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Ion–molecule reactions between the neutral ethyl- (EF), isopropyl- (IF), t-butyl- (TF) and phenyl formate (PF) and proton-bound water clusters W2H+ and W3H+ (W = H2O) showed that the major reaction product is water loss from the initial encounter complex, followed ultimately by the formation of the protonated formate. Collision-induced dissociation breakdown curves of the formate–water complexes were obtained as a function of collision energy and modeled to extract relative activation energies for the observed channels. Density functional theory calculations (B3LYP/6-311+G(d,p)) of the water loss reactions were consistent with reactions having no reverse energy barrier in each case. Overall, the results indicate that the interaction of formates with atmospheric water can form stable encounter complexes that will dissociate by sequential water loss to form protonated formates.

AlO is relatively abundant around oxygen-rich Asymptotic Giant Branch (AGB) stars, where it can react with major gas-phase species such as H2 and H2O to form AlOH. These Al-containing species are t...

ConspectusHomogeneous metal-mediated organometallic reactions represent a very large and diverse reaction class. Density functional theory calculations are now routinely carried out and reported for analyzing organometallic mechanisms and reaction pathways. While density functional theory calculations are extremely powerful to understand the energy and structure of organometallic reactions, there are several assumptions in their use and interpretation to define reaction mechanisms and to analyze reaction selectivity. Almost always it is assumed that potential energy structures calculated with density functional theory adequately describe mechanisms and selectivity within the framework of statistical theories, for example, transition state theory and RRKM theory. However, these static structures and corresponding energy landscapes do not provide atomic motion information during reactions that could reveal nonstatistical intermediates without complete intramolecular vibrational redistribution and nonintrinsic reaction coordinate (non-IRC) pathways. While nonstatistical intermediates and non-IRC reaction pathways are now relatively well established for organic reactions, these dynamic effects have heretofore been highly underexplored in organometallic reactions. Through a series of quasiclassical density functional theory direct dynamics trajectory studies, my group has recently demonstrated that dynamic effects occur in a variety of fundamental organometallic reactions, especially bond activation reactions. For example, in the C-H activation reaction between methane and [Cp*(PMe3)IrIII(CH3)]+, while the density functional theory energy landscape showed a two-step oxidative cleavage and reductive coupling mechanism, trajectories revealed a mixture of this two-step mechanism and a dynamic one-step mechanism that skipped the [Cp*(PMe3)IrV(H)(CH3)2]+ intermediate. This study also showed that despite a methane σ-complex being located on the density functional theory surface before oxidative cleavage and after reductive coupling, this intermediate is always skipped and should not be considered an intermediate during reactive trajectories. For non-IRC reaction pathways, quasiclassical direct dynamics trajectories showed that for the isomerization of [Tp(NO)(PMe3)W(η2-benzene)] to [Tp(NO)(PMe3)W(H)(Ph)], there are many dynamic reaction pathway connections due to a relatively flat energy landscape and π coordination is not necessary for C-H bond activation through oxidative cleavage. Trajectories also showed that dynamic effects are important in selectivity for ethylene C-H activation versus π coordination in reaction with Cp(PMe3)2Re, and trajectories provide a more quantitative model of selectivity than transition state theory. Quasiclassical trajectories examining Au-catalyzed monoallylic diol cyclizations showed dynamic coupling of several reaction steps that include alkoxylation π bond addition, proton shuttling, and water elimination reaction steps. Overall, these studies highlight the need to use direct dynamics trajectory simulations to consider atomic motion during reactions to understand organometallic reaction mechanisms and selectivity.

The oxidation mechanism of CH3CHCO initiated by OH is systematically studied using quantum theoretical methods. According to thermodynamic research, the dominant channel is the addition of CH3CHCO with OH generating C-IM1 (CH3CHOHCO) and C-IM2 (CH3CHCOOH), and then dissociate to the dominant products P1 (CH3CHO + CO) from C-IM1 with the lowest barrier. The rate constants for CH3CHCO + OH → C-IM1 → P1/CH3CHCO + OH → C-IM2 channels are computed through RRKM theory at 200–2000 K. The obtained overall rate constant at 298 K (7.60 × 10−11 cm3 molecule−1 s−1) is well consistent with the reported experimental value. The atmospheric lifetime of CH3CHCO is estimated to be around 1.83 h.

Decomposition of heptafluoro-iso-butyronitrile (C 3 F 7 CN), a novel eco-friendly dielectric gas for high voltage electrical applications, is investigated theoretically in the presence of water vapor using various computational methods including DFT, CBS-Q, G4, CCSD(T)-F12 and RRKM theory. Hydrolysis of C 3 F 7 CN proceeds dominantly via the stepwise addition/isomerization or elimination mechanisms to form amide or HF. The reaction pathways are bifurcated into cis and trans conformations. Activation barriers are strongly dependent on the number of water molecules in hydrolysis due to catalytic effect. The intermediate imidic acid prefers to produce formamido imine via recombination or secondary reaction with C 3 F 7 CN. Hydrolysis of CF 3 and CF 3 CFCN undergoes via the direct H-abstraction mechanism to form OH radicals enhancing hydrolysis of C 3 F 7 CN. Kinetic calculations predict that imidic acid is the major decomposition product below 500 K while amide is generated exclusively above 1000 K with a small fraction of HF. The present work not only provides a theoretical basis for the increasing risk of dielectric failure of C 3 F 7 CN with the increment of the high humidity levels, but also identifies the potential characteristic decomposition products to develop failure diagnostic techniques.

The aim of this study is to calculate the unimolecular rate coefficients for the unimolecular decomposition reaction of the industrially important molecule, SF6. The energies of stationary-points involved in the title reaction are calculated by the combination W1 method. Two main reaction paths are considered: SF6 → SF5 + F (R1) and SF6 → SF4 + F2 (R2). Having information on energies and molecular properties of reactants and transition-states, RRKM statistical rate theory is used to compute the rate coefficients as a function of temperature and pressure. For the bond dissociation process R1, special version of RRKM theory, i. e., Variable reaction coordinate-transition state theory (VRC-TST) is employed. Although the reaction R1 is the dominant process over a wide range of pressure and temperature, but the reaction R2 could be significant at high temperatures. The following Arrhenius expressions are obtained for high-pressure limiting rate constants of reaction paths R1 and R2: k∞,1 = 5.71 × 1016 s−1 exp (−429.8 kJ mol−1 /RT) k∞,2 = 2.14 × 1016 s−1 exp (−590.6 kJ mol−1 /RT).

A detailed chemical kinetic model of aluminum oxidation in gas phase has been constructed. Quantum chemical calculations are performed for obtaining molecular structures of intermediates on the reaction pathways. DFT level calculation is applied to search intermediates and transition states, and CBS-QB3 method is applied to calculate highly accurate potential energies. The rate coefficients of each reaction paths were also calculated based on VTST or RRKM theory for constructing the detailed chemical kinetic model. 76 elementary reactions were investigated for AlxOy intermediates, and chemical kinetics analysis was performed to find the major route of oxidation to Al8O12 as the final product of this model. The mainstream of oxidation and formation of large size AlxOy molecules were discussed on the model, and it was found that AlO, Al2O, and Al2O2 are important intermediates to form Al2O3 or Al2O4 as precursors for Al4O6.

A single-pulse shock tube study of the four pentene isomers is carried out at 2¿±¿0.16¿bar and 900¿1600¿K. C1 to C6 species profiles were recorded using gas chromatography mass spectrometry analyses. The species are identified using mass spectrometry and quantified by flame ionization detection. High-pressure limiting and pressure-dependent rate constants for 2M1B, 2M2B and 3M1B¿+¿¿ were calculated using RRKM theory with a Master Equation (ME) analysis using the Master Equation System Solver, MESS. A mechanism was formulated based on rate rules and theoretical calculations. Comparisons between experimental results and model simulations are provided for all of the five pentene isomers investigated with satisfactory agreement. Furthermore, an insight is provided into the influence of molecular structure on the reactivity of pyrolysis chemistry. Interestingly, it is found that the HACA mechanism is much less prominent for benzene formation compared to the role of cyclopentadienyl radical recombination with methyl radicals and also the recombination of propargyl radicals. The authors would like to acknowledge Science Foundation Ireland for funding via project numbers 15/IA/3177 and 16/SP/3829. The work at LLNL was performed under the auspices of the U.S. Department of Energy (DOE), Contract DE-AC52-07NA27344, and was supported by the U.S. Department of Energy, Vehicle Technologies Office, program managers, Mike Weismiller and Gurpreet Singh. peer-reviewed

2-Methyl-3-buten-2-ol (MBO232) is a biogenic volatile organic compound (BVOC), and has a large percentage of emission into the atmosphere. The vacuum ultraviolet (VUV) photochemistry of BVOCs is of great importance for atmospheric chemistry. Studies have been carried out on several BVOCs but have not extended to MBO232. In the present report, the photoionization and dissociation processes of MBO232 in the energy range of 8.0-15.0 eV have been studied by tunable VUV synchrotron radiation coupled with a time-of-flight mass spectrometer. By measuring the photoionization spectra, the adiabatic ionization energy (AIE) of MBO232 and the appearance energies (AEs) of the eight identified fragment ions (i.e., C4H7O+, C3H7O+, C5H9+, C3H6O+, CH3CO+, CH3O+, C4H5+, and C3H5+) were determined. High-level quantum chemistry calculations suggest that there are 3 direct channels and 5 indirect channels via transition states and intermediates accountable for these fragments. Among the reaction channels, the direct elimination of CH3 is the most dominant channel and produces the resonance-stabilized radical cation. Most interestingly, our results show that the CH3 selectively migrates towards the cation, which leads to the different indirect channels. The CH3 migration is a rare process in the dissociative photoionization of metal-free organic molecules. We explain the process by molecular orbital calculations and electron localization function analysis and explore the non-conventional dissociation channels via the CH3 roaming mechanism. We further perform kinetics analysis using RRKM theory for the channels of interest. The activation barrier, and rate constants are analyzed for the branching fractions of the products. These results provide important implications for the VUV photochemistry of BVOCs in the atmosphere.

The temperature- and pressure-dependence of rate constants for several radicals and unsaturated hydrocarbons reactions (1,3-C5H8/1,4-C5H8/cyC(5)H(8) + (H)over dot, C2H4 + (C)over dot(3)H5-a, C3H6 + (C)over dot(2)H(3)) are analyzed in this paper. The abstraction reactions of these systems are also calculated and compared with available literature data. (C)over dot(5)H(9) radicals can be produced via (H)over dot atom addition reactions to the pentadiene isomers and cyclopentene, and also by H-atom abstraction reactions from 1- and 2-pentene and cyclopentane. Comprehensive (C)over dot(5)H(9) potential energy surface (PES) analyses and high-pressure limiting rate constants for related reactions have been explored in part I of this work (J. Phys. Chem. A 2019, 123 (22), 9019-9052). In this work, a chemical kinetic model is constructed based on the computed thermochemistry and high-pressure limiting rate constants from part I, to further understand the chemistry of different C5H8 molecules. The most important channels for these addition reactions are discussed in the present work based on reaction pathway analyses. The dominant reaction pathways for these five systems are combined together to generate a simplified (C)over dot(5)H(9) PES including nine reactants, 25 transition states (TSs), and nine products. Spin-restricted single point energies are calculated for radicals and TSs on the simplified PES at the ROCCSD(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). Temperature- and pressure-dependent rate constants are calculated using RRKM theory with a Master Equation analysis, with restricted energies used for minima on the simplified (C)over dot(5)H(9) PES and unrestricted energies for other species, over a temperature range of 300-2000 K and in the pressure range 0.01-100 atm. The rate constants calculated are in good agreement with those in the literature. The chemical kinetic model is updated with pressure-dependent rate constants and is used to simulate the species concentration profiles for H. atom addition to cyclopentane and cyclopentene. Through detailed analyses and comparisons, this model can reproduce the experimental measurements of species qualitatively and quantitatively with reasonably good agreement. This study is supported by Science Foundation Ireland and the China Scholarship Council (CSC). The authors want to acknowledge the financial support of Science Foundation Ireland under Grant No. 15/IA/3177 and 16/SP/3829, and the provision of computational resources from ICHEC under the NUI Galway shared condominium accounts. The Computational resources are provided by the Irish Centre for High-End Computing (ICHEC), under project number ngche063c, ngcom006c and ngche058c. The authors are grateful to Stephen Klippenstein for the help with MESS code study. Chong-Wen Zhou acknowledges the support from National Science and Technology Major Project (2017-III-0004-0028) and Beihang University under the Fundamental Research Funds. peer-reviewed 2021-05-12

The traditional understanding is that the Hinshelwood-Lindemann mechanism for thermal unimolecular reactions, and the resulting unimolecular rate constant versus temperature and collision frequency ω (i.e., pressure), requires the Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant k(E) to represent the unimolecular reaction of the excited molecule versus energy. RRKM theory assumes an exponential N(t)/N(0) population for the excited molecule versus time, with decay given by RRKM microcanonical k(E), and agreement between experimental and Hinshelwood-Lindemann thermal kinetics is then deemed to identify the unimolecular reactant as a RRKM molecule. However, recent calculations of the Hinshelwood-Lindemann rate constant kuni(ω,E) has brought this assumption into question. It was found that a biexponential N(t)/N(0), for intrinsic non-RRKM dynamics, gives a Hinshelwood-Lindemann kuni(ω,E) curve very similar to that of RRKM theory, which assumes exponential dynamics. The RRKM kuni(ω,E) curve was brought into agreement with the biexponential kuni(ω,E) by multiplying ω in the RRKM expression for kuni(ω,E) by an energy transfer efficiency factor βc. Such scaling is often done in fitting Hinshelwood-Lindemann-RRKM thermal kinetics to experiment. This agreement between the RRKM and non-RRKM curves for kuni(ω,E) was only obtained previously by scaling and fitting. In the work presented here, it is shown that if ω in the RRKM kuni(ω,E) is scaled by a βc factor there is analytic agreement with the non-RRKM kuni(ω,E). The expression for the value of βc is derived.

The reaction between CFCl2CH2O2 radicals and ClO was studied using the B3LYP and CCSD(T) methods associated with the 6-311++G(d,p) and cc-pVTZ basis sets, and subsequently RRKM-TST theory was used to predict the thermal rate constants and product distributions. On the singlet PES, the dominant reaction is the addition of the ClO oxygen atom to the terminal-O of CFCl2CH2O2 to generate adduct IM1 (CFCl2CH2OOOCl), and then dissociation to final products P1 (CFCl2CHO + HO2 + Cl) occurs. RRKM theory is employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that the collision-stabilized IM1 (CFCl2CH2OOOCl) dominates the reaction at 200–500 K (accounting for about 60–100%) and the dominant products are P1 (CFCl2CHO + HO2 + Cl). The yields of the other products are very low and insignificant for the title reaction. The total rate constants exhibit typical “falloff” behavior. The pathways on the triplet PES are less competitive than that on the singlet PES. The calculated overall rate constants are in good agreement with the experimental data. The atmospheric lifetime of CFCl2CH2O2 in ClO is around 2.04 h. TD-DFT calculations imply that IM1 (CFCl2CH2OOOCl), IM2 (CFCl2CH2OOClO) and IM3 (CFCl2CH2OClO2) will photolyze under sunlight.

The kinetic and electron density flows are studied theoretically for the gas phase pyrolysis of cubane via its cage opening to reach bicyclooctatriene and then thermal rearrangement of bicyclooctatriene to produce [8]annulene which is the experimentally observed major product. The observed kinetic data at the MN15-L/maug-cc-pVTZ level of theory were in good agreement with the experimental results as compared to the CBS-QB3 method. The cage opening and the thermal rearrangement steps at the experimentally employed temperature of 520 K were exergonic and exothermic. The atmospheric rate constants calculated by means of the RRKM theory show that the cage opening is the rate-determining step. The temperature dependence of the rate constant for the cage opening step at the MN15-L level can be expressed as log(k/s−1)1barMN15-L = (15.63) − (48.99 kcal mol−1)/RT ln 10. The molecular mechanism of the reactions has been investigated by means of the bonding evolution theory (BET) at the B3LYP/6-311G (d,p) level of theory. The cage opening course is described topologically by cleaving of C1–C2, C4–C8, and C5–C6 single bonds and electron saturation of the C1–C4, C2–C6, and C5–C8 bonds, while the rearrangement of bicyclooctatriene is described by C3–C7 bond rupture, depopulation of C1–C4 and C5–C8 double bonds, and electron saturation of C1–C5, C3–C4, and C7–C8 bonds. Electron density rearrangement along the two successive steps are asynchronous and the sequence of catastrophes can be represented as: η-1-13-C†C†FFFC†C†FFFC†C†-2-6-[C]2C†[F]2[C†]2C†-0.

Criegee intermediates are reactive species formed in the ozonolysis of alkenes. Their subsequent chemistry is critical to an accounting of OH production, aerosol formation, and the oxidative capaci...

Tandem mass spectrometry was used to explore the trends in the unimolecular fragmentation of the ionized hydroxy-substituted polycyclic aromatic hydrocarbons 1-naphthol, 9-hydroxyphenanthrene, and 1-hydroxypyrene. The main dissociation reactions across all ring systems were CO- and HCO-losses, with ionized 1-naphthol also losing H atoms. Both ionized 1-naphthol and 9-hydroxyphenanthrene displayed the sequential loss of C2H2 and C4H2 from the [M-HCO]+ ions, reminiscent of unsubstituted PAH ions. CO-loss is slightly favored for 1-naphthol and 9-hydroxyphenanthrene, at low collision energy, but less so for 1-hydroxypyrene. Reaction mechanisms for HCO- and CO-losses from 1-hydroxypyrene were derived from CCSD/6-31G(d)//B3-LYP/6-31G(d) calculations. The CO-loss mechanism is dominated by two transition states: TS-A governing a 1,3-H shift in the molecular ion and TS-C which governs a ring-closing step to form a five-member ring in the product ion. HCO-loss occurs over a much flatter potential energy surface with the intermediate being the product ion bound to the carbon atom of HCO. Imaging photoelectron photoion coincidence spectroscopy of 1-hydroxypyrene yielded threshold photon-energy resolved breakdown curves and time-of-flight distributions that were modeled with RRKM theory to give 0 K reaction energies for HCO- and CO-losses of 3.92 ± 0.05 and 2.91 ± 0.05 eV, respectively. The entropies of activation for the two channels were very different, 14 and 95 JK-1 mol-1, respectively, a result consistent with the calculated mechanisms. The threshold photoelectron spectrum yielded an IE value of 7.14 ± 0.01 eV for 1-hydroxypyrene.

Oxygenated polycyclic aromatic hydrocarbons (OPAH) have received increasing attention due to their toxic effect on human health. This study comprehensively investigates the evolution of OPAH chemistry at flame temperatures. Jet-stirred reactor (JSR) experiments with benzene/phenol/C2H2/N2 and benzene/C2H2/O2/N2 revealed that OPAH with oxygenated heterocycle can be formed by the addition of C2H2 at 1400 K. To further clarify the evolution of OPAH chemistry in soot systems, OPAH formation and decomposition reaction pathways and kinetic parameters have been theoretically investigated. The potential energy surfaces of 1-naphtholate and 2-naphtholate growth, and thermal decomposition reactions, were calculated by combining the density functional theory B3LYP/6–311+G(d,p) and CCSD(T)/cc-pvdz methods. The reaction rate coefficients in the temperature range of 800–2500 K and pressure range of 0.1–100 atm were calculated using RRKM theory by solving the master equations. The potential energy surface of C2H2+1-naphtholate and C2H2+2-naphtholate growth reactions showed that the O atom could be locked in a naphthofuran molecule with the formation of a C O C oxygenated heterocycle; and the reaction rates were determined by adding the C2H2 elementary step with the energy barrier of 26.0 and 19.9 kcal/mol, respectively. Thermal decomposition reactions of 1-naphtholate and 2-naphtholate yielded an indenyl radical and CO. The thermal decomposition reaction rates were significantly sensitive to the zig-zag site structure next to the C O bond. The decomposition rate of 1-naphtholate at 1500 K, with a zig-zag site near the C O bond, was 14.8 times lower than that of 2-naphtholate with no zig-zag site near the C O bond. Rate comparison results indicate that the C O functional group rapidly converts to a C O C functional group with the addition of C2H2. The formation, growth and thermal decomposition reactions of 1-naphtholate and 2-naphtholate were added to a detailed PAH mechanism to check the effect of OPAH reactions on PAH formation chemistry. The concentration profile of naphthalene predicted by the updated PAH mechanism was lower than current PAH mechanism predictions by 29%, indicating that the OPAH reactions had a significant effect on PAH formation chemistry, and should be included in the PAH mechanism. However, due to the relatively low concentration of OPAH compared to PAH, it is possible to ignore the correlation between OPAH and soot nucleation at flame temperatures; therefore an OPAH evolution pathway (PAH → incipient soot → OPAH formation on soot particle → selective thermal decomposition of OPAH), is proposed to explain the high content of OPAH molecules (e.g., 9,10-anthraquinone, benz(a)anthracene-7,12-dione, and benzanthrone) adsorbed on the soot particle.

A comprehensive experimental and kinetic study of 2,4,4-trimethyl-1-pentene oxidation was conducted in this work. Oxidation in JSR was investigated at the temperature range of 675–1200 K and equivalence ratios of 0.5, 1.0, 2.0 under atmospheric pressure. The high-level quantum calculation was conducted on DLPNO-CCSD(T)/cc-pVTZ//M06-2X/6-311G(d,p) level to get some rate coefficients of high accuracy and choose important pathway. Temperature and pressure-dependent rate coefficients were obtained by solving master equation and RRKM theory. A detailed chemical kinetic model for 2,4,4-trimethyl-1-pentene was developed based on the high-level quantum calculation and validated against the mole fraction data measured in JSR in this work. Available experimental data including ignition delay times and laminar flame speeds in our previous work were also used for the validation of this model. Reaction pathway analysis and sensitivity analysis were performed using the Modified model. The results demonstrate the significance of H abstraction reactions in a wide range of temperature, while fuel unimolecular decomposition reaction are only dominant at high temperature. In addition, at low temperature, O 2 addition to octyl radical reactions are more important than radical decomposition and isomerization reactions. Furthermore, sensitivity analysis indicates that H abstraction reactions of fuel and isobutene have a strong inhibiting effect on low temperature reactivity.

The growth of polycyclic aromatic hydrocarbons (PAH) can proceed via multiple chemical mechanisms. The mechanism of naphthyl radical and vinylacetylene (C4H4) addition reaction has been systematically investigated in this computational study. A combination of DFT/B3LYP/6-311+G(d,p), CCSD/6-311+G(d,p) and CBS-QB3 methods were performed to calculate the potential energy surfaces. It revealed that the products, including phenanthrene, anthracene, a PAH with a five-membered ring structure, and PAH with a C4H3 radical substitution, can be formed in A2-1 (1-naphthyl)+C4H4 and A2-2 (2-naphthyl) +C4H4 reaction networks. The reaction rate constants at 0.1-100 atm were evaluated by RRKM theory by solving the master equation in the temperature range of 800–2500 K, which showed that the rate constants of reactions A2-1 (A2-2)+C4H4→product+H are highly temperature-dependent but nearly pressure-independent. The distribution of products was investigated in a 0-D batch reactor, wherein the initial reactant concentrations were taken from experimental measurements. The results showed that adduct intermediates were the main products at low temperature (T 1000 K). It was observed that a significant amount of phenanthrene is formed from PAH with a C4H3 radical substitution with the assistance of H atom. Reaction pathway sensitivity analysis for the PAH radical+C4H4 reaction system was performed and showed that the new benzene rings are more likely to be generated near the zig-zag edge surface site instead of the free edge. For the development of a PAH mechanism, the analogous treatment of rate constants for larger PAH radical + C4H4 reaction system are discussed. The formation rate of naphthalene from the reaction of phenyl+C4H4 was found to be very close to that of phenanthrene from the reaction of naphthyl+C4H4, suggesting that the analogous treatment of the rates is reasonable in PAH mechanisms.

The Journal of Physical Chemistry A, 125 (9), ISSN:1089-5639, ISSN:1520-5215

We have studied the decomposition and isomerization of furfural in the gas phase using quantum chemical and statistical reaction rate theory techniques. This work uncovers a variety of new reaction channels in furfural pyrolysis that lead to formation of the experimentally observed products, including CO2, which was previously unexplained. In addition to the known mechanism for furan + CO production, furfural is shown to isomerize directly to 2-pyrone, with a barrier height of 69 kcal mol-1, from where it can decompose to vinylketene + CO (highest barrier of 65 kcal mol-1) or to CO2 + 1,3-cyclobutadiene (highest barrier of 66 kcal mol-1). Alternative pathways to vinylketene + CO and 4-pyrone are also described. An RRKM theory/master equation model is developed to describe reactions on the C5O2H4 surface and used to simulate the decomposition kinetics of furfural and 2-pyrone. For both molecules, decomposition at 1400-2100 K is dominated by the formation of furan + CO, which represents around 75% of the total products, compared to around 19% and 6% for vinylketene + CO and total CO2, respectively. The model also predicts significant formation of stabilized 2-pyrone under these conditions. Rate coefficient expressions are reported as a function of both temperature and pressure for the main decomposition and isomerization channels identified in the pyrolysis of furfural and 2-pyrone, to facilitate detailed chemical kinetic modelling of these important oxygenated hydrocarbons.

In this work we report a joint experimental and computational study on the 2,5-dimethylfuran oxidation reaction in the gas phase initiated by atomic oxygen O(3P). The experiments have been performed by using vacuum-ultraviolet synchrotron radiation at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory (LBNL), at a temperature of 550 K and a pressure of 8 Torr. The experimental data were supported by quantum-chemical calculations along with a kinetic model, also taking into account the possible involvement of different magnetic states, performed in the framework of the RRKM theory. Propyne, acetaldehyde, methylglyoxal, dimethylglyoxal, 3-penten-2-one, 2,5-dimethylfuran-3(2H)-one, and 1,2-diacetyl ethylene have been identified as the main primary products arising under the conditions of the experiment. Our computational model suggests that these species can be formed at the concentration and branching ratio experimentally observed only in the presence of a non-negligible fraction of non-thermalized intermediates.

Secondary kinetic isotope effects arise as the result of transition-state zero-point vibrational energy differences. Unimolecular simple cleavage reactions of gas-phase ions in mass spectrometers allow detailed studies of isotope effects on competing reactions, particularly when examined in intramolecular competition experiments where interpretation requires very few simplifying assumptions. The zero-point energy differences reflect changes of isotope sensitive vibrational properties, and both α- and β-secondary deuterium isotope effects are related to the sp 3 → sp 2 hybridization changes that accompany bond cleavage. Deuterium substitution three bonds or more removed from the bond broken also gives rise to isotope effects, but their origin is less easily interpreted. The magnitude and variation of the observed effects depend not only on zero-point energy differences; a number of additional factors play a role. The influence of the critical energy, the excess energy, the size of the reactant, and the presence of competing reactions can be rationalized within a simple, qualitative RRKM framework. The distinction between kinetic and thermodynamic isotope effects is not always obvious.

The gas-phase reaction between CFCl2O2 and ClO was researched by means of quantum chemical methods. B3LYP method with the 6-311++G(d,p) basis set was used to obtain the geometric parameters of all stationary points including in the CFCl2O2 + ClO reaction. The singlet and triplet potential energy surfaces were characterized at the CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) level. Addition/elimination and SN2 displacement reaction mechanisms were identified on the singlet PES, and only substituent reaction mechanism was located on the triplet PES. The dominant reaction takes place on the singlet PES, and the major pathways are CFCl2O2 + ClO → IM1 (CFCl2OOOCl) → IM2 (CFCl2OOClO) → IM3 (CH3OClO2) → P6 (CFClO + (ClO)2). RRKM theory was used to calculate rate constants, which is consistent with the experimental data. The atmospheric lifetime of CFCl2O2 in ClO is around 8.84 h. TD-DFT computations imply that IM1 (CFCl2OOOCl), IM2 (CFCl2OOClO), and IM3 (CFCl2OClO2) will photolyze under the sunlight.

The PAHs-C2H2 pathway (PAHs + C2H2 → intermediate → product + H2) has been shown, in theory, to be the important contributor to the growth of polycyclic aromatic hydrocarbons (PAHs) and soot in the post-flame region where H atoms are rare. Calculations of the potential energy surface (PES) using the DFT B3LYP 6-311 + G(d,p) method, and the reaction rate coefficients using the RRKM theory, reveal that armchair and bridge surface sites share similar kinetic characteristics, and are more likely to be the targets of C2H2 molecules in flames compared to zig-zag and 5-membered ring surface sites. Results show that the energy barrier of a 2-H elimination reaction (14–23.8 kcal/mol) is much lower than that of a 1-H elimination (typically 30–40 kcal/mol) for some molecules. The formation of pyrene from phenanthrene via HACA (PAHs + H → PAHs radical (+ C2H2) → intermediate → product + H) and PAHs-C2H2 pathways is investigated using a closed homogeneous zero-dimensional reactor with combustion parameters abstracted from the premixed stagnation C2H4/O2/Ar sooting flame. Results show that the HACA pathway is the dominant pathway for the formation of PAHs and soot surface growth in the main-flame region where H atoms are abundant, but that the PAHs-C2H2 pathway is the preferred pathway in the post-flame region. Our study also suggests that the soot nucleation involving a chemical coalescence of moderate-sized PAHs into a crosslinked three-dimensional structure via the addition reactions of PAHs and PAH radicals in the main-flame region should be considered for inclusion in any soot modeling.

Volatile silicon compounds (VOSiCs) are air pollutants present in both indoor and outdoor environments. Here, tetramethylsilane (TMS) is selected as a model to study the photochemical oxidation mechanisms for VOSiCs using ab initio and RRKM theory / master equation kinetic modelling. Under tropospheric conditions the TMS radical (CH3)3SiCH2• reacts with O2 to produce a stabilized peroxyl radical which is expected to ultimately yield the alkoxyl radical (CH3)3SiCH2O•. At combustion-relevant temperatures, however, a well-skipping reaction to (CH3)3SiO• + HCHO dominates. Importantly, the (CH3)3SiCH2O• radical is predicted to rearrange to (CH3)3SiOCH2• with a very low reaction barrier, enabling an auto-oxidation process involving addition of a second O2. Subsequent oxidation reaction mechanisms of (CH3)3SiOCH2• have been developed, with the major product predicted to be the ester (CH3)3SiOCHO, an experimentally observed TMS oxidation product. The production of substantially oxygenated compounds following a single radical initiation reaction has implications for the ability of VOSiCs to contribute to ozone and particle formation in both outdoor and indoor environments.

Kinetics of the thermal decomposition reaction CH2F2 → CHF + HF was studied by laser absorption measurements in a shock tube. The rate constants were determined by analyzing temporal profiles of HF produced in pyrolysis of CH2F2 over the temperature range 1577–2214 K at pressure of ∼100 kPa in Ar bath. The results were well reproduced by master equation analysis based on RRKM theory and classical trajectory calculations, which gave the following rate constant expressions: k∞ = 2.34 × 1015 exp(−41,550 K/T) s−1, k0 = 5.33 × 1025 (T/K)−8.635 exp(−44,190 K/T) cm3 molecule−1 s−1, and Fcent = 0.13.

The ground-state rearrangement and decomposition of methyl vinyl ketone (MVK) and methacrolein (MACR) has been investigated using quantum chemical calculations and RRKM theory/master equation simul...

Direct dynamics simulations were performed to investigate unimolecular dynamics of the F − ⋯HCH 2 I pre-reaction complex for the F − + CH 3 I S N 2 reaction. The simulations were performed for a total energy commensurate with the 0.32 eV collision energy considered in previous experiments and simulations of this reaction. Two excitation patterns of the complex were considered for the simulations: one with the excitation energy primarily in vibration and the other with the energy primarily in rotation. For the latter equal amounts of energy were added about the three external rotation axes of the complex, giving rise to J and K rotation quantum numbers of 357 and 66 for the initial excitation. For the vibrational excitation the unimolecular dynamics agrees with RRKM theory and dissociation of the complex to the F − + CH 3 I reactants is negligible, since the barrier for this reaction is 20.2 kcal/mol in contrast to the 2.4 kcal/mol barrier for accessing the S N 2 transition state (TS). The unimolecular dynamics are much different for the rotational excitation simulation. They are non-RRKM and the instantaneous unimolecular rate constant for F − ⋯HCH 2 I decomposition increases with time. Apparently, this arises from K -mixing and vibration/rotation energy redistribution. At the end of the 10 ps simulation, the instantaneous simulation rate constant for accessing the S N 2 transition state is an order of magnitude smaller than the harmonic RRKM rate constant with the K quantum number treated as an active degree of freedom, indicating that K -mixing and vibration/rotation energy redistribution are not complete on this time scale. For rotational excitation, dissociation to the F − + CH 3 I reactants is the dominant unimolecular pathway, instead of forming the S N 2 products. This is a result of a much higher rotational barrier at the S N 2 TS than for the TS leading to F − + CH 3 I.

Guaiacol is the main representative of methoxyphenols generated from the pyrolysis of ligin. The gas-phase reaction mechanism of guaiacol with NO3 radical was researched by using density functional theory at M06-2X/6-31+g(d,p)//M06-2X/6-311+g(3df,2p) level in this work, and the reaction rate constants were calculated by using RRKM theory. The relative complete gas phase reaction mechanism of guaiacol with nitrate radicals was obtained. The calculated results showed that, NO3 addition and H abstraction are two dominant progresses for the initial reaction of guaiacol with NO3 radicals. The total reaction rate constant is 3.11 × 10−11 cm3 molecule−1 s−1 at 298 K and 1 atm. The atmospheric lifetime is 64 s under typical tropospheric concentration of NO3 radicals. The possibility of products (methoxybenzoquinone and nitro-guaiacol) is verified and new products (dialdehyde and diketone) are predicted.

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

The spin-forbidden quenching reaction, O(1D) + N2( X 1 Σ g + ) → N2( X 1 Σ g + ) + O(3P), has been studied by ab initio molecular orbital theory using different methods including CCSDTQ/CBS(TQ5)//CCSD(T)/aug-cc-pV5Z, whose energies were utilized for establishment of the singlet–triplet potential energy surface and prediction of thermal rate coefficients. The O(3P) + N2 formation via the long-lived singlet 1N2O intermediate has been identified to be dominant for the spin-forbidden crossing at the crossing point located at 60.3 kcal mol−1 above 1N2O, in good accordance with the literature values. The P,T-dependent rate constants predicted by the non-adiabatic RRKM theory agree closely with available results reported in the temperature range 50 – 700 K, over the pressure range of 1 – 250 Torr.

The thermal decomposition of 2-chloroethyl methyl ether (2-CEME) was studied in the temperatures between 1175 and 1467 K. The decomposition of 2-CEME happens predominantly via molecular elimination reactions than via C C and C O bond fission channels. The major decomposition products are methane, ethylene and methanol. The minor are acetaldehyde and ethane. The Arrhenius expression for the overall decomposition of 2-CEME was obtained to be k t o t a l e x p ( 1175 − 1467 K ) = ( 4.12 ± 0.42 ) × 10 11 exp ( − ( 52.2 kcal mo l − 1 ± 2.6 ) / RT ) s − 1 . To simulate the distribution of reactant and products over the experimentally studied temperatures between 1175 and 1467 K, a reaction scheme was constructed with 45 species and 71 elementary reactions. The pressure and temperature dependent rate coefficients were calculated for various unimolecular dissociation pathways in 2-CEME using RRKM theory. The high pressure limit temperature dependent rate coefficient for the total decomposition of 2-CEME was obtained to be k total CCSDT / / M 06 − 2 X (500–2000 K) = (2.55 ± 0.21) × 10 14 exp (−(67.6 kcal mol −1 ± 3.0)/RT) s −1 .

The study mainly focuses on the anharmonic effect of the reactions of CH3SCH2OO system. The geometries of the reactants and the transition states are optimized with Gaussian 09. The barrier heights are calculated with the energy of the reactants and the transition states. The RRKM theory is utilized to calculate the anharmonic and harmonic rate constants of the reactions. The anharmonic effect of these reactions can be clearly demonstrated by our results. Generally speaking, in the study, for most reactions, the rate constants increase with the temperature in the canonical case and the total energy in the microcanonical case, and the anharmonic effect of these reactions is significant and should not be neglected in high-temperature combustion. In CH3SCH2OO system, CH3SCH2OO → CH2SCH2OOH → CH2S + CH2O + OH is the main reaction channel. After a series of calculations, the anharmonic effect is remarkable, especially in high-temperature combustion. By analyzing other meaningful reactions that followed that channel above, the anharmonic effect of these reactions is generally obvious enough, especially for those reactions whose barrier heights are relatively low.

Singlet and triplet potential energy surfaces for the CH3O2 with I reaction have been investigated computationally to propose the reaction mechanisms and possible products. Multichannel RRKM theory and transition-state theory have been used to compute the overall and individual rate constants at 200-3000K and 10-14-1014Torr. On the singlet PES, addition-elimination, substitution and H-abstraction mechanisms are located, and the addition-elimination mechanism is dominant. At 70Torr with N2 as bath gas, IM1(CH3OOI) formed by collisional stabilization is dominated at 200-300K, whereas CH2O and HIO are the major products at the temperatures between 350 and 3000K; The title reaction exhibits the typical falloff behavior. The results show that temperature and pressure affect the yield of products. Furthermore, the predicted rate constants at 298K 70Torr of N2 agree well with the available experimental values. On the triplet PES, the most favorable product should be CH3I+O2(3Σ) at atmospheric condition. Other two pathways on the triplet PES will not compete with the pathways on the singlet PES in kinetically and thermodynamically.

The mechanisms and ionic product distributions related to multiple channels of the X− + CH3ONO2 (X = NCCH2, CH3C(O)CH2, and PhCH2) gas phase reactions were investigated at the B2PLYP/6–311 + G(3df,2p)//MP2/6–31 + G(d,p) level of theory. The reaction channels are bimolecular nucleophilic displacements at either the carbon (SN2@C) or nitrogen (SN2@N) centers and a proton abstraction followed by dissociation (ECO2). For the ambident NCCH2− and CH3C(O)CH2− nucleophiles, two additional pathways for each reaction channel become available through attacks via the methylenic carbon or nitrogen/oxygen centers. The large number of reaction channels for these ambident nucleophiles precludes the unique determination of the ECO2:SN2@C:SN2@N ratios and of the carbon or nitrogen/oxygen regioselectivity by mass spectrometry measurements. Thus, the relevance of performing detailed, accurate and reliable computational modeling, including determination of unimolecular rate constants with the RRKM theory. The nucleophilic displacements have the largest rate constants, which are: SN2@C for the NCCH2− and CH3C(O)CH2− nucleophiles via methylenic carbon and oxygen centers attacks, respectively, and SN2@N for the PhCH2− anion. The ionic products distributions 10:82:8, 2:97:1, and 0:18:82 related to the ECO2:SN2@C:SN2@N reaction pathways were calculated for the reactions with NCCH2−, CH3C(O)CH2−, and PhCH2−, respectively, which are in excellent quantitative agreement with the experimental data. These agreements suggest that these reactions with large nucleophiles and/or (de)localized charge have a statistical behavior, unlike the same reaction with smaller and localized charge nucleophiles (F− and OH−).

The thermal unimolecular decomposition of SiH4 + ion and its related reverse reactions, SiH3 + + H and SiH2 + + H2, have been investigated by ab initio molecular orbital and quantum statistical variational RRKM theory calculations. The potential energy surface has been calculated at different levels of theory; the results at the highest level, CCSD(T)/CBS//CCSD(T)/6-311++G(3df,2p), show that the decomposition of SiH4 + can mainly occur via a barrierless channel giving SiH2 + + H2 lying 11.8 kcal/mol above the reactant, or via a transition state forming SiH3 +···H complex to be followed by a barrierless decomposition yielding SiH3 + + H lying 23.5 kcal/mol above the reactant. Barrierless processes were calculated using the CASPT2 and CASSCF methods with the 6-311++G(3df,2p) basis set and fitted with Morse potentials. The rate constants were predicted by solving master equations based on the RRKM theory at the E,J-resolved level; the results show that the channel SiH4 + → SiH2 + + H2 is predominate under PEVCD conditions. For H- and H2-capturing by SiH3 + and SiH2 + ions, respectively, the rate constants were found to be weakly dependent on temperature at the high-pressure limit and decrease rapidly with pressure. The calculated heats of formation of the SiH x + (x = 2–4) ions are in close agreement with available thermochemical data.

The most significant and at the same time poorly studied stage for the formation of soot is the nucleation stage, when the young soot particles are formed from the molecules of the gas phase. The most likely mechanism for the particles nucleation is the nucleation through the formation of a strong chemical bond. In our study, the formation of the vinylidene bridge bond between naphthalene and acenaphthalene molecules was considered. Geometries of the reactants, products, transition states, and reaction intermediates have been optimized at the DFT B3LYP level of theory. Further, the B3LYP optimized geometries were utilized to refine single-point energies using the combined ab initio G3(MP2,CC) method. Temperature- and pressure-dependent rate constants for the reactions considered were evaluated within the framework of RRKM theory in combination with the Master Equation approach (RRKM–ME). The formation of the zig-zag - free-edge product is much more profitable than the zig-zag - zig-zag product, however, due to the low rates and much faster reverse reactions, the formation of both products is unlikely. Thus, the formation of the vinylidene bridge, according to the proposed mechanism and subsequent coagulation, is very unlikely in comparison with the nucleation of medium-size PAHs.

This chapter reviews the development of the statistical rate theory including the recent proposal for its further improvement, with emphasis on the development of microcanonical features. Since the birth of the statistical rate theory in the 1930s, a tremendous number of experiments have been conducted to prove the adequacy or inadequacy of the statistical treatment. The chapter discusses the fundamental assumptions originally embedded in the statistical rate theory. It summarizes the fundamental assumptions originally embedded in the statistical rate theory. The chapter describes both the canonical transition state theory and the microcanonical RRKM theory for unimolecular reactions. Typical restrictions that are due to the adiabaticity of the quantum state, incomplete energy randomization, and the vectorial correlation reflecting the shape of potential energy surface. The chapter also reviews the product energy distribution is another important element of chemical reactions and, of course, closely related to the statistical rate theory.

The self-rigidification of ionized π-conjugated systems based on two combinations of thiophene (T) and 3,4-Ethylenedioxythiophene (E) is investigated using mass-analyzed ion kinetic energy spectrometry (MIKES) of ions produced from electron impact ionization at 70 eV. The m/z 446 radical cations of the two isomers ETTE and TEET lead to detect m/z 418 and 390 daughter ions. The MIKE spectra differ only by the intensities of these fragment ions. As the m/z 418 daughter ion is produced through a same retro-Diels Alder reaction whatever the fragmenting isomer, the difference in daughter ion intensities is interpreted in term of unimolecular dissociation rate constants ( k( Eint)) ratios. Considering that the transition state (TS) of such reaction is attributed to a quinoid form, equivalent vibration modes are assumed for the TS of both dissociating ETTE and TEET radical cations. As a result, by using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory, the difference in daughter ion intensities is interpreted by considering that the fragmenting ion is more or less ordered in its ground state than at the transition state, resulting from the influence of the number of the S…O interactions in the planarization of the TEET ion toward the ETTE charged species. The comparison of this behavior in MIKES experiments is supported by the modeling of ion behavior in mass spectrometer and the calibration in internal energy of the radical cations produced in an EI source.

In this research, the reactions of two simple Criegee intermediates formaldehyde oxide and acetone oxide with dimethyl sulfide is explored theoretically. The well-tested meta-GGA density functional theory method M11-L is employed to locate the structures of stationary points on the potential energy surface of the title reactions. Statistical rate theories such as transition state theory (TST) and RRKM theory are employed to determine the dominant products of the reactions. It is found that both reactions occur via relatively strong van der Waals complexes leading to C S bonded adducts, IM1C and IM1D. These initial adducts decompose to yield other intermediates and products. According to the present theoretical thermodynamics and kinetics calculations, IM1C adduct could be formed in considerable amount in atmospheric condition. However, the reaction of (CH3)2COO with dimethyl sulfide is not of atmospheric significance.

The kinetics and mechanisms of pyrolysis of 1,3-dihydroisothianaphthene-2,2-dioxide toward benzocyclobutene have been theoretically studied using canonical transition state theory (CTST), statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and bonding evolution theory (BET) in conjugation with M06-2X/aug-cc-pVTZ calculations. The CTST slightly breaks down to estimate the reaction rate of the cheletropic extrusion. RRKM results indicated that the cheletropic extrusion and electrocyclic reaction require energy barriers of 171.3 and 122.2 kJ/mol to be overcome; and can be characterized respectively by 7 and 3 phases associated to the sequence of catastrophes C 8 H 8 SO 2 ( 1 ): 7-[FF]C † C † FFF-0: C 8 H 8 + SO 2 and C 8 H 8 ( 2 ): 3-[F † F † ]C-0: C 8 H 8 ( 3 ). For the cheletropic extrusion, breaking of the C 7 –S and C 8 –S bonds begins respectively at Rx = −2.7434 amu 1/2 Bohr and Rx = −1.7458 amu 1/2 Bohr, and formation of the sulfur dioxide is completed at Rx = −0.2494 amu 1/2 Bohr. For the electrocyclic reaction, formation of new C 7 –C 8 bond occurs at Rx = 1.6214 amu 1/2 Bohr from C- to C- coupling between the generated pseudoradical centers at Rx = 0.1474 amu 1/2 Bohr on the terminal carbon atoms.

The mechanism and kinetics for the O( 3 P) + CH 3 CH CHCN reaction has been investigated firstly. The BHandHLYP and M05-2X methods were employed to obtain the initial geometries. The triplet/singlet potential energy surfaces (PESs) were constructed with high-level BMC-CCSD method. The conventional transition-state theory (CTST) and multichannel RRKM theory were employed to calculate the total and individual rate constants over a wide range of temperatures under high-pressure limit. Our computed rates agree well with the available experimental results. The yield of IM1 is 0.9–0.5 from 200 to 1000 K, and the construction of h-P1(OH + CH 2 CHCHCN) is 0.42 at 2000 K under high-pressure limit. The yields of the predicted decomposition products P1(CH 3 + HCOHCCN), P2(HCCN + CH 3 CHO) and P3(H + CH 3 COHCCN) at 298 K and 1 atom are 0.81, 0.07, and 0.09, respectively.

Direct dynamics simulations were performed to study the SN2 double inversion mechanism SN2-DI, with retention of configuration, for the F− + CH3I reaction. Previous simulations identified a transition state (TS) structure, i.e. TS0, for the SN2-DI mechanism, including a reaction path. However, intrinsic reaction coordinate (IRC) calculations from TS0 show it is a proton transfer (PT) TS connected to the F−⋯HCH2I SN2 pre-reaction complex and the FH⋯CH2I− proton transfer post-reaction complex. Inclusion of TS0 in the SN2-DI mechanism would thus involve non-IRC atomistic dynamics. Indeed, trajectories initiated at TS0, with random ensembles of energies as assumed by RRKM theory, preferentially form the SN2-DI products and ∼70% follow the proposed SN2-DI pathway from TS0 to the products. In addition, the Sudden Vector Projection (SVP) method was used to identify which CH3I vibrational mode excitations promote access to TS0 and the SN2-DI mechanism. Results of F− + CH3I simulations, with SVP specified mode excitations, are disappointing. With the CH3 deformations of CH3I excited, the SN2 single inversion mechanism is the dominant pathway. If the CH stretch modes are also excited, proton transfer dominates the reaction. SN2-DI occurs, but with a very small probability of ∼1%. The reasons behind these results are discussed.

Density functional theory, along with the ωB97XD and UM06-2x exchange-correlation functional, has been used to study the reaction mechanisms and kinetics of the atmospheric oxidation of the main (kinetically dominant) thiophene-OH adduct [C4H4S-OH]• (R1) by molecular oxygen in its triplet electronic ground state. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been calculated by means of transition state theory (TST), variational transition state theory (VTST) and statistical Rice−Ramsperger−Kassel−Marcus (RRKM) theory. In line with the computed energy profiles, the dominant process under both the thermodynamic and kinetic control of the reaction is O2 addition at the C5 position in syn mode. The computed branching ratios indicate that the regioselectivity of the reaction decreases with increasing temperature and decreasing pressure. All calculations presented in this work have been performed at the Flemish Supercomputer Center (Vlaams Supercomputer Centrum). This cluster has been financed by budgets obtained from the Katholieke Universiteit Leuven, as well as from individual contributions by users, and funding obtained from the Hercules foundation and the Flemish government.

The cheletropic elimination process of N2 from (2,5-dihydro-1H-pyrrol-1-ium-1-ylidene) amide (C4H6N2) has been studied computationally using density functional theory, along with the M06-2X/aug-cc-pVTZ level of theory. The calculated energy profile has been supplemented with calculations of kinetic rate constants using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This elimination process takes place spontaneously with an activation energy around 33 kJ/mol. Pressure dependence of the rate constants revealed that the TST approximation breaks down and fall-off expression is necessary for the kinetic modeling. At temperatures ranging from 240 to 360 K and atmospheric pressure, the unimolecular rate constant is evaluated from RRKM theory as $$k_{{(240 - 360\,{\text{K}})}}^{{1.0{\text{atm}}}} = 1.0249 \times 10^{12} \times {\text{e}}^{{ - \frac{{33.11\;{\text{kJ}}/{\text{mol}}}}{RT}}} \,{\text{s}}^{ - 1}$$ . Bonding changes along the reaction coordinate have been studied using bonding evolution theory. Electron localization function topological analysis reveals that the cheletropic elimination is characterized topologically by four successive structural stability domains (SSDs). Breaking of C–N bonds (Rx = 0.1992 amu1/2 Bohr) and the other selected points separating the SSDs along the reaction coordinate occur in the vicinity of the transition state.

Hexachlorobenzene is highly persistent, bioaccumulative, toxic and globally distributed, a model persistent organic pollutant. The major atmospheric removal process for hexachlorobenzene is its oxidation by hydroxyl radicals. Unfortunately, there is no information on the reaction mechanism of this important atmospheric process and the respective degradation rates were measured in a narrow temperature range not of environmental relevance. Thus, the geometries and energies of all stationary points significant for the atmospheric oxidation of hexachlorobenzene are optimized using MP2/6-311G(d, p) method. Furthermore, the single point energies were calculated with G3 method on the optimized minima and transition-states. It was demonstrated for the first time that the addition of hydroxyl radicals to hexachlorobenzene proceeds indirectly, via a prereaction complex. In the prereaction complex the hydroxyl radical is almost perpendicular to the aromatic ring while oxygen is pointing to its center. In contrast, in the transition state it is nearly parallel with the aromatic ring. The reliable rate constants are calculated for the first time for the atmospheric oxidation of hexachlorobenzene for all environmentally relevant temperatures. It was also demonstrated for the first time that pentachlorophenol is the major stable product in the addition of hydroxyl radicals to hexachlorobenzene and that atmosphere seems to be a new global secondary source of pentachlorophenol.

Computational investigation on Cl-initiated oxidation of allyl acetate (AAC) including reaction mechanisms and kinetics are performed using quantum chemical method. Compared with experimental results, a more comprehensive and reasonable mechanism is proposed. For the primary reaction, seven channels (two Cl-additions and five H-abstractions) are discussed. The calculated results show that the two additional channels dominate the reaction of AAC with Cl. Further investigations of two Cl-adducts are performed in the presence of O2 and NO. The rate constants are calculated using RRKM theory by employing the MESMER program. The total rate constant (1.39 × 10−10 cm3 molecule−1 s−1) is well consistent with experimental data at 298 K and 760 Torr, and shows negative temperature dependence in the range of 200–500 K. Tropospheric half-life of AAC (τ1/2 = 4.6 h for Cl-initiated oxidation) is estimated to evaluate the atmospheric implications.

Based on the nitrogen-containing combustion, for the main reactions related to N2O in fuel combustion processes, the anharmonic and harmonic rate constant was calculated by using the method proposed by Yao and Lin according to the RRKM theory. For most reaction investigated in this study, the anharmonic effect was more obvious with increasing reaction temperature. Furthermore, with regard to some reactions, there was junction between the harmonic and anharmonic rate constant. Moreover, the study took advantage of the rate constant to fit the thermodynamic and kinetic parameters.