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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.

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

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 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.

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

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.

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.

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 .

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 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.

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.

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.

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.

A zero-point energy (ZPE) constraint model is proposed for classical trajectory simulations of unimolecular decomposition and applied to CH4* → H + CH3 decomposition. With this model trajectories are not allowed to dissociate unless they have ZPE in the CH3 product. If not, they are returned to the CH4* region of phase space and, if necessary, given additional opportunities to dissociate with ZPE. The lifetime for dissociation of an individual trajectory is the time it takes to dissociate with ZPE in CH3, including multiple possible returns to CH4*. With this ZPE constraint the dissociation of CH4* is exponential in time as expected for intrinsic RRKM dynamics and the resulting rate constant is in good agreement with the harmonic quantum value of RRKM theory. In contrast, a model that discards trajectories without ZPE in the reaction products gives a CH4* → H + CH3 rate constant that agrees with the classical and not quantum RRKM value. The rate constant for the purely classical simulation indicates that anharmonicity may be important and the rate constant from the ZPE constrained classical trajectory simulation may not represent the complete anharmonicity of the RRKM quantum dynamics. The ZPE constraint model proposed here is compared with previous models for restricting ZPE flow in intramolecular dynamics, and connecting product and reactant/product quantum energy levels in chemical dynamics simulations.

Benton Seymour Rabinovitch was one of the pioneers of chemical dynamics. His brilliant experiments performed during his four decades as a Professor of Chemistry at the University of Washington in Seattle provided most of our early quantitative measurements of the efficiency with which energy is transferred between molecules in gas-phase molecule–molecule collisions and in collisions of molecules with solid surfaces. More importantly, his work provided quantitative estimates of the rates with which vibrational energy deposited locally within a molecule is redistributed among the many vibrational modes within that molecule, proving that the equilibration of this vibrational energy among these modes almost always occurs in approximately one picosecond. He further showed that this validates (in most cases) the assumptions of Rice–Ramsperger–Kassel–Marcus (RRKM) theory. He also developed several widely used mathematical shortcuts for using RRKM theory to make important predictions about physical chemistry. These shortcuts greatly increased both the applications and impact of RRKM theory, so that it has become one of the most important theories of physical chemistry. It continues to guide much of our fundamental understanding of chemical dynamics and reaction kinetics even today. In addition to being a great scientist, Seymour Rabinovitch was a devoted husband and father. He raised four accomplished children, and later in life became an expert in the art of silversmithing, a writer of children's books, and a philanthropist. His offspring are following beautifully in his footsteps in their kindness to fellow human beings, their excellence in scholarship, science and art, and in their energetic dedication to improving the world through teaching, research, service and philanthropy. The same can be said for his academic offspring as well.

Methylation of [R(1)R(2)CNO2](-), where R(1) = R(2) = H (1), R(1) = CH3 and R(2) = H (2), R(1) = R(2) = CH3 (3), and R(1) + R(2) = c-(CH2)2 (4), by CH3I was studied by an ab initio MP2/CBS method, RRKM theory, and kinetic simulations. Contrary to a previous proposal for the reaction mechanism, C-methylation is the preferred pathway of thermodynamics and kinetics. This is corroborated by the agreement between the calculated and experimental reactivity trend 4 ≫ 321. The regioselectivity toward C-alkylation is explained by the much larger exothermicity of this reaction channel compared to that of O-alkylation. The increase in reactivity with an increase in the crowdedness of the central carbon atom is explained by differences in sp(3) character at this atom and the decrease in the vibrational frequency associated with pyramidalization around this carbon atom.

Anharmonic and harmonic rate constants for the unimolecular dissociation of CH3OOH and CD3OOD were calculated using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory at the MP2/6-311++G(3df,3pd) level of theory. The anharmonic effect of the reactions was investigated. Comparison of results for the decompositions of CH3OOH and CD3OOD shows that the direct bond dissociation channel, CH3(D3) O + OH (D), is the most dominant reaction. The anharmonic effect plays an important role in the unimolecular dissociation of both CH3OOH and CD3OOD. For channels CH3(D3) O + OH (D) and CH3(D3) + H (D) O2, the anharmonic effect of the unimolecular dissociation of CD3OOD is more pronounced than that of the unimolecular dissociation of CH3OOH. For channel H2(D2) CO + H2(D2) O, the anharmonic effect of the unimolecular dissociation of CH3OOH is more pronounced than that of the unimolecular dissociation of CD3OOD. The isotope effect is more distinct in the anharmonic oscillator model.

The reactions of NO2 with cis-/trans-CH3NHNH, CH3NNH2 and CH2NHNH2 have been studied theoretically by quantum chemical calculations and steady-state unimolecular master equation analysis based on RRKM theory. The barrier heights for the roaming transition states between nitro (RNO2) and nitrite (RONO) isomerization reactions and those for the concerted HONO and HNO2 elimination reactions from RNO2 and RONO, affect the pressure dependences of the product-specific rate coefficients. At ambient temperature and pressure, the dominant product of the reactions of NO2 with cis-/trans-CH3NHNH and CH2NHNH2 would be expected to be HONO with trans-CH3NNH and CH2NNH2, respectively, whereas it is CH3N(NH2)NO2 for CH3NNH2 + NO2. The product-specific rate coefficients for the titled and related reactions on the same potential energy surfaces were proposed for kinetics modeling., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1610065000

Detailed mechanisms of PAH growth involving methylene (CH2) were studied using accurate ab initio density functional theory B3LYP/6-311+G(d,p) calculations, as well as approximate QCISD(T,full)/6-311++G(3df,2pd) calculations. The PAH growth can be divided into five essential reaction steps, namely, addition C2H2 → intramolecular hydrogen migration → addition CH2 → cyclization → H-elimination. The aliphatic species of indene and 1H-phenalene are found in the pathways of PAH growth, which is in accord with the experimental results that reveal the formation of aliphatic species in flames. It was found that the simultaneous removal of two H atoms in one reaction step is feasible in PAH evolution, and this can reasonably interpret the absence of a H atom in the post-flame region. The corresponding rate coefficients at 1 atm were evaluated by using TST and RRKM theory by solving the master equations in the temperature range of 500-2500 K. The calculated branching ratios suggest that the pathways involving CH2 are competitive in PAH growth.

Rate constants for thermal decomposition of 1,1,1-trifluoroethane (CH3CF3) in the high-temperature falloff region were previously reported to have an unusual pressure dependence that could not be explained by Rice-Ramsperger-Kassel-Marcus (RRKM) theory in combination with unimolecular master equation analysis. This study investigates the dynamics of the CH3CF3 dissociation and the energy transfer of CH3CF3 in collisions with Ar and Kr by classical trajectory calculations on a global potential energy surface constructed from a large number of quantum chemical calculations. The simulations showed that the ensemble-averaged CH3CF3 populations decay with single exponential profiles that have rate constants close to those predicted by RRKM theory, indicating that the microcanonical ensemble is maintained during decomposition. The trajectory calculation also indicated that a significant portion of the HF product is formed in its vibrationally excited state. Such observation motivated this study to correct some of the reported rate constants for the CH3CF3 decomposition. With the correction applied, the experimental rate constants were well reproduced by the RRKM/master equation calculation using the collisional energy transfer parameters that were also obtained from trajectory calculations. Overall, the title reaction is demonstrated to be another successful example of RRKM/master equation modeling.

The rate constant for reaction of sulfur atoms with acetylene was measured. Laser flash photolysis of CS2 precursor was employed to generate ground-state S(3P) atoms, which were monitored with time-resolved resonance fluorescence as they reacted with C2H2 in a large excess of Ar bath gas. Temperatures from 295 to 1015 and pressures from 10 to 500 mbar were investigated. A pressure-dependence was observed at all temperatures, revealing that adduct formation is the dominant reaction channel. The necessary stability suggests H2CCS or possibly HCCSH are the products at high temperatures, so that the reaction is spin-forbidden. The fall-off curves may be represented with a broadening factor Fcent = 0.6, and low and high-pressure limiting rate constants of k0 = 1.0 × 10−18 (T/K)−3.55 exp(−1990 K/T) cm6 molecule−2 s−1 and k∞ = 2.1 × 10−11 exp(−11.2 kJ mol−1/RT) cm3 molecule−1 s−1, respectively. An entrance barrier to recombination of about 10 kJ mol−1 is proposed to arise where the singlet and triplet potential energy curves cross.