Unimolecular Decomposition Channels Research Articles

Pyrolysis of Methyl Formate and the Reaction of Methyl Formate with H Atoms: Shock Tube Experiments and Statistical Rate Theory.

Methyl formate (MF) is the smallest carboxylic ester and currently considered a promising alternative fuel. It can also serve as a model compound to study the combustion chemistry of the ester group, which is a typical structural feature in many biodiesel components. In the present work, the pyrolysis of MF was investigated behind reflected shock waves at temperatures between 1430 and 2070 K at a nominal pressure of 1.1 bar. Both time-resolved hydrogen atom resonance absorption spectroscopy (H-ARAS) and time-resolved time-of-flight mass spectrometry (TOF-MS) were used for species detection. Additionally, the reaction of MF and perdeuterated MF-d4 with H atoms was investigated at temperatures between 1000 and 1300 K at nominal pressures of 0.4 and 1.1 bar with H-ARAS. In the latter experiments, ethyl iodide served as precursor for H atoms. Rate coefficients of seven parallel unimolecular decomposition channels of MF and five parallel reaction channels of the MF + H reaction were calculated from statistical rate theory on the basis of molecular and transition state data from quantum chemical calculations. These calculated rate coefficients were implemented into an MF pyrolysis/oxidation mechanism from the literature, and the experimental concentration-time profiles of H (from ARAS) as well as MF, CH3OH, HCHO, and CO (from TOF-MS) were modeled. It turned out that the literature mechanism, which was originally validated against flow-reactor experiments, ignition delay times, and laminar burning velocities, was generally able to fit also the concentration-time profiles from the shock tube experiments reasonably well. The agreement could still be improved by substituting the original rate coefficients, which were estimated from structure-reactivity relationships, by the values calculated from statistical rate theory in the present work. Details of the channel branching are discussed, and the updated mechanism is given, also in machine-readable form.

Pressure Dependence of Rate Coefficients of Unimolecular and Chemical Activation Reactions Connected to the Potential Energy Wells of Si2H2Cl4, Si2Cl6, and Si2Cl4 via Rice-Ramsperger-Kassel-Marcus Calculations.

Rate coefficients for elementary reactions connected to the potential energy wells of Si2H2Cl4, Si2Cl6, and Si2Cl4, which are important Si2 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through the Rice-Ramsperger-Kassel-Marcus theory under various conditions of temperature and pressure. The optimized structures and vibrational frequencies of the reactants, products, and transition state were obtained using (U)B3LYP/6-31+G(d,p), and the single-point energies of the optimized structures were recalculated using the coupled cluster method with single and double excitations plus triple perturbation (U)CCSD(T) with complete basis set extrapolation. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall-off regime under subatmospheric to moderately high-pressure conditions so that it is expected that accurate modeling of the gas phase in the chlorosilane CVD reactor requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high-pressure limit rate coefficients. The rate coefficients determined here were expressed through Chebyshev coefficients and also modified Arrhenius parameters to be used in simulations of systems under a wide range of temperature and pressure conditions.

Unimolecular thermal decarbonylation of vanillin stifled by the bimolecular reactivity of methyl-loss intermediate

We studied the thermal decomposition of vanillin in a low-pressure pyrolysis microreactor by photoelectron photoion coincidence spectroscopy. The pyrolysis products were identified isomer-selectively by photoion mass-selected threshold photoelectron spectra (ms-TPES) based on Franck–Condon simulations and reference spectra. Methyl loss by homolytic C–O bond fission in the methoxy group is the initial unimolecular decomposition step. The primary intermediate, the 2-hydroxy-5-formyl-phenoxy radical (m/z 137), may undergo multiple decarbonylation steps and lose a hydrogen atom sequentially to finally form but-1-ene-3-yne at m/z 52. The ms-TPES and calculated reaction path energetics identified the product intermediates at masses of 109, 108, 81, and 80 amu. 3,4-Dihydroxybenzaldehyde (m/z 138) and 3,4-dioxocyclohexa-1,5-diene-1-carbaldehyde (m/z 136) peaks do not correspond to unimolecular decomposition products. Instead, they are formed in the hydrogen-transfer self-reaction of the primary methyl-loss fragment of vanillin at m/z 137. 3,4-Dihydroxybenzaldehyde is thermally more stable than vanillin, as also shown by its high activation energy to decomposition to m/z 137, 110 and 82 fragments. The lighter m/z 136 bimolecular product is prone to CO loss and its reaction path merges with the H-loss-containing unimolecular decomposition channel of vanillin. Further bimolecular reactions are also discussed. These mechanistic insights, especially the high (self-)reactivity of the methyl-loss intermediate and the resulting preponderance of bimolecular chemistry leading to inert dihydroxybenzaldehyde will guide process design involving vanillin and vanillin-like feedstocks.

Comparative Quantum Chemistry Study on the Unimolecular Decomposition Channels of Pyrazole and Imidazole Energetic Materials

The difference in the initial decomposition step of pyrazoles and imidazoles was explored using the M062X method for optimization and G4-MP2 and approximated CCSD(T) methods for energies. Laplacian bond order analysis was used to study the effect of the nitro group on the bond strength and predict the bond dissociation energy (BDE) of the ring. Thermochemistry results show that the most possible decay channel of 1H-pyrazole and 3-nitropyrazole is the N2 elimination, while the preferred initial step of 1H-imidazole is the CHN elimination. However, the nitro-nitrite isomerization dominates the decomposition of other nitro derivatives of 1H-pyrazole and 1H-imidazole. As for the formation of HO and HONO, the high energy barrier makes it difficult to take place. Based on the analysis of the lowest energy barrier and the BDE of NO2 loss, it can be concluded that imidazoles are more stable than pyrazoles. This work contributes to revealing the difference in the initial step of energetic isomers and the understanding of the decomposition mechanism of energetic azoles.

Shock tube study of ethanol pyrolysis II: Rate constant measurements and modeling

The rate constants of two unimolecular decomposition channels of ethanol, C2H5OH(+Ar) = CH3 + CH2OH(+Ar), and C2H5OH(+Ar) = C2H4 + H2O(+Ar) were measured near 1 atm and 10 atm behind reflected shock waves between 1190 and 1550 K by tracking the evolution of CO and C2H4, respectively. Use of sensitive laser diagnostics in conjunction with carefully selected test mixtures enabled the suppression of sensitivity of CO and C2H4 time-histories to secondary reactions, resulting in a significant reduction of uncertainty in the measured rate constants. Additionally, CH3CHO time-histories measured during the pyrolysis of 2% C2H5OH/Argon mixtures were used to determine the rate constant of the most dominant H-abstraction channel, C2H5OH + H = sC2H4OH + H2 between 1290 and 1530 K. The low temperature CH3CHO time-histories were also used to infer the rate constant of sC2H4OH = CH3CHO+H. The measured rate constants were used to generate an updated kinetic model, whose performance was evaluated against the species time-history data reported in recent shock tube pyrolysis studies. The performance of the updated model was also evaluated against ignition delay times (IDTs) of stochiometric C2H5OH/Oxygen/Nitrogen mixtures, measured between 1000 and 1500 K near 20 atm in this study. The updated kinetic model was found to show excellent agreement with both shock tube pyrolysis and IDT studies. Additionally, the reduction in the uncertainty of the rate constants studied in this work resulted in a significant reduction in the uncertainty of the model predictions. The rate constant expressions listed below were derived from the measurements in this work, and are valid in the temperature range of 1200–1600 K.kC2H5OH(+Ar)↔CH3+CH2OH(+Ar),1atm=2.125×1013e−34070Ts−1(±21%)kC2H5OH(+Ar)↔CH3+CH2OH(+Ar),10atm=3.079×1014T0.23e−38030Ts−1(±28%)kC2H5OH(+Ar)↔C2H4+H2O(+Ar),10atm=6.062×1036T−6.45e−42180Ts−1(±31%)ksC2H4OH↔CH3CHO+H,1atm=5.817×109e−11372Ts−1(±55%)kC2H5OH+H↔sC2H4OH+H2=1.780×107T1.94e−1835Ts−1(±28%)

Pressure dependence of rate coefficients of unimolecular and chemical activation reactions connected to the potential energy wells of chlorinated monosilanes by RRKM calculations

AbstractRate coefficients for elementary reactions connected to the potential energy wells of SiHCl3, SiH2Cl2, SiHCl2, and SiH3Cl, which are important Si1 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through Rice–Ramsperger–Kassel–Marcus (RRKM) theory for various conditions of temperature and pressure. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall‐off regime under subatmospheric to moderately high‐pressure conditions, so that it is expected that accurate modeling of the gas phase in chlorosilane CVD reactors requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high‐pressure limit rate coefficients. The rate coefficients determined here were tabulated using Chebyshev coefficients and can be used in simulations of systems under a wide range of temperature and pressure conditions.

Experimental and kinetic modeling studies of furfural pyrolysis at low and atmospheric pressures

Furfural (2-furaldehyde) is a promising platform chemical for lignocellulosic biofuels, but its thermal decomposition channels are not well understood. To better understand furfural thermal decomposition kinetics, flow tube reactor pyrolysis combined with synchrotron vacuum ultraviolet photoionization mass spectrometry at low and atmospheric pressures and temperatures from 929 to 1365 K was investigated. Carbon monoxide, methane, acetylene, propyne, allene, furan, and vinylketene as the main products were detected and measured. A detailed kinetic model for furfural pyrolysis consisting of 585 species and 3018 reactions was built based on the potential energy surface, and validated against the experimental data from the current and previous studies. Rate of production analysis shows that furfural mainly underwent unimolecular non-radical decomposition channel to form vinylketene and carbon monoxide. Sequential decomposition of vinylketene led to the production of allene, propyne and carbon monoxide, etc. H-atom addition reactions on the furan ring was the main source of furan. HCO, CH3 and COOCH3 groups influence the decomposition pathways of furanic fuels, and accelerate their decomposition with decomposition temperatures in the sequence of methyl 2-furoate < furfural < 2,5-dimethylfuran < 2-methylfuran < furan.

Thermal unimolecular reactivity pathways in dehydro‐diazines radicals

AbstractIn this work, we investigated the detailed reactivity of dehydro‐diazine radicals through unimolecular dissociation pathways. Aromatic six‐membered rings with two nitrogens as heteroatoms are classified as diazines, which include pyrimidine 1, pyridazine 2, and pyrazine 3. The C–H bond scission in pyrimidine, pyridazine, and pyrazine leads to three (1a–1c), two (2a and 2b), and one (3a) radical isomers, respectively. Through quantum chemical calculations, reactivity aspects of all the six radical isomers have been investigated by considering different modes of unimolecular reactions. On the basis of the computations at (U)B3LYP/cc‐pVTZ, (U)M06‐2X/cc‐pVTZ, and (U)CCSD(T)/cc‐pVTZ//(U)B3LYP/cc‐pVTZ levels of theory and analysis, we found out that ring‐opening pathways were kinetically more favorable compared with isomerization (1,2‐H shift) pathways or C–H bond dissociation leading to biradicals. In all the cases, ring‐opening via C–N bond cleavage has a lower energy barrier compared with C–C bond cleavage owing to the stabilizing interaction between radical electron and the nitrogen lone pair. Moreover, the unimolecular decomposition channels can lead to several fragmentation products, which are interesting in the astrochemical context. On the basis of these investigations and also the impact of three centered–five electrons (3c–5e) interactions on the reactivity aspects, we determined the kinetic stability order among the six isomeric dehydrodiazine radicals.

Insights on unimolecular and bimolecular reactivity patterns of pyridyl, pyridyl-N-oxide, and pyridinyl radicals through spin density

Spin density is very vital in understanding the thermodynamic stability of radicals, biradicals, etc. To obtain insights on reactivity aspects of radicals using it, we carried out theoretical investigations on the unimolecular and bimolecular reactions of pyridyl1a-c, pyridyl-N-oxide2a-c, and pyridinyl3a-cradical isomers. For all the radicals, we computed the CH bond dissociation energies (BDEs), isomerization reactions through the 1,2-H shift, unimolecular decomposition channels, and the bimolecular reactions with small molecules H2, H2O, CO, CO2, CH4, and CH3OH. Remarkably, spin density at the radical center or the transition state manifested the observed reactivity patterns.

A chemical kinetic model for the decomposition of perfluorinated sulfonic acids

Perfluorinated sulfonic acids (such as perfluorooctanesulfonic, PFOS, and short-chain analogues) are notorious halogenated pollutants that exhibit severe toxicity, even at minute levels. Limited number of experimental studies addressed their thermal decomposition at elevated temperatures. Such scenarios are particularly relevant to open fires and incineration of materials laden with perfluoroalkyl compounds (PFCs). Herein, we construct a detail kinetic model that illustrates major chemical reactions underpinning initial degradation of 1-butanesulfonic acid (CF3(CF2)3SO2OH), as a model compound of PFOS, and perfluorinated sulfonic acids in general. Reaction rate parameters were estimated based on an accurate density functional theory (DFT) formalism. The kinetic model incorporates four sets of reactions, namely, unimolecular decomposition channels, hydrofluorination, hydrolysis, and fragmentation of the alkyl chain. Results are discussed considering recent experimental measurements. Temperature-dependent profiles for a large array of perfluoroalkyl acyl fluorides, short perfluorinated cuts, and perfluorinated cyclic compounds, are presented. SO2 emerges as the main sulfur carrier, with a minor contribution from SO3. HF addition to double carbon bonds in alkenes, and to carbonyl bonds in aldehydic structures signifies a major sink pathway for hydrogen fluoride. Addition of moisture was shown to expedite the destruction of relatively large perfluoroalkyl acyl fluorides into C1 species. Construction of this model could aid in a better understanding of the fate and chemical transformation of PFCs under a pyrolytic environment pertinent to waste incineration and fluorine mineralization.

BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields.

By using a dc-slice imaging technique, photodissociation of 1,2-C2H4BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl+ in an 80 fs laser field. The occurrence of fragment ion BrCl+ in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl+ in this experiment. The relative quantum yield of the BrCl+ channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl+ and its partner ion C2H4+, we concluded that BrCl+ came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C2H4BrCl2+. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl+ detachment from 1,2-C2H4BrCl2+ has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C2H4BrCl2+ and the products BrCl+ + C2H4+. In this entire dissociation process, the C–Br and C–Cl bond lengths were observed to elongate asymmetrically, that is, the C–Br chemical bond broke firstly, and subsequently a new Br–Cl chemical bond started to emerge while the C–Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl+ detachment.

The thermal decomposition of furfural: molecular chemistry unraveled

The thermal decomposition of furfural is investigated experimentally and through theoretical calculations at the CBS-QB3 level of theory. Furfural is a major product observed during biomass pyrolysis, but despite its importance there are many speculations about the thermal decomposition channels of this compound. To address these open questions new experiments are performed in a jet-stirred reactor at atmospheric pressure and temperatures ranging from 900 to 1100 K with a furfural inlet mole fraction of 0.005 and He as diluent. The residence time is set to 2 s. The main products observed by GC analysis are CO, CO2, α-pyrone, furan, 3-furaldehyde, propyne, propadiene, acetylene, methane and benzene. Small amounts of other aromatics, e.g. toluene, styrene, benzaldehyde and phenol are observed as well.Theoretical calculations at the CBS-QB3 level are used to extend the furfural potential energy surface and to identify possible reaction pathways to the observed products. The unimolecular non-radical decomposition channel through α-pyrone as proposed in literature is confirmed as the main channel, but carbene pathways are found to make small contributions as well. Furthermore, pericyclic reactions are suggested to contribute to the molecular elimination of CO in open-chain molecules and Diels Alder reactions are found to be important for the formation of CO2 and aromatics. Finally, even radical chemistry initiated by homolytic scission of the approximately 380 kJ/mol strong CH bond in the furfural carbonyl group has a non-negligible influence.

Products from the Oxidation of n-Butane from 298 to 735 K Using Either Cl Atom or Thermal Initiation: Formation of Acetone and Acetic Acid-Possible Roaming Reactions?

The oxidation of 2-butyl radicals (and to a lesser extent 1-butyl radicals) has been studied over the temperature range of 298-735 K. The reaction of Cl atoms (formed by 360 nm irradiation of Cl2) with n-butane generated the 2-butyl radicals in mixtures of n-C4H10, O2, and Cl2 at temperatures below 600 K. Above 600 K, 2-butyl radicals were produced by thermal combustion reactions in the absence of chlorine. The yields of the products were measured by gas chromatography using a flame ionization detector. Major products quantified include acetone, acetic acid, acetaldehyde, butanone, 2-butanol, butanal, 1- and 2- chlorobutane, 1-butene, trans-2-butene, and cis-2-butene. At 298 K, the major oxygenated products are those expected from bimolecular reactions of 2-butylperoxy radicals (butanone, 2-butanol, and acetaldehyde). As the temperature rises to 390 K, the butanone decreases while acetaldehyde increases because of the increased rate of 2-butoxy radical decomposition. Acetone and acetic acid first appear in significant yield near 400 K, and these species rise slowly at first and then sharply, peaking near 525 K at yields of ∼25 and ∼20 mol %, respectively. In the same temperature range (400-525 K), butanone, acetaldehyde, and 2-butanol decrease rapidly. This suggests that acetone and acetic acid may be formed by previously unknown reaction channels of the 2-butylperoxy radical, which are in competition with those that lead to butanone, acetaldehyde, and 2-butanol. Above 570 K, the yields of acetone and acetic acid fall rapidly as the yields of the butenes rise. Experiments varying the Cl atom density, which in turn controls the entire radical pool density, were performed in the temperature range of 410-440 K. Decreasing the Cl atom density increased the yields of acetone and acetic acid while the yields of butanone, acetaldehyde, and 2-butanol decreased. This is consistent with the formation of acetone and acetic acid by unimolecular decomposition channels of the 2-butylperoxy radical, which are in competition with the bimolecular channels that form butanone, acetaldehyde, and 2-butanol. Such unimolecular decomposition channels would be unlikely to proceed through conventional transition states because those states would be very constrained. Therefore, the possibility that these decomposition channels proceed via roaming should be considered. In addition, we investigated and were unable to fit our data trends by a simplified ketohydroperoxide mechanism.

Shock Tube Study of Dimethylamine Oxidation

ABSTRACTDimethylamine (DMA) ignition delay times and OH time histories during the oxidation process were investigated behind reflected shock waves. The ignition delay time measurements cover the temperature range of 1181–1498 K, with pressures near 0.9, 1.5, and 2.8 atm, and equivalence ratios of 0.5, 1, and 2, in 4% oxygen/argon. The ignition delay time data feature low scatter and can be correlated to a single expression with 2 ∼ 0.99: τign = 7.30 × 10−4 −0.68 Φ0.45 exp(18,265/), where τign is in μs, in atm, and in K. OH time histories were measured using laser absorption of the R1(5) line of the A‐X(0,0) transition near 306.7 nm, in stoichiometric mixtures of 500 ppm DMA/O2/argon. The mechanism developed by Li et al. was used initially to simulate the measured DMA ignition delay times and the OH time histories. The Li et al. mechanism was then updated by adding the DMA unimolecular decomposition channel: DMA = CH3NH + CH3, with the reaction rate constant estimated by analogy to dimethyl ether decomposition, previously investigated by Cook et al. The reactions of DMA + OH were also updated based on recent work in our laboratory. The simulation results using the modified Li et al. mechanism are in good agreement with both the ignition delay times and OH time‐history data.

Theoretical Studies on the Unimolecular Decomposition of Propanediols and Glycerol

Polyols, a typical type of alcohol containing multiple hydroxyl groups, are being regarded as a new generation of a green energy platform. In this paper, the decomposition mechanisms for three polyol molecules, i.e., 1,2-propanediol, 1,3-propanediol, and glycerol, have been investigated by quantum chemistry calculations. The potential energy surfaces of propanediols and glycerol have been built by the QCISD(T) and CBS-QB3 methods, respectively. For the three molecules studied, the H(2)O-elimination and C-C bond dissociation reactions show great importance among all of the unimolecular decomposition channels. Rate constant calculations further demonstrate that the H(2)O-elimination reactions are predominant at low temperature and pressure, whereas the direct C-C bond dissociation reactions prevail at high temperature and pressure. The temperature and pressure dependence of calculated rate constants was demonstrated by the fitted Arrhenius equations. This work aims to better understand the thermal decomposition process of polyols and provide useful thermochemical and kinetic data for kinetic modeling of polyols-derived fuel combustion.

Theoretical Investigation of 1,2-Interchange of a Chlorine Atom and Methyl Group in 1,1-Dichloroacetone

A recent photofragment translational spectroscopy study of 1,1-dichloroacetone at 193 nm reported two primary unimolecular decomposition channels: C-Cl bond cleavage and elimination of HCl in a 9:1 ratio, respectively. The HCl translational energy distribution was bimodal suggesting two distinct decomposition pathways that were assumed to be 1,1-HCl loss forming a carbene and a 1,3-HCl elimination reaction forming a biradical ( Butler , L. J. ; Liu , Y. ; Lau , K. ; McCunn , L. R. ; Fitzpatrick , B. L. ; Bell , J. M. ; Krisch , M. J. J. Phys. Chem. A 2007 , 111 , 5968. ). An alternative two-step mechanism for HCl loss has been proposed involving interchange of a chlorine atom and a CH(3) group converting 1,1-dichloroacetone into 2-chloropropanoyl chloride followed by either a 1,2-HCl or 2,3-HCl elimination reaction. This alternative mechanism was computationally explored with density functional theory using B3PW91/6-31G(d',p') and unimolecular rate constants were calculated. The theoretical rate constant ratio for loss of HCl and the mean HCl translation energy for each elimination channel were in excellent agreement with the experimental results.

Neutralization–reionization study of the allyloxide anion

The charge-reversal (CR) and neutralization–reionization (NR) mass spectra of the mass-selected allyloxide anion CH 2 CHCH 2O − are reported. Similar to other alkoxide ions, the allyloxide anion shows only small abundances of the survivor cation in the CR and NR spectra. In combination with theoretical studies, the differences between the CR and NR experiments allow to explain why loss of atomic hydrogen corresponds to the major unimolecular decomposition channel of the neutral allyloxy radical. In addition, indirect evidence for an intramolecular rearrangement of the allyloxy radical to a carbon-centered oxiranyl methyl radical is provided. Interestingly, the patterns of the C 3H n O + ions in the NR spectra show similarities to those of a recently reported photoionization study of oxygenated products formed upon interaction of V 4O 11 − ions with propene. By extrapolation of our findings, we suggest that the reactions of the vanadium-oxide cluster anion V 4O 11 − are initiated by allylic C–H bond activation of propene followed by oxygenation of the hydrocarbon fragment.

A Kinetic and Spectroscopic Study of the CH3I−Cl and ICH2I−Cl Adducts

Laser-induced fluorescence from the CH3I-Cl and ICH2I-Cl adducts formed in association reactions between chlorine atoms and CH3I and CH2I2 has been observed for the first time. The LIF excitation and dispersed fluorescence spectra have been measured in the range 345-375 nm and 380-480 nm, respectively, at 204 and 296 K. The excitation spectra exhibit vibrational fine structure, and a semiquantitative analysis of the spectra yields a similar binding energy for both adducts of approximately 60 kJ mol(-1). The adduct fluorescence is efficiently quenched by N2 and exhibits a zero-pressure lifetime of approximately 25-30 ns. Using LIF excited from the CH3I-Cl and ICH2I-Cl adducts, the kinetics of the reactions of atomic chlorine with methyl iodide and diiodomethane have been investigated, the results showing that both reactions proceed via two independent channels, an association reaction to form the adduct and a bimolecular abstraction reaction. At T approximately 200 K, the association reaction is predominant, and CH3I-Cl formation is irreversible, with rate coefficients for adduct formation found to be pressure-dependent and in reasonable agreement with the literature. At approximately 200 K, removal of the adduct is dominated by reaction with radical species (CH3 and ClSO) and by self-reaction, which proceed at close to the gas kinetic limit. At 296 K, CH3I-Cl formation is reversible, and the equilibrium constant, K(p) = (70.9 +/- 27.4) x 10(3) atm(-1), was determined, which is in excellent agreement with the literature, and the adduct does not significantly react with CH3I. The uncertainty is at the 95% confidence level (2sigma) and includes systematic errors. At approximately 200 K, the ICH2I-Cl adduct is again stabilized, with pressure-dependent rate coefficients reaching the high pressure limit at lower pressures than for the Cl + CH3I reaction. At room temperature, the ICH2I-Cl adduct is removed via an additional unimolecular decomposition channel, which dominates over the reversible decomposition channel to reform Cl + CH2I2. Neither adduct was observed to undergo significant reaction with molecular oxygen at approximately 200 or 296 K, with an upper limit rate coefficient determined as k < 10(-16) cm(3) molecule(-1) s(-1).

Theoretical study of the ground‐state gas‐phase unimolecular decomposition channels of propynoic acid

AbstractTheoretical studies have been carried out to elucidate the ground‐state unimolecular decomposition channels for propynoic acid (PA) in the gas phase. In this paper we present details of mechanisms for the decarboxylation and decarbonylation channels of PA. The transformations of PA result in the formation of carbon dioxide and acetylene from the decarboxylation channel while carbon monoxide and hydroxyacetylene, ultimately the ketene, are formed from the decarbonylation pathway. Equilibrium structures and the principal transition states (TSs) for each pathway have been identified and characterized. The Gaussian 98 suite of programs and MOLDEN were used for computation and visualization. All computations of equilibrium and TS structures relevant to the two competing unimolecular decomposition channels (decarboxylation and decarbonylation) were performed with second‐order Møller–Plesset (MP2) and Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP) levels using the 6‐31G(d,p), 6‐311++G (d, p) and Dunning's correlation‐consistent polarized valence basis set (aug‐cc‐pVDZ). The geometries were fully optimized and characterized as minima (0 imaginary frequencies) or first‐order saddle points (1 imaginary frequency) by harmonic vibrational analysis at the MP2/6‐311++G(d,p) and B3LYP/6‐311++G(d,p) levels. All the calculations indicate that the lowest energy decomposition pathway for PA is decarboxylation. Decarboxylation of PA occurs most easily through a three‐step mechanism: conversion of s‐cis‐HCCCO2H to s‐trans‐HCCCO2H, followed by a 1, 3‐hydrogen migration of hydroxyl hydrogen, resulting in a four‐center TS that decays easily to acetylene and carbon dioxide, with an activation barrier of 61.3–79.9 kcal/mol (average 65.67 kcal/mol). Decarbonylation, leading to hydroxyacetylene (and subsequently the ketene) and carbon monoxide, occurs easily via a direct (one‐step) three‐center TS. The direct (one‐step) elimination of carbon monoxide from propynoic acid has a calculated average activation barrier of 67.9–81.7 kcal/mol (average, 74.1 kcal/mol). The decarboxylation pathway is shown to occur with a maximum exothermicity of 28 kcal/mol. On the contrary, the decarbonylation of PA is predicted to occur with an overall endothermicity of ≈29.2 kcal/mol for formation of hydroxyacetylene and carbon monoxide and an average exothermicity of −7.69 kcal/mol for formation of the ketene and carbon monoxide. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Photodissociation dynamics of pyrrole: evidence for mode specific dynamics from conical intersections.

The H and D atom elimination mechanisms in the photodissociation of jet cooled pyrrole and pyrrole-d1 have been studied by photofragment velocity map imaging. The molecules were excited to the 1 1A2 (pi sigma*) state at lambda = 243 nm and to the 1 1B2 (pi pi*) state at lambda = 217 nm. H/D atoms were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI) at lambda = 243 nm. The analysis of the images and the resulting translational energy distributions from the 1 1A2 state demonstrates the existence of two decay pathways, fast mode-specific cleavage of the NH bond in the excited state (channel A) and internal conversion (IC) to the electronic ground state (S0) followed by unimolecular decomposition of the vibrationally hot S0 molecules (channel B). The angular distributions of the H/D atoms from the direct dissociation in the excited state are strongly anisotropic, whereas the decay of the S0 molecules leads to spatially isotropic distributions. The results at lambda = 217 nm indicate that the 1 1B2 state undergoes an ultrafast radiationless transition to 1 1A2 followed by the abovementioned direct mode-specific NH bond fission on the 1 1A2 potential energy surface (channel A') or conversion to S0 and subsequent unimolecular decomposition (channel B'). The latter pathway may also be initiated by a direct relaxation from 1 1B2 to S0. The anisotropy parameter of beta approximately -1 for the direct NH bond fission at lambda = 217 nm is in accordance with the expectations for a perpendicular electronic excitation and a dissociation lifetime that is short compared to the rotational period of the molecules. The fast decay dynamics of both excited electronic states can be rationalized with reference to the theoretically predicted conical intersections between the pi pi*, pi sigma*, and S0 potential energy surfaces and the antibonding nature of the pi sigma* potential energy surface with respect to the NH bond [A. L. Sobolewski, W. Domcke. C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys. 2002, 4, 1093].