Thermal Unimolecular Decomposition Research Articles

The Unimolecular Decomposition Mechanism of Trimethyl Phosphate.

Trimethyl phosphate (TMP), an organophosphorus compound (OPC), is a promising fire-retardant candidate for lithium-ion battery (LIB) electrolytes to mitigate fire spread. This study aims to understand the mechanism of TMP unimolecular thermal decomposition to support the integration of a TMP chemical kinetic model into a LIB electrolyte surrogate model. Reactive intermediates and products of TMP thermal decomposition were experimentally detected using vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Phosphorus-containing intermediates such as PO, HPO and HPO2 were identified. Sampling effects could successfully be obviated thanks to photoion imaging, which also showed evidence for isomerization reactions upon wall collisions in the ionization chamber. Quantum chemical calculations performed for the unimolecular decomposition of TMP revealed for the first time that isomerization channels via hydrogen and methyl transfer (barrier heights of 65.9 and 72.6 kcal/mol, respectively) are the lowest-energy primary steps of TMP decomposition followed by CH3OH/CH3/CH2O or dimethyl ether (DME) production, respectively. We found an analogous DME production channel in the unimolecular decomposition of dimethyl methylphosphonate (DMMP), another important OPC fire-retardant additive with a similar molecular structure to TMP, which are not included in currently available chemical kinetic models.

Kinetic and Experimental Investigations on the Unimolecular Thermal Decomposition of an Eco-friendly Insulating Gas HFO-1336mzz(E)

Kinetic and Experimental Investigations on the Unimolecular Thermal Decomposition of an Eco-friendly Insulating Gas HFO-1336mzz(E)

Beyond vanilla: The dissociation mechanism of vanillin in four charge states

We compare the unimolecular decay mechanism of vanillin in four charge states. The unimolecular thermal decomposition of the neutral and dissociative ionization reactions in the cation have already been addressed in synchrotron-based photoionization studies. The picture is completed here by studying the collision-induced dissociation mechanism of protonated vanillin cations and deprotonated vanillin anions in the 5–30 eV collision energy range. The mass spectrometric observations are rationalized by ab initio calculations, which reveal the mechanism and the energetics of dissociation pathways. In the positive ion mode, the aldehyde oxygen is protonated, after which CO is lost. In contrast to previous results, CO loss is found to be the sole primary fragmentation pathway of protonated vanillin. Two hydrogen migration steps to the benzene ring open sequential CH3OH and CH3 loss channels to yield C6H5O+ and C6H6O2+, respectively. The former fragment ion can continue to lose CO, associated with ring contraction and producing the cyclopentadiene cation, C5H5+. In the negative ion mode, the proton is removed from the hydroxyl group. The primary dissociation channel of deprotonated vanillin corresponds to (C)OCH3 bond breaking in the methoxy group to form C7H4O3− at m/z 136. This species goes on to produce C6H4O2− and C6H4O− anions via CO and CO2 loss, respectively, in sequential dissociation processes at higher collision energies. We compare the protonated and deprotonated vanillin fragmentation pathways with those observed in the decomposition of neutral vanillin and with the dissociation pathways of the vanillin cation. The initial decomposition step for neutral, cationic, and deprotonated vanillin is methyl loss by direct CO bond breaking, often followed by loss of CO. In contrast, protonated vanillin first loses CO after isomerization.

Mechanism of Interaction between Ammonium Perchlorate and Aluminum.

There is an interactive effect between ammonium perchlorate (AP) and aluminum (Al) powder during the combustion process of composite solid propellants, but the mechanism of this effect is still lacking. Using quantum chemical methods, we investigated this mechanism from a molecular perspective. The interaction process between Al and AP was analyzed by comparing the chemical bond changes between the atoms during the reaction process of the Al/AP system and the AP unimolecular thermal decomposition system. The results show that Al atoms alter the reaction mechanism of AP thermal decomposition, significantly decreasing the activation energy of AP decomposition at high temperature but increasing that at low temperature. Meanwhile, the temperature-dependent rate constant of each basic reaction was calculated by transition state theory. The rate constants increase with temperature. Under high temperature and pressure, Al can increase the high-temperature decomposition rate of AP by up to 1-3 orders of magnitude.

Pyrolytic elimination of ethylene from ethoxyquinolines and ethoxyisoquinolines: a computational study

This work reports a thermo-kinetic study on unimolecular thermal decomposition of some ethoxyquinolines and ethoxyisoquinolines derivatives (1-ethoxyisoquinoline (1-EisoQ), 2-ethoxyquinoline (2-EQ), 3-ethoxyquinoline (3-EQ), 3-ethoxyisoquinoline (3-EisoQ), 4-ethoxyquinoline (4-EQ), 4-ethoxyisoquinoline (4-EisoQ), 5-ethoxyquinoline (5-EQ), 5-ethoxyisoquinoline (5-EisoQ), 8-ethoxyquinoline (8-EQ) and 8-ethoxyisoquinoline (8-EisoQ)) using density functional theory DFT (BMK, MPW1B95, M06-2X) and ab initio complete basis set-quadratic Becke3 (CBS-QB3) calculations. In the course of the decomposition of the investigated systems, ethylene is eliminated with the production of either keto or enol tautomer. The six-membered transition state structure encountered in the path of keto formation is much lower in energy than the four-membered transition state required to give enol form. Rate constants and activation energies for the decomposition of 1-EisoQ, 2-EQ, 3-EQ, 3-EisoQ, 4-EQ, 4-EisoQ, 5-EQ, 5-EisoQ, 8-EQ, and 8-EisoQ have been estimated at different temperatures and pressures using conventional transition state theory combined with Eckart tunneling and the unimolecular statistical Rice–Ramsperger–Kassel–Marcus theories. The tunneling correction is significant at temperatures up to 1000 K. Rate constants results reveal that ethylene elimination and keto production are favored kinetically and thermodynamically over the whole temperature range of 400–1200 K and the rates of the processes under study increase with the rising of pressure up to 1 atm.

Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study.

To handle energetic materials safely, it is important to have knowledge about their sensitivity. Density functional theory (DFT) has proven a valuable tool in the study of energetic materials, and in the current work, DFT is employed to study the thermal unimolecular decomposition of 2,4,6-trinitrophenol (picric acid, PA), 3-methyl-2,4,6-trinitrophenol (methyl picric acid, mPA), and 3,5-dimethyl-2,4,6-trinitrophenol (dimethyl picric acid, dmPA). These compounds have similar molecular structures, but according to the literature, mPA is far less sensitive to impact than the other two compounds. Three pathways believed important for the initiation reactions are investigated at 0 and 298.15 K. We compare the computed energetics of the reaction pathways with the objective of rationalizing the unexpected sensitivity behavior. Our results reveal a few if any significant differences in the energetics of the three molecules, and thus do not reflect the sensitivity deviations observed in experiments. These findings point toward the potential importance of crystal structure, crystal morphology, bimolecular reactions, or combinations thereof on the impact sensitivity of nitroaromatics.

Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Bromoacetaldehyde (BrCH2CHO) is a major stable brominated organic intermediate of the bromine-ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH2CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH2CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surface-dominated photodynamics, the primary photodissociation yields of BrCH2CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm-1 < ⟨ΔEdown⟩ < 450 cm-1, 78-33% of BrCH2CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔEdown⟩ = 300 cm-1 and an excitation wavelength of 320 nm, with 27% for C-C bond fission, 11% for C-Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O-Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C-Br and C-C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.

Thermal Decomposition of CH3O: A Curious Case of Pressure-Dependent Tunneling Effects.

The thermal unimolecular decomposition of a methoxy radical (CH3O), a key intermediate in the combustion of methane, methanol, and other hydrocarbons, was studied using high-level coupled-cluster calculations, followed by E,J-resolved master equation analyses. The experimental results available for a wide range of temperature and pressure are in striking agreement with the calculations. In line with a previous theoretical study that used a one-dimensional master equation, the tunneling correction is found to exhibit a marked pressure dependence, being the largest at low pressure. This curious effect on the tunneling enhancement also affects the calculated kinetic isotope effect, which falls initially with pressure but is predicted to rise again at high pressures. These findings serve to reconcile a set of conflicting results regarding the importance of tunneling in this prototype unimolecular reaction and also motivate further experimental investigation. This study also exemplifies how changes in the energy redistribution due to collisions manifest in the tunneling rates.

Thermal Decomposition of 2-Methyltetrahydrofuran behind Reflected Shock Waves over the Temperature Range of 1179–1361 K

The thermal unimolecular decomposition of 2-methyltetrahydrofuran (2-MTHF) was studied behind reflected shock waves in a single-pulse shock tube over the temperature range of 1179-1361 K and pressure range of 9-17 atm. Methane, ethylene, ethane, 1,3-butadiene, propylene, acetaldehyde, and acetylene were identified as products in the decomposition of 2-MTHF. A reaction scheme was proposed to explain the mechanism for the observed products. The experimentally determined rate coefficients were best fit to an Arrhenius expression for the overall decomposition and is represented as ktotalexp(1179-1361 K) = (3.23 ± 0.59) × 1011 s-1 exp(-51.3 ± 1.4 kcal mol-1/RT). Quantum chemistry methods were used to calculate the energetics and kinetics of various possible unimolecular dissociation pathways involved in the thermal decomposition of 2-MTHF. The initial decomposition of 2-MTHF occurs predominantly via ring-methyl (C-CH3) single bond fission, leading to the formation of tetrahydrofuran (C4H7O) radical, and methyl radical was found to be the major reaction compared to all the possible initial bond fission, ring opening, and molecular elimination channels. The temperature-dependent rate coefficients for the unimolecular dissociation of 2-MTHF were calculated using the RRKM (Rice-Ramsperger-Kassel-Marcus) theory in combination with the CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of electronic structure calculations over the temperature range of 800-1500 K. The computed high-pressure limiting rate coefficients for the initial decomposition of 2-MTHF through C-CH3 single bond fission channel were found to be ∼2 times higher in the temperatures between 800 and 900 K, and above this temperature, they agree well with the values reported in the literature.

Ab initio and kinetics study of the thermal unimolecular decomposition of 2-furfuryl alcohol

To uncover the fundamental chemistry of thermal unimolecular decomposition of 2-furfuryl alcohol, the quantum chemical calculations and reaction kinetics analysis are performed. The temperature and pressure-dependent rate constants of 2-furfuryl alcohol pyrolysis were calculated using Rice–Ramsperger–Kassel–Marcus/Master Equation method. Our calculations reveal that the preliminarily main reaction pathway occurs via 1,2 hydrogen shift to form a carbine intermediate (labelled as INT2) at high pressure limit. The temperature- and pressure- dependent rate constants for these main reaction pathways were discussed in details. The reaction pathway to produce bimolecular products (3-butyn-1-ol plus CO) has a main contribution at high temperatures and at P ≤ 1 atm. Morever, the thermodynamics and kinetics data determined in the present work would be useful for chemical kinetics modelers.

Thermal unimolecular decomposition of ethyl 2‐furoate and its reactivity toward OH radicals: A theoretical study

AbstractMechanism and kinetics of the thermal pyrolysis of ethyl 2‐furoate were studied in a wide range of conditions (ie, 500‐1500 K and 1‐7600 Torr) using the accurate dual‐level theory CCSD(T)&amp;MP2(FC)//B3LYP/aug‐cc‐pVTZ and state‐of‐the‐art Rice‐Ramsperger‐Kassel‐Marcus based master equation rate model including hindered internal rotation and tunneling treatments. The predicted rate coefficients are found to be in accordance with the experimental data. The mechanism details are revealed as (i) the major decomposition channel occurs via a six‐center transition state with the barrier energy of 48.3 kcal/mol at 0 K, leading to the formation of C2H4 and 2‐furoic acid and (ii) pressure has a slightly positive effect on the rate constant, particularly at the high temperature. Also, the reactivity of ethyl 2‐furoate toward OH radicals was discussed in the first place.

Direct Kinetic Measurements and Master Equation Modelling of the Unimolecular Decomposition of Resonantly-Stabilized CH2CHCHC(O)OCH3 Radical and an Upper Limit Determination for CH2CHCHC(O)OCH3 + O2 Reaction

Abstract Methyl-Crotonate (MC, (E)-methylbut-2-enoate, CH3CHCHC(O)OCH3) is a potential component of surrogate fuels that aim to emulate the combustion of fatty acid methyl ester (FAME) biodiesels with significant unsaturated FAME content. MC has three allylic hydrogens that can be readily abstracted under autoignition and combustion conditions to form a resonantly-stabilized CH2CHCHC(O)OCH3 radical. In this study we have utilized photoionization mass spectrometry to investigate the O2 addition kinetics and thermal unimolecular decomposition of CH2CHCHC(O)OCH3 radical. First we determined an upper limit for the bimolecular rate coefficient of CH2CHCHC(O)OCH3 + O2 reaction at 600 K (k ≤ 7.5 × 10−17 cm3 molecule−1 s−1). Such a small rate coefficient suggest this reaction is unlikely to be important under combustion conditions and subsequent efforts were directed towards measuring thermal unimolecular decomposition kinetics of CH2CHCHC(O)OCH3 radical. These measurements were performed between 750 and 869 K temperatures at low pressures (&lt;9 Torr) using both helium and nitrogen bath gases. The potential energy surface of the unimolecular decomposition reaction was probed at density functional (MN15/cc-pVTZ) level of theory and the electronic energies of the stationary points obtained were then refined using the DLPNO-CCSD(T) method with the cc-pVTZ and cc-pVQZ basis sets. Master equation simulations were subsequently carried out using MESMER code along the kinetically important reaction pathway. The master equation model was first optimized by fitting the zero-point energy corrected reaction barriers and the collisional energy transfer parameters Δ E down , ref $\Delta{E_{{\text{down}},\;{\text{ref}}}}$ and n to the measured rate coefficients data and then utilize the constrained model to extrapolate the decomposition kinetics to higher pressures and temperatures. Both the experimental results and the MESMER simulations show that the current experiments for the thermal unimolecular decomposition of CH2CHCHC(O)OCH3 radical are in the fall-off region. The experiments did not provide definite evidence about the primary decomposition products.

Ab-initio studies of thermal unimolecular decomposition of furan: A complementary deterministic and stochastic master equation model

The detailed reaction mechanism for the thermal unimolecular decomposition of furan was comprehensively investigated in a wide range of conditions (T = 800–2000 K and P = 0.001–100 atm). The main reaction pathways were explored using different composite electronic structure methods including W1U, CBS-APNO, CBS-QB3, G3, G3B3, and G4. The temperature-/pressure-dependent dynamic behaviors of the furan pyrolysis were characterized using the combined deterministic and stochastic Master Equation/Rice–Ramsperger–Kassel–Marcus (ME/RRKM) model. These calculations embodied the hindered internal rotation and quantum tunneling corrections. Besides the C–H bond fission channels, the pyrolysis mechanism is found to involve the H-transfer reactions yielding α-carbene and β-carbene as intermediates which eventually decompose and/or isomerize producing three final products, viz. C2H2 + H2CO (P1), CH3CCH + CO (P2), and CH2CCH + HCO (P3). While P1 and P2 appear to be the primary products at T > 1300 K, channels P3, 2-furyl + H (P4) and 3-furyl + H (P5) are found insignificant. Our calculations reveal that the title reaction occurs via β-carbene with a contribution of 85–91% between 1600 K and 2000 K and at 1 atm pressure, which agrees well with the recent measurements (Urness et al., J. Chem. Phys., 2013, 139, 124305). The calculated rate coefficients, k(T, P), and the thermodynamic properties of the species involved are found to be in good agreement with the experimental results. Therefore, the reported data in this work are highly recommended for future modeling and simulation of furan-related combustion applications. The performance of the considered electronic structure methods for kinetic purposes was also discussed.

Thermal Decomposition of Diphenyl Tetroxane in Chlorobenzene Solution

The thermal decomposition of Cyclic Diperoxide of Benzaldehyde 3,6-diphenyl-1,2,4,5-tetroxane, (DFT) in chlorobenzene solution in the studied temperature range (130°C - 166°C) satisfactorily satisfies a first order law up to 60% conversions of diperoxide. DFT would decompose through a mechanism in stages and initiated by the homolytic breakdown of one of the peroxidic bonds of the molecule, with the formation of the corresponding intermediate biradical. The concentration studied was very low, so that the effects of secondary reactions of decomposition induced by free radicals originated in the reaction medium can be considered minimal or negligible. The activation parameters for the unimolecular thermal decomposition reaction of the DFT are ΔH# = 30.52 ± 0.3 kcal·mol-1 and ΔS# = -6.38 ± 0.6 cal·mol-1 K-1. The support for a step-by-step mechanism instead of a process concerted is made by comparison with the theoretically calculated activation energy for the thermal decomposition of 1,2,4,5-tetroxane.

Direct measurements of C3F7I dissociation rate constants using a shock tube ARAS technique

AbstractThis work presents the first direct experimental study on the thermal unimolecular decomposition of n‐C3F7I. Experiments were performed behind incident and reflected shock waves using the atomic resonance absorption spectroscopy (ARAS) technique on a resonant line of atomic iodine at 183.04 nm. The reaction C3F7I + Ar → C3F7 + I + Ar (1) was studied at specific temperature (800–1200 K) and pressure (0.6–8.3 bar) ranges. Under experimental conditions, the obtained values of the rate constant at temperatures below 950 K are close to the high‐pressure limit; however, considering theoretical calculations, the influence of pressure on the rate constant at elevated temperatures remains noticeable. The resulting value of the experimental rate constant of reaction 1 is presented in the following Arrhenius form: urn:x-wiley:05388066:media:kin21244:kin21244-math-0001Experimental data were found to correlate with the results of the Rice–Ramsperger–Kassel–Marcus –master equation analysis based on quantum‐chemical calculations. The following low‐ and high‐pressure limiting rate coefficients were obtained over the temperature range T = 300–3000 K: urn:x-wiley:05388066:media:kin21244:kin21244-math-0002with the center broadening factor Fc = 0.119.

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH2 with ketene CH2 CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice-Ramsperger-Kassel-Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH3 and C2 H4 + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C2 H4 + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH2 COCH2 intermediate or along the pathway of CO elimination from the initial CH2 CH2 CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C2 H4 + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH3 CHCO and cyclic CH2 COCH2 intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300-3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.

Reaction mechanism, rate constants, and product yields for the oxidation of Cyclopentadienyl and embedded five-member ring radicals with hydroxyl

Potential energy surfaces for the C5H5 + OH and C15H9 + OH reactions have been studied by ab initio calculations at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) and G3(MP2,CC)//B3LYP/6-311G(d,p) levels of theory, respectively, in order to unravel the mechanism of oxidation of the cyclopentadienyl radical and five-member-ring radicals embedded in a sheet of six-member rings with OH. The VRC-TST approach has been employed to compute high-pressure-limit rate constants for barrierless entrance and exit reaction steps and multichannel/multiwell RRKM-ME calculations have been utilized to produce phenomenological pressure- and temperature-dependent absolute and individual-channel reaction rate constants. The calculations allowed us to quantify relative yields of various products in a broad range of conditions relevant to combustion and to generate rate expressions applicable for kinetic models of oxidation of aromatics. The C5H5 + OH reaction is shown to proceed either by well-skipping pathways without stabilization of C5H6O intermediates leading to the bimolecular products ortho-C5H5O + H, C5H4OH (hydroxycyclopentadienyl) + H, and C4H6 (1,3-butadiene) + CO, or via stabilization of the C5H6O intermediates, which then undergo unimolecular thermal decomposition to ortho-C5H5O + H and C4H6 + CO. The well-skipping and stabilization/dissociation pathways compete depending on the reaction conditions; higher pressures favor the stabilization/dissociation and higher temperature favor the well-skipping channels. For the C15H9 + OH reactions, the results demonstrate that embedding decreases the oxidation rate constants and hinder the decarbonylation process; the removal of CO grows less likely as the number of common edges of the five-member ring with the surrounding six-member rings increases.

A high temperature kinetic study for the thermal unimolecular decomposition of diethyl carbonate

Thermal unimolecular decomposition of diethyl carbonate (DEC) was investigated in a shock tube by measuring ethylene concentration with a CO2 gas laser over 900–1200K and 1.2–2.8bar. Rate coefficients were extracted using a simple kinetic scheme comprising of thermal decomposition of DEC as initial step followed by rapid thermal decomposition of the intermediate ethyl hydrogen carbonate. Our results were further analysed using ab initio and master equation calculations to obtain pressure- and temperature-dependence of rate coefficients. Similar to alkyl esters, unimolecular decomposition of DEC is found to undergo six-center retro-ene elimination of ethylene in a concerted manner.

Ab Initio Chemical Kinetics for the Thermal Decomposition of SiH4 + Ion and Related Reverse Ion–Molecule Reactions of Interest to PECVD of a-Si:H Films

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.

Reaction mechanism, rate constants, and product yields for unimolecular and H-assisted decomposition of 2,4-cyclopentadienone and oxidation of cyclopentadienyl with atomic oxygen

Ab initio calculations of potential energy surfaces in conjunction with the RRKM-Master Equation theoretical approach have been employed to evaluate temperature- and pressure-dependent total and product specific rate constants and product branching ratios for unimolecular thermal decomposition of 2,4-cyclopentadienone C5H4O and for the C5H4O+H and C5H5+O reactions. The formation of the cyclobutadiene+CO products via a ring contraction/CO elimination mechanism is shown to be the prevailing channel for the unimolecular decomposition of C5H4O. The unimolecular reaction is found to be relatively slow, but decomposition of cyclopentadienone can be greatly facilitated through bimolecular encounters with H atoms. The C5H4O+H reaction is predicted to be fast, with rate constants ranging from 4.6 × 10−12 to 1.8 × 10−10cm3 molecule−1 s−1 at T = 500–2500K and finite pressures. Cyclic C5H5O intermediates formed after the initial H addition undergo ring openings by β-scissions and then decompose to either butadienyl C4H5+CO or 1-oxoprop-2-enyl H2CCHCO+C2H2, which are respectively predicted as the major and the minor reaction products. The calculations predict that thermal decomposition of the ortho and meta C5H5O radicals as well as pyranyl nearly exclusively forms the C4H5+CO products, whereas decomposition of hydroxycyclopentadienyl C5H4OH predominantly produces cyclopentadienone+H. The C5H5+O reaction is shown to proceed by barrierless oxygen addition to the ring followed by fast H migration, ring opening, and dissociation to C4H5+CO. The C5H5+O rate constant is calculated to be close to 1 × 10−10cm3 molecule−1 s−1 and to be pressure-independent and nearly independent of temperature. Modified Arrhenius expressions for rate constants for all considered reactions at the high-pressure limit and at finite pressures are generated for kinetic modeling.