Effect of temperature on the gas-phase reaction of CH3CN with OH radicals: experimental (T = 11.7-177.5 K) and computational (T = 10-400 K) kinetic study

Acetonitrile (CH3CN) is present in the interstellar medium (ISM) in a variety of environments. However, at the ultracold temperatures of the ISM, radical-molecule reactions are not widely investigated because of the experimental handicap of getting organic molecules in the gas phase by conventional techniques. The CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique solves this problem. For this reason, we present in this work the kinetic study of the gas-phase reaction of CH3CN with one of the most ubiquitous radicals, the hydroxyl (OH) radical, as a function of temperature (11.7-177.5 K). The kinetic technique employed to investigate the CH3CN + OH reaction was the pulsed laser photolysis-laser induced fluorescence. The rate coefficient for this reaction k(T) has been observed to drastically increase from 177.5 K to 107.0 K (about 2 orders of magnitude), while the increase in k(T) from 107.0 K to 11.7 K was milder (around 4 times). The temperature dependent expressions for k(T) are provided in the two distinct T-ranges, excluding the upper limit obtained for k(177.5 K): In addition, the rate coefficients estimated by the canonical competitive unified statistical (CCUS) theory show a similar behaviour to the experimental results, when evaluated within the high-pressure limit. This is consistent with the experimentally observed independence of k(T) with total gas density at selected temperatures. Astrochemical networks, such as the KIDA database or UMIST, do not include the CH3CN + OH reaction as a potential depletion process for acetonitrile in the ISM because the current studies predict very low rate coefficients at IS temperatures. According to the model (T = 10 K), the impact of the titled reaction on the abundances of CH3CN appears to be negligible in dark molecular clouds of the ISM (∼1% of the total depletion reactions included in UMIST network). With respect to the potential formation of the CH2CN radical in those environments, even in the most favourable scenario, where this radical could be formed in a 100% yield from the CH3CN + OH reaction, this route would only contribute around 2% to the current assumed formation routes by the UMIST network.

New prebiotic molecules in the interstellar medium from the reaction between vinyl alcohol and CN radicals: unsupervised reaction mechanism discovery, accurate electronic structure calculations and kinetic simulations

Vinyl alcohol (VyA) and cyanide (CN) radicals are relatively abundant in the interstellar medium (ISM). VyA is the enolic tautomer of acetaldehyde and has two low-lying conformers, characterized by the syn or anti placement of hydroxyl hydrogen with respect to the double bond. In this paper, we present a gas-phase model of the barrierless reactions of both VyA's conformers with CN employing accurate quantum chemical computations in the framework of a master equation approach based on the transition state theory. Our results indicate that both VyA conformers feature a similar reactivity with CN, starting with a barrierless addition to the double bond and followed by different isomerization, dissociation, and/or hydrogen elimination steps. The rate constants computed for temperatures up to 600 K show that several reaction channels are open even under the harsh conditions of the ISM, with the favoured one providing the first feasible formation route of a prebiotic molecule not yet detected in the ISM, namely cyanoacetaldehyde. This finding suggests looking for cyanoacetaldehyde in regions where both VyA and CN have already been detected, like, e.g., Sagittarius B2N or G+0.693-0.027.

Gas-phase formation of glycolonitrile in the interstellar medium

Our automated reaction discovery program, AutoMeKin, has been utilized to investigate the formation of glycolonitrile (HOCH2CN) in the gas phase under the low temperatures of the interstellar medium (ISM). The feasibility of a proposed pathway depends on the absence of barriers above the energy of reactants and the availability of the suggested precursors in the ISM. Based on these criteria, several radical-radical reactions and a radical-molecule reaction have been identified as viable formation routes in the ISM. Among the radical-radical reactions, OH + CH2CN appears to be the most relevant, considering the energy of the radicals and its ability to produce glycolonitrile in a single step. However, our analysis reveals that this reaction produces hydrogen isocyanide (HNC) and formaldehyde (CH2O), with rate coefficients ranging from (7.3-11.5) × 10-10 cm3 molecule-1 s-1 across the temperature range of 10-150 K. Furthermore, the identification of this remarkably efficient pathway for HNC elimination from glycolonitrile significantly broadens the possibilities for any radical-radical mechanism proposed in our research to be considered as a feasible pathway for the formation of HNC in the ISM. This finding is particularly interesing given the persistently unexplained overabundance of hydrogen isocyanide in the ISM. Among the radical-molecule reactions investigated, the most promising one is OH + CH2CHNH, which forms glycolonitrile and atomic hydrogen with rate coefficients in the range (0.3-6.6) × 10-10 cm3 molecule-1 s-1 within the 10-150 K temperature range. Our calculations indicate that the formation of both hydrogen isocyanide and glycolonitrile is efficient under the harsh conditions of the ISM.

Reaction of OH radicals with CH3NH2 in the gas phase: experimental (11.7-177.5 K) and computed rate coefficients (10-1000 K)

Nitrogen-bearing molecules, like methylamine (CH₃NH₂), can be the building blocks of amino acids in the interstellar medium (ISM). At the ultralow temperatures of the ISM, it is important to know its gas-phase reactivity towards interstellar radicals and the products formed. In this work, the kinetics of the OH + CH₃NH₂ reaction was experimentally and theoretically investigated at low- and high-pressure limits (LPL and HPL) between 10 and 1000 K. Moreover, the CH₂NH₂ and CH₃NH yields were computed in the same temperature range for both pressure regimes. A pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus was employed to determine the rate coefficient, k(T), in the 11.7-177.5 K range. A drastic increase of k(T) when the temperature is lowered was observed in agreement with theoretical calculations, evaluated by the competitive canonical unified statistical (CCUS) theory, below 300 K in the LPL regime. The same trend was observed in the HPL regime below 350 K, but the theoretical k(T) values were higher than the experimental ones. Above 200 K, the calculated rate coefficients are improved with respect to previous computational studies and are in excellent agreement with the experimental literature data. In the LPL, the formation of CH₃NH becomes largely dominant below ca. 100 K. Conversely, in the HPL regime, CH₂NH₂ is the only product below 100 K, whereas CH₃NH becomes dominant at 298 K with a branching ratio similar to the one found in the LPL regime (≈70%). At T > 300 K, both reaction channels are competitive independently of the pressure regime.

New Tools for Taming Complex Reaction Networks: The Unimolecular Decomposition of Indole Revisited

The level of detail attained in the computational description of reaction mechanisms can be vastly improved through tools for automated chemical space exploration, particularly for systems of small to medium size. Under this approach, the unimolecular decomposition landscape for indole was explored through the automated reaction mechanism discovery program AutoMeKin. Nevertheless, the sheer complexity of the obtained mechanisms might be a hindrance regarding their chemical interpretation. In this spirit, the new Python library amk-tools has been designed to read and manipulate complex reaction networks, greatly simplifying their overall analysis. The package provides interactive dashboards featuring visualizations of the network, the three-dimensional (3D) molecular structures and vibrational normal modes of all chemical species, and the corresponding energy profiles for selected pathways. The combination of the joined mechanism generation and postprocessing workflow with the rich chemistry of indole decomposition enabled us to find new details of the reaction (obtained at the CCSD(T)/aug-cc-pVTZ//M06-2X/MG3S level of theory) that were not reported before: (i) 16 pathways leading to the formation of HCN and NH₃ (via amino radical); (ii) a barrierless reaction between methylene radical and phenyl isocyanide, which might be an operative mechanism under the conditions of the interstellar medium; and (iii) reaction channels leading to both hydrogen cyanide and hydrogen isocyanide, of potential astrochemical interest as the computed HNC/HCN ratios greatly exceed the calculated equilibrium value at very low temperatures. The reported reaction networks can be very valuable to supplement databases of kinetic data, which is of remarkable interest for pyrolysis and astrochemical studies.

Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study

The photoisomerization behavior of styryl 9M, a common dye used in material sciences, is investigated using tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy. Styryl 9M has two alkene linkages, potentially allowing for four geometric isomers. IMS measurements demonstrate that at least three geometric isomers are generated using electrospray ionization with the most abundant forms assigned to a combination of EE (major) and ZE (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections. Two additional but minor isomers are generated by collisional excitation of the electrosprayed styryl 9M ions and are assigned to the EZ and ZZ geometric isomers, with the latter predicted to have a π-stacked configuration. The isomer assignments are supported through calculations of equilibrium structures, collision cross sections, and statistical isomerization rates. Photoexcitation of selected isomers using an IMS-photo-IMS strategy shows that each geometric isomer photoisomerizes following absorption of near-infrared and visible light, with the EE isomer possessing a S₁ ← S₀ electronic transition with a band maximum near 680 nm and shorter wavelength S₂ ← S₀ electronic transition with a band maximum near 430 nm. The study demonstrates the utility of the IMS-photo-IMS strategy for providing fundamental gas-phase photochemical information on molecular systems with multiple isomerizable bonds.

New Approach for Correcting Noncovalent Interactions in Semiempirical Quantum Mechanical Methods: The Importance of Multiple-Orientation Sampling

A new approach is presented to improve the performance of semiempirical quantum mechanical (SQM) methods in the description of noncovalent interactions. To show the strategy, the PM6 Hamiltonian was selected, although, in general, the procedure can be applied to other semiempirical Hamiltonians and to different methodologies. A set of small molecules were selected as representative of various functional groups, and intermolecular potential energy curves (IPECs) were evaluated for the most relevant orientations of interacting molecular pairs. Then, analytical corrections to PM6 were derived from fits to B3LYP-D3/def2-TZVP reference–PM6 interaction energy differences. IPECs provided by the B3LYP-D3/def2-TZVP combination of the electronic structure method and basis set were chosen as the reference because they are in excellent agreement with CCSD(T)/aug-cc-pVTZ curves for the studied systems. The resulting method, called PM6-FGC (from functional group corrections), significantly improves the performance of PM6 and shows the importance of including a sufficient number of orientations of the interacting molecules in the reference data set in order to obtain well-balanced descriptions.

AutoMeKin2021: An open-source program for automated reaction discovery

AutoMeKin2021 is an updated version of tsscds2018, a program for the automated discovery of reaction mechanisms (J. Comput. Chem. 2018, 39, 1922). This release features a number of new capabilities: rare-event molecular dynamics simulations to enhance reaction discovery, extension of the original search algorithm to study van der Waals complexes, use of chemical knowledge, a new search algorithm based on bond-order time series analysis, statistics of the chemical reaction networks, a web application to submit jobs, and other features. The source code, manual, installation instructions and the website link are available at: https://rxnkin.usc.es/index.php/AutoMeKin.

ChemDyME: Kinetically Steered, Automated Mechanism Generation through Combined Molecular Dynamics and Master Equation Calculations

In many scientific fields, there is an interest in understanding the way in which chemical networks evolve. The chemical networks which researchers focus upon have become increasingly complex, and this has motivated the development of automated methods for exploring chemical reactivity or conformational change in a "black-box" manner, harnessing modern computing resources to automate mechanism discovery. In this work, we present a new approach to automated mechanism generation which couples molecular dynamics and statistical rate theory to automatically find kinetically important reactions and then solve the time evolution of the species in the evolving network. The key to this chemical network mapping through combined dynamics and ME simulation approach is the concept of "kinetic convergence", whereby the search for new reactions is constrained to those species which are kinetically favorable at the conditions of interest. We demonstrate the capability of the new approach for two systems, a well-studied combustion system and a multiple oxygen addition system relevant to atmospheric aerosol formation.

Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF): Automatic Generation of Sum-of-Products Form Potential Energy Surfaces for Quantum Dynamical Calculations

We present Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF), an automated methodology for the generation of global potential energy surfaces (PES), molecular properties surfaces, e.g., dipole, polarizabilities, etc. using a single random geometry as input. The SRP-MGPF workflow integrates: (i) a fully automated procedure for the global topographical characterization of a (intermolecular) PES based on the Transition State Search Using Chemical Dynamical Simulations (TSSCDS) family of methods;i (ii) the global optimization of the parameters of a semiempirical Hamiltonian in order to reproduce a given level of electronic structure theory; and (iii) a tensor decomposition algorithm which turns the resulting SRP-PES into sum of products (Tucker) form with the Multigrid POTFIT algorithm. The latter is necessary for quantum dynamical studies within the Multiconfiguration Time-Dependent Hartree (MCTDH) quantum dynamics method. To demonstrate our approach, we have applied our methodology to the cis-trans isomerization reaction in HONO in full dimensionality (6D). The resulting SRP-PES has been validated through the computation of classical on-the-fly dynamical calculations as well as calculations of the lowest vibrational eigenstates of HONO as well as high-energy wavepacket propagations.

Reply to the 'Comment on "Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature"' by D. Heard, R. Shannon, J. Gomez Martin, R. Caravan, M. Blitz, J. Plane, M. Antiñolo, M. Agundez, E. Jimenez, B. Ballesteros, A. Canosa, G. El Dib, J. Albaladejo and J. Cernicharo, Phys. Chem. Chem. Phys., 2018, 20, DOI: 10.1039/C7CP04561A

In this Reply we answer the two main arguments raised in the Comment. The first argument is related to the binding energy of the methanol dimer and its influence on the dimerization rate constant. We show that the dimerization rate constants calculated in the Comment are unphysically low. We report values that are about two orders of magnitude higher than the values of the Comment, which confirm the conclusions of the original article that dimers can be present in a small amount. The second argument based on the dependence of the pseudo-first order rates on the methanol concentration was already explained in detail in the Supporting Information of the original article.

Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature

The kinetics of the reaction of methanol with hydroxyl radicals is revisited in light of the reported new kinetic data, measured in cold expansion beams. The rate constants exhibit an approximately 10(2)-fold increase when the temperature decreases from 200 to 50 K, a result that cannot be fully explained by tunneling, as we confirm by new calculations. These calculations also show that methanol dimers are much more reactive to hydroxyl than monomers and imply that a dimer concentration of about 30% of the equilibrium concentration can account quantitatively for the observed rates. The assumed presence of dimers is supported by the observation of cluster formation in these and other cold beams of molecules subject to hydrogen bonding. The calculations imply an important caveat with respect to the use of cold expansion beams for the study of interstellar chemistry.

An automated method to find reaction mechanisms and solve the kinetics in organometallic catalysis

A novel computational method is proposed in this work for use in discovering reaction mechanisms and solving the kinetics of transition metal-catalyzed reactions. The method does not rely on either chemical intuition or assumed a priori mechanisms, and it works in a fully automated fashion. Its core is a procedure, recently developed by one of the authors, that combines accelerated direct dynamics with an efficient geometry-based post-processing algorithm to find transition states (Martinez-Nunez, E., J. Comput. Chem.2015, 36, 222-234). In the present work, several auxiliary tools have been added to deal with the specific features of transition metal catalytic reactions. As a test case, we chose the cobalt-catalyzed hydroformylation of ethylene because of its well-established mechanism, and the fact that it has already been used in previous automated computational studies. Besides the generally accepted mechanism of Heck and Breslow, several side reactions, such as hydrogenation of the alkene, emerged from our calculations. Additionally, the calculated rate law for the hydroformylation reaction agrees reasonably well with those obtained in previous experimental and theoretical studies.

Effect of the Metal Ion on the Enantioselectivity and Linkage Isomerization of Thiosemicarbazone Helicates

The effect of the metal ion and ligand design on the enantioselectivity and linkage isomerization of neutral cobalt and zinc bisthiosemicarbazone metallohelicates has been investigated in this work. The electrochemical synthesis has afforded the enantioselective formation of chirally pure cobalt helicates, and the ΛΛ isomer of a single enantiomer has been crystallized as only product for the cobalt methyl-substituted thiosemicarbazone helicate. Interestingly linkage isomers have been formed from zinc ethyl-substituted thiosemicarbazone helicate enantiomers for the first time. The co-existence of these isomers has been evaluated from the point of view of both experimental results and computational calculations.

Photodissociation of acryloyl chloride at 193 nm: interpretation of the product energy distributions, and new elimination pathways

The use of an automated TS search method leads to the finding of novel HCl elimination pathways.

On the gas phase fragmentation of protonated uracil: a statistical perspective

The major fragmentation pathways of protonated uracil calculated in this work using statistical methods agree with mass spectrometry experiments.

HCN elimination from vinyl cyanide: product energy partitioning, the role of hydrogen–deuterium exchange reactions and a new pathway

A schematic diagram of HCN elimination channels from vinyl cyanide including a new CCdiss pathway.

An automated transition state search using classical trajectories initialized at multiple minima

TS finding using iterative TSSCDS with trajectories initialized at different minima.

Semiempirical Hamiltonian for simulation of azobenzene photochemistry

We present a semiempirical Hamiltonian that provides an accurate description of the first singlet and triplet potential energy surfaces of azobenzene for use in direct simulations of the excited-state dynamics. The parameterization made use of spectroscopic and thermochemical data and the best ab initio results available to date. Two-dimensional potential energy surfaces based on constrained geometry optimizations are presented for the states that are most relevant for the photochemistry of azobenzene, namely, S(0), S(1), and S(2). In order to run simulations of the photodynamics of azobenzene in hydrocarbons or hydroxylic solvents, we determined the interactions of methane and methanol with the azo group by ab initio calculations and fitted the interactions with a QM/MM interaction Hamiltonian.

Dynamics of Mg+ + H2O + He: capture, collisional stabilization and collision-induced dissociation

A laser flash photolysis technique and quasi-classical trajectory (QCT) calculations have been used to determine the rate coefficients for the title process. The experimental high-pressure-limiting rate coefficient is 7.0 x 10(-11) cm(3) s(-1) at T = 300 K, which compares with the computed QCT value for the Mg(+) + H(2)O capture rate of 2.75 +/- 0.08 x 10(-9) cm(3) s(-1) at the same temperature. The 39-fold difference between the experimental and simulation results is explained by further QCT calculations for the He + Mg(+).H(2)O* collision process. In particular, our simulation results indicate that collision-induced dissociation (CID) of the Mg(+).H(2)O* excited adduct is very likely compared with collisional stabilization (CS), which is an order of magnitude less likely. Including the relative rates of CID and CS in the calculation and assuming that those Mg(+).H(2)O* complexes that perform only one inner turning point in the dissociation coordinate are unlikely to be stabilized by CS, the computed rate coefficient compares well with the high-pressure experimental value.

Internal energy of HCl upon photolysis of 2-chloropropene at 193 nm investigated with time-resolved Fourier-transform spectroscopy and quasiclassical trajectories

Following photodissociation of 2-chloropropene (H(2)CCClCH(3)) at 193 nm, vibration-rotationally resolved emission spectra of HCl (upsilon < or = 6) in the spectral region of 1900-2900 cm(-1) were recorded with a step-scan time-resolved Fourier-transform spectrometer. All vibrational levels show a small low-J component corresponding to approximately 400 K and a major high-J component corresponding to 7100-18,700 K with average rotational energy of 39+/-(3)(11) kJ mol(-1). The vibrational population of HCl is inverted at upsilon = 2, and the average vibrational energy is 86+/-5 kJ mol(-1). Two possible channels of molecular elimination producing HCl + propyne or HCl + allene cannot be distinguished positively based on the observed internal energy distribution of HCl. The observed rotational distributions fit qualitatively with the distributions of both channels obtained with quasiclassical trajectories (QCTs), but the QCT calculations predict negligible populations for states at small J. The observed vibrational distribution agrees satisfactorily with the total QCT distribution obtained as a weighted sum of contributions from both four-center elimination channels. Internal energy distributions of HCl from 2-chloropropene and vinyl chloride are compared.

Representing and Selecting Vibrational Angular Momentum States for Quasiclassical Trajectory Chemical Dynamics Simulations

Linear molecules with degenerate bending modes have states, which may be represented by the quantum numbers N and L. The former gives the total energy for these modes and the latter identifies their vibrational angular momentum jz. In this work, the classical mechanical analog of the N,L-quantum states is reviewed, and an algorithm is presented for selecting initial conditions for these states in quasiclassical trajectory chemical dynamics simulations. The algorithm is illustrated by choosing initial conditions for the N = 3 and L = 3 and 1 states of CO2. Applications of this algorithm are considered for initial conditions without and with zero-point energy (zpe) included in the vibrational angular momentum states and the C-O stretching modes. The O-atom motions in the x,y-plane are determined for these states from classical trajectories in Cartesian coordinates and are compared with the motion predicted by the normal-mode model. They are only in agreement for the N = L = 3 state without vibrational angular momentum zpe. For the remaining states, the Cartesian O-atom motions are considerably different from the elliptical motion predicted by the normal-mode model. This arises from bend-stretch coupling, including centrifugal distortion, in the Cartesian trajectories, which results in tubular instead of elliptical motion. Including zpe in the C-O stretch modes introduces considerable complexity into the O-atom motions for the vibrational angular momentum states. The short-time O-atom motions for these trajectories are highly irregular and do not appear to have any identifiable characteristics. However, the O-atom motions for trajectories integrated for substantially longer period of times acquire unique properties. With C-O stretch zpe included, the long-time O-atom motion becomes tubular for trajectories integrated to approximately 14 ps for the L = 3 states and to approximately 44 ps for the L = 1 states.

Hydrogen Transfer vs Proton Transfer in 7-Hydroxy-quinoline·(NH3)3: A CASSCF/CASPT2 Study

Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol --> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under Cs-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1pipi* state and a pisigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for Cs-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1pipi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.

Direct-Dynamics VTST Study of the [1,7] Hydrogen Shift in 7-Methylocta-1,3(Z),5(Z)-triene. A Model System for the Hydrogen Transfer Reaction in Previtamin D3

Direct-dynamics canonical variational transition-state theory calculations with microcanonically optimized multidimensional transmission coefficient (CVT/muOMT) for tunneling were carried out at the MPWB1K/6-31+G(d,p) level to study the [1,7] sigmatropic hydrogen rearrangement in 7-methylocta-1,3(Z),5(Z)-triene. This compound has seven conformers, of which only one leads to products, although all of them have to be included in the theoretical treatment. The calculated CVT/muOMT rate constants are in good agreement with the available experimental data. To try to understand the role of tunneling in the hydrogen shift reaction, we have also calculated the thermal rate constants for the monodeuterated compound in the interval T = 333.2-388.2 K. This allowed us to evaluate primary kinetic isotope effects (KIEs) and make a direct comparison with the experiment. Our calculations show that both the large measured KIE and the large measured difference in the activation energies between the deuterated and root compounds are due to the quantum tunneling. The tunneling contribution to the KIE becomes noticeable only when the coupling between the reaction coordinate and the transverse modes is taken into account. Our results confirm previous experimental and theoretical works, which guessed that the obtained kinetic parameters pointed to a reaction with an important contribution due to tunneling. The above conclusion would be essentially valid for the case of the [1,7] hydrogen shift in previtamin D3 because of the similarity to the studied model system.

Rovibrational distributions of HF in the photodissociation of vinyl fluoride at 193 nm: a direct MP2 quasiclassical trajectory study

Quasiclassical trajectory calculations were performed to calculate rovibrational distributions of the nascent HF fragment in the photodissociation of vinyl fluoride at 193 nm. The trajectories were initiated at the transition states of the four-center (4C) and three-center (3C) HF elimination channels, using a microcanonical, quasiclassical normal-mode sampling. In general, the calculated distributions are in reasonably good agreement with experiment. In particular, the trajectory distributions show bimodal character, although not as pronounced as that observed experimentally. The calculations predict that the 3C and 4C distributions are rather similar to each other, which suggests that the low-J and high-J components of the rotational distributions cannot be specifically assigned to each of these channels.

Trajectory dynamics study of collision-induced dissociation of the Ar + CH4 reaction at hyperthermal conditions: vibrational excitation and isotope substitution

We investigate the role of vibrational energy excitation of methane and two deuterated species (CD(4) and CH(2)D(2)) in the collision-induced dissociation (CID) process with argon at hyperthermal energies. The quasi-classical trajectory method has been applied, and the reactive Ar + CH(4) system has been modeled by using a modified version of the CH(4) potential energy surface of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339) and the Ar-CH(4) intermolecular potential function obtained by Troya (J. Phys. Chem. A 2005, 109, 5814). This study clearly shows that CID is markedly enhanced with vibrational excitation and, to a lesser degree, with collision energy. In general, CID increases by exciting stretch vibrational modes of the reactant molecule. For the direct dissociation of CH(4), however, the CID cross sections appear to be essentially independent of which vibrational mode is initially excited. In all situations studied, the CID cross sections are always greater for the Ar + CD(4) reaction than for the Ar + CH(4) one, the Ar + CH(2)D(2) being an intermediate situation. A detailed analysis of the energy transfer processes, including their relation with CID, is also presented.

Quasiclassical trajectory calculations on the photodissociation of CF2CHCl at 193 nm: product energy distributions for the HF and HCl eliminations

Quasiclassical trajectory calculations were carried out to determine product energy distributions for the HCl and HF eliminations that take place in the photodissociation of 2-chloro-1,1-difluoroethylene at 193 nm. The trajectories were initiated at the transition states of the HCl and HF elimination channels under microcanonical, quasiclassical conditions, and were propagated with the energies and gradients taken directly from density functional theory calculations. Good agreement with experiment is found, except for the translational energy distribution of the HF elimination channel and the average vibrational energy of the HCl fragment. Possible sources of disagreement are discussed.

Quasiclassical Trajectory Study of the Collision-Induced Dissociation Dynamics of Ar + CH3SH+ Using an Ab Initio Interpolated Potential Energy Surface

Classical trajectory calculations have been performed to investigate the collision-induced dissociation (CID) of the CH(3)SH(+) cation with Ar atoms. A new intramolecular potential energy surface for the CH(3)SH(+) cation is evaluated by interpolation of 3000 ab initio data points calculated at the MP2/6-311G(d,p) level of theory. The new potential energy surface includes seven accessible dissociation channels of the cation. The present QCT calculations show that migration of hydrogen atoms, leading to the rearrangement CH(3)SH(+) <--> CH(2)SH(2)(+), is significant at the collision energies considered (6.5-34.7 eV) and that the formation of CH(3)(+), CH(3)S(+), and CH(2)(+) cations takes place primarily by a "shattering" mechanism in which the products are formed just after the collision. The theoretical product abundances are found to be in qualitative agreement with the experimental data. However, at high collision energies, the calculated total cross sections for the formation of CH(3)(+) and CH(2)SH(+) cations are noticeably larger than the experimental determinations. Several features of the dynamics of the CID processes are discussed.

Quasiclassical dynamics simulation of the collision-induced dissociation of Cr(CO)6+ with Xe

Quasiclassical trajectory calculations are employed to investigate the dynamics of collision-induced dissociation (CID) of Cr(CO)6 + with Xe atoms at collision energies ranging from 1.3 to 5.0 eV. The trajectory simulations show that direct elimination of CO ligands, during the collision, becomes increasingly important as the collision energy increases. In a significant number of cases, this shattering mechanism is accompanied with a concomitant formation of a transient Xe-Cr(CO)x +(x<6) complex. The calculated results are in very good agreement with the experimental results presented previously [F. Muntean and P. B. Armentrout, J. Chem. Phys. 115, 1213 (2001)]. In particular, the computed cross sections and scattering maps for the product ions Cr(CO)x +(x=3-5) compare very favorably with the reported experimental data. However, in contrast with the conclusions of the previous study, the present calculations suggest that CID dynamics for this system exhibits a significant impulsive character rather than proceeding via a complex surviving more than a rotational period.

Trajectory Dynamics Study of the Ar + CH4Dissociation Reaction at High Temperatures: the Importance of Zero-Point-Energy Effects

Large-scale classical trajectory calculations have been performed to study the reaction Ar + CH4--> CH3 +H + Ar in the temperature range 2500 < or = T/K < or = 4500. The potential energy surface used for ArCH4 is the sum of the nonbonding pairwise potentials of Hase and collaborators (J. Chem. Phys. 2001, 114, 535) that models the intermolecular interaction and the CH4 intramolecular potential of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339), which has been modified to account for the H-H repulsion at small bending angles. The thermal rate coefficient has been calculated, and the zero-point energy (ZPE) of the CH3 product molecule has been taken into account in the analysis of the results; also, two approaches have been applied for discarding predissociative trajectories. In both cases, good agreement is observed between the experimental and trajectory results after imposing the ZPE of CH3. The energy-transfer parameters have also been obtained from trajectory calculations and compared with available values estimated from experiment using the master equation formalism; in general, the agreement is good.

On the Conformational Memory in the Photodissociation of Formic Acid

The photodissociation of formic acid at 248 and 193 nm was investigated by classical trajectory and RRKM calculations using an interpolated potential energy surface, iteratively constructed using the B3LYP/aug-cc-pVDZ level of calculation. Several sampling schemes in the ground electronic state were employed to explore the possibility of conformational memory in formic acid. The CO/CO2 branching ratios obtained from trajectories initiated at the cis and at the trans conformers are almost identical to each other and in very good accordance with the RRKM results. In addition, when a specific initial excitation that simulates more rigorously the internal conversion process is used, the calculated branching ratio does not vary with respect to those obtained from cis and trans initializations. This result is at odds with the idea of conformational memory in the ground state proposed recently for the interpretation of the experimental results. It was also found that the calculated CO vibrational distributions after dissociation of the parent molecule at 248 nm are in agreement with the experimental available data.

Quasiclassical trajectory study of the collision-induced dissociation of CH[sub 3]SH[sup +]+Ar

Quasiclassical trajectory calculations were carried out to study the dynamics of energy transfer and collision-induced dissociation (CID) of CH(3)SH(+) + Ar at collision energies ranging from 4.34 to 34.7 eV. The relative abundances calculated for the most relevant product ions are found to be in good agreement with experiment, except for the lowest energies investigated. In general, the dissociation to form CH(3)(+) + SH is the dominant channel, even though it is not among the energetically favored reaction pathways. The results corroborate that this selective dissociation observed upon collisional activation arises from a more efficient translational to vibrational energy transfer for the low-frequency C-S stretching mode than for the high-frequency C-H stretching modes, together with weak couplings between the low- and high-frequency modes of vibration. The calculations suggest that CID takes place preferentially by a direct CH(3)(+) + SH detachment, and more efficiently when the Ar atom collides with the methyl group-side of CH(3)SH(+).