The Dissociation-Recombination System CH4 + M⇌CH3 + H + M: Reevaluated Experiments from 300 to 3000 K

Reconciling Accuracy and Feasibility for Barrierless Reaction Steps by the PCS/DDCI/MC-PDFT Protocol: Methane and Ethylene Dissociations as Case Studies

Several enhancements have been introduced into state-of-the-art computational protocols for the treatment of barrierless reaction steps in the framework of variable reaction coordinate variational transition state theory. The first step is the synergistic integration of the Iterative Difference Dedicated Configuration Interaction (I-DDCI) and Pisa Composite Scheme, which defines a reduced cost, yet very accurate, computational workflow. This approach provides a near black box tool for obtaining 1D reference potentials. Then, a general strategy has been devised for tuning the level of theory used in Monte Carlo (MC) sampling, employing Multiconfiguration Pair Density Functional Theory (MC-PDFT) with dynamically adjusted Hartree-Fock exchange. Concurrently, partial geometry optimizations during the MC simulations account for the coupling between the reaction coordinates and conserved modes. The protocol closely approaches full size consistency and yields highly accurate results, with several test computations suggesting rapid convergence of the I-DDCI correction with the basis set dimensions. The capabilities of the new platform are illustrated by two case studies (the hydrogen dissociation from CH4 and C2H4), which highlight its flexibility in handling different carbon hybridizations (sp3 and sp2). The remarkable accuracy of the computed rate constants confirms the robustness of the proposed method. Together with their intrinsic interest, these results pave the way for systematic investigations of complex gas-phase reactions through a reliable, user-friendly tool accessible to specialists and nonspecialists alike.

Quantum versus classical unimolecular fragmentation rate constants and activation energies at finite temperature from direct dynamics simulations

In the present work, we investigate how nuclear quantum effects modify the temperature dependent rate constants and, consequently, the activation energies in unimolecular reactions. In the reactions under study, nuclear quantum effects mainly stem from the presence of a large zero point energy. Thus, we investigate the behavior of methods compatible with direct dynamics simulations, the quantum thermal bath (QTB) and ring polymer molecular dynamics (RPMD). To this end, we first compare them with quantum reaction theory for a model Morse potential before extending this comparison to molecular models. Our results show that, in particular in the temperature range comparable with or lower than the zero point energy of the system, the RPMD method is able to correctly capture nuclear quantum effects on rate constants and activation energies. On the other hand, although the QTB provides a good description of equilibrium properties including zero-point energy effects, it largely overestimates the rate constants. The origin of the different behaviours is in the different distance distributions provided by the two methods and in particular how they differently describe the tails of such distributions. The comparison with transition state theory shows that RPMD can be used to study fragmentation of complex systems for which it may be difficult to determine the multiple reaction pathways and associated transition states.

Conversion of methane to benzene in CVI by density functional theory study

A density functional theory (DFT) study was employed to explore the mechanism of the conversion of methane to benzene in chemical vapor infiltration (CVI) based on the concluded reaction pathways from C₁-species to C₆-species. The geometry optimization and vibrational frequency analysis of all the chemical species and transition states (TS) were performed with B3LYP along with a basis set of 6–311 +G(d, p), and Gaussian 09 software was used to perform the study. The rate constants were calculated by KiSThelP according to the conventional transition state theory (TST), and the Wigner method was applied to acquire the tunneling correction factors. Then the rate constants were fitted to the modified Arrhenius expression in the temperature range of 800–2000 K. As for the barrierless reactions calculated in this paper, the rate constants were selected from the relating references. Through the energetic and kinetic calculations, the most favorable reaction pathway for benzene formation from methane was determined, which were mainly made of the unimolecular dissociation. The conversion trend from C₁-species to C₄-species is mainly guided by a strong tendency to dehydrogenation and the pathways from C₄-species to C₆-species are all presumed to be able to produce C₆H₆ molecule.

A theoretical study of three gas-phase reactions involving the production or loss of methane cations

Hydrocarbon ions are important species in flames, spectroscopy and the interstellar medium. Their importance is reflected in the extensive body of literature on the structure and reactivity of carbocations. However, the geometry, electronic structure and reactivity of carbocations are difficult to assess. This study aims to contribute to the current knowledge of this subject by presenting a quantum mechanics description of methane cation dissociation using multiconfigurational methods. The geometric and electronic parameters of the minimum structure were determined for three main reaction paths: the dissociation CH4(+)→ CH2(+) + H2 and the dissociation-recombination processes CH4(+)↔ CH3(+) + H. The electronic and energetic effects of these reactions were analyzed, and it was found that each reaction path has a strong dependence on the methodology used as well as a strong multiconfigurational character during dissociation. The first doublet excited states are inner-shell excited states and may correspond to the ions that are expected to be formed after electron detachment. The rate coefficient for each reaction path was determined using variational transition state theory and RRKM/master equation calculations. The major dissociation paths, with their rate coefficients at the high-pressure limit, are CH4(+)(X(~)(2)B1) → CH3(+)(A(2)A1') + H((2)S) (k∞(T) = 1.42 × 10(+14) s(-1) exp(-37.12/RT)) and CH4(+)(X(~)(2)B1) → CH2(+)(A(2)A1) + H2((2)Σg(+)) (k∞(T) = 9.18 × 10(+14) s(-1) exp(-55.77/RT)). Our findings help to explain the abundance of ions formed from CH4 in the interstellar medium and to build models of chemical evolution.

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.

The role of the radical-complex mechanism in the ozone recombination/dissociation reaction

The data bases for low-pressure rate coefficients of the dissociation of O3 and the reverse recombination of O with O2 in the bath gases M=He, Ar, N2, CO2 and SF6 are carefully analyzed. At very high temperatures, the rate constants have to correspond solely to the energy transfer (ET) mechanism. On condition that this holds for Ar and N2 near 800 K, average energies transferred per collision of -DeltaE/hc=18 and 25 cm-1 are derived, respectively. Assuming an only weak temperature dependence of DeltaE as known in similar systems, rate coefficients for the ET-mechanism are extrapolated to lower temperatures and compared with the experiments. The difference between measured and extrapolated rate coefficients is attributed to the radical complex (RC) mechanism. The derived rate coefficients for the RC-mechanism are rationalized in terms of equilibrium constants for equilibria of van der Waals complexes of O (or O2) with the bath gases and with rate coefficients for oxygen abstraction from these complexes. The latter are of similar magnitude as rate coefficients for oxygen isotope exchange which provides support for the present interpretation of the reaction in terms of a superposition of RC- and ET-mechanisms. We obtained rate coefficients for the ET-mechanism of k/[Ar]=2.3x10(-34) (T/300)(-1.5) and k/[N2]=3.5x10(-34) (T/300)(-1.5) cm6 molecule-2 s-1 and rate coefficients for the RC-mechanism of k/[Ar]=1.7x10(-34) (T/300)(-3.2) and k/[N2]=2.5x10(-34) (T/300)(-3.3) cm6 molecule-2 s-1. The data bases for M=He, CO2 and SF6 are less complete and only approximate separations of RC- and ET-mechanism were possible. The consequences of the present analysis for an analysis of isotope effects in ozone recombination are emphasized.

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.