Theoretical study of the dynamics of the H+CH4 and H+C2H6 reactions using a specific-reaction-parameter semiempirical Hamiltonian

We present a theoretical study of the reactions of hydrogen atoms with methane and ethane molecules and isotopomers. High-accuracy electronic-structure calculations have been carried out to characterize representative regions of the potential-energy surface (PES) of various reaction pathways, including H abstraction and H exchange. These ab initio calculations have been subsequently employed to derive an improved set of parameters for the modified symmetrically-orthogonalized intermediate neglect of differential overlap (MSINDO) semiempirical Hamiltonian, which are specific to the H+alkane family of reactions. The specific-reaction-parameter (SRP) Hamiltonian has then been used to perform a quasiclassical-trajectory study of both the H+CH4 and H+C2H6 reactions. The calculated values of dynamics properties of the H+CH4-->H2+CH3 reaction and isotopologues, including alkyl product speed distributions, diatomic product internal-state distributions, and cross sections, are generally in good agreement with experiment and with the results provided by the ZBB3 PES [Z. Xie et al., J. Chem. Phys. 125, 133120 (2006)]. The results of trajectories propagated with the SRP Hamiltonian for the H+C2H6-->H2+C2H5 reaction also agree with experiment. The level of agreement between the results calculated with the SRP Hamiltonian and experiment in both the H+methane and H+ethane reactions indicates that semiempirical Hamiltonians can be improved for not only a specific reaction but also a family of reactions.     

自由基HO2与C2H2反应势能面的理论研究

应用量子化学从头计算和密度泛函理论(DFT)对HO2+C2H2反应体系的反应机理进行了研究.在B3LYP/6-311G**和CCSD(T)/6-311G**水平上计算了HO2+ C2H2反应的二重态反应势能面.计算结果表明,主要反应方式为自由基HO2的H原子和C2H2分子中的C原子结合,经过一系列异构化,最后分解得到主要产物P1 (CH2O+ HCO).此反应是放热反应,化学反应热为-321.99 kJ·mol-1.次要产物为P2 (CO2 +CH3),也是放热反应.     

Ab initio investigation of the potential energy surfaces of C2H2F2 and C2H2F2+

Ab initio MO calculations have been performed for neutral and cationic C₂H₂F₂ structures. Olefinic and carbene structures are investigated for the neutral isomers, while olefinic, carbene, and fluoronium-type cations are found. Stability orders and rotational barriers are discussed in terms of orbital and Coulomb interaction. Contrary to previous studies, the higher stability of the geminal isomers is interpreted to be caused by Coulomb attraction.     

Multiple photon induced chemistry in mixtures of C2F6 with H2 or C6H14

CO₂ laser induced chemistry is demonstrated to occur efficiently in mixtures of C₂F₆ with H₂ or C₆H₁₄ at a fluence of 6 J/cm² and for a 30 cm^-1 red-shift from the C₂F₆ absorption maximum. H₂ or C₆H₁₄ pressure may be used to control the distribution of product species.     

Theoretical study of the reaction of CN with C2H2+

 A theoretical study of the reaction of CN with C₂H₂ ⁺ has been carried out at three levels of theory, namely G2, B3LYP and CCSD(T). The main conclusion is that this is a feasible process under interstellar conditions, but only linear species may be produced. The most favourable product is HCCCN⁺, followed by CCCNH⁺. Production of HCCNC⁺ is predicted to be slightly endothermic; therefore, the reaction of CN + C₂H₂ ⁺ may produce precursors of HC₃N and C₃N in space. Furthermore, the B3LYP level is found to perform rather well compared with G2 and even better than CCSD(T). Received: 14 September 1999 / Accepted: 3 February 2000 / Published online: 12 May 2000     

Photodissociation dynamics of fluorobenzene (C6H5F) at 157 and 193 nm: Branching ratios and distributions of kinetic energy

Following photodissociation of fluorobenzene (C6H5F) at 193 and 157 nm, we detected the products with fragmentation-translational spectroscopy by utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Between two primary dissociation channels observed upon irradiation at 193 (157) nm, the HF-elimination channel C6H5F --> HF + C6H4 dominates, with a branching ratio of 0.94+/-0.02 (0.61+/-0.05) and an average release of kinetic energy of 103 (108) kJ mol(-1); the H-elimination channel C6H5F --> H + C6H4F has a branching ratio of 0.06+/-0.02 (0.39+/-0.05) and an average release of kinetic energy of 18.6 (26.8) kJ mol(-1). Photofragments H, HF, C6H4, and C6H4F produced via the one-photon process have nearly isotropic angular distributions. Both the HF-elimination and the H-elimination channels likely proceed via the ground-state electronic surface following internal conversion of C6H5F; these channels exhibit small fractions of kinetic energy release from the available energy, indicating that the molecular fragments are highly internally excited. We also determined the ionization energy of C6H4F to be 8.6+/-0.2 eV.     

Charge transfer in S2++H collisions at intermediate energy

Partial cross-sections for the charge transfer process of S²⁺ ions in collision with atomic hydrogen at impact energies up to 8 keV have been calculated by means of a semi-classical method using ab initio potential energy curves and couplings. The results are in relatively good agreement with experiment and improve significantly previous Landau-Zener calculations.     

Theoretical study of the low-lying States of Fe2-(C6H6)3

Using the gradient-corrected BPW91 method and 6311++G(2d,2p) basis sets, it was found that adsorption of benzene by iron atoms forms a multiple-decker sandwich (MDS) geometry for the ground state (GS) of Fe(2)-(C(6)H(6))(3), as Fe(2) is broken. Though decoordination occurs, the ligands are bonded symmetrically to the Fe sites by η(6) (for the two external rings) and two η(3) (for the central ring) Fe-C coordinations; this big amount of Fe-C bonds enhances the stability of the MDS GS. This is unexpected, as the experiment suggests MDS for TM(n)-(C(6)H(6))(m) species of earlier transition metals (TMs) and clusters covered with benzene, i.e., rice-ball (RB) structures, for late 3d atoms. However, preserving Fe(2), an RB state was found, quasi-degenerate with the GS, with a smaller amount of Fe-C contacts and a stronger Fe(2) bond. The MDS shows higher stability for electron attachment and deletion events, but its adiabatic electron affinity, 1.11 eV, differs more from the experiment (0.80 ± 0.1 eV) than the one (0.97 eV) for RB. Thus, MDS and RB states can appear in a sample of Fe(2)-(C(6)H(6))(3). Like the electron affinity, the ionization energy of the complex is also smaller than those of the metal and benzene moieties, but closer to the former, signifying that the electron, delocalized through the 3d-π bonds, is mainly deleted from the metallic units.     

An ab-initio study of the C3H6-HX, C2H4-HX and C2H2-HX hydrogen-bonded complexes with X=F or Cl.

MP2/6-31G** ab-initio molecular orbital calculations have been performed to obtain geometries, H-bond energies and vibrational properties of the C3H6-HX, C2H4-HX and C2H2-HX H-bonded complexes with X=F or Cl. The more pronounced effects on the structural parameters of the isolated molecules due to complexation are verified to the CC and HX bond lengths, which are directly involved in the H-bond formation. They are increased after complexation. The calculated H-bond lengths for the hydrogen complexes for X=F are shorter than those for x-Cl by about 0.55 A, whereas the corresponding experimental value is 0.58 A. The H-bond energies are essentially determined by the nature of the proton donor molecule. For X=F, the AE mean value is 20 kJ/mol, whereas it is approximately 14.5 kJ/mol for X-Cl. The H-bond energies including zero-point corrections show a good correlation with the H-bond lengths. The more pronounced effect on the normal modes of the isolated molecules after complexation occurs to the H-X stretching mode. The H-X stretching frequency is shifted downward, whereas its IR intensity is much enhanced upon H-bond formation. The new vibrational modes arising from complexation show several interesting features.     

Theoretical studies of hyperthermal O(3P) collisions with hydrocarbon self-assembled monolayers

We present a dynamics study of inelastic and reactive scattering processes in collisions of hyperthermal (5 eV) O(3P) atoms with a hydrocarbon self-assembled monolayer (SAM). Molecular-dynamics simulations are carried out using a quantum mechanics/molecular mechanics (QM/MM) interaction potential that uses a high quality semiempirical Hamiltonian for the QM part and the MM3 force field for the MM part. A variety of products coming from reaction are identified, including H abstraction to generate OH, O atom addition to the SAM with subsequent elimination of H atoms, and direct C-C breakage. The C-C breakage mechanism provides a pathway for significant surface mass loss in single reactive events whereas the O addition-H elimination channel leads to surface oxidation. Reaction probabilities, product energy, and angular distributions are examined to gain insight on polymer erosion in low Earth orbit conditions and on fundamentals of inelastic and reactive hyperthermal gas-surface interactions.     

Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane

Crossed molecular beams experiments and classical trajectory calculations have been used to study the dynamics of Ar+ethane collisions at hyperthermal collision energies. Experimental time-of-flight and angular distributions of ethane molecules that scatter into the backward hemisphere (with respect to their original direction in the center-of-mass frame) have been collected. Translational energy distributions, derived from the time-of-flight distributions, reveal that a substantial fraction of the collisions transfer abnormally large amounts of energy to internal excitation of ethane. The flux of the scattered ethane molecules increased only slightly from directly backward scattering to sideways scattering. Theoretical calculations show angular and translational energy distributions which are in reasonable agreement with the experimental results. These calculations have been used to examine the microscopic mechanism for large energy transfer collisions ("supercollisions"). Collinear ("head-on") or perpendicular ("side-on") approaches of Ar to the C-C axis of ethane do not promote energy transfer as much as bent approaches, and collisions in which the H atom is "sandwiched" in a bent Ar...H-C configuration lead to the largest energy transfer. The sensitivity of collisional energy transfer to the intramolecular potential energy of ethane has also been examined.     

An eight-degree-of-freedom quantum dynamics study for the H2+C2H system

An eight-degree-of-freedom (8DOF) time-dependent wave-packet approach has been developed to study the H(2)+C(2)H-->H+C(2)H(2) reaction system. The 8DOF model is obtained by fixing one of the Jacobi torsion angle in the nine-degree-of-freedom AB+CDE reaction system. This study is an extension of the previous seven-degree-of-freedom (7DOF) computation [J. Chem. Phys. 119, 12057 (2003)] of this reaction system. This study shows that vibrational excitations of H(2) enhance the reaction probability, whereas the stretching vibrational excitations of C(2)H have only a small effect on the reactivity. Furthermore, the bending excitation of C(2)H, compared to the ground-state reaction probability, hinders the reactivity. A comparison of the rate constant between the 7DOF calculation and the present 8DOF results has been made. The theoretical and experimental results agree with each other very well when the present 8DOF results are adjusted to account for the lower transition state barrier heights found in recent ab initio calculations.     

Theoretical study of HCN(+) + C2H2 reaction

A detailed theoretical investigation for the ion-molecule reaction of HCN (+) with C 2H 2 is performed at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are probed. It is shown that eight dissociation products P 1 (H 2C 3N (+)+H), P 2 (CN+C 2H 3 (+)), P 3 (HC 3N (+)+H 2), P 4 (HCCCNH (+)+H), P 5 (H 2NCCC (+)+H), P 6 (HCNCCH (+)+H), P 7 (C 2H 2 (+)+HCN), and P 8 (C 2H 2 (+)+HNC) are both thermodynamically and kinetically accessible. Among the eight dissociation products, P 1 is the most abundant product. P 7 and P 3 are the second and third feasible products but much less competitive than P 1 , followed by the almost negligible product P 2 . Other products, P 4 (HCCCNH (+)+H), P 5 (HCNCCH (+)+H), P 6 (H 2NCCC (+)+H), and P 8 (C 2H 2 (+)+HNC) may become feasible at high temperatures. Because the intermediates and transition states involved in the reaction HCN (+) + C 2H 2 are all lower than the reactant in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. The present calculation results may provide a useful guide for understanding the mechanism of HCN (+) toward other pi-bonded molecules.     

A Theoretical and Experimental Study of the Interaction of C6F6 with Electron Donors

The NMR effects produced on the nitrogen absolute shieldings in a series of electron donors when they interact with hexafluorobenzene, C₆F₆, have been theoretically studied. The complexes have been optimized at the B3LYP/6-311++G** level and the NMR shieldings have been calculated using the GIAO method. The results obtained have allowed devising an experiment (C₆F₆···NCCH₃ complex) that is compatible with the theoretical calculations.     

On the position of electronic states of the C6H6 and C6H5F+ ions

The sigma ionization potentials of benzene are shown to be 10.3 and 10.8 eV. These states are not detected by photoelectron spectroscopy using 21.21 eV photons. An assignment for the lowest electronic states of C₆H⁺₆ and C₆H₅F⁺ is given.     

Close-coupling study of rotational energy transfer and differential scattering in H2O collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer (RET) of rotationally excited H(2)O due to collisions with He are presented for collision energies between 10(-6) and 1000 cm(-1) with para-H(2)O initially in levels 1(1,1), 2(0,2), 2(1,1), and 2(2,0) and ortho-H(2)O in levels 1(1,0), 2(1,2), and 2(2,1). Quenching cross sections and rate coefficients for state-to-state RET were computed. Both elastic and inelastic differential cross sections are also calculated and compared with relative experimental results giving generally good agreement in all cases, but less so for inelastic results. Significant differences in the computed collisional parameters, obtained on three different potential energy surfaces (PESs), were found particularly in the ultracold regime. In the thermal regime, the rate coefficients calculated on each of the surfaces are generally in better agreement and comparable, but typically larger, than those obtained in a previous calculation. Unfortunately, a lack of absolute differential or integral inelastic experimental data prevents firm determination of a preferred PES.     

Mixed quantum-classical reaction path dynamics of C2H5F --> C2H4 + HF

A mixed quantum-classical method for calculating product energy partitioning based on a reaction path Hamiltonian is presented and applied to HF elimination from fluoroethane. The goal is to describe the effect of the potential energy release on the product energies using a simple model of quantized transverse vibrational modes coupled to a classical reaction path via the path curvature. Calculations of the minimum energy path were done at the B3LYP/6-311++G(2d,2p) and MP2/6-311++G** levels of theory, followed by energy-partitioning dynamics calculations. The results for the final HF vibrational state distribution were found to be in good qualitative agreement with both experimental studies and quasiclassical trajectory simulations.