Tropospheric Photochemistry of 2-Butenedial: Role of the Triplet States, CO and Acrolein Formation, and the Experimentally Unidentified Carbonyl Compound-Theoretical Study.

Solar irradiation of 2-butenedial in the lower troposphere mainly produces isomeric ketene-enol (a key intermediate product), furanones, and maleic anhydride, the formation pathways of which were investigated in a previous study. The other main products were carbon monoxide and an experimentally unidentified carbonyl compound. This was the subject of the present study.

Mechanism of the Photochemical Isomerization and Oxidation of 2-Butenedial: A Theoretical Study.

Under tropospheric conditions, 2-butenedial is photochemically removed to produce secondary organic aerosol. Upon solar irradiation in the lower troposphere, the main photochemical products are ketene-enol (a key intermediate product), furanones, and maleic anhydride. The oxidative reaction mechanism was studied using the multireference method CASSCF to explore the hypersurface of the two most accessible singlet excited states, and by DFT for the ground state.

Multireference Study of the HCOO (Criegee Intermediate) + O Addition: A Reaction of Possible Tropospheric Interest.

The addition of carbonyl oxides to ozone could have an effect on the tropospheric HO nocturnal formation. Its mechanistic description has provided so far conflicting results. CASPT2 (and CASSCF) geometry optimizations, focused on the initial addition step, show that the most likely pathway is not a concerted cycloaddition to give directly a c-HCO intermediate, as had been proposed.

Comprehensive study on the degradation of ochratoxin A in water by spectroscopic techniques and DFT calculations.

Ochratoxin A (OTA) is one of the most important dietary risk factors and is classified as a possible carcinogen to humans. Assessing the conditions to remove it from foodstuffs in a simple and effective way is of the utmost importance. OTA behaviour in water in the pH range 1.

Effects of collision energy and vibrational excitation of CH cations on its reactivity with hydrocarbons: But-2-yne CHCCCH as reagent partner.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH + CHCCCH (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan.

Tuning of the Electronic Properties of Armchair Graphene Nanoribbons through Functionalization: Theoretical Study of (1)Δg O2 Border Addition.

We report the results of a DFT study of the border oxidation by (1) Δg O2 of molecular models of armchair graphene nanoribbons (a-GNRs). The aim of this work is to propose a new method, as an alternative or complementary method to the tuning of the size, to modify the electronic properties of a-GNRs. Here, we investigate modification of the HOMO and LUMO energies, which are some of the most important parameters to be controlled in the design of organic electronic devices.

Combustive, Postcombustive, and Tropospheric Butadiyne Oxidation by O2, Following Initial HO Attack. Theoretical Study.

Butadiyne (diacetylene, HC(4)H) is produced during combustions, and has been quantified in different flames as well as a biomass burning emission. Its reaction with the hydroxyl radical, HO((2)Π(3/2)), under combustion conditions, was investigated in a thorough RRKM study by J. P.

Antagonistic functionalized nucleation and oxidative degradation in combustive formation of pyrene-based clusters mediated by triplet O and O2 : theoretical study.

Polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanoparticles can be oxidized right from their inception and all through their growth. Oxidation can also promote their degradation. This modelistic density functional theory (DFT) study explores, in a descriptive manner, if oxidation can mediate the earliest stages of nucleation ("functionalized nucleation"), though contrasted by mass declension triggered by oxidation itself.

o-Benzyne fragmentation and isomerization pathways: a CASPT2 study.

The mechanisms of the fragmentation and isomerization pathways of o-benzyne were studied at the multi-configurational second-order perturbative level [CAS(12,12)-PT2]. The direct fragmentation of o-benzyne to C2H2 + C4H2 follows two mechanisms: a concerted mechanism and a stepwise mechanism. Although the concerted mechanism is characterized by a single closed-shell transition structure, the stepwise pathway is more complex and structures with a strong diradical character are seen.

Density functional theory study of the interaction of vinyl radical, ethyne, and ethene with benzene, aimed to define an affordable computational level to investigate stability trends in large van der Waals complexes.

Our purpose is to identify a computational level sufficiently dependable and affordable to assess trends in the interaction of a variety of radical or closed shell unsaturated hydro-carbons A adsorbed on soot platelet models B. These systems, of environmental interest, would unavoidably have rather large sizes, thus prompting to explore in this paper the performances of relatively low-level computational methods and compare them with higher-level reference results. To this end, the interaction of three complexes between non-polar species, vinyl radical, ethyne, or ethene (A) with benzene (B) is studied, since these species, involved themselves in growth processes of polycyclic aromatic hydrocarbons (PAHs) and soot particles, are small enough to allow high-level reference calculations of the interaction energy ΔEAB.

First ring formation by radical addition of propargyl to but-1-ene-3-yne in combustion. Theoretical study of the C7H7 radical system.

Combustive formation of a first carbon ring is an important step in the growth of polycyclic aromatic hydrocarbons (PAHs) and soot platelets. Propargyl radical addition to but-1-ene-3-yne (vinylacetylene) can start this process, possibly forming 5-, 6-, and 7-membered rings. A variety of partially intertwined reaction pathways results from density functional theory (DFT), which indicates three C7H7 radicals, benzyl, tropyl, and vinylcyclopentadienyl, as particularly stable.

Memory effects in carbocation rearrangements: structural and dynamic study of the norborn-2-en-7-ylmethyl-X solvolysis case.

The solvolysis of two diastereomers may give the same two products, but in different ratios, notwithstanding the fact that the two reaction pathways share an apparently identical intermediate carbocation. This has been dubbed the "memory effect", since the initial carbocation seems to "remember" its origin when undergoing further evolutions through multistep rearrangements. This puzzling result was studied theoretically for the case of the solvolysis of norborn-2-en-7-ylmethyl-X systems by defining the reaction potential energy surface (PES) and then carrying out a dynamical study.

Growth of polyphenyls via ion-molecule reactions: an experimental and theoretical mechanistic study.

The reactivity of biphenylium cations C12H9(+) with benzene C6H6 is investigated in a joint experimental and theoretical approach. Experiments are performed by using a triple quadruple mass spectrometer equipped with an atmospheric pressure chemical ion source to generate C12H9(+) via dissociative ionization of various isomers of the neutral precursor hydroxybiphenyl (C12H10O). C-C coupling reactions leading to hydrocarbon growth are observed.

Synthesis of five-membered cyclic ethers by reaction of 1,4-diols with dimethyl carbonate.

The reaction of 1,4-diols with dimethyl carbonate in the presence of a base led to selective and high-yielding syntheses of related five-membered cyclic ethers. This synthetic pathway has the potential for a wide range of applications. Distinctive cyclic ethers and industrially relevant compounds were synthesized in quantitative yield.

Border reactivity of polycyclic aromatic hydrocarbons and soot platelets toward ozone. A theoretical study.

PAH-based models, with an even or odd number of unsaturated carbon atoms and π electrons (even and odd PAHs for short), are selected to investigate, by molecular and periodic methods, their electron distribution and border reactivity toward ozone, and also to represent local features and edge reactivity of even or odd soot platelets. These results will contrast those previously collected for the internal positions of similar even (J. Phys.

Growth of polyaromatic molecules via ion-molecule reactions: an experimental and theoretical mechanistic study.

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C(10)H(7)(+) with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed.

Polycyclic aromatic hydrocarbon formation mechanism in the "particle phase". A theoretical study.

The synthesis of polycyclic aromatic hydrocarbons (PAHs) and the formation of soot platelets occur both during combustion at relatively low [O(2)], or under pyrolysis conditions. When the PAH size grows beyond the number of three-four condensed cycles, the partitioning of PAHs between the gas and particle phases favours the latter (i.e.

The mechanism of the Stevens and Sommelet-Hauser rearrangements. A theoretical study.

The [1,2] and [2,3] migration steps in the Stevens and Sommelet-Hauser rearrangements which occur in the ylides of quaternary ammonium salts have been studied at M05-2x levels. The Stevens migration has been found to take place through a diradical pathway in several cases (tetramethylammonium, benzyltrimethylammonium, benzylphenacyldimethylammonium ylides). By contrast, in the phenyltrimethylammonium ylide this reaction takes place through a concerted process.

The reaction of N2O with phenylium ions C6(H,D)5(+): an integrated experimental and theoretical mechanistic study.

The reaction of N(2)O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C(6)H(5)(+), and its perdeuterated derivative C(6)D(5)(+) are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C(5)(H,D)(5)(+), C(6)(H,D)(5)O(+), and C(3)(H,D)(3)(+) are experimentally detected as the most relevant reaction products.

Oxidation of ethyne and But-2-yne. 2. Master equation simulations.

The aim of this study is to improve understanding of the tropospheric oxidation of ethyne (acetylene, C2H2) and but-2-yne, which takes place in the presence of HO and O2. The details of the potential energy hypersurface have been discussed in a previous article [Maranzana et al., J.

Tropospheric oxidation of ethyne and But-2-yne. 1. Theoretical mechanistic study.

This paper (part 1) and the following one (part 2) aim to assess the viability of some tropospheric oxidation channels for two symmetrical alkynes, ethyne (acetylene) and but-2-yne. Paper 1 defines the features of the DFT(B3LYP)/6-311G(3df,2p) energy hypersurface and qualitatively considers the practicability of different pathways through the estimate of free energy barriers. Paper 2 will assess this in more detail by way of master equation simulations.

Soot platelets and PAHs with an odd number of unsaturated carbon atoms and pi electrons: theoretical study of their spin properties and interaction with ozone.

PAHs made from an odd number of unsaturated carbon atoms and pi electrons (odd PAHs) have been detected in flames and flank the more familiar even PAHs, having approximately the same quantitative importance, particularly for PAHs containing more than 25 carbon atoms. Similarly, soot platelets containing an odd number of carbon atoms can be reasonably assumed to form during combustion. PAHs are intended here as small models for the investigation of some of their local features.

Master equation simulations of competing unimolecular and bimolecular reactions: application to OH production in the reaction of acetyl radical with O2.

Master equation calculations were carried out to simulate the production of hydroxyl free radicals initiated by the reaction of acetyl free radicals (CH3(C=O).) with molecular oxygen. In particular, the competition between the unimolecular reactions and bimolecular reactions of vibrationally excited intermediates was modeled by using a single master equation.

Aromatic hydrocarbon nitration under tropospheric and combustion conditions. A theoretical mechanistic study.

The viability of some nitration pathways is explored for benzene (B), naphthalene (N), and in part pyrene (P). In principle, functionalization can either take place by direct nitration (NO2 or N2O5 attack) or be initiated by more reactive species, as the nitrate and hydroxyl radicals. The direct attack of the NO2 radical on B and N, followed by abstraction of the H geminal to the nitro group (most likely accomplished by 3O2) could yield the final nitro-derivatives.

The oxidized soot surface: theoretical study of desorption mechanisms involving oxygenated functionalities and comparison with temperature programed desorption experiments.

The desorption mechanism for oxygenated functionalities on soot is investigated by quantum mechanical calculations on functionalized polycyclic aromatic hydrocarbon (PAH) models and compared with recently published temperature programed desorption-mass spectrometry results. Substituents on PAHs of increasing size (up to 46 carbon atoms in the parent PAH) are chosen to reproduce the local features of an oxidized graphenic soot platelet. Initially, the study is carried out on unimolecular fragmentation (extrusion, in some cases) processes producing HO, CO, or CO2, in model ketones, carboxylic acids, lactones, anhydrides, in one aldehyde, one peroxyacid, one hydroperoxide, one secondary alcohol, and one phenol.