼30%, consistent with the theoretical estimates given their uncertainties. This makes the title reaction a major methanol source (115 Tg per year), comparable to global terrestrial emissions., The high observed abundance of atmospheric methanol over remote oceans is still not well-explained. Here the authors use quantum calculations and atmospheric modelling to show the reaction of methyl peroxy and hydroxyl radicals is a major methanol source (115 Tg/yr), comparable to global terrestrial emissions.

In a theoretical investigation using the CBS-QB3//UB3LYP/6-31+G** method supported by higher-level computations such as CBS-QB3//UQCISD/6-31+G**, the 1,6-H shifts of the enolic hydrogen in peroxy radicals of the type Z-HO-CH═CH-CH(2)-OO(•) were found to face exceptionally low energy barriers of only about 11 kcal mol(-1)--i.e., 6-9 kcal mol(-1) lower than the barriers for similar shifts of alkane hydrogens--such that they can proceed at unequaled rates of order 10(5) to 10(6) s(-1) at ambient temperatures. The unusually low barriers for enolic 1,6-H shifts in peroxy radicals, characterized here for the first time to our knowledge, are rationalized. As cases in point, the secondary peroxy radicals Z-HO-CH═C(CH(3))-CH(OO(•))-CH(2)OH (case A) and Z-HO-CH═CH-C(CH(3))(OO(•))-CH(2)OH (case B) derived from the primary Z-δ-hydroxy-peroxy radicals in the oxidation of isoprene, are predicted to undergo 1,6-H shifts of their enolic hydrogens at TST-calculated rates in the range 270-320 K of k(T)(A) = 5.4 × 10(-4) × T(5.04) × exp(-1990/T) s(-1) and k(T)(B) = 109 × T(3.13) × exp(-3420/T) s(-1), respectively, i.e., 2.0 × 10(6) and 6.2 × 10(4) s(-1), respectively, at 298 K, far outrunning in all relevant atmospheric and laboratory conditions their reactions with NO proposed earlier as their dominant pathways (Dibble J. Phys. Chem. A 2004, 108, 2199). These fast enolic-H shifts are shown to provide the explanation for the first-generation formation of methylglyoxal + glycolaldehyde, and glyoxal + hydroxyacetone in the oxidation of isoprene under high-NO conditions, recently determined by several groups. However, under moderate- and low-NO atmospheric conditions, the fast interconversion and equilibration of the various thermally labile, initial peroxy conformers/isomers from isoprene and the isomerization of the initial Z-δ-hydroxy-peroxy radicals, both recently proposed by us (Peeters et al. Phys. Chem. Chem. Phys. 2009, 11, 5935), are expected to substantially reduce the yields of the small carbonyls at issue.

Absolute rate constants for the gas-phase reaction C 2 H + SO 2 are experimentally determined over the temperature range 295–800 K. C 2 H radicals are generated by pulsed 193-nm photolysis of C 2 H 2 in the presence of SO 2 and He or N 2 buffer gas. The temperature dependence of the rate constants is established as k SO 2 ( T ) = (0.86 ± 0.08) T 3.80±0.13 exp[−(1222 ± 79)/ T ] cm 3 s −1 . The rate constants are moderately high at these temperatures and show negative temperature dependence. CCSD(T)/6-311++G(3df,2p) calculations of the potential energy surface confirm experimental findings and show that the reaction products are HCCO + SO.

BOREAM, a detailed model for the gas-phase oxidation of α-pinene and its subsequent formation of Secondary Organic Aerosol (SOA), is tested against a large set of SOA yield measurements obtained in dark ozonolysis experiments. For the majority of experiments, modelled SOA yields are found to agree with measured yields to within a factor 2. However, the comparisons point to a general underestimation of modelled SOA yields at high temperatures (above 30 °C), reaching an order of magnitude or more in the worst cases, whereas modelled SOA yields are often overestimated at lower temperature (by a factor of about 2). Comparisons of results obtained using four different vapour pressure prediction methods indicate a strong sensitivity to the choice of the method, although the overestimated temperature dependence of the yields is found in all cases. Accounting for non-ideality of the aerosol mixture (based on an adapted UNIFAC method) has significant effects, especially at low yields. Our simulations show that the formation of oligomers through the gas-phase reactions of Stabilised Criegee Intermediates (SCI) with other molecular organic products could increase the SOA yield significantly only at very low relative humidity (below 1%). Further tests show that the agreement between model and measurements is improved when the ozonolysis mechanism includes additional production of non-volatile compounds.

Stimulated by recent important developments regarding the oxidation chemistry of isoprene, this study evaluates and quantifies the impacts of different mechanism updates on the boundary layer concentrations of OH and HO2 radicals using the IMAGESv2 global chemistry transport model. The model results for HOx, isoprene, NO, and ozone are evaluated against air-based observations from the GABRIEL campaign, conducted over the Guyanas in October 2005, and from the INTEX-A campaign over the Eastern US in summer 2004. The version 2 of the Mainz Isoprene Mechanism (MIM2, Taraborrelli et al., 2009) used as reference mechanism in our simulations, has been modified to test (i) the artificial OH recycling proposed by Lelieveld et al. (2008), (ii) the epoxide formation mechanism proposed by Paulot et al. (2009b), and finally (iii) the HOx regeneration of the Leuven Isoprene Mechanism (LIM0) proposed by Peeters and Müller (2010). The simulations show that the LIM0 scheme holds by far the largest potential impact on HOx concentrations over densely vegetated areas in the Tropics as well as at mid-latitudes. Strong increases, by up to a factor of 4 in the modelled OH concentrations, and by a factor of 2.5–3 in the HO2 abundances are estimated through the LIM0 mechanism compared to the traditional isoprene degradation schemes. Comparatively much smaller OH increases (

As a sequel to our communication on a proposed new isoprene oxidation mechanism aiming to rationalize the unexpectedly high OH and HO(2) levels observed in isoprene-rich areas (J. Peeters, T. L. Nguyen, L. Vereecken, Phys. Chem. Chem. Phys. 2009, 11, 5935), we report herein the detailed quantum chemical and statistical kinetics characterization of the crucial 1,6-H shifts in the two Z-δ-hydroxy-peroxy radicals from isoprene. Geometries, energies and vibration frequencies of all conformers of the reactant radicals and transition states are computed at the B3LYP/6-31+G(d,p) level of theory and the energies of the lowest-lying conformers are then refined at various higher levels of theory, including CBS-QB3, IRCMax(CBS-QB3//B3LYP) and CBS-APNO. The rate coefficients over a wide temperature range are calculated using multi-conformer transition state theory with WKB tunneling factors evaluated for the barrier shape found by CBS-QB3//B3LYP IRC analyses. The WKB tunneling factors for these allyl-stabilisation-assisted reactions are about 25 at ambient temperatures. The rate coefficients can be represented by Arrhenius expressions over the 250-350 K range: k(T)=1.4×10(9) exp(-6380/T) s(-1) for the Z-1-OH-4-OO(·)-isoprene radical, and k(T)=0.72×10(9) exp(-5520/T) s(-1) for Z-1-OH-4-OO(·)-isoprene. With the k(1,6-H) of order 1 s(-1) at ambient temperatures, these isomerisations can compete with and even outrun the traditional peroxy reactions at low and moderate NO levels. The importance of these reactions as key processes in the newly proposed, OH-regenerating isoprene oxidation scheme is discussed.

The HO2 • + HOCH2OO• reaction was theoretically investigated, using various high-level, single-reference Complete Basis Set methods including CBS-QB3, CBS-QCI/APNO and CBS-Q(MPW1B95) and a new multi-reference CI-PT2 approach. Three major product channels under atmospheric conditions were identified and their molecular mechanisms elucidated in great detail by Intrinsic Reaction Coordinate Analyses (IRC) at the B3LYP/6–311G(d,p) level: (i) Direct head-to-tail H-atom abstraction from the hydroperoxy radical by the alkylperoxy, occurring on the triplet Potential Energy Surface (PES) leading to HOCH2OOH + O2; (ii) A two-step rearrangement of the initial singlet HOCH2OOOOH tetroxide complex to form HC(O)OH + •OH + HO2 •; (iii) A multi-step rearrangement of the initial HOCH2OOOOH tetroxide to yield HC(O)OH + O2(1Δ) + H2O, about twice as fast as the former channel on the singlet-surface. The findings provide an explanation for the observed hydroxyl radical formation in this reaction (Jenkin et al., Phys. Chem. Chem. Phys. 9 (2007) 3149) and rationalize the high overall rate and its pronounced negative temperature dependence (Veyret et al., J. Phys. Chem. 93 (1989) 2368).

During the autoxidation of cyclohexane, abstraction of the αH-atom of the hydroperoxide product by chain-carrying peroxyl radicals produces both the desired alcohol and ketone products, as well as the majority of by-products. Rationalizing the impact of this reaction, one should aim for a (catalytic) destruction of this hydroperoxide without the intervention of peroxyl chain-carriers. Starting from these new insights in the molecular mechanism, attempts for rational catalyst design are initiated.

Herein we report a new, one-step procedure for the metal-free ketonization of unsaturated fatty acid methyl esters and triacylglycerol mixtures with nitrous oxide (N 2 O). The conversion of various substrates can be tuned by parameters such as temperature, reaction time and N 2 O partial pressure. This ketonization chemistry offers various advantages over the classic Wacker catalytic process and a state-of-the-art two-step procedure via intermediate epoxides.

The reaction of CF2(a3B1) with NO(X2Pi) was theoretically investigated using the B3LYP, MP2, CCSD(T), G2M, CASSCF, and CASPT2 quantum chemical methods with various basis sets including 6-31G(d), 6-311G(d), 6-311+G(3df), cc-pVDZ, and cc-pVTZ. In agreement with the experimental kinetic data, the CF2(a3B1)+NO(X2Pi) reaction is found to proceed via a fast, barrier-free combination. This process, occurring on the doublet potential energy surface, leads to the electronically excited adduct F2C-NO(22A''), which readily undergoes a surface hopping to the 12A' electronic surface, with a Landau-Zener transition probability estimated to be close to 90% per C-N vibration. The metastable adduct F2C-NO(12A') can then either spontaneously decompose into CF2(X1A1)+NO(X2Pi) in a direct chemical quenching mechanism or relax to its ground-state equilibrium structure F2CNO(X2A'). The product distribution resulting from the latter, chemically activated intermediate was evaluated by solution of the master equation (ME), under different reaction conditions, using the exact stochastic simulation method; microcanonical rate constants were computed using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, based on the potential energy surfaces (PESs) constructed using both G2M and CASPT2 methods. The RRKM/ME analysis reveals that the hot F2CNO(X2A') rapidly fragments almost exclusively to the same products as above, CF2(X1A1)+NO(X2Pi), which amounts to an indirect chemical quenching mechanism. The reaction on the quartet PES is unlikely to be significant except at very high temperatures. The high crossing probability (up to 90%) between the two "avoided" doublet PESs points out the inherent difficulty in treating chemically activated reactions with fast-moving nuclei within the Born-Oppenheimer approximation.

The lowest-lying triplet and singlet potential energy surfaces for the O(3P) + C6H6 reaction were theoretically characterized using the "complete basis set" CBS-QB3 model chemistry. The primary product distributions for the multistate multiwell reactions on the individual surfaces were then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. It is newly found that electrophilic O-addition onto a carbon atom in benzene can occur in parallel on two triplet surfaces, 3A' and 3A' '; the results predict O-addition to be dominant up to combustion temperatures. Major expected end-products of the addition routes include phenoxy radical + H*, phenol and/or benzene oxide/oxepin, in agreement with the experimental evidence. While c-C6H5O* + H* are nearly exclusively formed via a spin-conservation mechanism on the lowest-lying triplet surface, phenol and/or benzene oxide/oxepin are mainly generated from the lowest-lying singlet surface after inter-system crossing from the initial triplet surface. CO + c-C5H6 are predicted to be minor products in flame conditions, with a yieldor = 5%. The O + C6H6 --c-C5H5* + *CHO channel is found to be unimportant under all relevant combustion conditions, in contrast with previous theoretical conclusions (J. Phys. Chem. A 2001, 105, 4316). Efficient H-abstraction pathways are newly identified, occurring on two different electronic state surfaces, 3B1 and 3B2, resulting in hydroxyl plus phenyl radicals; they are predicted to play an important role at higher temperatures in hydrocarbon combustion, with estimated contributions of ca. 50% at 2000 K. The overall thermal rate coefficient k(O + C6H6) at 300-800 K was computed using multistate transition state theory: k(T) = 3.7 x 10-16 x T 1.66 x exp(-1830 K/T) cm(3) molecule(-1) s(-1), in good agreement with the experimental data available.

Absolute rate coefficients for the title reaction, HO+HOCH2CHO--products (R1), were measured over the temperature range 240-362 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. Within experimental error, the rate coefficient, k1, is independent of temperature over the range covered and is given by k1(240-362 K)=(8.0+/-0.8)x10(-12) cm3 molecule-1 s-1. The effects of the hydroxy substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for HOCH2CHO throughout the troposphere. As part of this work, the absorption cross-section of HOCH2CHO at 184.9 nm was determined to be (3.85+/-0.2)x10(-18) cm2 molecule-1, and the quantum yield of HO formation from the photolysis of HOCH2CHO at 248 nm was found to be (7.0+/-1.5)x10(-2).

In this work, a complementary experimental and theoretical approach is used to unravel the formation of byproducts in the autoxidation of cyclohexane. The widely accepted vision that cyclohexanone would be the most important precursor of undesired products was found inconsistent with several experimental observations. However, the propagation reaction of cyclohexyl hydroperoxide, which we recently put forward as the missing source of cyclohexanol and cyclohexanone, is now unambiguously identified also as the dominant path leading to byproducts. Indeed, this overlooked reaction produces large amounts of cyclohexoxy radicals, able to ring-open via a beta-C--C cleavage to omega-formyl radicals. The pathway by which these radicals are converted into the observed and quantified byproducts is derived in this work. In this liquid-phase reaction, solvent cages were found very important, steering the fate of nascent species.

The atmospheric oxidation of vinyl alcohol (VA) produced by photoisomerization of acetaldehyde (AA) is thought to be a source of formic acid (FA). Nevertheless, a recent theoretical study predicted a high rate coefficient k1(298 K) of ≈10**–14 cm3 molecule–1 s–1 for the FA-catalyzed tautomerization reaction 1 of VA back into AA, which suggests that FA buffers its own production from VA. However, the unusually high frequency factor implied by that study prompted us to reinvestigate reaction 1. On the basis of a high-level ab initio potential energy profile, we first established that transition state theory is applicable, and derived a k1(298 K) of only ≈2 × 10**–20 cm3 molecule–1 s–1, concluding that the reaction is negligible. Instead, we propose and rationalize another important VA sink: its uptake by aqueous aerosol and cloud droplets followed by fast liquid-phase tautomerization to AA; global modeling puts the average lifetime by this sink at a few hours, similar to oxidation by OH. ispartof: Journal of Physical Chemistry Letters vol:6 issue:20 pages:4005-4011 ispartof: location:United States status: published

The rate constants for the reaction OH + CH 3 C(O)OH → products (1) were determined over the temperature range 287-802 K at 50 and 100 Torr of Ar or N 2 bath gas using pulsed laser photolysis generation of OH by CH 3 C(O)OH photolysis at 193 nm coupled with OH detection by pulsed laser-induced fluorescence. The rate coefficient displays a complex temperature dependence with a sharp minimum at 530 K, indicating the competition between a reaction proceeding through a pre-reactive H-bonded complex to form CH 3 C(O)O + H 2 O, expected to prevail at low temperatures, and a direct methyl-H abstraction channel leading to CH 2 C-(O)OH + H 2 O, which should dominate at high temperatures. The temperature dependence of the rate constant can be described adequately by k 1 (287-802 K) = 2.9 x 10 -9 exp{-6030 K/T} + 1.50 × 10 -13 exp{515 K/T} cm 3 molecule -1 s -1 , with a value of (8.5 ± 0.9) x 10 -13 cm 3 molecule -1 s -1 at 298 K. The steep increase in rate constant in the range 550-800 K, which is reported for the first time, implies that direct ion of a methyl-H becomes the dominant pathway at temperatures greater than 550 K. However, the data indicates that up to about 800 K direct methyl-H abstraction remains adversely affected by the long-range H-bonding attraction between the approaching OH radical and the carboxyl -C(O)OH functionality.

This work deals with a theoretical study of the interaction between CH 2 X 2 and CHX 3 (X=F, Cl, Br) and hydrogen peroxide (HP). The optimized geometries, binding energies and harmonic vibrational frequencies are calculated at the MP2/6-311++G(3df, 2p)//MP2/6-31G(d,p) level. The results of a natural bond orbital analysis (NBO) are reported as well. All the complexes are characterized by a cyclic structure, the complexes being stabilized by CH⋯O and OH⋯X interactions. Complex formation results in a contraction of the CH bond and an elongation of the OH bond. The binding energies range between 11.7 and 13.8 kJ mol −1 . The ν (CH) stretching vibrations show the characteristic features of the blue-shifted hydrogen bonds. In contrast, the ν (OH) vibrations are red-shifted. The NBO analysis shows that the charge transfer taking place from the CH 2 X 2 or CHX 3 molecules to HP is small and that complex formation induces an electronic reorganization mainly in the XCH part of the molecule involved in the interaction. In most of the complexes, there is a very small increase of the occupation of the σ *(CH) orbital and an increase in s-character of the C hybrid orbital in the CH⋯O bond. A quantitative correlation between the contraction of the CH bond and the variation of these two parameters is presented. In contrast, the lengthening of the OH bond mainly depends on the occupation of the σ *(OH) orbital. The results are compared with theoretical data on other blue-shifted hydrogen bonds.

Herein, we demonstrate that the chain-initiating dissociation of cyclohexyl hydroperoxide, CyOOH, is substantially accelerated by H-bond acceptors (e.g. Teflon), which assist O-O bond breaking by stabilising the leaving *OH radical. This is a completely new approach to boost the chain-propagating radical concentration. Indeed, up to now, literature has remained focussed on transition metal catalysis. In addition to this initiation effect, we demonstrate how inert perfluorinated compounds are also able to steer the selectivity at the molecular level, by promoting the conversion of the intermediate cyclohexyl hydroperoxide to the most desired end-product, cyclohexanone. This effect is explained by an enhanced, H-bond-assisted, hydroperoxide propagation. This hitherto overlooked hydroperoxide propagation was recently presented by us as the dominant cyclohexanone and cyclohexanol source. We herein thus confirm our previously reported autoxidation scheme, and illustrate its usefulness as a solid basis for designing new catalytic systems.

In this article, we present a thorough study on the autoxidation of cyclohexane, a model substrate for other (saturated) hydrocarbons. Despite the industrial impact of autoxidation reactions, a detailed mechanism is still missing. We present a combined experimental and computational study on the formation of both the major products (cyclohexylhydroperoxide, cyclohexanol and cyclohexanone), and the formation of ring-opened side-products. Up to now, these by-products, mainly adipic acid, were thought to originate from cyclohexanone. However, we found strong evidence that the subsequent propagation of ketone is much slower than assumed, and can only account for some 25% of ring-opened products. On the other hand, the hitherto completely overlooked propagation of the hydroperoxide, via fast αH-abstraction by chain-carrying peroxyl radicals, is identified as the major source of not only alcohol and ketone, but also by-products. In the case of N-hydroxyphthalimide (NHPI) catalysed oxidations, where mostly phthalimide N-oxyl (PINO ) radicals are propagating the chain, the situation is slightly different, as PINO reacts more selectively with the alkane substrate than peroxyl radicals. This results in an increase in hydroperoxide selectivity. Lowering of the ROOH concentration by its, e.g. cobalt-catalyzed decomposition, leads to an enhanced catalytic efficiency, as a result of the shift in the ROO + NHPI ⇌ ROOH + PINO equilibrium to the more efficient PINO chain carrier.

In this work, the C(2)F(4)(X(1)A(g)) + O((3)P) reaction was investigated experimentally using molecular beam-threshold ionization mass spectrometry (MB-TIMS). The major primary products were observed to be CF(2)O (+ CF(2)) and CF(3) (+ CFO), with measured approximate yields of % versus %, respectively, neglecting minor products. Furthermore, the lowest-lying triplet and singlet potential energy surfaces for this reaction were constructed theoretically using B3LYP, G2M(UCC, MP2), CBS-QB3, and G3 methods in combination with various basis sets such as 6-31G(d), 6-311+G(3df), and cc-pVDZ. The primary product distribution for the multiwell multichannel reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis. It was found that the observed production of CF(3) (+ CFO) can only occur on the singlet surface, in parallel with formation of ca. 5 times more CF(2)O(X) + CF(2)(X(1)A(1)). This requires fast intersystem crossing (ISC) from the triplet to the singlet surface at a rate of ca. 4 x 10(12) s(-1). The theoretical calculations combined with the experimental results thus indicate that the yield of triplet CF(2)(ã(3)B(1)) + CF(2)O formed on the triplet surface prior to ISC isor =35%, whereas singlet CF(2)(X(1)A(1)) + CF(2)O is produced with yieldor =60%, after ISC. In addition, the thermal rate coefficients k(O + C(2)F(4)) in the T = 150-1500 K range were computed using multistate transition state theory and can be expressed as k(T) = 1.67 x 10(-16) x T(1.48) cm(3) molecule(-1) s(-1); they are in agreement with the available experimental results in the T = 298-500 K range.

Quantum chemical calculations were applied to investigate the electronic structure of the parent 1,3,5-tridehydrobenzene triradical (C6H3, TDB) and its anion ( C 6 H 3 - ) , cation ( C 6 H 3 + ) and protonated form ( C 6 H 4 + ) . Our results obtained using the state-averaged complete active space self-consistent-field (CASSCF) followed by second-order multi-state multi-configuration perturbation theory, MS-CASPT2, and MRMP2 in conjunction with the large ANO-L and 6-311++G(3df,2p) basis set, confirm and reveal the followings: (i) TDB has a doublet 2A1 ground state with a 4B2–2A1 energy gap of 29 kcal/mol, (ii) the ground state of the C 6 H 3 - anion in the triplet 3B2 being 4 kcal/mol below the 1A1 state. (iii) the electron affinity (EA), ionization energy (IE) and proton affinity (PA) are computed to be: EA = 1.6 eV, IE = 7.2 eV, PA = 227 kcal/mol using UB3LYP/6-311++G(3df,2p) + ZPE; standard heat of formation ΔHf(298 K, 1 atm)(TDB) = 179 ± 2 kcal/mol was calculated with CBS-QB3 method. An atoms-in-molecules (AIM) analysis of the structure reveals that the topology of the electron density is similar in all compounds: hydrogens connect to a six-membered ring, except for the case of the 2A2 state of C 6 H 4 + (MBZ+) which is bicyclic with fused five- and three-membered rings. Properties of the chemical bonds were characterized with Electron Localization Function (ELF) analysis, as well as Wiberg indices, Laplacian and spin density maps. We found that the radicals form separate monosynaptic basins on the ELF space, however its pair character remains high. In the 2A1 state of TDB, the radical center is mainly localized on the C1 atom, while in the 2B2 state it is equally distributed between the C3 and C5 atoms and, due to the symmetry, in the 4B2 state the C1, C2 and C3 atoms have the same radical character. There is no C3–C5 bond in the 2A1 state of TDB, but the interaction between these atoms is strong. The ground state of cation C 6 H 3 + (DHP), 1A1, is not a diradical and has a doubly aromatic character. Aromaticity of the different compounds was studied within the ELF framework and the standard deviation of the bond lengths and bond orders. The Jahn–Teller distorted 2A1 and 2B2(C2v) states of TDB were found to exhibit an aromaticity comparable to that of benzene. Overall protonation of the TDB reinforces the stability of the low-spin doublet states, the classical Hund’s rule is not obeyed. In a view, these species could better be regarded as radicals than triradicals.

The potential energy surface for the O((3)P) + C(2)H(4) reaction, which plays an important role in C(2)H(4)/O(2) flames and in hydrocarbon combustion in general, was theoretically reinvestigated using various quantum chemical methods, including G3, CBS-QB3, G2M(CC,MP2), and MRCI. The energy surfaces of both the lowest-lying triplet and singlet electronic states were constructed. The primary product distribution for the multiwell multichannel reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. Intersystem crossing of the "hot" CH(2)CH(2)O triplet adduct to the singlet surface, shown to account for about half of the products, was estimated to proceed at a rate of approximately 1.5 x 10(11) s(-1). In addition, the thermal rate coefficients k(O + C(2)H(4)) in the T = 200-2000 K range were computed using multistate transition state theory and fitted by a modified Arrhenius expression as k(T) = 1.69 x 10(-16) x T(1.66) x exp(-331 K/T) . Our computed rates and product distributions agree well with the available experimental results. Product yields are found to show a monotonic dependence on temperature. The major products (with predicted yields at T = 300 K/2000 K) are: CH(3) + CHO (48/37%), H + CH(2)CHO (40/19%), and CH(2)(X(3)B(1)) + H(2)CO (5/29%), whereas H + CH(3)CO, H(2) + H(2)CCO, and CH(4) + CO are all minor (< or =5%).

In spite of its industrial importance, the detailed reaction mechanism of cyclohexane autoxidation by O2 is still insufficiently known. Based on quantum chemical potential energy surfaces, rate coefficients of the primary and secondary chain propagation steps involving the cyclohexylperoxyl (CyOO) radical were evaluated using multiconformer transition-state theory. Including tunneling and hindered-internal-rotation effects, the rate coefficient for hydrogen-atom abstraction from cyclohexane (CyH) by CyOO was calculated to be k(T)= 1.46 x 10(-11) x exp(-17.8 kcal mol(-1)/ RT) cm3s(-1) (300-600K), close to the experimental data. A "Franck-Rabinowitch cage" reaction between the nascent cyclohexylhydroperoxide (CyOOH) and cyclohexyl radical, products from CyOO + CyH, is put forward as an initially important cyclohexanol (CyOH) formation channel. alphaH abstraction by CyOO. from cyclohexanone was calculated to be only about five times faster than that from CyH, too slow to explain all the observed side products. The a-hydrogen (alphaH) abstractions from CyOH and CyOOH by CyOO. are predicted to be about 10 and 40 times faster, respectively, than the CyOO. +CyH reaction. The very fast CyOO.+CyOOH reaction proceeds through the unstable Cy-alphaH .OOH radical that decomposes spontaneously into the ketone (Q=O) plus the OH radical; the "hot" .OH is found to produce the bulk of the alcohol via a second, "activated cage" reaction analogous to that above. It is thus shown how the very reactive CyOOH intermediate is the predominant source of ketone and alcohol, while it also leads to some side products. The alpha-hydroxycyclohexylperoxyl radical formed during the moderately fast oxidation of CyOH is shown to decompose fast into HO2 + cyclohexanone in a rapidly equilibrated reaction, which constitutes a smaller, second ketone source. These two fast cyclohexanone forming routes avoid the need for unfavorable molecular routes hitherto invoked as ketone sources. The theoretical predictions are supported and complemented by experimental findings. The newly proposed scheme is also largely applicable to the oxidation of other hydrocarbons, such as toluene, xylene, and ethylbenzene.

A comparative theoretical study is presented on the formation and decomposition of alpha-hydroxy-alkylperoxyl radicals, Q(OH)OO* (Q = RR'C:), important intermediates in the oxidation of several classes of oxygenated organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons. Detailed potential energy surfaces (PESs) were computed for the HOCH2O2*==HO2* + CH2O reaction and its analogues for the alkyl-substituted RCH(OH)OO* and R2C(OH)OO* and the cyclic cyclo-C6H10(OH)OO*. The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the approximately equal to 15 kcal/mol endothermal alpha-hydroxy-alkylperoxyl decompositions and of the reverse HO2*+ ketone/aldehyde reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available experimental (kinetic) data validates our methodologies. Contrary to current views, HO2* is found to react as fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)OO*==HO2* + carbonyl quasi-equilibrium. The sizable [Q(OH)OO*]/[carbonyl] ratios predicted for formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OO* reactions with NO and HO2*. IMAGES model calculations indicate that at the tropical tropopause the HO2*-initiated oxidation of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls, thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an RRKM-master equation analysis shows that hot alpha-hydroxy-alkylperoxyls formed by the addition of O(2) to C(1)-, C(2)-, and C(3)-alpha-hydroxy-alkyl radicals will quasi-uniquely fragment to HO2* plus the carbonyl under all atmospheric conditions.

We investigated the electronic structure of 1-methylene-3,5-didehydrobenzene triradical (MDB, C 7 H 5 ) containing a σ 1 σ 1 biradical benzene ring coupled with an exocyclic π 1 CH 2 . MS-CASPT2(9,9)/ANO-L calculations reveal that MDB exhibits a low-spin doublet 2 B 1 ( 2 Π) ground state, followed by an open-shell doublet 2 A 2 state with a 2 A 2 – 2 B 1 gap of 17 ± 3 kcal mol −1 . The energy ordering of MDB is (kcal mol −1 ): X 2 B 1 (0) 2 A 2 (17) 4 A 2 (20) 2 B 2 (39) 4 A 1 (59) 2 A 1 (61) 4 B 1 (75) 4 B 2 (79). The doublet states are lower in energy than the quartets, violating Hund’s rule, due to a coupling of the two unpaired σ-electrons transforming the triradical into a radical.

The product distribution of the reaction of acetic acid, CH(3)COOH, with hydroxyl radicals, OH, was studied experimentally and theoretically. Mass-spectrometric measurements at 290 K and 2 Torr of He of the CO(2) yield versus the loss of acetic acid yielded a branching fraction of 64 +/- 14% for the abstraction of the acidic hydrogen as follows: CH(3)COOH + OH --CH(3)COO + H(2)O --CH(3) + CO(2) + H(2)O. A quantum chemical and theoretical kinetic analysis showed that the abstraction of the acidic hydrogen is enhanced relative to the abstraction of -CH(3) hydrogens because of the formation of a strong pre-reactive H-bonded complex, where the H-bonds are retained in the H-abstraction transition state. The potential energy surface of the reaction is explored in detail, and the reaction products of the individual channels are identified. The theoretical product branching is found to be critically dependent on the energetic and rovibrational differences between the H-abstraction transition states.

The stationary points of H-atom abstraction reactions of triplet CF2(3B1) with XHn (n = 1–4: X = H, F, Cl, Br, O, S, N, P, C and Si) were computed using UCCSD(T) methods with 6-311++G(3df,2p) and aug-cc-pVTZ basis sets. Covalent surface crossing heights, calculated using the X–H and C–H bond dissociation energies of XHn and of the CHF2 product, correlate well with the computed classical barrier heights. Within each group of co-reactants, the barrier heights increase with increasing X–H bond dissociation energy, whereas the C–H bond lengths of the transition structures decrease. H-abstractions remain energy-demanding processes for second-row X atoms, but become more facile for their third-row X counterparts.

The rate constants for the combination reactions CF3 + CF3 and CF3 + F at 290 K and helium pressures of ≈1–6 Torr have been determined, using clean chemical sources of CF3, by means of discharge flow-molecular beam sampling-threshold ionisation mass spectrometry (DF/MB-TIMS). For the mutual reaction of CF3, no pressure dependence could be observed over the 1–6 Torr pressure range, indicating that the obtained rate constant of k1∞ = (1.8 ± 0.6) × 10−12 cm3 s−1 is the high pressure limit. This result, which agrees with the lowest values in literature but is ca. five times smaller than the most recent data, is fully in line with the known trend in the mutual reaction rate constant for the series CH3; CH2F; and CHF2. The reaction of CF3 with F was found to exhibit a clear pressure dependence in the 0.5 to 6 Torr range. Using a Troe fall-off formalism, the low-pressure limit rate constant was determined as k20(He) = (1.47 ± 0.24) × 10−28 cm6 s−1, differing substantially from the only available previous determination; a variational transition state theoretical treatment is shown to support our data.

MP2/6-31+G(d,p) calculations are used to analyze the interaction between CH3X (X = F, Cl, or Br) and hydrogen peroxide (HP). Two stable structures, A and B, are found on each potential energy surface. The A complexes are characterized by a six-membered structure and the B complexes, having a lower stability, by a five-membered structure. In both complexes, the molecules are held together by both OH· · ·X and CH1· · ·O hydrogen bonds. The binding energies range between 2.0 and 3.2 kcal mol-1 for the A complexes and between 1.5 and 1.7 kcal mol-1 for the B complexes. The frequency shifts are calculated for the CH1D2D3X isotopomers. Both A and B complexes exhibit simultaneously an elongation of the OH bond and a red shift and an infrared intensity increase of the corresponding OH stretching vibration along with a contraction of the CH1 bond, a blue shift, and an infrared intensity decrease of the CH1 stretching vibration. The interaction of CH3F and CH3Cl with HP also induces a contraction of the external CH...

Degradation mechanisms of peroxide-containing compounds, either stable or transients in the degradation of volatile organic compounds, are still poorly understood, despite their importance in atmospheric chemistry. A theoretical DFT and Coupled Cluster theory analysis of the stability of alkyl radicals with hydroperoxyl- (HOO–) or alkylperoxyl (ROO–) substituents on the radical carbon atom revealed that such radicals are unstable, dissociating by O–O scission to a carbonyl compound and a hydroxy or alkoxy radical even for multiple substituted compounds. This dissociation occurs sequentially after completion of the reaction forming the α-peroxyl-substituted radicals, such that the latter is independent of the O–O scission.

In the atmospheric oxidation of organic compounds, free peroxy and oxy radicals play a crucial role. The traditional view is that peroxy radicals react with NO or with other (hydro-)peroxy radicals, whereas oxy radicals decompose, undergo a hydrogen shift, or react with O2. However, we show in this quantum chemical and statistical-rate investigation that the presence of a double CC bond in many (per)oxy radicals formed from isoprene and monoterpenes, the most abundant nonmethane hydrocarbons emitted into the atmosphere, can result in hitherto neglected ring-closure isomerizations. These processes are shown to be competitive under atmospheric conditions and can substantially alter the predicted oxidation products. As an illustration, the major pathways in the OH-initiated oxidation of β-pinene are discussed. It is shown that the predominance of the (per)oxy ring-closure routes offers a consistent rationalization for the “anomalously” low observed yields of traditional-pathway oxidation products from this c...

A novel and readily applicable Structure-Activity Relationship (SAR) for predicting the barrier height Eb to decomposition by β C-C scission of (substituted) alkoxy radicals is presented. Alkoxy radicals are pivotal intermediates in the atmospheric oxidation of (biogenic) volatile organic compounds, and their fate is therefore of crucial importance to the understanding of atmospheric VOC degradation mechanisms. The SAR is based on available theoretical energy barriers and validated against barriers derived from experimental data. The SAR is expressed solely in terms of the number(s) Ni of alkyl-, hydroxy- and/or oxo-substituents on the α- and β-carbons of the breaking bond: Eb(kcal/mol) =17.5 − 2.1 × Nα(alk) − 3.1 ×Nβ(alk) − 8.0 × Nα,β(OH) − 8.0 × Nβ(O=) − 12 × Nα(O=). For barriers below 7 kcal/mol, an additional, second-order term accounts for the curvature. The SAR reproduces the available experimental and theoretical data within 0.5 to 1 kcal/mol. The SAR generally allows conclusive predictions as to the fate of alkoxy radicals; several examples concerning oxy radicals from prominent atmospheric VOC are presented. Specific limitations of the SAR are also discussed. Using the predicted barrier height Eb, the high-pressure rate coefficient for alkoxy decomposition k diss ∞ (298 K) can be obtained from k diss ∞ (298 K) = L ×1.8 × 1013 exp(−Eb/RT) s−1, with L the reaction path degeneracy.