The Gas Phase HO-Initiated Oxidation of Furan: A Theoretical Investigation on the Reaction Mechanism

14 pages, 7 figures, 1 scheme, 2 tables.The reaction mechanism of the gas-phase HO-initiated oxidation of furan has been investigated by means of high level theoretical methods. The reaction is a complex process that begins with the formation of a pre-reactive hydrogen bonded complex, previous to the addition of the HO radical to furan, forming the 2 and 3-HO-adducts. In the prereactive complex, the hydrogen bond is formed by interaction between the hydrogen of the hydroxyl radical and the π system of furan and its stability is computed to be 1.6 kcal·mol-1 (including the BSSE corrections). The 2 and 3-HO-adducts are computed to be 30.5 and 12.5 kcal·mol-1 respectively, more stable than the reactants. The transition state leading to the formation of the 2-HO-adduct lies below the energy of the separate reactants (0.5 kcal·mol-1) and the transition state producing the 3-HO-adduct that is computed to lie 3.4 kcal·mol-1 above the sum of the energies of furan and hydroxyl radical. There are four reaction paths for the ring-opening of the 2-HO-adduct, leading to the formation of different conformers of 4-hydroxybutenaldehyde radical. The most stable of these conformers presents a cyclic symmetric (C2V) structure and can be characterized as a low-barrier hydrogen bond. The geometry optimizations and characterizations done in this work were carried out at MP2/6-311G(d,p), MP2/6-311+G(2df,2p) and QCISD/6-311G(d,p) levels of theory, whereas the relative energies are obtained at CCSD(T)/cc-pVTZ level of theory.The financial support for this research was provided by the Spanish Dirección General de Investigación Científica y Técnica (DGYCIT, Grant CTQ2005-07790) and by the Generalitat de Catalunya (Grant 2005SGR00111).Peer reviewe

Complex mechanism of the gas phase reaction between formic acid and hydroxyl radical. Proton coupled electron transfer versus radical hydrogen abstraction mechanisms

12, pages, 5 figures, 2 schemes, 2 tables.-- PMID: 15291585 [PubMed].-- Printed version published Aug 11, 2004.-- Supporting information available at: http://pubs.acs.org/doi/suppl/10.1021/ja0481169The gas phase reaction between formic acid and hydroxyl radical has been investigated with high level quantum mechanical calculations using DFT-B3LYP, MP2, CASSCF, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction has a very complex mechanism involving several elementary processes, which begin with the formation of a reactant complex before the hydrogen abstraction by hydroxyl radical. The results obtained in this investigation explain the unexpected experimental fact that hydroxyl radical extracts predominantly the acidic hydrogen of formic acid. This is due to a mechanism involving a proton coupled electron-transfer process. The calculations show also that the abstraction of formyl hydrogen has an increased contribution at higher temperatures, which is due to a conventional hydrogen abstraction radical type mechanism. The overall rate constant computed at 298 K is 6.24 × 10-13 cm3 molecules-1 s-1, and compares quite well with the range from 3.2 ± 1 to 4.9 ± 1.2 × 10-13 cm3 molecules-1 s-1, reported experimentally.The financial support for this work was provided by the Dirección General de Investigación Científica y Técnica (DGYCIT, Grant No. BQU2002-0485-C02-01) and by the Generalitat de Catalunya (Grant No. 2001SGR00048).Peer reviewe

Theoretical studies of the isoprene ozonolysis under trropospheric conditions. 2. Unimolecular and water-assisted decomposition of the α-hydroxy hydroperoxides

9 paeges, 6 figures, 1 scheme, 3 tables.-- Printed version published Jul, 31, 2003.-- Supporting information available at: http://pubs.acs.org/doi/suppl/10.1021/jp034203wTheoretical studies of the unimolecular and water-assisted decomposition reactions of the α-hydroxy hydroperoxides compounds produced during the isoprene ozonolysis have been investigated in this paper. Geometrical parameters of all the stationary points as well as energies and rate constants have been computed by means of several ab initio and DFT methods (B3LYP, CCSD(T), and G2M-RCC5 levels). Our calculations indicate that only the water-assisted decomposition of the α-hydroxy hydroperoxides could be active in the atmosphere. The main reaction products are predicted to be H2O2 plus methyl vinyl ketone (MVK) or methacroleine (MAC), depending on the particular α-hydroxy hydroperoxide considered. In both cases the reaction is endothermic by about 19 kcal mol-1.The financial support for this work was provided by the Dirección General de Investigación Científica y Técnica (DGICYT, Grant BQU2002-0485-C02-01) and by the Generalitat de Catalunya (Grant 2001SGR00048).Peer reviewe

Theoretical Characterization of the Gas-Phase O3HO Hydrogen-Bonded Complex

6 pages, 2 tables, 3 figures.We report a theoretical study on two gas-phase hydrogen-bonded complexes formed between ozone and hydroxyl radical that have relevance to atmospheric chemistry. This study was carried out by using CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches in conjunction with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. Both complexes have a planar structure and differ from each other in the orientation of the electronic density of the unpaired electron associated with the HO radical moiety. Our calculations predict their stabilities to be 0.87 and 0.67 kcal mol-1, respectively, at 0 K and show the importance of anharmonic effects in computing the red shift of the HO stretch originating from the hydrogen-bonding interaction. We also report two transition states involving the movement of the HO moiety on the potential energy surfaces of these hydrogen-bonded complexes.The financial support for this research was provided by the Spanish Dirección General de Investigación Científica y Técnica (DGYCIT, grant CTQ2005-07790) and by the Generalitat de Catalunya (Grant 2005SGR00111). The calculations described in this work were carried out at the Centre de Supercomputació de Catalunya (CESCA), whose services are gratefully acknowledged, and on an AMD Opteron cluster of our group. A.M. thanks the Spanish Ministerio de Educación y Ciencia for a fellowship (BES-2003-1352).Peer reviewe

Reaction Mechanism between Carbonyl Oxide and Hydroxyl Radical: A Theoretical Study

11 pages, 1 scheme, 5 figures, 2 tables.The reaction mechanism of carbonyl oxide with hydroxyl radical was investigated by using CASSCF, B3LYP, QCISD, CASPT2, and CCSD(T) theoretical approaches with the 6-311+G(d,p), 6-311+G(2df, 2p), and aug-cc-pVTZ basis sets. This reaction involves the formation of H2CO + HO2 radical in a process that is computed to be exothermic by 57 kcal/mol. However, the reaction mechanism is very complex and begins with the formation of a pre-reactive hydrogen-bonded complex and follows by the addition of HO radical to the carbon atom of H2COO, forming the intermediate peroxy-radical H2C(OO)OH before producing formaldehyde and hydroperoxy radical. Our calculations predict that both the pre-reactive hydrogen-bonded complex and the transition state of the addition process lie energetically below the enthalpy of the separate reactants (ΔH(298K) = −6.1 and −2.5 kcal/mol, respectively) and the formation of the H2C(OO)OH adduct is exothermic by about 74 kcal/mol. Beyond this addition process, further reaction mechanisms have also been investigated, which involve the abstraction of a hydrogen of carbonyl oxide by HO radical, but the computed activation barriers suggest that they will not contribute to the gas-phase reaction of H2COO + HOThe financial support for this research was provided by the Spanish Dirección General de Investigación Científica y Técnica (DGYCIT, Grant CTQ2005-07790) and by the Generalitat de Catalunya (Grant 2005SGR00111). The calculations described in this work were carried out at the Centre de Supercomputació de Catalunya (CESCA) and the Centro de Supercomputación de Galicia (CESGA), whose services are gratefully acknowledged, and at an AMD Opteron cluster of our group. A. Mansergas thanks the Spanish Ministerio de Educación y Ciencia for a fellowship (BES-2003-1352).Peer reviewe

Impact of the water dimer on the atmospheric reactivity of carbonyl oxides

The reactions of twelve carbonyl oxides or Criegee intermediates with the water monomer and with the water dimer have been investigated employing high level theoretical methods. The study includes all possible carbonyl oxides arising from the isoprene ozonolysis and the methyl and dimethyl carbonyl oxides that originated from the reaction of ozone with several hydrocarbons. These reactions have great significance in the chemistry of the atmosphere because Criegee intermediates have recently been identified as important oxidants in the troposphere and as precursors of secondary organic aerosols. Moreover, water vapor is one of the most abundant trace gases in the atmosphere and the water dimer can trigger the atmospheric decomposition of Criegee intermediates. Our calculations show that the nature and position of the substituents in carbonyl oxides play a very important role in the reactivity of these species with both the water monomer and the water dimer. This fact results in differences in rate constants of up to six orders of magnitude depending on the carbonyl oxide. In this work we have defined an effective rate constant (keff) for the atmospheric reaction of carbonyl oxides with water vapor, which depends on the temperature and on the relative humidity as well. With this keff we show that the water dimer, despite its low tropospheric concentration, enhances the atmospheric reactivity of Criegee intermediates, but its effect changes with the nature of carbonyl oxide, ranging between 59 and 295 times in the most favorable case (syn-methyl carbonyl oxide), and between 1.4 and 3 times only in the most unfavorable case. © 2016 the Owner Societies.This work was supported by the Spanish Secretaria de Estado de Investigación, Desarrollo e Innovación (CTQ2014-59768-P) and the Generalitat de Catalunya (Grant 2014SGR139). We also thank the Consorci de Serveis Universitaris de Catalunya (CSUC) for providing computational resources.Peer reviewe

Role of vibrational anharmonicity in atmospheric radical hydrogen-bonded complexes

12 pages, 2 figures, 8 tables.-- PMID: 19809669 [PubMed].-- Available online Jun 19, 2009.Harmonic and anharmonic vibrational frequency calculations are reported for the most stable hydrogen bonded complexes formed between the hydroperoxyl radical and formic, acetic, nitric, and sulfuric acids which are of atmospheric interest. A comparison between the calculated IR spectra of the hydrogen bonded complexes with the corresponding separate monomers is also reported with the aim to facilitate a possible experimental identification of these complexes. The calculations have been carried out using the second-order vibrational perturbative treatment implemented by Barone applied to the PES obtained with the B3LYP functional using the 6-31+G(d,p) and 6-311+G(2d,2p) basis sets. Our calculations for the separate monomers predict vibrational frequencies with quite a good agreement with the experimental values. The anharmonic contribution results in differences of around 40 cm-1 with respect to the harmonic values; although in some cases involving highly anharmonic modes, these differences can rise up to 300 and 450 cm-1.M.T. acknowledges the European Community for financial help through postdoctoral grant MEIF-CT-2006-025362. The financial support of the Spanish MEC and the Catalan Departament d’Universitats, Recerca i Societat de la Informació (DURSI), respectively, is acknowledged according to the following programmes: CTQ2008-06696/BQU and 2005SGR- 00238 (J.M.L.); and CTQ2005-07790 and 2005SGR00111 (J.M.A.).Peer reviewe

The reactions of SO3 with HO2 radical and H2O...HO2 radical complex. Theoretical study on the atmospheric formation of HSO5 and H2SO4

10 pages, 6 figures, 2 tables.-- PMID: 20165760 [PubMed].-- Available online Jan 14, 2010.-- Supporting information available at: http://dx.doi.org/10.1039/b916659aThe influence of a single water molecule on the gas-phase reactivity of the HO2 radical has been investigated by studying the reactions of SO3 with the HO2 radical and with the H2OHO2 radical complex. The naked reaction leads to the formation of the HSO5 radical, with a computed binding energy of 13.81 kcal mol−1. The reaction with the H2OHO2 radical complex can give two different products, namely (a) HSO5 + H2O, which has a binding energy that is computed to be 4.76 kcal mol−1 more stable than the SO3 + H2OHO2 reactants ((E + ZPE) at 0K) and an estimated branching ratio of about 34% at 298K and (b) sulfuric acid and the hydroperoxyl radical, which is computed to be 10.51 kcal mol−1 below the energy of the reactants ((E + ZPE) at 0K), with an estimated branching ratio of about 66% at 298K. The fact that one of the products is H2SO4 may have relevance in the chemistry of the atmosphere. Interestingly, the water molecule acts as a catalyst, [as it occurs in (a)] or as a reactant [as it occurs in (b)]. For a sake of completeness we have also calculated the anharmonic vibrational frequencies for HO2, HSO5, the HSO5H2O hydrogen bonded complex, H2SO4, and two H2SO4H2O complexes, in order to help with the possible experimental identification of some of these species.This research has been supported by the Spanish Dirección General de Investigación Científica y Técnica (DGYCIT, grant CTQ2008-06536/BQU) and by the Generalitat de Catalunya (Grant 2009SGR01472).Peer reviewe