Quantum chemistry and kinetics of hydrogen sulphide oxidation.

A fundamental understanding of the acid gas (HS and CO) chemistry is key to efficiently implement the desulphurisation process and even the production of clean fuels such as hydrogen or syngas. In this work, we developed a new kinetic model for the pyrolysis and oxidation of hydrogen sulphide by merging two previously reported models with the goal of covering a wider range of conditions and including the effect of carbon dioxide. The resulting model, which consists of 75 species and 514 reactions, was used to conduct rate of production and sensitivity analysis in plug flow reactor simulations, and the results were used to determine the most prominent reactions in which hydrogen sulphide, molecular hydrogen, and sulphur monoxide are involved.

The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study.

The presence of two functional groups (OH and double bond) in C methyl-substituted enols (, isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (, below ∼700 K), and H-atom abstraction dominates at higher temperatures.

Early Chemistry of Nicotine Degradation in Heat-Not-Burn Smoking Devices and Conventional Cigarettes: Implications for Users and Second- and Third-Hand Smokers.

Nicotine exposure results in health risks not only for smokers but also for second- and third-hand smokers. Unraveling nicotine's degradation mechanism and the harmful chemicals that are produced under different conditions is vital to assess exposure risks. We performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation.

Efficient alkane oxidation under combustion engine and atmospheric conditions.

Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol.

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory.

The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory ( , , 2690) is suitable to determine rate constants below the high-pressure limit. Its current implementation allows incorporating variational effects, multidimensional tunneling, and multistructural torsional anharmonicity in rate constant calculations. Master equation solvers offer a more rigorous approach to compute pressure-dependent rate constants, but several implementations available in the literature do not incorporate the aforementioned effects.

Data Science Approach to Estimate Enthalpy of Formation of Cyclic Hydrocarbons.

In spite of increasing importance of cyclic hydrocarbons in various chemical systems, studies on the fundamental properties of these compounds, such as enthalpy of formation, are still scarce. One of the reasons for this is the fact that the estimation of the thermodynamic properties of cyclic hydrocarbon species via cost-effective computational approaches, such as group additivity (GA), has several limitations and challenges. In this study, a machine learning (ML) approach is proposed using a support vector regression (SVR) algorithm to predict the standard enthalpy of formation of cyclic hydrocarbon species.

Kinetics of the benzyl + HO and benzoxyl + OH barrierless association reactions: fate of the benzyl hydroperoxide adduct under combustion and atmospheric conditions.

Radical-radical association reactions are challenging to address theoretically due to difficulties finding the bottleneck that variationally minimizes the reactive flux. For this purpose, the variable reaction coordinate (VRC) formulation of the variational transition state theory (VTST) represents an appropriate tool. In this work, we revisited the kinetics of two radical-radical association reactions of importance in combustion modelling and poly-aromatic hydrocarbon (PAH) chemistry by performing VRC calculations: benzyl + HO2 and benzoxyl + OH, both forming the adduct benzyl hydroperoxide.

Machine Learning To Predict Standard Enthalpy of Formation of Hydrocarbons.

Thermodynamic properites of molecules are used widely in the study of reactive processes. Such properties are typically measured via experiments or calculated by a variety of computational chemistry methods. In this work, machine learning (ML) models for estimation of standard enthalpy of formation at 298.

Identifying Collisions of Various Molecularities in Molecular Dynamics Simulations.

We present a method based on kinetic molecular theory that identifies reactions of various molecularities in molecular dynamics (MD) simulations of bulk gases. The method allows characterization of the thermodynamic conditions at which higher than bimolecular reactions are a factor in the mechanisms of complex gas-phase chemistry. Starting with Bodenstein's definition of termolecular collisions we derive analytical expressions for the frequency of higher molecularity collisions.

Ab Initio, Transition State Theory, and Kinetic Modeling Study of the HO-Assisted Keto-Enol Tautomerism Propen-2-ol + HO ⇔ Acetone + HO under Combustion, Atmospheric, and Interstellar Conditions.

Keto-enol tautomerisms are important reactions in gaseous and liquid systems with implications in different chemical environments, but their kinetics have not been widely investigated. These reactions can proceed via a unimolecular process or may be catalyzed by another molecule. This work presents a theoretical study of the HO-catalyzed tautomerism that converts propen-2-ol into acetone at conditions relevant to combustion, atmospheric and interstellar chemistry.

Theoretical Kinetic Study of the Unimolecular Keto-Enol Tautomerism Propen-2-ol ↔ Acetone. Pressure Effects and Implications in the Pyrolysis of tert- and 2-Butanol.

The need for renewable and cleaner sources of energy has made biofuels an interesting alternative to fossil fuels, especially in the case of butanol isomers, with its favorable blend properties and low hygroscopicity. Although C alcohols are prospective fuels, some key reactions governing their pyrolysis and combustion have not been adequately studied, leading to incomplete kinetic models. Enols are important intermediates in the combustion of C alcohols, as well as in atmospheric processes.

Theoretical kinetic study of the formic acid catalyzed Criegee intermediate isomerization: multistructural anharmonicity and atmospheric implications.

We performed a theoretical study on the double hydrogen shift isomerization reaction of a six carbon atom Criegee intermediate (C6-CI), catalyzed by formic acid (HCOOH), to produce vinylhydroperoxide (VHP), C6-CI + HCOOH → VHP + HCOOH. This Criegee intermediate can serve as a surrogate for larger CIs derived from important volatile organic compounds like monoterpenes, whose reactivity is not well understood and which are difficult to handle computationally. The reactant HCOOH exerts a pronounced catalytic effect on the studied reaction by lowering the barrier height, but the kinetic enhancement is hindered by the multistructural anharmonicity.

Ab initio and transition state theory study of the OH + HO → HO + O(Σ)/O(Δ) reactions: yield and role of O(Δ) in HO decomposition and in combustion of H.

Reactions of hydroxyl (OH) and hydroperoxyl (HO) are important for governing the reactivity of combustion systems. We performed post-CCSD(T) ab initio calculations at the W3X-L//CCSD = FC/cc-pVTZ level to explore the triplet ground-state and singlet excited-state potential energy surfaces of the OH + HO → HO + O(Σ)/O(Δ) reactions. Using microcanonical and multistructural canonical transition state theories, we calculated the rate constant for the triplet and singlet channels over the temperature range 200-2500 K, represented by k(T) = 3.

On the role of the termolecular reactions 2O + H → 2HO and 2O + H → H + HO + O in formation of the first radicals in hydrogen combustion: ab initio predictions of energy barriers.

We have investigated the role of termolecular reactions in the early chemistry of hydrogen combustion. We performed molecular chemical dynamics simulations using ReaxFF in LAMMPS to identify potential initial reactions for a 1 : 4 mixture of H : O in the NVT ensemble at density 276.3 kg m and ∼3000 K (∼4000 atm) and ∼4000 K (∼5000 atm), and then characterized the saddle points for those reactions using ab initio methods: CCSD(T) = FC/cc-pVTZ//MP2/6-31G, CCSD(T) = FULL/aug-cc-pVTZ//CCSD = FC/cc-pVTZ and CASSCF MP2/6-31G//MP2/6-31G.

Quasiclassical trajectory study of the effect of antisymmetric stretch mode excitation on the O(³P) + CH₄(ν₃ = 1) → OH + CH₃ reaction on an analytical potential energy surface. Comparison with experiment.

Motivated by a recent crossed-beam experiment on the title reaction reported by Pan and Liu [J. Chem. Phys.

The rate constant for radiative association of HF: comparing quantum and classical dynamics.

Radiative association for the formation of hydrogen fluoride through the A(1)Π → X(1)Σ(+) and X(1)Σ(+) → X(1)Σ(+) transitions is studied using quantum and classical dynamics. The total thermal rate constant is obtained for temperatures from 10 K to 20,000 K. Agreement between semiclassical and quantum approaches is observed for the A(1)Π → X(1)Σ(+) rate constant above 2000 K.

Bond and mode selectivity in the OH + NH2D reaction: a quasi-classical trajectory calculation.

A state-to-state dynamics study was performed to analyze the effects of vibrational excitation on the dynamics of the OH + NH2D gas-phase reaction, which are connected to issues such as bond and mode selectivity. This reaction can evolve along two channels: H-abstraction, H2O(ν) + NHD(ν); and D-abstraction, HOD(ν) + NH2(ν). Based on an analytical potential energy surface previously developed by our group, quasi-classical trajectory calculations and subsequent normal mode analysis were performed.

Dynamics study of the OH + NH3 hydrogen abstraction reaction using QCT calculations based on an analytical potential energy surface.

To understand the reactivity and mechanism of the OH + NH3 → H2O + NH2 gas-phase reaction, which evolves through wells in the entrance and exit channels, a detailed dynamics study was carried out using quasi-classical trajectory calculations. The calculations were performed on an analytical potential energy surface (PES) recently developed by our group, PES-2012 [Monge-Palacios et al. J.

Role of vibrational and translational energy in the OH + NH3 reaction: a quasi-classical trajectory study.

Issues such as mode selectivity and Polanyi rules are connected to the effects of vibrational and translational energy in dynamics studies. Using the heavy-light-heavy OH(ν) + NH3(ν) gas-phase reaction, these effects were analyzed by performing quasi-classical trajectory calculations, at low and high collision energies (3.0 and 10.

Ab initio based potential energy surface and kinetics study of the OH + NH3 hydrogen abstraction reaction.

A full-dimensional analytical potential energy surface (PES) for the OH + NH3 → H2O + NH2 gas-phase reaction was developed based exclusively on high-level ab initio calculations. This reaction presents a very complicated shape with wells along the reaction path. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction path, and points on the reaction swath) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the coupled-cluster single double triple (CCSD(T)) = FULL/aug-cc-pVTZ//CCSD(T) = FC/cc-pVTZ single point level, which represents a severe test of the new surface.

Quasi-classical trajectory study of the role of vibrational and translational energy in the Cl(2P) + NH3 reaction.

A detailed state-to-state dynamics study was performed to analyze the effects of vibrational excitation and translational energy on the dynamics of the Cl((2)P) + NH(3)(v) gas-phase reaction, effects which are connected to such issues as mode selectivity and Polanyi's rules. This reaction evolves along two deep wells in the entry and exit channels. At low and high collision energies quasi-classical trajectory calculations were performed on an analytical potential energy surface previously developed by our group, together with a simplified model surface in which the reactant well is removed to analyze the influence of this well.

QCT and QM calculations of the Cl(2P) + NH3 reaction: influence of the reactant well on the dynamics.

A detailed dynamics study, using both quasi-classical trajectory (QCT) and reduced-dimensional quantum mechanical (QM) calculations, was carried out to understand the reactivity and mechanism of the Cl((2)P) + NH(3)→ HCl + NH(2) gas-phase reaction, which evolves through deep wells in the entry and exit channels. The calculations were performed on an analytical potential energy surface recently developed by our group, PES-2010 [M. Monge-Palacios, C.

Theoretical study of the F + NH3 and F + ND3 reactions: mechanism and comparison with experiment.

We study the barrierless and highly exothermic F + NH(3) and F + ND(3) abstraction reactions using quasiclassical trajectory calculations based on an analytical potential energy surface developed in our research group. The calculations correctly reproduce the experimental evidence that the vibrational fraction deposited into the DF product for the F + ND(3) reaction is greater than into HF for the F + NH(3) reaction and that the vibrational distribution is inverted in the HF(v') and the DF(v') products. Of special interest is that recent crossed-beam experiments reported by Yang and co-workers at 4.

Reaction-path dynamics calculations of the Cl + NH(3) hydrogen abstraction reaction: the role of the intermediate complexes.

Using ab initio information at the CCSD(T)/cc-pVTZ level, the reaction path for the Cl + NH(3) hydrogen abstraction reaction was traced, and the coupling terms between the reaction coordinate and the normal modes were analyzed along it. Two intermediate complexes were located in the entry channel and characterized close to the reactants. One of them presents a typical Cl.