RRKM Rate Constants Research Articles

Kinetics and Dynamics of Cyclopentanone Thermal Decomposition in Gas Phase.

Cyclopentanone is a potential bio-fuel which can be produced from bio-mass. Its gas phase dissociation chemistry has attracted several experimental and theoretical investigations. In the photochemical and thermal decomposition studies of cyclopentanone, ethylene and carbon monoxide were found to be dominant reaction products along with several other compounds in smaller quantities. For the formation of ethylene and carbon monoxide, a concerted mechanism has been proposed as primary reaction pathway. In addition, a step-wise mechanism involving ring-opened radical intermediate has also been considered. The present work reports gas phase thermal decomposition of cyclopentanone at high temperatures investigated using electronic structure theory methods, Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant calculations, and Born-Oppenheimer direct classical trajectory simulations. The trajectory calculations were performed on density functional PBE96/6-31+G* potential energy surface using initial conditions selected from fixed energy normal mode distributions. Simulations showed that ethylene and carbon monoxide formed primarily via the concerted mechanism confirming the earlier predictions. In addition, stepwise pathways were also observed for same products in lower fraction of trajectories. Furthermore, several other reaction products in smaller quantities and new mechanistic pathways were observed. The computed RRKM rate constants and simulation data are agreement with experimental results and detailed atomic level dissociation mechanisms presented.

Investigation of Methylcyclopentadiene Reactivity: Abstraction Reactions and Methylcyclopentadienyl Radical Unimolecular Decomposition.

Understanding the reactivities of methylcyclopentadiene and the methylcyclopentadienyl radical is important in order to improve our comprehension of the chemical kinetics leading to the formation, decomposition, and growth of the first aromatic ring, as it has been shown that five-membered-ring species are important intermediates in the reaction kinetics of aromatic species. In this work, the rate constants of some key H-abstraction reactions from methylcyclopentadiene to produce the methylcyclopentadienyl radical and the formation of fulvene and benzene from the latter are theoretically determined. Rate constants are evaluated using the ab initio transition state theory-based master equation approach, determining structures and Hessians of all stationary points at the ωB97X-D/aug-cc-pVTZ level, energies at the CCSD(T) level extrapolated to the complete basis set limit, RRKM rate constants using conventional and variational transition state theory, and phenomenological rate constants through the solution of the one-dimensional master equation. Variational corrections are determined in both internal and Cartesian coordinates, and it is found that the choice of the coordinate system can impact the accuracy of the calculated rate constants by up to a factor of 4 for H-abstraction reactions and 2 for the unimolecular decomposition of the methylcyclopentadienyl radical. The calculated rate constants are in good agreement with the available literature data. Prompt dissociation of methylcyclopentadienyl radicals accessed following H-abstraction from methylcyclopentadiene was also investigated, and the corresponding rate constants were determined; the results show that prompt dissociation plays a key role under combustion conditions. Finally, lumping of theoretically derived rate constants to account for methylcyclopentadiene ⇄ methylcyclopentadienyl tautomerism allowed the derivation of a simplified set of rate constants suitable to be inserted into kinetic mechanisms.

Calculating RRKM Rate Constants from Vibrational Frequencies and Their Dynamic Interpretation

Calculating RRKM Rate Constants from Vibrational Frequencies and Their Dynamic Interpretation

Theoretical study on reaction mechanism of ground-state cyano radical with 1,3-butadiene: prospect of pyridine formation.

The reaction of ground-state cyano radicals, CN(X(2)Σ(+)), with the simplest polyene, 1,3-butadiene (C4H6(X(1)Ag)), is investigated to explore probable routes and feasibility to form pyridine at ultralow temperatures. The isomerization and dissociation channels for each of the seven initial collision complexes are characterized by utilizing the unrestricted B3LYP/cc-pVTZ and the CCSD(T)/cc-pVTZ calculations. With facilitation of RRKM rate constants, through ab initio paths composed of 7 collision complexes, 331 intermediates, 62 hydrogen atom, 71 hydrogen molecule, and 3 hydrogen cyanide dissociated products, the most probable paths at collision energies up to 10 kcal/mol, and thus the reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the concentration evolutions of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are determined. The low-energy routes for the formation of most thermodynamically stable product, pyridine, are identified. This study, however, predicts that seven collision complexes would produce predominately 1-cyano-1,3-butadiene, CH2CHCHCHCN (p2) plus atomic hydrogen via the collision complex c1(CH2CHCHCH2CN) and intermediate i2(CH2CHCH2CHCN), with a very minor amount of pyridine. Our scheme also effectively excludes the presence of 2-cyano-1,3-butadiene, which has energy near-degenerate to 1-cyano-1,3-butadiene, as supported by experimental findings.

Comparisons of classical chemical dynamics simulations of the unimolecular decomposition of classical and quantum microcanonical ensembles

Previous studies have shown that classical trajectory simulations often give accurate results for short-time intramolecular and unimolecular dynamics, particularly for initial non-random energy distributions. To obtain such agreement between experiment and simulation, the appropriate distributions must be sampled to choose initial coordinates and momenta for the ensemble of trajectories. If a molecule's classical phase space is sampled randomly, its initial decomposition will give the classical anharmonic microcanonical (RRKM) unimolecular rate constant for its decomposition. For the work presented here, classical trajectory simulations of the unimolecular decomposition of quantum and classical microcanonical ensembles, at the same fixed total energy, are compared. In contrast to the classical microcanonical ensemble, the quantum microcanonical ensemble does not sample the phase space randomly. The simulations were performed for CH(4), C(2)H(5), and Cl(-)---CH(3)Br using both analytic potential energy surfaces and direct dynamics methods. Previous studies identified intrinsic RRKM dynamics for CH(4) and C(2)H(5), but intrinsic non-RRKM dynamics for Cl(-)---CH(3)Br. Rate constants calculated from trajectories obtained by the time propagation of the classical and quantum microcanonical ensembles are compared with the corresponding harmonic RRKM estimates to obtain anharmonic corrections to the RRKM rate constants. The relevance and accuracy of the classical trajectory simulation of the quantum microcanonical ensemble, for obtaining the quantum anharmonic RRKM rate constant, is discussed.

Collision induced dissociation of doubly-charged ions: Coulomb explosion vs. neutral loss in [Ca(urea)]2+ gas phase unimolecular reactivity via chemical dynamics simulations

In this paper we report different theoretical approaches to study the gas-phase unimolecular dissociation of the doubly-charged cation [Ca(urea)](2+), in order to rationalize recent experimental findings. Quantum mechanical plus molecular mechanical (QM/MM) direct chemical dynamics simulations were used to investigate collision induced dissociation (CID) and rotational-vibrational energy transfer for Ar + [Ca(urea)](2+) collisions. For the picosecond time-domain of the simulations, both neutral loss and Coulomb explosion reactions were found and the differences in their mechanisms elucidated. The loss of neutral urea subsequent to collision with Ar occurs via a shattering mechanism, while the formation of two singly-charged cations follows statistical (or almost statistical) dynamics. Vibrational-rotational energy transfer efficiencies obtained for trajectories that do not dissociate during the trajectory integration were used in conjunction with RRKM rate constants to approximate dissociation pathways assuming complete intramolecular vibrational energy redistribution (IVR) and statistical dynamics. This statistical limit predicts, as expected, that at long time the most stable species on the potential energy surface (PES) dominate. These results, coupled with experimental CID from which both neutral loss and Coulomb explosion products were obtained, show that the gas phase dissociation of this ion occurs by multiple mechanisms leading to different products and that reactivity on the complicated PES is dynamically complex.

Intramolecular competition in the photodissociation of C3D3 radicals at 248 and 193 nm

Motivated by recent experimental work, a theoretical study of the photodissociation of perdeuterated propargyl (D(2)CCD) and propynyl (D(3)CCC) radicals has been carried out, focusing on the C-C bond cleavage and D(2) loss channels. High-level ab initio calculations were carried out, and RRKM rate constants were calculated for isomerization and dissociation pathways. The resulting reaction barriers, microcanonical rate constants and product branching ratios are consistent with the experimental findings, supporting the overall mechanism of internal conversion followed by statistical dissociation on the ground state surface. We found loose transition states and very low exit barriers for two of the C-C bond cleavage channels and an additional CD(2) + CCD channel, which had not been reported previously. Our results probe the extent of propargyl and propynyl isomerization prior to dissociation at 248 and 193 nm and deliver a comprehensive picture of all ongoing molecular dynamics.

Theoretical Study on the Reaction of Ground State Cyano Radical with Propylene in Titan’s Atmosphere

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.

Synthesis of interstellar 1,3,5-heptatriynylidyne, C7H(X Π2), via the neutral-neutral reaction of ground state carbon atom, C(P3), with triacetylene, HC6H (X Σ1g+)

The reaction of ground-state carbon atom with a polyyne, triacetylene (HC6H) is investigated theoretically by combining ab initio calculations for predicting reaction paths, RRKM theory to yield rate constant for each path, and a modified Langevin model for estimating capturing cross sections. The isomerization and dissociation channels for each of the five collision complexes are characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. Navigating with the aid of RRKM rate constants through web of ab initio paths composed of 5 collision complexes, 108 intermediates, and 20 H-dissociated products, the most probable paths, reduced to around ten species at collision energies of 0 and 10 kcal/mol, respectively, are identified and adopted as the reaction mechanisms. The rate equations for the reaction mechanisms are solved numerically such that the evolutions of concentrations with time for all species involved are obtained and their lifetimes deduced. This study predicts that the five collision complexes, c1–c5, would produce a single final product, C7H (p1)+H, via the most stable intermediate, carbon chain HC7H (i1); namely, C+HC6H→HC7H→C7H+H. Our investigation indicates that the title reaction is efficient to form astronomically observed C7H in cold molecular clouds, where a typical translational temperature is 10 K.

Characterization of the Unimolecular Water Elimination Reaction from 1-Propanol, 3,3,3-Propan-1-ol-d3, 3,3,3-Trifluoropropan-1-ol, and 3-Chloropropan-1-ol

The unimolecular reactions of 1-propanol, 3,3,3-propan-1-ol-d3, 3,3,3-trifluoropropan-1-ol, and 3-chloropropan-1-ol have been studied by the chemical activation technique. The recombination of CH3, CD3, CF3, and CH2Cl radicals with CH2CH2OH radicals at room temperature was used to generate vibrationally excited CH3CH2CH2OH, CD3CH2CH2OH, CF3CH2CH2OH, and CH2ClCH2CH2OH molecules. The principal unimolecular reaction for propanol and propanol-d3 with 90 kcal mol(-1) of vibrational energy is 1,2-H2O elimination with rate constants of 3.4 x 10(5) and 1.4 x 10(5) s(-1), respectively. For CH2ClCH2CH2OH also with 90 kcal mol(-1) of energy, 2,3-HCl elimination with a rate constant of 3.0 x 10(7) s(-1) is more important than 1,2-H2O elimination; the branching fractions are 0.95 and 0.05. For CF3CH2CH2OH with an energy of 102 kcal mol(-1), 1,2-H2O elimination has a rate constant of 7.9 x 10(5) and 2,3-HF elimination has a rate constant of 2.6 x 10(5) s(-1). Density functional theory was used to obtain models for the molecules and their transition states. The frequencies and moments of inertia from these models were used to calculate RRKM rate constants, which were used to assign threshold energies by comparing calculated and experimental rate constants. This comparison gives the threshold energy for H2O elimination from 1-propanol as 64 kcal mol(-1). The threshold energies for 1,2-H2O and 2,3-HCl elimination from CH2ClCH2CH2OH were 59 and 54 kcal mol(-1), respectively. The threshold energies for H2O and HF elimination from CF3CH2CH2OH are 62 and 70 kcal mol(-1), respectively. The structures of the transition states for elimination of HF, HCl, and H2O are compared.

Potential energy surface and unimolecular dynamics of stretched n-butane

The potential energy surface (PES) and unimolecular reaction dynamics of stretched n-butane are investigated, as a model for a stretched "normal" alkane or straight chain polymer. The nature of the PES for stretched n-butane depends on the extent of stretching. If it is less than that required to reach the inflection points in the C[Single Bond]C stretch potentials and the C[Single Bond]C torsions are considered free rotors, there is only one potential energy minimum, with each bond elongated. However, for stretching past these inflection points, the PES has three minima and each has one bond longer than the other two, i.e., C[Single Bond]C[Single Bond]C[Single Bond]C, C[Single Bond]C[Single Bond]C[Single Bond]C, and C[Single Bond]C[Single Bond]C[Single Bond]C. There are three transition states (TSs) connecting these minima. A linear alkane, consisting of n carbon atoms and stretched past its C[Single Bond]C inflection points, has (n-1) minima and (n-1)(n-2)2 TSs connecting them. For stretching less than that required to reach the C[Single Bond]C inflection points, the only unimolecular pathways are dissociations to form the C+C[Single Bond]C[Single Bond]C, C[Single Bond]C+C[Single Bond]C, and C[Single Bond]C[Single Bond]C+C products. However, with stretching past the C[Single Bond]C inflection points, isomerizations between the three potential energy minima may also occur. The relative importance of isomerization versus dissociation depends on the relative size of their barriers. For slight stretching past the C[Single Bond]C inflection points, the isomerization barriers are much lower than those for dissociation and relaxation between the minima is much faster than dissociation. Thus, the molecule samples these minima randomly during its dissociation, with a density of states that comprises the complete PES. With extensive stretching past the inflection points, isomerizations between the potential energy minima do not occur and only dissociation for the excited minima occurs, e.g., C[Single Bond]C[Single Bond]C[Single Bond]C-->C+C[Single Bond]C[Single Bond]C. For intermediate stretching past the C[Single Bond]C inflection points, the rates for the isomerization and dissociation pathways are competitive and both must be considered in modeling the dissociation kinetics. Microcanonical chemical dynamics simulations are performed to study the unimolecular kinetics of n-butane in three stretched configurations: Stretched less than the C[Single Bond]C inflection point; stretched slightly beyond the C[Single Bond]C inflection point; and stretched significantly beyond the C[Single Bond]C inflection point. The resulting unimolecular dynamics were found to be in excellent agreement with RRKM theory. Frequency nu factors, determined by fitting the trajectory unimolecular rate constants to the classical harmonic RRKM rate constant expression, depend upon the extent of stretching and vary from 1.0 x 10(12)-8.4 x 10(16) s(-1). For a molecule with a large number of vibrational degrees of freedom and high excess energy, it is shown that the classical harmonic RRKM and classical harmonic transition state theory rate constants, k(E) and k(T), are equivalent.

Formation of interstellar 2,4-pentadiynylidyne, HCCCCC(XΠ2), via the neutral-neutral reaction of ground state carbon atom, C(P3), with diacetylene, HCCCCH(XΣg+1)

The interstellar reaction of ground-state carbon atom with the simplest polyyne, diacetylene (HCCCCH), is investigated theoretically to explore probable routes to form hydrogen-deficient carbon clusters at ultralow temperature in cold molecular clouds. The isomerization and dissociation channels for each of the three collision complexes are characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. With facilitation of RRKM and variational RRKM rate constants at collision energies of 0-10 kcalmol, the most probable paths, thus reaction mechanism, are determined. Subsequently, the corresponding rate equations are solved that the evolutions of concentrations of collision complexes, intermediates, and products versus time are obtained. As a result, the final products and yields are identified. This study predicts that three collision complexes, c1, c2, and c3, would produce a single final product, 2,4-pentadiynylidyne, HCCCCC(X (2)Pi), C(5)H (p1)+H, via the most stable intermediate, carbon chain HC(5)H (i4). Our investigation indicates the title reaction is efficient to form astronomically observed 2,4-pentadiynylidyne in cold molecular clouds, where a typical translational temperature is 10 K, via a single bimolecular gas phase reaction.

Fragmentation kinetics of a Morse oscillator chain under tension

The bond dissociation kinetics of tethered atomic (Morse potential) chains under tensile stress is studied. Both RRKM (fully anharmonic, Monte Carlo) and RRK (harmonic appproximation) theory are applied to predict bond dissociation rate constants as a function of energy and tensile force. For chains with N ⩾ 3 atoms a hybrid statistical theory is used involving a harmonic approximation for motion in the transition state for bond dissociation. For chains with N = 2–5 atoms, while the RRK approximation significantly overestimates the dissociation rate constant, the fully anharmonic RRKM rate is quite close to simulation results. For the N = 2 chain, a novel approach to the extraction of decay rate constants based on the classical spectral theorem is implemented. Good agreement between the RRKM and dynamical rate constants is obtained for N = 2 despite the fact that the reactant phase space contains a significant fraction of relatively short-lived trajectories.

Improved Whitten−Rabinovitch Approximation for the Rice−Ramsperger−Kassel−Marcus Calculation of Unimolecular Reaction Rate Constants for Proteins

The Whitten-Rabinovitch (WR) approximation used in the semi-classical calculation of the Rice-Ramsperger-Kassel-Marcus (RRKM) unimolecular reaction rate constant was improved for reliable application to protein reactions. The state sum data for the 10-mer of each amino acid calculated by the accurate Beyer-Swinehart (BS) algorithm were used to obtain the residue-specific correction functions (w). The correction functions were obtained down to a much lower internal energy range than reported in the original work, and the cubic, rather than quadratic, polynomial was used for data fitting. For a specified sequence of amino acid residues in a protein, an average was made over these functions to obtain the sequence-specific correction function to be used in the rate constant calculation. Reliability of the improved method was tested for dissociation of various peptides and proteins. Even at low internal energies corresponding to the RRKM rate constant as small as 0.1 s-1, the rate constant calculated by the present method differed from the accurate BS result by 60% only. In contrast, the result from the original WR calculation differed from the accurate result by a factor of 3000. Compared to the BS method, which is difficult to use for proteins, the main advantage of the present method is that the RRKM rate constant can be calculated instantly regardless of the protein mass.

Reaction of cyanoacetylene HCCCN(XΣ+1) with ground-state carbon atoms C(P3) in cold molecular clouds

The reaction of the simplest cyanopolyyne, cyanoacetylene [HCCCN(X (1)Sigma(+))], with ground-state atomic carbon C((3)P) is investigated theoretically to explore the probable routes for the depletion of the famed interstellar molecule HCCCN, and the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Six collision complexes (c1-c6) without entrance barrier as a result of the carbon atom addition to the pi systems of HCCCN are located. The optimized geometries and harmonic frequencies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex are obtained by utilizing the unrestricted B3YLP6-311G(d,p) level of theory, and the corresponding CCSD(T)/cc-pVTZ energies are calculated. Subsequently, with the facilitation of Rice-Ramsperger-Kassel-Marcus (RRKM) and variational RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for the titled reaction are determined, and the product yields are estimated. Five collision complexes (c1-c3, c5, and c6) are predicted to give the same products, a chained CCCCN (p2)+H, via the linear and most stable intermediate, HCCCCN (i2), while collision complex c4 is likely to dissociate back to C+HCCCN. The study suggests that this class of reaction is an important route to the destruction of cyanoacetylene and cyanopolyynes in general, and to the synthesis of linear carbon-chained nitriles at the temperature as low as 10 K to be incorporated in future chemical models of interstellar clouds.

Unimolecular Rate Constants, Kinetic Isotope Effects and Threshold Energies for FH and FD Elimination from CF3CHFCH3 and CF3CHFCD3

The combination of CF(3)CHF and CH(3) or CD(3) radicals was used to prepare vibrationally excited CF(3)CHFCH(3) or CF(3)CHFCD(3) molecules with 97 kcal mol(-1) of internal energy. The experimental unimolecular rate constants were 3.7 x 10(6) s(-1) for 2,3-FH elimination from CF(3)CHFCH(3) and 1.3 x 10(6) s(-1) for 2,3-DF elimination from CF(3)CHFCD(3). Unimolecular rate constants for 1,2-FH elimination reaction were approximately 230 and 98 times smaller for CF(3)CHFCH(3) and CF(3)CHFCD(3), respectively, than the corresponding rate constants for 2,3-FH elimination. Density functional theory (DFT) was used to calculate the structures and vibrational frequencies of the molecules and transition states; this information was subsequently employed for calculations of RRKM rate constants. Comparison of the experimental and calculated rate constants gave a threshold energy of 73 +/- 2 kcal mol(-1) for the 1,2-FH elimination process and 60.5 +/- 1.5 kcal mol(-1) for the 2,3-FH elimination reaction from CF(3)CHFCH(3). The calculated kinetic-isotope effects agree with the experimental results. The experimentally derived threshold energies for 1,2-FH and 2,3-FH elimination reactions from several fluoropropanes and fluorochloropropanes are summarized and compared to those from DFT calculations.

Unimolecular Rate Constants for HX or DX Elimination (X = F, Cl) from Chemically Activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl: Threshold Energies for HF and HCl Elimination

Vibrationally activated CF(3)CH(2)CH(2)Cl molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF(3)CH(2) and CH(2)Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF(3) and CH(2)CH(2)Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF(3)CH(2)CH(2)Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH(3)CH(2)CH(2)Cl and CD(3)CD(2)CH(2)Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination of C(2)H(5) (or C(2)D(5)) radicals with CH(2)Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF(3)CH(2)CH(2)Cl and CH(3)CH(2)CH(2)Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E(0), of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF(3)CH(2)CH(2)Cl and 54 kcal mol(-1) for HCl elimination from CH(3)CH(2)CH(2)Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF(3)CH(2)CH(3,) CF(3)CH(2)CF(3), CH(3)CH(2)CH(2)F, CH(3)CHFCH(3), and CH(3)CF(2)CH(3). In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C(2)H(5)Cl, C(2)H(5)F, and CH(3)CF(3).

A theoretical study for the reaction of vinyl cyanide C2H3CN(XA′1) with the ground state carbon atom C(P3) in cold molecular clouds

The reaction of the ground state atomic carbon, C(3P), with simple unsaturated nitrile, C2H3CN(X1A' (vinyl cyanide), is investigated theoretically to explore the probable routes for the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Five collision complexes without entrance barrier as a result of the carbon atom addition to the pi systems of C2H3CN are characterized. The B3YLP/6-311G(d,p) level of theory is utilized in obtaining the optimized geometries, harmonic frequencies, and energies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex. Subsequently, with the facilitation of computed RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for each collision complexes are determined, of which the CCSD(T)/6-311G(d,p) energies are calculated. The major products predicted are exclusively due to the hydrogen atom dissociations, while the products of H2, CN, and CH2 decompositions are found negligible. Among many possible H-elimination products, cyano propargyl (p4) and 3-cyano propargyl (p5) are the most probable, in which p5 can be formed via two intermediates, cyano allene (i8) and cyano vinylmethylene (i6), while p4 is yielded from i8. The study suggests this class of reaction is an important route to the synthesis of unsaturated nitriles at the temperature as low as 10 K, and the results are valuable for future chemical models of interstellar clouds.

A theoretical study of the O( 1D) + CH 4 reaction II

The titled reaction is assumed to proceed via an insertion mechanism. Statistical branching ratios for various product channels are predicted with RRKM rate constants that are obtained based on ab initio calculations. A modified collision-complex model is utilized in the estimation of absolute reaction cross-sections for primary channels. The product energy partitions for H and H 2 formation pathways are obtained. Subsequently, the RRKM rate constants for secondary reactions are calculated with correction for the translational and rotational energies of primary products. The results give insight to reaction mechanism and provide probable explanations to disputes among previous experimental findings.

Reaction Coordinate and Rate Constants for Nitrous Acid cis−trans Isomerization

Nitrous acid is one of the simplest molecules that undergoes geometric isomerization. The reaction path properties and rate constants of isomerization have been determined at the MP2 and DFT levels of theory. Good agreement is obtained with experimental data. The calculated isomerization threshold energy is 10.8−12.0 kcal/mol, and the experimental value is 11.6 ± 0.2 kcal/mol. The microcanonical RRKM rate constants, which include the effects of the variational transition state and the J rotational quantum number, are calculated. High-pressure rate constants are determined for the 223−323 K temperature range.