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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

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.

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.

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.

Four molecules were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy: 1-propynylbenzene, indene, ethynylbenzene, and benzocyclobutene. Their threshold photoelectron spectrum was obtained and electronic transitions were assigned by OVGF (outer valence Green's function) calculations. Vibrational progressions observed in the electronic ground and excited states were simulated by calculating Franck-Condon factors based on the neutral as well as the cation ground and excited state geometries. iPEPICO was used to obtain ion dissociation data in threshold photoionization as a function of photon energy, which were modeled with RRKM theory to extract kinetic parameters for the reactions C9H8(+•) (1-propynylbezene) → C9H7(+) + H (R1); C9H8(+•) (indene) → C9H7(+) + H (R2); C8H8(+•) (benzocyclobutene) → C8H7(+) + H (R3); C8H8(+•) (benzocyclobutene) → C6H6(+) + C2H2 (R4); C8H6(+•) (1-ethynylbenzene, aka phenylacetylene) → C6H4(+) + C2H2 (R5). These results were compared to G3 level calculations. In addition, the enthalpy of formation of the indenyl radical was estimated to be ΔfH°0K = 249 ± 50 kJ mol(-1) based on a previously measured IE and a cation ΔfH°0K = 976 kJ mol(-1), determined herein.

International audience; By combining chemical dynamics simulations and RRKM statistical theory we have characterized collision induced dissociation (CID) mechanisms of [M(formamide)]2+ ions (M = Ca, Sr) at different timescales, from few femtoseconds to microseconds. Chemical dynamics simulations account for the short-time and dynamically driven reactivity, such as impulsive collision mechanism for formamide neutral loss. From the simulations, we also got the amounts of energy transferred during the collision and, especially important, the vibrational and rotational energy distributions of the ions that did not react during the simulation time length of 2.5 ps. These internal energy distributions were in turn used in combination with RRKM theory to estimate the rate constants of the possible reactive pathways. Hence, we performed a statistical analysis of the CID dynamics accounting for the long-time and statistical reactivity (i.e., through an IVR mechanism). This multiscale approach allowed us to account for all the products observed in the CID experimental spectra of [Ca(formamide)]2+ and [Sr(formamide)]2+ doubly charged cations, as well as the differences between them.

The kinetics and mechanisms for SiH2 + Si2H6 and SiH3 + Si2H5 reactions and the related unimolecular decomposition of Si3H8 have been investigated by ab initio molecular orbital theory based on the QCISD(T)/CBS//QCISD/6-311++G(d,p) method in conjunction with quantum statistical variational Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. For the barrierless radical association processes, their variational transition states have been characterized by the CASPT2//CASSCF method. The species involved in the study are known to coexist under CVD conditions. The results show that the association reaction of SiH2 and Si2H6 producing Si3H8 occurs by insertion via its lowest-energy path forming a loose hydrogen-bonding molecular complex with 8.3 kcal/mol binding energy; the reaction is exothermic by 55.0 kcal/mol. The chemically activated Si3H8 adduct can fragment by several paths, producing SiH4 + SiH3SiH (-0.7 kcal/mol), Si(SiH3)2 + H2 (-1.4 kcal/mol), and SiH3SiH2SiH + H2 (-1.4 kcal/mol). The predicted enthalpy changes as given agree well with available thermochemical data. Three other decomposition channels of Si3H8 occurring by Si-H or Si-Si breaking were found to be highly endothermic, and the reactions take place without a well-defined barrier. The heats of formation of Si3H8, SiH2SiH, Si2H4, i-Si3H7, n-Si3H7, Si(SiH3)2, and SiH3SiH2SiH have been predicted and found to be in close agreement with those available data in the literature. The product branching rate constants for SiH2 + Si2H6 and SiH3 + Si2H5 reactions and the thermal unimolecular decomposition of Si3H8 for all low-energy paths have been calculated with multichannel variational RRKM theory covering varying P,T conditions typically employed in PECVD and Cat-CVD processes for hydrogenated amorphous silicon (a-Si/H) film growth. The results were also found to be in good agreement with available kinetic data. Our kinetic results may be employed to model and control very large-area a-Si/H film growth for a new generation of solar cell applications.

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

The thermal decomposition of the CH(3)N(•)NH(2), cis-CH(3)NHN(•)H, trans-CH(3)NHN(•)H, and C(•)H(2)NNH(2) radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the β-scission of NH(2) to form methyleneimine is the predominant channel for the decomposition of the C(•)H(2)NNH(2) radical due to its small energy barrier of 13.8 kcal mol(-1). The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10(19)T(-1.672) exp(-9665.13/T) s(-1). In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH(3)NHN(•)H and trans-CH(3)NHN(•)H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol(-1) for their isomerization. The β-scission of CH(3) from the cis-CH(3)NHN(•)H radical to form trans-diazene has an energy barrier of 35.2 kcal mol(-1), and the β-scission of CH(3) from the trans-CH(3)NHN(•)H radical to form cis-diazene has an energy barrier of 39.8 kcal mol(-1). The CH(3)N(•)NH(2) radical undergoes the β-scission of methyl hydrogen and amine hydrogen to form CH(2)═NNH(2), trans-CH(3)N═NH, and cis-CH(3)N═NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol(-1), respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01-100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.

While the decomposition kinetics of the benzyl radical has been studied in depth both from the experimental and the theoretical standpoint, much less is known about the reactivity of what is likely to be its main decomposition product, fulvenallene. In this work the high temperature reactivity of fulvenallene was investigated on a Potential Energy Surface (PES) consisting of 10 wells interconnected through 11 transition states using a 1 D Master Equation (ME). Rate constants were calculated using RRKM theory and the ME was integrated using a stochastic kinetic Monte Carlo code. It was found that two main decomposition channels are possible, the first is active on the singlet PES and leads to the formation of the fulvenallenyl radical and atomic hydrogen. The second requires intersystem crossing to the triplet PES and leads to acetylene and cyclopentadienylidene. ME simulations were performed calculating the microcanonical intersystem crossing frequency using Landau-Zener theory convoluting the crossing probability with RRKM rates evaluated at the conical intersection. It was found that the reaction channel leading to the cyclopentadienylidene diradical is only slightly faster than that leading to the fulvenallenyl radical, so that it can be concluded that both reactions are likely to be active in the investigated temperature (1500-2000 K) and pressure (0.05-50 bar) ranges. However, the simulations show that intersystem crossing is rate limiting for the first reaction channel, as the removal of this barrier leads to an increase of the rate constant by a factor of 2-3. Channel specific rate constants are reported as a function of temperature and pressure.

The dissociation dynamics of gas phase formic acid ions (HCOOH(+), DCOOD(+), HCOOD(+), DCOOH(+)) are investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy and high level ab initio calculations. The slow rate constants for this seemingly simple H loss reaction and the large onset energy shifts due to isotopic substitution point to a substantial exit barrier through which the H or D atoms tunnel. Modeling of the HCOOH(+) experimental data using RRKM theory with tunneling through an Eckart potential are best fitted with a barrier of about 17 kJ mol(-1). High level ab initio calculations support the experimental findings with a computed barrier of 15.9 kJ mol(-1), which is associated with the substantial geometry change between the product HOCO(+) cation and the corresponding HCOOH(+) molecular ion. Because of this exit channel barrier, the formic acid ion dissociation does not provide a route for determination of the HOCO(+) heat of formation. Rather, the most accurate value comes from the calculations employing the high accuracy extrapolated ab initio thermochemistry (HEAT) scheme, which yields a Δ(f)H(o)(0K)[HOCO(+)] = 600.3 ± 1.0 kJ mol(-1) (Δ(f)H(o)(298K)[HOCO(+)] = 597.3 ± 1.0 kJ mol(-1)). The calculated proton affinity of CO(2) is thus 534.7 ± 1.0 kJ mol(-1) at 0 K and 539.3 ± 1.0 kJ mol(-1) at 298.15 K.

Investigations on the dissociation kinetics of hydrated protonium ions, (H(2)O)(2)H(+) and their deuterated species (D(2)O)(2)D(+), are reported based on the harmonic and anharmonic oscillator model using the transition state theory and ab initio calculations. We find that the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+) exhibits a distinct threshold behavior due to the existence of activation energies. Moreover, the deviation between the harmonic and anharmonic dissociation rate constants becomes larger in the high energy or temperature range, with the rate constants becoming unreasonably large under the harmonic oscillator model. The isotope effect is found to become more distinct but only in the case of the anharmonic oscillator model. These results show that the anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) theory can provide a reasonably good description for the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+). Furthermore, a theoretical model to demonstrate the principle of vibrational predissociation spectroscopy (VPS) is established from the viewpoint of RRKM theory and applied in determining the experimental conditions and understanding the role of the dissociation rate constant k(E) played in the VPS experiment, using (H(2)O)(2)H(+) and (D(2)O)(2)D(+) as examples.

The unimolecular dissociation of 1,1-dimethylhydrazine ions was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Time-of-flight distributions and breakdown curves were recorded in the photon energy range of 9.5-10.4 eV. The 0 K appearance energies of the fragment ions were extracted by modeling the experimental data with rigid activated complex (RAC-) RRKM theory. It was found that the data could be well-reproduced with a single TS for each dissociation channel if two different H-loss channels were assumed, one corresponding to a C-H and the other to a N-H bond dissociation. Once the appearance energies were established, heats of formation of the fragment ions could be derived. The heat of formation of the neutral molecule was computed by applying composite ab initio methods (G3, CBS-APNO, W1U) on a series of isodesmic reactions between methyl hydrazines and methyl amines.

The reaction pathways for the ClO + HOCO reaction have been explored using the coupled-cluster method to locate and optimize the critical points on the ground-state potential-energy surface. Results show that the ClO + HOCO reaction can produce Cl + HOC(O)O, HOCl + CO(2), HCl + CO(3), and HClO + CO(2) via an addition or a direct hydrogen abstraction reaction mechanism. The reaction kinetics has been studied using the variational RRKM theory. It is found that the ClO + HOCO reaction is fast and has a negative temperature dependence at low temperatures. At room temperature, the thermal rate coefficient is obtained as 4.26 x 10(-12) cm(3) molecules(-1) s(-1) with product branching fractions of Cl (0.518), HOCl (0.469), HCl (0.01), and HClO (0.003) at zero pressure. The Cl + HOC(O)O products are major, compared to the HOCl + CO(2) products, because of the loose transition state along the dissociation pathway to eliminate Cl. In addition, the RRKM/master equation simulations indicate that the stabilization of the HOC(O)OCl intermediates is noticeable at moderate pressures as its thermal rate constants reach about 6.0 x 10(-13) cm(3) molecules(-1) s(-1). In contrast, the other product branching ratios for the ClO + HOCO reaction are weakly dependent on pressure.

The bimolecular reaction of ground state cyano radical with propylene under the condition of single collision is investigated by combining ab initio electronic structure calculations for predicting reaction paths and RRKM theory to yield rate constant for each path. The isomerization and dissociation channels for each of the seven collision complexes are characterized by utilizing the unrestricted B3LYP/cc-pVTZ level of theory and the CCSD(T)/cc-pVTZ calculations. Sifting with the facilitation of RRKM rate constants through web of ab initio paths composed of 8 collision complexes, 37 intermediates, and 12 H-, 23 H(2)-, 3 HCN-, and 4 CH(3)-dissociated products, we identify the most probable paths down to 7-9 species at collisions energies of 0 and 5 kcal/mol as the reaction mechanisms. The rate equations of the reaction mechanisms are solved numerically such that the concentration evolutions for all species involved are obtained. This study predicts that CN + C(2)H(3)CH(3) reaction via any of the seven collision complex, c1-c5, c7, and c8, would produce p1(CH(3)CHCHCN) + H, p2(CH(2)CHCH(2)CN) + H, and mostly p43(vinyl cyanide) + CH(3) for collision energy within 0-5 kcal/mol. In addition to the insertion mechanism through collision complex, the direct H-abstraction of propylene by CN radical might occur. Our investigation indicates that the barrierless and exoergic CN(X(2)Sigma(+)) + C(2)H(3)CH(3)(X(1)A') reaction would be an efficient route for the p1, p2, and p43, and likely HCN formation in cold molecular clouds and in the atmosphere of Saturn's satellite Titan.

A survey is presented of theoretical models and computational studies for unimolecular reaction dynamics. Intrinsic RRKM and non-RRKM dynamics are described, and properties of the unimolecular reactant's classical phase space giving rise to these dynamics are discussed. Quantum dynamical calculations of isolated resonances and state-specific decomposition are reviewed, and the resulting possible mode-specific or statistical state-specific decomposition is delineated. The relationship between the latter and RRKM theory is described. Computational studies give the probability that a molecule dissociates in a time interval of t --t + dt, that is, the lifetime distribution P(t), and determining unimolecular rate constants versus pressure, energy, and temperature from P(t) is outlined. Non-RRKM behavior evident in P(t) is not always present in the rate constants. The need to include anharmonicity and the proper treatment of the K quantum number, in calculating the RRKM unimolecular rate constant, is explained. The possibility of observing "steps" in unimolecular rate constants is considered. The extensive experimental non-RRKM dynamics found for several classes of chemical reactions are surveyed. The direct coupling of chemical dynamics with electronic structure theory, that is, direct dynamics, has allowed one to study the atomic-level dynamics for numerous unimolecular reactions, and extensive non-RRKM and nonintrinsic reaction coordinate (IRC) dynamics have been discovered. These dynamics for OH(-) + CH(3)F and F(-) + CH(3)OOH are reviewed.

The dissociation of pentane-2,4-dione radical cation has been studied by ab initio direct classical trajectory calculations at the MP2/6-31G(d) level of theory. A bond additivity correction has been used to improve the MP2 potential energy surface (BAC-MP2). A microcanonical ensemble was constructed using quasiclassical normal-mode sampling by distributing 10 kcal/mol of excess energy above ZPE for the transition state for the tautomerization of the enol with a terminal double bond, 4-hydroxypent-4-en-2-one radical cation, to the diketo form. A total of 244 trajectories were run starting from this transition state, yielding pentane-2,4-dione radical cation and depositing energy in the terminal CC bond. As a result, the branching ratio for dissociation of the terminal CC bond versus the interior CC bonds is significantly larger than expected from RRKM theory. The branching ratio for the dissociation of the two interior CC bonds is ∼20:1, with the one closest to the activated methyl breaking more often. Since the two interior bonds are equivalent and should dissociate with equal probability, this branching ratio represents a very large deviation from statistical behavior. A simple kinetic scheme has been constructed to model the dissociation rates. The nonstatistical behavior is seen because the rate of energy flow within the molecule is comparable to or less than the rates of dissociation for the activated system. In addition to the expected dissociation products, some of the trajectories also lead to the formation of an ester-like product, prop-1-en-2-yl acetate radical cation.

We report data from real-time FTIR temperature programmed reaction spectroscopy on a cryogenic zinc selenide window revealing the intermediates from ozonation of 2,3-dimethyl-2-butene (TME). We have found convincing evidence of a 1,2,3-trioxolane (the primary ozonide, POZ), which decomposes at 185 K to yield a 1,2,4-trioxolane product (the secondary ozonide, SOZ). Computational infrared spectra confirmed the presence of the POZ and SOZ. The barrier height for POZ decomposition, determined experimentally, was found to be 13.8 +/- 1.0 kcal mol(-1), and the A factor calculated with RRKM theory based on density functional reactant and transition state frequencies was found to be 4.16 x 10(13) s(-1). The TME SOZ has not previously been observed without the presence of a polyethylene surface. SOZ formation kinetics from the reaction of the POZ decomposition products along with the competing reaction pathways were examined with computational chemistry calculations using DFT. These calculations confirm our experimental observation of SOZ formation.

In this article we analyze quantitatively and discuss in detail a number of reactions that take place on a C3H5 potential. These reactions include the reaction of hydrogen atoms with allene and propyne, the reaction of methyl with acetylene, the isomerization of cyclopropyl to allyl, and the dissociation of allyl, 1-propenyl, and 2-propenyl. The theory employs high-level electronic-structure methods to characterize the potential energy surface, RRKM theory to calculate microcanonical, J-resolved rate coefficients, and master-equation methods to determine phenomenological rate coefficients, k(T,p). The agreement between our theory and the experimental results available is very good. The final theoretical results are cast in a form that is convenient for use in chemical kinetics modeling.

Vibrationally excited CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) molecules were prepared in the gas phase at 300 K with approximately 93 kcal mol(-1) of energy by recombination of CF(2)ClCHF and C(2)H(5) or C(2)D(5) radicals. Three unimolecular reactions were observed. 1,2-ClF interchange converts CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) into CF(3)CHClC(2)H(5)(CF(3)CHClC(2)D(5)), and subsequent 2,3-ClH (ClD) elimination gives CF(3)CH=CHCH(3) (CF(3)CH=CDCD(3)). 2,3-FH(FD) elimination gives cis- and trans-CF(2)ClCH=CHCH(3) (CF(2)ClCH=CDCD(3)), and 1,2-ClH elimination gives CF(2)=CFCH(2)CH(3) (CF(2)=CFCD(2)CD(3)). The experimental rate constants for CF(2)ClCHFC(2)H(5) (CF(2)ClCHFC(2)D(5)) were 1.3 x 10(4) (0.63 x 10(4)) s(-1) for 1,2-FCl interchange and 2.1 x 10(4) (0.61 x 10(4)) s(-1) with a trans/cis ratio of 3.7 for 2,3-FH(FD) elimination. The 1,2-ClH process was the least important with a branching fraction of only 0.08 +/- 0.04. The rate constants for 2,3-ClH (ClD) elimination from CF(3)CHClC(2)H(5) (CF(3)CHClC(2)D(5)) were 1.8 x 10(6) (0.49 x 10(6)) s(-1) with a trans/cis ratio of 2.4. Density functional theory was used to compute vibrational frequencies and structures needed to obtain rate constants from RRKM theory. Matching theoretical and experimental rate constants provides estimates of the threshold energies, E0, for the three reaction pathways; 1,2-FCl interchange has the lowest E0. The unimolecular reactions of CF(2)ClCHFC(2)H(5) are compared to those of CF(2)ClCHFCH(3). Both of these systems are compared to CH(3)CHFC(2)H(5) to illustrate the influence of a CF(2)Cl group on the E0 for FH elimination.

The kinetics and mechanisms for the unimolecular decomposition reactions of formic acid and oxalic acid have been studied computationally by the high-level G2M(CC1) method and microcanonical RRKM theory. There are two reaction pathways in the decomposition of formic acid: The dehydration process starting from the Z conformer is found to be the dominant, whereas the decarboxylation reaction starting from the E conformer is less competitive. The predicted rate constants for the dehydration and decarboxylation reactions are in good agreement with the experimental data. The calculated CO/CO2 ratio, 13.6-13.9 between 1300 and 2000 K, is in close agreement with the ratio of 10 measured experimentally by Hsu et al. (In The 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1983; p 89). For oxalic acid, its isomer with two intramolecular hydrogen bonds is the most stable structure, similar to earlier reports. Two primary decomposition channels of oxalic acid producing CO2+HOCOH with barriers of 33-36 kcal/mol and CO2+CO+H2O with a barrier of 39 kcal/mol were found. At high temperatures, the latter process becomes more competitive. The rate constant predicted for the formation of CO2 and HOCOH (the precursor of HCOOH) agrees well with available experimental data. The mechanism for the isomerization of HOCOH to HCOOH is also discussed.

The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta(f)H degrees 298K[P(C(2)H(5))3] = -152.7 +/- 2.8 kJ/mol, Delta(f)H degrees 298K[P(C(2)H(5))3+] = 571.6 +/- 4.0 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2] = -89.6 +/- 2.1 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2+] = 669.9 +/- 2.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)] = -36.5 +/- 1.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.

The potential energy surface for the CH(2)O + ClO reaction was calculated at the QCISD(T)/6-311G(2d,2p)//B3LYP/6-311G(d,p) level of theory. The rate constants for the lower barrier reaction channels producing HOCl + HCO, H atom, OCH(2)OCl, cis-HC(O)OCl and trans-HC(O)OCl have been calculated by TST and multichannel RRKM theory. Over the temperature range of 200-2000 K, the overall rate constants were k(200-2000K) = 1.19 x 10(-13)T(0.79) exp(-3000.00/T). At 250 K, the calculated overall rate constant was 5.80 x 10(-17) cm(3) molecule(-1) s(-1), which was in good agreement with the experimental upper limit data. The calculated results demonstrated that the formation of HOCl + HCO was the dominant reaction channel and was exothermic by 9.7 kcal/mol with a barrier of 5.0 kcal/mol. When it retrograded to the reactants CH(2)O + ClO, an energy barrier of 14.7 kcal/mol is required. Furthermore, when HOCl decomposed into H + ClO, the energy required was 93.3 kcal/mol. These results suggest that the decomposition in both the forward and backward directions for HOCl would be difficult in the ground electronic state.

The radical reaction mechanism of FCO + NO on the ground electronic state energy surface has been studied at the G2M level of theory based on the geometric parameters optimized at the B3LYP/6-311+G(d) level of theory. The two kinds of reaction pathways include the direct fluorine abstraction channel producing CO + FNO and the association channel forming the FC(O)NO complex. The former has a distinct barrier of 8.9 kcal mol(-1), while the latter is a barrierless association process. The rate constant of this reaction system in the temperature range 200-3000 K has been calculated by the microcanonical VTST/RRKM theory. The theoretical result shows that the predicted total rate constants exhibit a negative-temperature dependence and positive-pressure effect at lower temperatures. Under the experimental conditions, the predicted values are in good agreement with the experimental results. In addition, the predicted branching ratios clearly indicate that the dominant product channel is the formation of FC(O)NO at low temperatures and FNO + CO at high temperatures (>500 K).

The energy-resolved competitive collision-induced dissociation of the proton-bound complex [HS.H.CN](-) is studied in a guided ion beam tandem mass spectrometer. H(2)S and HCN have nearly identical gas-phase acidities, and therefore, the HS(-) + HCN and the CN(-) + H(2)S product channels exhibit nearly the same threshold energies, as expected. However, the HS(-) + HCN channel has a cross section up to a factor of 50 larger than CN(-) + H(2)S at higher energies. The cross sections are modeled using RRKM theory and phase space theory. The complex dissociates to HS(-)+ HCN via a loose transition state, and it dissociates to CN(-) + H(2)S via a tight transition state. Theoretical calculations show that the proton-transfer potential energy surface has a single minimum and that the hydrogen bonding in the complex is strongly unsymmetrical, with an ion-molecule complex of the form HS(-)..HCN rather than CN(-)..H(2)S or an intermediate structure. The requirement for proton transfer before dissociation and curvature along the reaction path impedes the CN(-) + H(2)S product channel.

The reaction mechanism, rate constants, and product branching ratios for the O(1D) + NH3 reaction have been studied using ab initio/RRKM methods. The reaction is shown to occur mainly through the i...

Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidity of phenol relative to hydrogen cyanide. The competitive dissociation of the [C6H5O·H·CN]- complex into C6H5OH + CN- and C6H5O- + HCN is studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions are measured as a function of collision energy. The enthalpy difference between the two reaction channels is found by modeling the reaction cross sections near threshold using RRKM theory to account for the energy-dependent product branching ratio and kinetic shift. From the enthalpy difference, the phenol gas-phase acidity, ΔacidH0(C6H5OH) = 1448 ± 8 kJ/mol, is determined relative to the established literature value of hydrogen cyanide, ΔacidH0(HCN) = 1462.3 ± 0.9 kJ/mol. We then derive ΔacidH298(C6H5OH) = 1454 ± 8 kJ/mol and the bond dissociation energy of D298(C6H5O−H) = 359 ± 8 kJ/mol.

Rate coefficients for the pyrolysis of SO2 in Ar in the temperature range 2188−4249 K were determined using a diaphragmless shock tube. The concentration of O atoms was monitored with resonance absorption. Rate coefficients determined in this work show Arrhenius behavior, with k1a(T) = (4.86 ± 1.31) × 10-9 exp [−(50450 ± 730)/T] cm3 molecule-1 s-1; listed errors represent one standard deviation in fitting. These values are consistent with some previous measurements that show a preexponential factor and activation energy greater than other measurements. Theoretical calculations at the G2M(RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with a microcannonical variational RRKM theory agree well with experimental observations; contributions from electronically excited states of SO2 are significant. Rate coefficients for the recombination O + SO → SO2 are predicted to dec...

The reflected shock tube technique with D atom atomic resonance absorption spectrometry (ARAS) detection has been used to study the bimolecular reaction, D + C{sub 2}H{sub 2} {yields} C{sub 2}HD + H. D atoms were produced from the thermal decomposition of C{sub 2}D{sub 5}I above {approx}1150 K. The initially formed C{sub 2}D{sub 5} radicals rapidly decompose to give D + C{sub 2}D{sub 4}. Rate constant values were obtained from both reactant and product hydrogen atom measurements, and these were found to be identical within experimental error. The title reaction proceeds through a vibrationally excited vinyl radical, and the equivalence of results based on reactant and product measurements suggests that radical stabilization is negligible over the temperature and pressure ranges of the experiments. For 1100 {

We apply multireference ab initio quantum chemistry and microcanonical transition state (RRKM) theory with quantum energy flow corrections from local random matrix theory (LRMT) to determine the kinetics of trans-stilbene photoisomerization. With a single ab initio potential energy surface and no adjustable parameters, simultaneous agreement with experiment of the microcanonical isomerization rates for the d0, d2, d10, and d12 isotopomers is obtained. We are also able to reproduce the pressure dependence of the thermal rate. Laser cooling effects on the isomerization rate are calculated and found to be quite small. The S1/S2 energy gap at the transition state is found to be quite large (0.86 eV), suggesting that nonadiabatic effects are negligible. Using the ab initio results in a simple RRKM theory without corrections for finite quantum energy flow does not lead to agreement with experiment. We conclude that non-RRKM effects are essential to understand photoisomerization of trans-stilbene and that these ...

A threshold collision-induced dissociation (TCID) study was performed on the n-butylbenzene cation, investigating the competitive dissociation leading to propyl and propene elimination. Ab initio calculations at the B3LYP/6-311++G(2d,2p)//B3LYP/6-31G* level were performed on the system. Reaction path calculations were performed for both dissociation channels to provide details of the dissociation mechanisms and to identify rate-determining transition states. Unimolecular dissociation rates as a function of energy from literature PEPICO and PD-MIKES experiments are modeled using RRKM statistical unimolecular decay theory to select the best calculated transition states for the two dissociation channels. The effects of kinetic and competitive shifts on the CID threshold determinations are investigated and explained using a model that incorporates RRKM theory. Final analysis of the TCID data yields 0 K dissociation energies of 1.70 ± 0.09 eV for the propyl elimination channel and 1.28 ± 0.06 eV for the propen...

The gas-phase removal rate constants for the reaction of ground-state palladium (a1S0) with oxygen in argon buffer are reported as a function of temperature and pressure. Palladium atoms were produced by the photodissociation of palladium(II) trifluoroacetate and detected by laser-induced fluorescence. The reaction is pressure dependent, indicating recombination to form PdO2. Falloff behavior is observed at pressures above about 20 Torr. Quantum calculations indicate that the reaction proceeds on a triplet surface to form bent PdO2(3A‘ ‘), which has a low frequency bending mode of only 129 cm-1 and a bond energy D0 = 95 kJ mol-1. RRKM theory, fitted to the experimental data, shows that D0 should lie below 155 kJ mol-1 and predicts the following expression for the rate coefficient from 200 to 600 K: krec,0 = 1.09 × 10-30 (T/300 K)-2.76 cm6 molecule-2 s-1, krec,∞= 2.52 × 10-10 exp(−69.3/T) cm3 molecule-1 s-1, and Fc = 0.61.

Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been carried out to investigate the potential energy surface for the C(3Pj) + C3H5(X 2A1) reaction. The results show that C(3Pj) can add without a barrier either to a terminal CH2 group of the allyl radical to form a metastable intermediate INT1 or between two CH2 groups to produce a bicyclic structure INT2. INT1 immediately isomerizes to buta-1,3-dien-4-yl (INT3), which can decompose to vinylacetylene + H or to the vinyl radical + acetylene. Buta-1,3-dien-4-yl (INT3) can also rearrange to 1,2-dien-4-yl (INT4), and the latter fragments by the H atom loss to vinylacetylene or butatriene. The alternative pathway from the reactants to 1,2-dien-4-yl (INT4) involves two sequential ring openings in the bicyclic intermediate INT2. 1,2-dien-4-yl (INT4) can also rearrange to but-2-yn-1-yl (INT7), which in turn decomposes to butatriene + H. The RRKM theory has been applied to compute rate constants for unimolecular reaction steps and the product branching ratios....

Using various forms of electronic-structure theory to characterize the important features of the potential energy surface, RRKM theory to calculate microcanonical rate coeffients, and several formulations of the master equation to predict phenomenological rate coefficients, we have studied a number of reactions that occur on the C3H4 potential. We discuss the results in some detail and compare them with experiment when possible. Generally, the agreement with experiment is excellent. “Multiple-well effects” are emphasized throughout the discussion. We cast our results in the form of modified Arrhenius functions for use in chemical kinetics modeling.

The mechanism for the OH + OClO reaction on the singlet and triplet surfaces and its rate constants for formation of various products have been investigated by means of ab initio molecular orbital theory and variational RRKM theory calculations. The geometric parameters of stationary points were optimized at the B3LYP level of theory with the 6-311G(d,p) and 6-311+G(3df,2p) basis sets, and the potential energy surfaces were evaluated at the G2M(CC2)//B3LYP/6-311+G(3df,2p) level of theory. Three main product channels, all located on the singlet PES, have been identified: (1) HOO + ClO, (2) HOCl + 1O2, and (3) HOClO2, the association product. The predicted results show that the rate constants for channels 1 and 2 are pressure-independent up to 1000 atm and that for channel 3 is strongly pressure dependent. Below 1000 K, all rate constants were found to vary negatively with temperature. The individual and total rate constants in the temperature range from 200 to 1000 K at 1 Torr He pressure can be represent...

Previous work demonstrates that rates of H . losses generally increase much less rapidly with increasing internal energy than do those of losses of polyatomic fragments. To determine whether this is also the case for the losses of other atoms, the dependencies of the rates of Cl . and CH 3 . losses from CH 3 CHClCH 3 + . on ion internal energy are compared. These dependencies were established experimentally by photoionization mass spectrometry. The reactant ion and the transition states were characterized by B3LYP/6-31G(d), B3LYP/6-311G(d,p), and MP2/6-31G(d) theories. The transition states located are at very long bond lengths and close in energy to the corresponding dissociation products. Rate constants as a function of internal energy were obtained by RRKM theory. According to both experiment and theory, above its onset, methyl loss increases much more rapidly with internal energy than does loss of Cl . . This stems from three vibrational frequencies being substantially lowered in the transition state for the former but not in the latter reaction as they are being transformed into rotations of the separated products. This produces a much more rapid increase in the RRKM sum of states for the transition state to CH 3 CHCl + + CH 3 . , and thus also in the rate of that reaction. This, together with previous work, establishes that losses of polyatomic fragments from ions in the gas phase generally increase much faster with increasing internal energy than do losses of atoms.

The recombination reaction between IO and NO 2 was studied over a large range of temperature (216-474 K) and pressure (20-760 Torr). IO, produced by the pulsed photolysis at 193 nm of NO 2 to yield O in the presence of CF 3 I, was probed at 445.0 nm [IO(A 2 Π 3 / 2 - X 2 Π 3 / 2 ), (2,0)] and monitored by nonresonant LIF at 458.6 nm [IO(A 2 Π 3 / 2 - X 2 II 3 / 2 ). (2,1)]. The resulting rate coefficients were then fitted by RRKM theory, using molecular parameters and a heat of formation for INO 3 (ΔH° f , 0 = 70 ′ 16 kJ mol - 1 , best fit value 80 kJ mol - 1 ) derived from quantum calculations at the B3LYP/6-31 1+g(2d,p) level. This yielded k r e c , 0 = (1.3 ′ 0.2) × 10 - 3 0 (T/300 K) - ( 4 . 5 γ 0 . 6 ) cm 6 molecule - 2 s - 1 , k r e c , ∞ = (6.5 ′ 1.0) x 10 - 1 2 (T/300 K) - ( 1 . 3 γ 0 . 8 ) cm 3 molecule - 1 s - 1 , and F c = 0.57, over the experimental range of temperature and pressure. The RRKM calculations, in conjunction with measurements made at 474 K, were used to determine the rate of thermal dissociation of INO 3 for the conditions of the lower atmosphere: k d i s s (240-305 K, 760 Torr) = 1.1 x 10 1 5 exp(-12060/T s - 1 , with an upper limit about a factor of 2 higher. The atmospheric significance of these results is discussed briefly.

Density functional theory, DFT, and high-level conventional ab initio calculations, together with RRKM calculations, have been employed to study the nature of the transition state geometry for 1,2 elimination of HF from 1,1,1-trifluoroethane-d0,-d3; these serve as test cases for 1,2-HF elimination from fluorocarbons. Quantities calculated include structural parameters, bond indices, energies, atomic charges, vibrational frequencies, and moments of inertia for the reactant and the transition state geometry. The threshold energies for HF and DF elimination were computed and the vibrational frequencies and moments of inertia data were used with the RRKM theory to calculate the entropies of activation, preexponential factors for thermal activation, and also rate constants and the kinetic isotope effect for both thermally and chemically activated 1,1,1-trifluoroethane-d0,-d3. Of all the methods employed, the hybrid DFT methods incorporating either the three-parameter exchange functional of Becke with the corre...

Quantum mechanical electronic structure calculations were carried out to determine equilibrium geometries, energetics, and normal-mode frequencies for stationary points along the minimum energy reaction path for the reaction of methyl radicals with chlorine molecules. The results are used to calculate the rate coefficient, employing both extended RRKM theory and quasi-classical trajectory techniques. The results of both methods agree well with each other and with the experimental measurements. The reactivity is investigated and is shown to be greatly enhanced by increasing the initial translational energy of the reactants. A large part of the total available energy is shown to be directed into product vibration, in good consistency with the experimental observations.

Specific rate consts. k(E) of the photoisomerization of four isotopomers of trans-stilbene, of 4-chloro-(4CS) and 4-methyl-trans-stilbene (4MS), of all-trans-1,4-diphenyl-1,3-butadiene (DPB), and of 4-(dimethylamino)-4'-cyanostilbene (DCS) are represented by conventional RRKM theory, using new ab initio calcns. of excited state frequencies. Data from supersonic beam expts. are reproduced quant. when the barrier heights are fitted to the expts. and activated complex frequencies are taken unchanged from the parent mols. The main difference to earlier work lies in the smaller frequencies now calcd. for the reaction coordinates. The results of this work clearly support an adiabatic mechanism of the reaction which can very well be quantified by statistical unimol. rate theory. [on SciFinder(R)]

Translational energy dependence of reaction kinetics and internal energy depositions in reaction products were examined for gas-phase SN2 reactions of Cl- and F- with NCCO2CH3 and CF3CO2CH3 in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). The energy dependence of the kinetics was examined by measuring the SN2 rate constants as a function of reactant ion translational energies and by comparing the experimental results with those calculated using RRKM theory. Negative kinetic energy dependence was observed, and the changes of rate constants agree with RRKM theory predictions. The internal energy deposition in the SN2 products was examined by measuring the fraction of secondary fragments resulting from dissociation of the ionic SN2 product. A statistical model was used to calculate the statistical fraction of internally excited SN2 product ion under thermal conditions. For the least exothermic reaction (

We have investigated theoretically the recombination reaction between two propargyl (C3H3) radicals using previously published BAC-MP4 calculations (supplemented by DFT-B3LYP results) to characterize the potential energy surface, RRKM theory to compute microcanonical rate coefficients, and solutions to the time-dependent, multiple-well master equation to predict thermal rate coefficients and product distributions as a function of temperature and pressure. The thermal rate coefficient k(T,p) drops off precipitously at high temperature, regardless of the pressure. Below 500 K, k(T,p) ≈ k∞(T), the high-pressure limit rate coefficient for initial complex formation, independent of p. For 500 K < T < 2000 K, the rate coefficient increases with increasing pressure, as one would normally expect. At 2000 K, the “coalescence temperature” for this reaction, k(T,p) = k0(T), the zero-pressure rate coefficient, and only bimolecular products (phenyl + H) are predicted, no matter how high we make the pressure. The latter...

Photodissociation dynamics of acetic acid and trifluoroacetic acid at 193 nm have been investigated by measuring laser-induced fluorescence spectra of OH fragments. The OH fragments are produced exclusively in the ground electronic state. The measured energy distributions among the fragments are fr(OH) = 0.05, ft = 0.42, fint(Ac) = 0.53 and fr(OH) = 0.05, ft = 0.33, and fint(FAc) = 0.62, for acetic and trifluoroacetic acid, respectively, and negligible vibrational excitation in the OH fragments was observed. The dissociation does not depend on the polarization of the dissociating light. It was concluded from the measured energy distribution and no polarization dependence that the electronic transition at 193 nm leads the parent molecule to the singlet excited surface, and the dissociation takes place along the triplet surface with an exit channel barrier. From the estimated internal energies in the acetyl radicals, the lifetimes of the acetyl radicals are estimated from the RRKM theory.

The reaction of the CH3 radical with HO2 has been investigated by means of ab initio molecular orbital theory and variational RRKM theory calculations. The reaction can take place by several product channels producing (a) CH4 + O2 (3Σg-) and (b) CH4 + O2 (1Δ) by direct H abstraction and (c) CH3O + OH and (d) CH2O + H2O by an association/decomposition mechanism via CH3OOH. The bimolecular reaction rate constants for the formation of these products have been calculated for the temperature range 300−3000 K and found to be pressure independent up to 50 atm. The Arrhenius equations for the two major channels a and c were found to be strongly curved; they can be represented by ka = 4.23 × 10-16T1.25 exp(828/T) for 300−800 K, ka = 3.02 × 10-21T2.83 exp(1877/T) for 800−3000 K, and kc = 2.97 × 10-10T-0.24 exp(182/T) for 300−1000 K and 1.02 × 10-13T0.76 exp(1195/T) for 1000−3000 K, in units of cm3 molecule-1 s-1. In the abstraction channel a, the effect of multiple reflections above its van der Waals complex (CH3··...

Ab initio MO calculations have been carried out for the C(3P) + N2 reaction system. The reaction has been shown to pass through CNN, NCN, and cyclic c-NCN intermediates. The rate constants for the forward and reverse directions have been calculated with multichannel variational RRKM theory. We have also calculated the unimolecular rate constants for the NCN decomposition. The theoretically predicted heats of formation for NCN and CNN are in good agreement with recent experimental measurements.

The mechanism of chemiluminescence of 1,2-dioxetane (DO) and 3,3,4,4-tetramethyl-1,2-dioxetane (TMDO) are investigated by the ab initio molecular orbital calculation. The rate-determining step of the chemiluminescent reaction is the O−O bond breaking to form the biradicals. The potential energies along the reaction path are calculated by uB3LYP and uB3P86 methods with 6-31+G(d); the calculated potential barriers are in reasonable agreement with experimental activation energies. An overview of the potential surfaces for overall reaction is obtained. The intersystem crossing mechanism from the singlet biradical to the triplet state is investigated, and the reaction path is followed to the 3(nπ*) excited states of the carbonyl group of respective fragment molecules. The mechanism of promotion to the 1(nπ*) excited state of formaldehyde is investigated by the MCSCF method on DO. The reaction rates and the yield of chemiluminescence are discussed by the RRKM theory of unimolecular reaction.

Unimolecular pathways for the isomerization and/or dissociation of HSOO, HOSO, HSO2, and HOOS to H + SO2 and OH + SO have been investigated computationally, as well as HSO formation via an HSOO intermediate. The atmospheric lifetime of HSO2 is discussed. Some pathways have no barrier, including OH + SO → HOSO and H + SOO → HSOO and SOOH, while structures and vibrational frequencies of transition states for HOSO → H + SO2, HOOS → OH + SO, HOSO → HSO2, HSOO → HS + O2, and HSO2 → H + SO2 have been characterized at the MP2=FULL/6-31G(d) level. Some geometries were further refined at the QCISD/6-311G(d,p) level. Gaussian-2 theory was employed to calculate approximate QCISD(T)/6-311+G(3df,2p) energy barriers, and the kinetics were analyzed by RRKM theory. Rate constant expressions at the high and low-pressure limits and thermochemical properties for transient intermediates are tabulated, and the results are discussed in the context of atmospheric and combustion chemistry. A revised theoretical H−OSO bond streng...

The effect of pressure on the cross-radical reactions of vinyl and methyl radicals has been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH{sub 3}COC{sub 2}H{sub 3}) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.28 kPa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C{sub 3}H{sub 6}), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C{sub 2}H{sub 6}), the methyl radical combination product. At 27 kPa [C{sub 3}H{sub 6}]/[C{sub 2}H{sub 6}] = 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to bout 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C{sub 2}H{sub 3} + CH{sub 2}+ M {r{underscore}arrow} C{sub 3}H{sub 6} + M to the total cross-radical reactions is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C{sub 2}H{sub 3} + CH{sub 3} {r{underscore}arrow}C{sub 3}H{sub 5} + H, becomes available, producing a host of allyl radicalmore » reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5-hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C{sub 3}H{sub 6}. Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. It has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The apparent pathological behavior in this unsaturated system is attributed to the formation of a stronger C-C bond as contrasted to the weaker C-C bond formed from combination of saturated hydrocarbon radicals. This C-C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C-H bond or isomerize to cyclopropane.« less