Η Επίδραση του Σχηματισμού Ενδιαμέσων Προϊόντων Πρόσθεσης Ατόμων Χλωρίου και Ριζών Υδροξυλίου στην Κινητική των Αντιδράσεων τους με Κεκορεσμένες Πτητικές Οργανικές Ενώσεις

Η συγκεκριμένη διατριβή είχε σαν στόχο τη διερεύνηση της σταθερότητας των ενδιαμέσων προϊόντων πρόσθεσης (adducts) και την συσχέτιση τους τόσο με τους συντελεστές ταχύτητας όσο και τους μηχανισμούς αντιδράσεων μιάς σειράς κορεσμένων πτητικών οργανικών ενώσεων (Volatile Organic Compounds, VOC), με ρίζες υδροξυλίου και άτομα χλωρίου στην αέρια φάση, χρησιμοποιώντας μοριακούς κβαντομηχανικούς υπολογισμούς. Οι αντιδράσεις των ριζών ΟΗ είναι ιδιαίτερα σημαντικές στη χημεία της ατμόσφαιρας διότι οι ρίζες ΟΗ αποτελούν το κυρίαρχο δραστικό συστατικό της τροπόσφαιρας. Ωστόσο, τα τελευταία χρόνια, σχετικά αυξημένα επίπεδα συγκεντρώσεων ατόμων Cl που παρατηρήθηκαν, ιδιαίτερα σε παράκτιες περιοχές, κατέστησαν τη μελέτη της συμβολής τους στην τροποσφαιρική αποικοδόμηση των VOC απαραίτητη. Ο τρόπος δράσης και των δύο απορρυπαντικών της ατμόσφαιρας είναι παρεμφερής και συγκεκριμένα, ως επί το πλείστον, εκκινούν αντιδράσεις απαγωγής ατόμων υδρογόνου: Στην παρούσα εργασία μελετάται, με τη βοήθεια του Προγράμματος Υπολογιστικής Χημείας Gaussian 98, η δομή και η σταθερότητα των ενδιαμέσων προϊόντων πρόσθεσης των δύο δραστικών συστατικών (ΟΗ, Cl) στα μόρια CH4, CH3F, CHF3, CH3OCH3 και CH3OH. Επιπλέον, η σταθερότητα των ενδιαμέσων συσχετίζεται με τους αντίστοιχους συντελεστές ταχύτητας kCl και kOH και αναζητούνται οι αιτίες της φαινομενικής ιδιομορφίας στην εξάρτηση των συντελεστών ταχύτητας από τη μοριακή δομή. Οι δομές και οι δονητικές συχνότητες των ενδιαμέσων προσδιορίστηκαν σε διάφορα επίπεδα Θεωρίας, περιλαμβάνοντα την Θεωρία Διαταραχών δεύτερης τάξης (MP2) καθώς και την Θεωρία Συζευγμένου Σμήνους (Coupled-Cluster) η οποία περιλαμβάνει απλές, διπλές και τριπλές διεγέρσεις, CCSD(T). Η σταθερότητα των ενδιαμέσων προσδιορίστηκε χρησιμοποιώντας την Θεωρία CCSD(T), στο όριο πλήρους συνόλου βάσης, το οποίο προσεγγίστηκε χρησιμοποιώντας σύνολα βάσης συνεπών με την ηλεκτρονιακή συσχέτιση (correlation-consistent basis sets).The present work attempts to investigate the stability of a series of weakly bound complexes and their correlation with the rate coefficients, as well as the reaction mechanism, for five different molecules, categorized as Volatile Organic Compounds (VOC) with hydroxyl radicals and chlorine atoms in the gas-phase, using ab initio calculations. Reactions of hydroxyl radicals are very important in Atmospheric Chemistry, as they are considered to be the dominant tropospheric component. However, recent studies implied that chlorine atoms should be included, as far as the tropospheric degradation of Volatile Organic Compounds is concerned. That is due to the relatively high concentration of chlorine atoms, especially in coastal areas, along with the fact that generally, chlorine atoms tend to react faster than hydroxyl radicals. Both hydroxyl radicals and chlorine atoms, which are denoted as tropospheric detergents, react by a similar way with Volatile Organic Compounds, that include an available hydrogen atom, via direct hydrogen abstraction: In this thesis, we use the program suite Gaussian 98, in order to determine the conformation (geometries) and binding energy of weakly bound complexes that consist either of ΟΗ radicals, or Cl atoms and methane, fluoromethane, trifluoromethane, methanol and dimethylether. Moreover, we examine if there is any correlation between the stability of these complexes and the rate coefficients (kOH and kCl) for the corresponding reactions. Optimized geometries as well as the vibrational frequencies of these complexes, were obtained by using second order Møller-Plesset Perturbation Theory, MP2 and Coupled-Cluster Theory, which handles single, double and partly triple excitations by a quasipertubative treatment CCSD(T), in combination with cc-pVDZ and AUG-cc-pVDZ basis sets. In addition, single point calculations were carried out, using CCSD(T) with correlation-consistent basis sets (cc-pVnZ, n = D, T, Q), in order to approach the complete basis set limit, by using two, as well as three-point extrapolation

Chemical processes of atmospheric interest in water clusters

Two of the most contemporary issues facing humanity, are the enhancement of the greenhouse effect and the depletion of the ozone layer in the stratosphere. The rapid industrial development and excessive use of chemical contaminants in a variety of technological applications are main causes of air pollution. Therefore, the design of environmentally friendly compounds and control emissions is imperative to protect the environment. Besides the photolysis and gas-phase chemical degradation by the active components (OH, Cl, NO3 and O3), lies the suggestion that water in the Atmosphere, can form complexes through strong hydrogen bonds with atmospheric pollutants, which may alter the kinetics of their chemical conversion processes. The possible change in the gas phase reaction rates was the springboard for the focus of interest in chemical processes during the presence of water clusters. This study investigated the effect of the presence of water clusters [(H2O)n where n: 1,2,3], in the rate coefficients of reactions OH radicals and Cl atoms with the simplest air pollutants: CH4, CH3OH, CH3F, CH2F2 and CHF3, using theoretical calculations, with the computational chemistry package Gaussian 03. Main concern was to determine the change in the reaction rate coefficient when water is involved as a substrate, in comparison with the reaction that occurs in the gas phase. The geometry optimization as well as the vibrational frequencies of the structures was carried out at two levels: a) MP2/cc-pVDZ and b) BH&HLYP/cc-pVDZ. The optimized geometries of the components were further refined by extra single point calculations using the method G3, in order to obtain even more reliable energetic results.The results of this study showed that the presence of water clusters, introduces a change in the kinetics of Cl atoms and the OH radicals reactions, with most frequent effect their acceleration. As OH radicals and Cl atoms are considered as main tropospheric detergents and given the notable atmospheric abundance of water, its tendency to form water clusters as well as their considerable concentrations, it appears that the acceleration may modify the atmospheric lifetimes of the investigated molecules. These molecules are expected to exhibit shorter lifetimes and hence smaller global warming potentials (GWP).Σύγχρονα ζητήματα που απασχολούν την ανθρωπότητα είναι η ενίσχυση του φαινομένου του θερμοκηπίου και η αραίωση του στρατοσφαιρικού όζοντος. Η βιομηχανική ανάπτυξη και η χρήση ρυπαντών, σε τεχνολογικές εφαρμογές αποτελούν βασικές αιτίες ρύπανσης της Ατμόσφαιρας. Η σχεδίαση φιλικών προς το περιβάλλον ενώσεων και ο έλεγχος εκπομπών τους αποτελούν προϋπόθεση για την προάσπιση του Περιβάλλοντος. Πέραν της φωτόλυσης και της χημικής αποικοδόμησης αέριας φάσης (μηχανισμοί αποικοδόμησης), μέσω των δραστικών συστατικών της ατμόσφαιρας (OH, Cl, NO3 και O3) προτάθηκε ότι οι υδρατμοί, ενδέχεται να σχηματίζουν σύμπλοκα μέσω δεσμών υδρογόνου με ατμοσφαιρικούς ρύπους, μεταβάλλοντας κινητικά τη χημική αποικοδόμησή τους. Η μεταβολή των συντελεστών ταχύτητας αντίδρασης, αποτέλεσε το εφαλτήριο για την επικέντρωση ενδιαφέροντος σε χημικές διαδικασίες παρουσία νερού και συσσωματωμάτων του. Στην παρούσα μελέτη διερευνήθηκε η επίδραση παρουσίας συμπλόκων νερού [(H2O)n n:1,2,3], στους συντελεστές ταχύτητας των αντιδράσεων ριζών ΟΗ και ατόμων Cl με απλούς ατμοσφαιρικούς ρύπους: CH4, CH3OH, CH3F, CH2F2 και CHF3, χρησιμοποιώντας θεωρητικούς υπολογισμούς, με το πακέτο Υπολογιστικής Χημείας Gaussian 03. Προσδιορίζεται η μεταβολή του συντελεστή ταχύτητας αντίδρασης, όταν το νερό εμπλέκεται, σε σύγκριση με την αντίδραση αέριας φάσης. Για τη βελτιστοποίηση της γεωμετρίας και των δονητικών συχνοτήτων των χρησιμοποιήθηκαν δυο επίπεδα θεωρίας:α)ΜP2/cc-pVDZ και β)BH&HLYP/ccpVDZ. Επιπλέον, έγιναν υπολογισμοί απλού σημείου με τη μέθοδο G3, προκειμένου να ληφθούν ακόμη πιο αξιόπιστα ενεργειακά αποτελέσματα.Τα αποτελέσματα της διατριβής έδειξαν πως η πλειονότητα των αντιδράσεων με τα άτομα Cl και με τις ρίζες OH, παρουσία συσσωματωμάτων νερού, συμβαίνει ταχύτερα. Δεδομένου ότι αποτελούν κύρια απορρυπαντικά της τροπόσφαιρας, σε συνδυασμό με την σχετική αφθονία του νερού, την τάση να δημιουργεί σύπλοκα και των αξιοσημείωτων συγκεντρώσεων αυτών, η επιτάχυνση ενδέχεται να τροποποιήσει τους χρόνους ζωής των μελετώμενων μορίων. Τα μόρια αυτά αναμένεται να εμφανίζουν μικρότερους χρόνους ζωής και μικρότερο δείκτη παγκόσμιας θέρμανσης GWP

Computational Kinetic Study for the Unimolecular Decomposition of Cyclopentanone

International audienc

Computational Kinetic Study for the Unimolecular Decomposition Pathways of Cyclohexanone

There has been evidence lately that several endophytic fungi can convert lignocellulosic biomass into ketones among other oxygenated compounds. Such compounds could prove useful as biofuels for internal combustion engines. Therefore, their combustion properties are of high interest. Cyclohexanone was identified as an interesting second-generation biofuel (Boot, M.; et al. Cyclic Oxygenates: A New Class of Second-Generation Biofuels for Diesel Engines? Energy Fuels 2009, 23, 1808−1817; Klein-Douwel, R. J. H.; et al. Soot and Chemiluminescence in Diesel Combustion of Bio-Derived, Oxygenated and Reference Fuels. Proc. Combust. Inst. 2009, 32, 2817–2825). However, until recently (Serinyel, Z.; et al. Kinetics of Oxidation of Cyclohexanone in a Jet- Stirred Reactor: Experimental and Modeling. Proc. Combust. Inst. 2014; DOI: 10.1016/j.proci.2014.06.150), no previous studies on the kinetics of oxidation of that fuel could be found in the literature. In this work, we present the first theoretical kinetic study of the unimolecular decomposition pathways of cyclohexanone, a cyclic ketone that could demonstrate important fuel potential. Using the quantum composite G3B3 method, we identified six different decomposition pathways for cyclohexanone and computed the corresponding rate constants. The rate constants were calculated using the G3B3 method coupled with Rice–Ramsperger–Kassel–Marcus theory in the temperature range of 800–2000 K. Our calculations show that the kinetically more favorable channel for thermal decomposition is pathway 2 that produces 1,3-butadien-2-ol, which in turn can isomerize easily to methyl vinyl ketone through a small barrier. The results presented here can be used in a future kinetic combustion mechanism

A Chemical Kinetic Investigation on Butyl Formate Oxidation: Ab Initio Calculations and Experiments in a Jet-Stirred Reactor

Biofuels are expected to play a significant role in the quest for greener energy generation. In this perspective, esters produced from biomass are promising candidates. This work presents the first computational kinetic study on n-butyl formate (BF) oxidation under combustion conditions coupled to an experimental study in a jet-stirred reactor. Absolute rate constants for hydrogen abstraction reactions by the OH radical were calculated using the G3//MP2/aug-cc-pVDZ model chemistry, in conjunction with statistical rate theory (TST). Subsequently, the fate of the butyl formate radicals was also investigated by calculating absolute rate constants for combustion relevant decomposition channels such as β-scission and hydrogen transfer reactions. The derived rate expressions were used in the presently developed detailed kinetic mechanism, which was validated over experimental data obtained in a jet-stirred reactor at 10 atm and for three different mixtures (φ = 0.45, 0.9, and 1.8). Rate of production analyses were finally used to understand the oxidation kinetics of butyl formate over the temperature range of 500–1300 K and highlighted the importance of the unimolecular decomposition reactions of the fuel, producing formic acid and 1-butene

The atmospheric impact of the reaction of N2O with NO3: A theoretical study

International audienceThe reaction of N2O with NO3 is studied for the first time using high-level quantum chemical calculations, followed by statistical rate coefficient estimations. Two reaction pathways giving NO2+N2+O2 and NO2+2NO have been explored. The formation of NO2+N2+O2 is exothermic by 30 kcal mol−1 while that of NO2+2NO is endothermic by 13 kcal mol−1. Both mechanisms have significant reaction barriers, and the reaction rate constants are very low: about 6×10–43 cm3 s−1 at 298 K. Therefore, this reaction is not expected to affect the lifetime of N2O in the atmosphere

Mineral Oxides Change the Atmospheric Reactivity of Soot: NO 2 Uptake under Dark and UV Irradiation Conditions

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Mineral Oxides Change the Atmospheric Reactivity of Soot: NO₂ Uptake under Dark and UV Irradiation Conditions

The heterogeneous reactions between trace gases and aerosol surfaces have been widely studied over the past decades, revealing the crucial role of these reactions in atmospheric chemistry. However, existing knowledge on the reactivity of mixed aerosols is limited, even though they have been observed in field measurements. In the current study, the heterogeneous interaction of NO₂ with solid surfaces of Al₂O₃ covered with kerosene soot was investigated under dark conditions and in the presence of UV light. Experiments were performed at 293 K using a low-pressure flow-tube reactor coupled with a quadrupole mass spectrometer. The steady-state uptake coefficient, γ_ss, and the distribution of the gas-phase products were determined as functions of the Al₂O₃ mass; soot mass; NO₂ concentration, varied in the range of (0.2–10) × 10¹² molecules cm^–3; photon flux; and relative humidity, ranging from 0.0032% to 32%. On Al₂O₃/soot surfaces, the reaction rate was substantially increased, and the formation of HONO was favored compared with that on individual pure soot and pure Al₂O₃ surfaces. Uptake of NO₂ was enhanced in the presence of H₂O under both dark and UV irradiation conditions, and the following empirical expressions were obtained: γ_ss,BET,dark = (7.3 ± 0.9) × 10^–7 + (3.2 ± 0.5) × 10^–8 × RH and γ_ss,BET,UV = (1.4 ± 0.2) × 10^–6 + (4.0 ± 0.9) × 10^–8 × RH. Specific experiments, with solid sample preheating and doping with polycyclic aromatic hydrocarbons (PAHs), showed that UV-absorbing organic compounds significantly affect the chemical reactivity of the mixed mineral/soot surfaces. A mechanistic scheme is proposed, in which Al₂O₃ can either collect electrons, initiating a sequence of redox reactions, or prevent the charge-recombination process, extending the lifetime of the excited state and enhancing the reactivity of the organics. Finally, the atmospheric implications of the observed results are briefly discussed

A Chemical Kinetic Investigation on Butyl Formate Oxidation: <i>Ab Initio</i> Calculations and Experiments in a Jet-Stirred Reactor

Biofuels are expected to play a significant role in the quest for greener energy generation. In this perspective, esters produced from biomass are promising candidates. This work presents the first computational kinetic study on <i>n</i>-butyl formate (BF) oxidation under combustion conditions coupled to an experimental study in a jet-stirred reactor. Absolute rate constants for hydrogen abstraction reactions by the OH radical were calculated using the G3//MP2/aug-cc-pVDZ model chemistry, in conjunction with statistical rate theory (TST). Subsequently, the fate of the butyl formate radicals was also investigated by calculating absolute rate constants for combustion relevant decomposition channels such as β-scission and hydrogen transfer reactions. The derived rate expressions were used in the presently developed detailed kinetic mechanism, which was validated over experimental data obtained in a jet-stirred reactor at 10 atm and for three different mixtures (φ = 0.45, 0.9, and 1.8). Rate of production analyses were finally used to understand the oxidation kinetics of butyl formate over the temperature range of 500–1300 K and highlighted the importance of the unimolecular decomposition reactions of the fuel, producing formic acid and 1-butene