A time-dependent wave-packet quantum scattering study of the reaction H2+(v = 0-2,4,6;j = 1) + He--> HeH+ + H

The quantum scattering dynamics calculation was carried out for the titled reaction in the collision energy range of 0.0-2.4 eV with reactant H(2) (+) in the rotational state j = 1 and vibrational states v = 0-2, 4, and 6. The present time-dependent wave-packet calculation takes into account the Coriolis coupling (CC) and uses the accurate ab initio potential-energy surface of Palmieri et al. [Mol. Phys. 98, 1835 (2000)]. The importance of including the CC quantum scattering calculation has been revealed by the comparison between the CC calculation and the previous coupled state (CS) calculation. The CC total cross sections for the v = 2, 4, and 6 states show collision energy-dependent behaviors different from those based on the CS calculation. Furthermore, the collision energy dependence of the total cross sections obtained in the present CC calculation only exhibits minor oscillations, indicating that the chance is slim for reactive resonances in total cross sections to survive through the partial-wave averaging. The magnitude and profile of the CC total cross sections for v = 0-2 in the collision energy range of 0.0-2.5 eV are found to be consistent with experimental cross sections obtained recently by Tang et al. [J. Chem. Phys. 122, 164301 (2005)] after taking into account the experimental uncertainties.     

Finite element investigation of the ground states of the helium trimers 4He3 and 4He2–3He

A three‐dimensional finite element method is applied to the ground states of the symmetric and asymmetric atomic helium trimers ⁴He₃ and ⁴He₂–³He. Three different He–He interaction potentials of hard‐core nature were studied. Two extrapolation procedures based on the convergence properties of the finite element method are investigated. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Nonadiabatic dynamics in the CH3+HCl-->CH4+Cl(2PJ) reaction

Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl(2P(3/2)) and Cl*(2P(1/2)) products. Reaction was initiated by photodissociation of CH(3)I at 266 nm within a single expansion of a dilute mixture of CH(3)I and HCl in argon, giving a mean collision energy of 7800 cm(-1) in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl(2P(J)) atoms formed in the J=1/2 level at this collision energy was 0.150+/-0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH(4)+Cl(2P(3/2)) products.     

Charge transfer mechanism in N4+ + He and B3+ + He/H2 ion–atom/molecule collisions

Ab initio potential energy curves and coupling matrix elements of the molecular states involved in the collision of the N⁴⁺(2s)²S and B³⁺(1s²)¹S multicharged ions on the helium atom or molecular hydrogen have been determined by means of configuration interaction methods. The total and partial electron capture cross-sections have been evaluated in the frame of a semiclassic approach in the 1- to 100-keV laboratory energy range and compared to experimental data. A comparative study of both targets is performed in the case of the B³⁺ ion. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Dynamical studies of the reaction H~++He+He→HHe~++He

The classical trajectory method is employed to calculate the rate coefficient kr for the reaction H++He+He-HHe++He at the temperatures ranging 200-350 K, based on an ab initio potential energy surface. The results show that kr is strongly dependent on the temperature, which can be well fitted by the function kr=ATDr-3 with A=4.192x10-31 cm6/s and the reaction dimension Dr=2.706. The product molecules HHe+ are found in high vibrational states.     

Accurate potential energy surface and quantum reaction rate calculations for the H+CH4-->H2+CH3 reaction

Calculations for the cumulative reaction probability N(E) (for J=0) and the thermal rate constant k(T) of the H+CH(4)-->H(2)+CH(3) reaction are presented. Accurate electronic structure calculations and a converged Shepard-interpolation approach are used to construct a potential energy surface which is specifically designed to allow the precise calculation of k(T) and N(E). Accurate quantum dynamics calculations employing flux correlation functions and multiconfigurational time-dependent Hartree wave packet propagation compute N(E) and k(T) based on this potential energy surface. The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step. These tests demonstrate the predictive power of the present calculations. In addition, approximate approaches for reaction rate calculations are discussed. A quite accurate approximation can be obtained from a potential energy surface which includes only interpolation points on the minimum energy path.     

Hybrid QM/MM molecular dynamics simulations for an ionic SN2 reaction in the supercritical water: OH- + CH3Cl --> CH3OH + Cl-

A hybrid real space quantum mechanical/molecular mechanical (RS-QM/MM) method has been applied to an ionic S(N)2 reaction (OH- + CH3Cl --> CH3OH + Cl-) in water solution to investigate dynamic solvation effects of the supercritical water (SCW) on the reaction. It has been demonstrated that the approaching process of OH- to methyl group is prevented by water molecules in the ambient water (AW), while the reaction takes place easily in the gas phase. Almost the same solvation effect on the dynamics of OH- is observed in the SCW, though the bulk density of water is substantially reduced compared with that of the AW. It has been shown that the solvation of the SCW around the OH anion is locally identical to that of the AW due to the strong ion-dipole interactions between OH- and water molecules. At the transition state, the QM/MM simulations have revealed that the excess electron is quite flexible, and the charge volume, as well as the fractional charges on atoms, vary seriously depending on the instantaneous solvent configurations. However, it has been found that the solvation energy in the SCW can be qualitatively related to the HOMO volume of the system by Born's equation.     

Modeling the fragmentation dynamics of ionic clusters inside helium nanodroplets: the case of He100Ne4+

We present simulation results on the effect of a helium nanodroplet environment on the fragmentation dynamics of embedded molecular systems. The helium atoms are treated explicitly, with zero-point effects taken into account through an effective helium-helium interaction potential. The ionized neon tetramer is used as a model molecular system because, like all the small rare-gas clusters, it fragments extensively upon ionization. All the nonadiabatic effects between electronic states of the ionized neon cluster are taken into account. The results reveal a predominance of Ne2+ and HepNe2+ fragments and the absence of bare Ne+ fragments, in agreement with available experimental data. The neutral monomer fragments exhibit a rather wide kinetic energy distribution that can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experimental results. Purely classical calculations are shown to strongly overestimate the amount of cage effect (cooling), clearly indicating the need to take into account zero-point effects.     

The homogeneous reaction CD4 + NH3 → CD3H + NH2D: The effect of ethane impurities

The homogeneous exchange reaction between tetradeutero methane and ammonia was studied behind reflected shocks in a single-pulse shock tube over the temperature range of 1300–1800°K. The rate of production of CD₃H at the early stages of the reaction in mixtures ranging between 1-4.5% NH₃ and 1–4.3% CD₄ in argon is given by d[CD₃H]/dt=k_b [CD₄]₀[NH₃]₀, where k_b=8 × 10¹⁶ exp (?65.3 × 10³/RT) cm³/mole·sec. This activation energy is considerably lower than the one that may be expected on the basis of a pure free radical mechanism. It is rationalized by C₂D₆ impurities in the methane. No clear answer can be obtained regarding the role of a four-center intermediate in this reaction.     

Ab initio study of the mechanism of the atmospheric reaction: NO2 + O3 --> NO3 + O2

The atmospheric reaction NO2 + O3 --> NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single-reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5-6.1 kcal mol(-1), in reasonable agreement with experimental data. The calculated reaction enthalpy is -24.6 kcal mol(-1) and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 x 10(-18) cm(3) molecule(-1) s(-1). Both results can be regarded also as accurate predictions of the methodology employed in this article.     

Mechanism and kinetics of the reaction NO3 + C2H4

The reaction of NO(3) radical with C(2)H(4) was characterized using the B3LYP, MP2, B97-1, CCSD(T), and CBS-QB3 methods in combination with various basis sets, followed by statistical kinetic analyses and direct dynamics trajectory calculations to predict product distributions and thermal rate constants. The results show that the first step of the reaction is electrophilic addition of an O atom from NO(3) to an olefinic C atom from C(2)H(4) to form an open-chain adduct. A concerted addition reaction mechanism forming a five-membered ring intermediate was investigated, but is not supported by the highly accurate CCSD(T) level of theory. Master-equation calculations for tropospheric conditions predict that the collisionally stabilized NO(3)-C(2)H(4) free-radical adduct constitutes 80-90% of the reaction yield and the remaining products consist mostly of NO(2) and oxirane; the other products are produced in very minor yields. By empirically reducing the barrier height for the initial addition step by 1 kcal mol(-1) from that predicted at the CBS-QB3 level of theory and treating the torsional modes explicitly as one-dimensional hindered internal rotations (instead of harmonic oscillators), the computed thermal rate constants (including quantum tunneling) can be brought into very good agreement with the experimental data for the overall reaction rate constant.     

Product angular distribution for the H + CD4 --> HD + CD3 reaction

Using an analytical potential energy surface previously developed by our group, namely PES-2002, we analyzed the gas-phase reaction between a hydrogen atom and perdeuterated methane. We studied the effect of quasiclassical trajectory (QCT) and reduced dimensionality quantum-scattering (QM) calculations, with their respective limitations, on CD3 product angular distributions in the collision energy range 16.1-46.1 kcal x mol(-1). It was found that at low collision energy, 16.1 kcal x mol(-1), both the QCT and QM calculations yielded forward scattered CD3 products, i.e., a rebound mechanism. However, at high energies only the QM calculations on the PES-2002 surface reproduced the angular scattering found experimentally.     

Homopolymerization and [2 + 4]/[3 + 2] cycloaddition in the reaction of nitroethylene with vinyl ethers

The reaction of nitroethylene with isobutyl vinyl ether was reinvestigated. In agreement with the report of Kushibiki, Irie, and Hayashi (J Polym Sci Polym Chem Ed 1975, 13, 77), cycloaddition accompanied the anionic homopolymerization of nitroethylene. However, the cycloadduct was not a cyclobutane but a bicyclic oxazolidine, in keeping with the more recent report by Denmark and Hurd (J Org Chem 1998, 63, 3045). Cycloaddition accompanied by the anionic homopolymerization of nitroethylene was general for a series of vinyl and silyl ethers. Mechanistically, the cycloadducts formed by a cis–syn concerted approach of the reactants, whereas a trans–anti approach gave a zwitterionic tetramethylene that initiated anionic homopolymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1886–1891, 2001     

Mode correlation of product pairs in the reaction OH+CD4-->HOD+CD3

The hydrogen abstraction reaction from methane by a hydroxyl radical produces two polyatomic molecules. Each product has several vibrational modes that characterize distinct, concerted motions of the constituent atoms of the molecule. This communication describes the first measurement that maps out the coincident information on how the mode of excitation of one product varies with that of the other co-product. Such information on mode correlation of product pairs is particularly appealing in that it provides intuitively a glimpse of the reaction paths by which the chemical transformation occurs.     

Imaging photon-initiated reactions: a study of the Cl(2P(3/2))+CH4-->HCl+CH3 reaction

The hydrogen or deuterium atom abstraction reactions between Cl((2)P(3/2)) and methane, or its deuterated analogues CD(4) and CH(2)D(2), have been studied at mean collision energies around 0.34 eV. The experiments were performed in a coexpansion of molecular chlorine and methane in helium, with the atomic Cl reactants generated by polarized laser photodissociation of Cl(2) at 308 nm. The Cl-atom reactants and the methyl radical products were detected using (2+1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. Analysis of the ion images reveals that in single-beam experiments of this type, careful consideration must be given to the spread of reagent velocities and collision energies. Using the reactions of Cl with CH(4), CD(4), and CH(2)D(2), as examples, it is shown that the data can be fitted well if the reagent motion is correctly described, and the angular scattering distributions can be obtained with confidence. New evidence is also provided that the CD(3) radicals from the Cl+CD(4) reaction possess significant rotational alignment under the conditions of the present study. The results are compared with previous experimental and theoretical works, where these are available.     

CAS SCF/CI calculations of potential energy surfaces of He3+ and He2+

Complete active space MC SCF (CAS SCF) calculations followed by second-order configuration interaction (SOCI) calculations are carried out on the potential energy surfaces (bending surface, linear surfaces) of the ²Σ_g⁺ ground state of He₃⁺. The potential minimum for the ²Σ_g⁺ state occurs at a linear geometry with HeHe bond length of 1.248 Å. The binding energy of He₃⁺ with respect to He + He⁺ + He was calculated to be 2.47 eV at the SOCI level. The energy required to dissociate He₃⁺ (²Σ_g⁺) into He₂⁺ (²Σ_u⁺) and He(¹S) is calculated to be 0.14 eV. The same level of SOCI calculations of He₂⁺ yield a D_e value of 2.36 eV.     

Angle-energy distributions of Penning ions in crossed molecular beams. IV. He*(21 S,2 3S)+H2-->He+H2+ +e-

Relative doubly differential cross sections for the Penning ionization of H(2) by spin-state-selected metastable He (1s2s) are reported at center-of-mass collision energies E of 3.1 and 4.2 kcal/mol in a crossed supersonic beam experiment employing a rotatable mass spectrometer detector. The measurements are sufficiently dense in velocity space as to avoid having to functionalize the differential cross sections in order to transform the intensities into the c.m. The H(2) (+) product is scattered sharply forward, c.m. Deltatheta<10 degrees half-width at half-maximum, with respect to the incident direction of H(2) at both energies for both spin states. On the average the products have lost energy upon recoil, mean recoil energy E(')相似文献   

Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.     

Quantifying the non-RRKM effect in the H + O2 ⇄ OH + O reaction

In the present investigation the non-RRKM behavior in the title reaction is quantified in two different ways: (1) Quasiclassical trajectory calculations of the thermal rate coefficient are compared with results from a microcanonical variational transition-state theory/RRKM model. Results on both the Varandas DMBE IV and Melius-Blint potentials indicate that the non-RRKM behavior acts to reduce the thermal rate coefficient by about a factor of two, independent of temperature from 250 K to 5500 K. The QCT thermal rate coefficients on the two potentials are in remarkably good agreement with experiment and with each other over the entire temperature range. (2) The non-RRKM behavior as a classical phenomenon is demonstrated and quantified on both potentials by a direct test of the fundamental assumption. Complex-forming classical trajectories, started as either O + OH or H + O₂, are shown preferentially to return to the region of configuration space from which they were started. This test is discussed in detail in the text. The transition of the non-RRKM behavior from classical to quantum mechanics is also discussed. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 275–287, 1997.