Ab Initio Calculations of Hydrocarbon Thermochemistry and Reaction Kinetics

In the framework of the SFB 551 "Carbon from the Gas Phase: Elementary Reactions, Structures, Materials" several areas of carbon related chemistry have been studied with help of computational tools. They include the exploration of different ways of building PAHs, the attempt to check the limits of quantum chemistry methods in hydrocarbon chemistry using explicitly-correlated methods and the calculation of accurate reaction rates

A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients

HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50 to 400 K. We present 15 new reaction rate coefficients with no previously known value; 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to be inefficient and suggest that the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.Comment: 34 pages, 8 figures, 14 tables, accepted for publication in J Phys Chem

Elementary reactions involved in pollutant-forming mechanisms

The reactions of the hydroxyl radical (OH) with molecular chlorine (Reaction 1), methane (Reaction 2), and propane (Reaction 3) have been studied experimentally using a pulsed laser photolysis/pulsed-laser-induced fluorescence technique over wide ranges of temperatures (297-826, 298-1009, and 296-908 K, respectively) and at pressures between 6.68 and 24.15 kPascals. The rate coefficients for these reactions exhibit no dependence on pressure and exhibit positive temperature dependences that can be represented with modified three-parameter Arrhenius expressions within their corresponding temperature ranges: k1 = 3.59 x 10-16T1.35exp(-745K/T)cm3molecule-1sec-1, k2 = 3.82 x 10-19T2.38 exp(-1136K/T)cm3molecule-1sec-1, and k3 = 6.64 x 10-16T1.46 exp(-271K/T)cm3molecule-1sec-1. For the OH + Cl2 reaction, the potential energy surface has been studied using quantum chemical methods which suggests OH + Cl2 à HOCl + Cl as the main channel of this reaction. Density Functional Theory (DFT) along with Quadratic Configuration Interaction (QCISD(T)//DFT) calculations, with single, double, and triple electronic excitations, for the energetics of formation, stability, and reactivity of ortho-semiquinone, para-semiquinone, and the chloro-phenoxyl radicals have been performed using the 6-31G(d,p) basis set. Formation of these radicals from potential molecular precursors catechol, hydroquinone, and the chloro-phenols is readily achieved under combustion conditions through unimolecular scission of the phenoxyl-hydrogen bond or abstraction of the phenoxyl hydrogen by a hydrogen atom or hydroxyl radical. The resulting radicals are resonance stabilized and resist decomposition and oxidation. The calculations strongly suggest that combustion-generated semiquinone and chloro-phenoxyl radicals are sufficiently stable and resistant to oxidation to be considered persistent in the atmospheric environment. Semiquinone radicals (ortho- and para-hydroxy substituted phenoxyl radicals and various derivatives) are suspected to be biologically active and may lead to DNA damage, pulmonary disease, cardiovascular disease, and liver dysfunction. These radicals thought to be highly stable with low reactivity due to resonance stabilization including both carbon-centered and oxygen-centered radical resonance structures and been reported in cigarette tar. Chloro-phenoxyl radicals, on the other hand, are implicated in polychlorinated-dibenzodioxin and -dibenzofuran formation mechanisms, EPA pollutants, in the low temperature sections of hazardous waste combustion

Speciation of gaseous oxidized mercury molecules relevant to atmospheric and combustion environments

Mercury is a pervasive and highly toxic environmental pollutant. Major anthropogenic sources of mercury emissions include artisanal gold mining, cement production, and combustion of coal. These sources release mostly gaseous elemental mercury (GEM), which upon entering the atmosphere can travel long distances before depositing to environmental waters and landforms. The deposition of GEM is relatively slow, but becomes greatly accelerated when GEM is converted to gaseous oxidized mercury (GOM) because the latter has significantly higher water solubility and lower volatility. Modeling GOM deposition requires the knowledge of its molecular identities, which are poorly known because ultra-trace (tens to hundreds part per quadrillion) level of GOM in the atmosphere makes its experimental detection and analysis a formidable task. It is here where computational methods can help address the GOM molecular identity problem. Accordingly, the two major goals of this work are to (a) develop a computationally inexpensive approach for assessing accurate thermochemistry of GOM species and (b) investigate ion-molecule reactions of GOM species in order to assist experimentalists in the development of a novel detection method. The first goal addresses the question of what are some of the molecular identities of GOM species that could be present in combustion and atmospheric environments. Ab initio and density functional theory calculations are used in combination with the methods of isodesmic and isogyric work reactions in order to calculate accurate heats of formation for GOM species that can form in reactions of GEM with atomic halogens, OH, OCl, and OBr. The accuracy of the calculations is assessed by comparing the calculated values against experimental data and also data from rigorous and computationally expensive state-of-the-art ab initio calculations. Bond dissociation energies (BDE) are determined from the heats of formation and used as a measure of the stability of the GOM species studied. The second goal of this work addresses the question of how can GOM species be measured in the atmosphere in real-time while retaining speciation information, using chemical ionization mass spectrometry. Ab initio and density functional theory calculations are used to determine structures of products of ion-molecule reactions and calculate associated reaction enthalpies and Gibbs free energies. The obtained data are used to identify reagent ions that can be used for atmospheric detection of GOM. The calculations provide an understanding of the complex ion-molecule chemistry that occurs during the chemical ionization process. The implications of this body of work are as follows. A low computational cost methodology is established that can be used to study a wide range of GOM species outside the scope of this work. The thermochemistry of the GOM species calculated in this work can serve as the foundation for future kinetic studies with the goal of improving the reaction mechanism in global transport models to provide a better understanding of the global mercury budget. Reagent ions identified in this work can be used for real-time speciation of GOM in the atmosphere, using chemical ionization mass spectrometry

Kinetic modeling of sulfur oxides formation from fuels and biofuels

LAUREA MAGISTRALELa continua ricerca verso la miscelazione di combustibili fossili (benzina, diesel, ecc.) con biocarburanti ottenuti da fonti rinnovabili deve sempre affrontare normative sempre più severe, che stanno spingendo verso maggiori efficienze e minori emissioni di composti inquinanti. Inoltre, se da un lato i biocarburanti, ottenuti da biomasse, possono ridurre efficacemente le emissioni inquinanti, dall'altro è importante studiarne la compatibilità con le tecnologie dei motori esistenti. L'acido solfidrico (H2S) è un prodotto comune nelle raffinerie di petrolio; inoltre, è presente anche in piccole quantità nel biogas e nel bio-olio, che sono possibili sostituti ai combustibili fossili poiché sono ottenuti da fonti rinnovabili. Generalmente, le specie a base di zolfo sono inquinanti comuni che possono creare le cosiddette "piogge acide" dopo essere state rilasciate nell'atmosfera. Le emissioni di acido solfidrico sono estremamente limitate dal punto di vista legislativo a causa della sua letale conseguenza sulla salute umana. Per questo motivo, è fondamentale comprendere e prioritizzare la sua chimica di pirolisi e ossidazione per fornire gli sviluppi necessari alle tecnologie di rimozione dello zolfo. Partendo dagli esperimenti all'avanguardia e dai calcoli teorici, e sfruttando i principi gerarchici e modulari del meccanismo POLIMI sviluppato dal gruppo CRECK, questa tesi mira ad aggiornare un meccanismo cinetico dettagliato che descrive la pirolisi e l'ossidazione delle specie contenenti zolfo. A tal fine, sarà adottato un approccio di primo principio per ricavare le velocità di reazione delle reazioni selezionate (ad es. Reazioni di estrazione di H), che sono state scelte grazie a delle analisi di sensitività preliminari. In altre parole, le costanti cinetiche di ogni reazione selezionata sono completamente determinate a priori, cioè ab initio, a partire dalla struttura elettronica. Il meccanismo cinetico ottenuto verrà quindi convalidato rispetto agli esperimenti disponibili raccolti negli ultimi decenni nei reattori ideali. Inoltre, verranno evidenziate specie chiave e percorsi di reazione per comprendere quali reazioni richiedono ulteriori analisi teoriche per il futuro miglioramento del modello cinetico.The continuous research towards the blending of fossil fuels (gasoline, diesel, etc.) with biofuels obtained from renewable sources has always to deal with increasingly stringent regulations, which are pushing towards higher efficiencies and lower emissions of pollutant compounds. Moreover, if from one side biofuels, obtained from biomasses, can effectively decrease pollutants emissions, from the other side, it’s important to study their compatibility with the existing engines technologies. Hydrogen sulfide (H2S) is a common by product in oil refineries; moreover, it’s also present in trace in biogas and bio-oil, which are possible substitute to fossil fuels since they are obtained from renewable sources. Generally, sulfur species are common pollutants which can create the so-called “acid rains” after being released into the atmosphere. Hydrogen sulfide emissions are extremely limited from a legislative point of view because of its lethal consequence on human health. For this reason, it’s fundamental to understand and underlie its chemistry of pyrolysis and oxidation to provide the necessary developments to sulfur removal technologies. Starting from the state-of-art experiments and theoretical calculations, and leveraging the hierarchy and modularity principles of the POLIMI mechanism developed by the CRECK group, this thesis is targeted to upgrade a detailed kinetic mechanism describing the pyrolysis and oxidation of sulfur-containing species. To the purpose, a first-principles approach will be adopted to derive the reaction rates of selected reactions (e.g. H-abstraction reactions), which have been chosen thanks to preliminary sensitivity analysis. In other words, the kinetic constants of each reaction selected are completely determined a priori, i.e. ab initio, starting from electronic structure. The upgraded kinetic mechanism is then validated against the available experiments collected in the latest decades in ideal reactors. Furthermore, key species and reaction pathways will be highlighted to understand which reactions require further theoretical analysis for future improvement of the kinetic model

Thermochemistry and kinetic analysis on radicals of acetaldehyde + O2, allyl radical + O2 and diethyl and chlorodiethyl sulfides

Thermochemical properties for reactants, intermediates, products and transition states important in the radicals of acetaldehyde + O2 and allyl radical + O2 reaction systems are analyzed with density functional and ab initic calculations, to evaluate the reaction paths and kinetics in oxidation and pyrolysis. Ketene is one important product resulting from acetaldehyde oxidation; thus thermochemistry plus isomerization and conversion reactions of ketene are also analyzed. Enthalpies of formation are determined using isodesmic reaction analysis at the CBSQ composite and density functional levels. Entropies and heat capacities are determined using geometric parameters and vibration frequencies obtained at the HF/6-31G(d\u27) or B3LYP/6-31G(d,p) level of theory. Internal rotor contributions are included in calculation of entropy, S°298, and heat capacities, Cp(T). Rate constants are estimated as a function of pressure and temperature using multifrequency quantum Rice-Ramsperger-Kassel analysis for k(E) and master equation analysis for falloff. A mechanism for pyrolysis and oxidation of acetaldehyde and its\u27 corresponding radicals is constructed. The competition between reactions of radicals of acetaldehyde with O2 versus unimolecular decomposition is evaluated versus temperature and pressure. Thermodynamic parameters, enthalpies, entropies and heat capacities are evaluated for C1 and C2 chlorocarbon molecules and radicals. These thermodynamic properties are used in evaluation and comparison of Cl2 + R. \u3c--\u3e Cl. + RCl reaction rate constants from the kinetics literature for comparison with empirical analysis. Data from some 20 reactions in the literature show linearity on a plot of Eafwd vs Δrxn, fwd, yielding a slope of (0.38 ± 0.04) and an intercept of (10.12 ± 0.81) kcal/mol. The use of Density Functional Theory, B3LYP/6-31g(d,p), with isodesmic working reactions for enthalpy of formation of sulfur hydrocarbons is evaluated using a set of known sulfur hydrocarbon / radical species. Thermodynamic and kinetic parameters for reactants, transition states, and products from unimolecular dissociations of sulfur species related to the chemical agent: CH3CH2SCH2CH2, CH3CH2SCH2CH2Cl, and CH2ClCH2SCH2CH2Cl and corresponding radicals are analyzed. Standard enthalpy, ΔHf°298, for the molecules and radicals are determined using isodesmic reaction analysis at the B3LYP/6-31G(d,p) level, with S°298 and Cp(T) determined using geometric parameters and vibrational frequencies obtained at this same level of theory. Potential barriers for the internal rotor potentials are also calculated at the B3LYP/6-31G(d,p) level, and the hindered rotation contributions to S°298 and Cp(T) are calculated

Plasma Nanoscience: from Nano-Solids in Plasmas to Nano-Plasmas in Solids

The unique plasma-specific features and physical phenomena in the organization of nanoscale solid-state systems in a broad range of elemental composition, structure, and dimensionality are critically reviewed. These effects lead to the possibility to localize and control energy and matter at nanoscales and to produce self-organized nano-solids with highly unusual and superior properties. A unifying conceptual framework based on the control of production, transport, and self-organization of precursor species is introduced and a variety of plasma-specific non-equilibrium and kinetics-driven phenomena across the many temporal and spatial scales is explained. When the plasma is localized to micrometer and nanometer dimensions, new emergent phenomena arise. The examples range from semiconducting quantum dots and nanowires, chirality control of single-walled carbon nanotubes, ultra-fine manipulation of graphenes, nano-diamond, and organic matter, to nano-plasma effects and nano-plasmas of different states of matter.Comment: This is an essential interdisciplinary reference which can be used by both advanced and early career researchers as well as in undergraduate teaching and postgraduate research trainin