Competition between dissociation and exchange processes in a collinear A + BC collision. I. Exact quantum results

Exact quantum results for collision-induced dissociation on a reactive surface are presented. A modified LEPS potential-energy surface modeling the H + HD → H₂ + D system has been used. HD and H₂ bearing respectively 7 and 6 bound states. This system has been chosen because it displays significant reactive scattering for total energies above the dissociation threshold. Calculations have been performed using the time-dependent wavepacket method for two initial vibrational quantum numbers of the HD molecule (v = 0, 2). For each vibrational quantum number, two wavepackets with overlapping energy distributions have been run, covering a total energy range up to more than three times the dissociation energy. Comparison with previous collision-induced dissociation calculations shows that the dissociation is greatly enhanced by the presence of concomitant reactive scattering. A vibrational enhancement effect is also observed above the dissociation threshold; for higher energies the system exhibits a pronounced vibrational inhibition effect.     

Long-range interactions in molecules and rotational coupling

We report a case study on Na₂ of the influence of rotational coupling for molecular states directly below the dissociation limit, where the electronic binding energy and the hyperfine interactions are of similar magnitude and the rotational energy can be varied from small to large compared to the former energies. The experimental observation and the theoretical analysis are important for obtaining precise data concerning long-range interactions and extrapolation to the dissociation limit, which are required for describing cold collisions in atomic traps. A consistent model for all observations with rotational quantum number J′ up to 41 is developed which involves few atomic parameters and demonstrates that these are sufficient to describe molecular levels few μeV below the dissociation energy.     

Quantum dynamics of dissociative adsorption of H_2 and D_2:The time-dependent wave packet method

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Quantum dynamics of dissociative adsorption of H2 and D2: The time-dependent wave packet method

A time-dependent quantum wave packet method was used to study the dynamics of dissociative adsorption of H₂ and D₂ on a flat and static surface. The molecule-surface interaction is described using a modified London-Eyring-Polanyi-Sato (LEPS) type potential for the H₂/Ni(100) system. The three-dimensional (3-D) dissociation probabilities were calculated for different initial rovibrational states as a function of initial incident energies. Our results show that the dissociation of the diatomic rotational states whose quantum numbers satisfyj+m = odd is forbidden at low energies for the homonuclear H_z and D₂ due to the selection rule. The effect of the rotational orientation of diatoms on adsorption predicts that the in-plane rotation (m = j) is more favorable for dissociation than the out-of-plane rotation (m = 0). Enhanced dissociation for vibrationally excited molecules and the significant enhancement of the dissociation probability of H2 when compared to D₂ were explained reasonably in terms of quantum mechanical zero-point energies, the tunneling effect and the reflection from an activation barrier. Project supported by the National Natural Science Foundation of China (Grant No. 19694033) and partially by the Science Foundation for Overseas Chinese Scholars and Students, administered by the State Education Commission of China (Grant No. 1992), by the State Key Laboratory of Theoretical and Computational Chemistry of Jilin University at Changchun (Grant No. 98011, and by the Natural Science Foundation of Shandong Province (Grant No. Y96B03022)     

D_2在Ni(100)表面离解吸附的量子动力学研究

用量子含时波包法研究了Ｄ２在镍表面顶－桥位上离解吸附量子动力学．计算了不同入射动能及初始振转态的离解几率．讨论了分子的同核对称性、转动取向和振动激发对离解几率的影响，并与其他理论计算结果做了比较．     

Spectroscopic calculations of CH and OH bond dissociation energies for aldehydes, ketones, acids, and alcohols

CH and OH bond dissociation energies were calculated by the spectroscopic and quantum-chemical methods for aldehydes, ketones, acids, and alcohols. The spectroscopic values of CH and OH bond dissociation energies were obtained from the fundamental absorption bands by the variational method in an anharmonic approximation using the Morse-anharmonic basis set. Quantum-chemical calculations were carried out using the 6-311G(3df,3pd)/B3LYP basis set. It is discussed how the bond dissociation energies change with the structure of the molecule.     

A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET‐LRP of vinyl monomers

A quantum‐chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron‐withdrawing groups as well as for a nonactivated monomer containing an electron‐donor group. Two examples of sulfonyl and N‐halide initiators are also reported. The homolytic inner‐sphere electron‐transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET‐LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron‐withdrawing groups in SET‐LRP. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1607–1618, 2007     

Spectroscopic calculations of CH bond dissociation energies for ethene, propene, and benzene chlorine derivatives

CH bond dissociation energies have been determined by spectroscopic and quantum-chemical calculations for ethane, propene, and benzene chlorine derivatives. The spectroscopic values of CH bond dissociation energies were obtained from the fundamental absorption bands in an anharmonic approximation using the variation method and the Morse harmonic basis. Quantum-chemical calculations were carried out using the 6-311G(3df,3pd)/B3LYP basis. The resulting tendencies of variation of bond dissociation energies are discussed in relation to changes in the structure of the molecule.     

Striving to understand the photophysics and photochemistry of thiophosgene: a combined CASSCF and MR-CI study

The potential energy surfaces for Cl(2)CS dissociation into ClCS + Cl in the five lowest electronic states have been determined with the combined complete active space self-consistent field (CASSCF) and MR-CI method. The wavelength-dependent photodissociation dynamics of Cl(2)CS have been characterized through computed potential energy surfaces, surface crossing points, and CASSCF molecular dynamics calculations. Irradiation of the Cl(2)CS molecules at 360-450 nm does not provide sufficient internal energy to overcome the barrier on S(1) dissociation, and the S(1)/T(2) intersection region is energetically inaccessible at this wavelength region; therefore, S(1) --> T(1) intersystem crossing is the dominant process, which is the main reason S(1)-S(0) fluorescence breaks off at excess energies of 3484-9284 cm(-1). Also, the S(1) --> T(2) intersystem crossing process can take place via the S(1)-T(2) vibronic interaction in this range of excess energies, which is mainly responsible for the quantum beats observed in the S(1) emission. Both S(2) direct dissociation and S(2) --> S(3) internal conversion are responsible for the abrupt breakoff of S(2)-S(0) fluorescence at higher excess energies. S(2) direct dissociation leads to the formation of the fragments of Cl(X(2)P) + ClCS(A(2)A' ') in excited electronic states, while S(2) --> S(3) internal conversion followed by direct internal conversion to the ground electronic state results in the fragments produced in the ground state.     

Heats of formation for Fe(CO)n(n=1–4)

The dissociation energies of Fe(CO)_n (n=2–4) are computed using correlation consistent basis sets and the CCSD(T) approach. The dissociation energies are extrapolated to the CBS limit and are corrected for core–valence (CV), scalar relativistic, spin–orbit, zero-point, and thermal effects. Our iron carbonyl bond strengths agree with experiment within the respective error bars. We use our dissociations energies at 298 K to obtain the heats of formation of Fe(CO)_n (n=1–4).     

Classical trajectory study of rotationally inelastic scattering of two HF molecules

Classical trajectory calculations of the partial opacities and integral cross sections for rotationally inelastic collisions of HF—HF were carried out for the j₁ = 0,j₂ = 0 → (11), (02), (22) transitions at initial relative translational energies of 500, 1000, and 8000 cm^?1 and for the (11) → (02) transition at 1000 cm^?1. Three different methods of relating the initial and final quantum rotational levels to classical distributions were used. The results were compared to the quantum calculations of DePristo and Alexander. It was found that the classical method using a random distribution of initial rotational energies was in poor agreement with the quantum results, while the other two methods which assigned definite classical energies to the quantum levels were in good agreement with the quantum results.     

Quantum study of the active sites of the γ alumina surface (II): QM/MM (LSCF) approach to water, hydrogen disulfide and carbon monoxide adsorption

The mixed quantum mechanics/molecular mechanics (QM/MM) local self consistent field (LSCF) method is applied to study the adsorption of water, hydrogen sulfide, and carbon monoxide molecules on γ alumina surfaces. The effect of the long-range contributions included in the LSCF adsorption/dissociation energies are compared to cluster results. For the carbon monoxide, the long-range contributions do not change the adsorption energies in comparison with the cluster approach. In opposition, the long-range contributions lower the adsorption and dissociation energies of water and hydrogen disulfide. Cautions to be taken on the application of the LSCF method to γ alumina are also discussed.     

Direct measurement of near-threshold rate constants for unimolecular dissociation of CF3I after IR multiphoton excitation

Excited-state populations of CF₃I after IR multiphoton excitation were monitored by time-resolved hot-band UV absorption spectroscopy. Using a calibration of the spectrum by shock-wave experiments, the absorption changes during the laser pulse are analyzed with respect to excited-state populations and dissociation at higher excitation energies. Dissociation of molecules near threshold is detected under collision-free conditions by absorption changes after the laser pulse. At higher pressures, absorption signals after the pulse are markedly influenced by energy transfer between excited and cold molecules. The measured dissociation rate constants near threshold are consistent with statistical calculations of k(E,J), showing pronounced rotational dependence.     

Spectroscopic calculation of CH bond dissociation energies for the bromo derivatives of alkanes,alkenes, and arenes

The CH bond dissociation energies were determined for the bromo derivatives of methane, ethane, propane, cyclopropane, ethane, propene, and benzene by the spectroscopic and quantum-chemical methods. The spectroscopic values of the CH bond dissociation energies were obtained from the fundamental absorption bands by the variational method in an anharmonic approximation using the Morse-harmonic basis. Quantum-chemical calculations were performed using the 6-311G(3df, 3pd)/B3LYP basis. The resulting tendencies of variation of the bond dissociation energies due to changes in the molecular structure are discussed.     

Quantum chemical simulation of 1,3-dipolar cycloaddition of nitrones to alkenes and their (η6-arene)(tricarbonyl)-chromium complexes

Charge distribution and frontier orbital energies of styrene, C,N-diphenylnitrone, and their (arene)-(tricarbonyl)chromium complexes were calculated by quantum chemical methods. The difference in the HOMO and LUMO energies of the chromium complexes was found to be smaller than in the free ligands, and the reactions with (arene)(tricarbonyl)chromium complexes were characterized by higher rate and selectivity.     

Classical rotational cross sections for H2—HD and HD—HD collisions

Classical trajectory calculations of integral cross sections for rotationally inelastic collisions of HD-para H₂ and HD—HD were carried out for a wide variety of transitions over a wide range of initial relative translational energies. The results of the HD—H₂ calculations were compared with the quantum effective potential calculations of Chu. It was found that the classical method is in reasonably good agreement with the quantum method for the calculation of rotational transitions of HD at the higher initial translational energies, but the classical method is in poor agreement with quantum results for HD excitation at low energies and for H₂ excitations at all energies.     

A comparative study of lithium,sodium and hydrogen bonds formed by 1:1 interaction of electron donor molecules with lithium,sodium and hydrogen halides

Dissociation energies of 1:1 complexes of hydrogen, lithium and sodium halides with water, methanol and ammonia vary in the order LiX > NaX > HX, the dissociation energy in the case of lithium bonds being of the order of 200 kJ mol^?1 or more. The dissociation energy of sodium bonds lies in the range 80–120 kJ mol^?1. The magnitude of charge transfer, Δq, between the donor and acceptor molecules is also highest in the case of lithium bonds. The dissociation energies and Δq have been related with various properties of the complexes.     

Sodium-water clusters and their role in radiation chemistry

Studies of sodium-water clusters are presented which could serve as models for the recently suggested intermediate species in the radiation chemistry of water. The ionization potentials and the lower excited states of sodium with n-water molecules are calculated by ab initio quantum chemistry methods. The ionization potential calculated at the SCF level for the water monomer is 4.10 eV, which becomes 4.34 at the MP2 correlation level. The experimental value is 4.379 ± 0.002 eV. Structural data is presented for the lower members of the sodium with n-water clusters. In addition the Hartree-Fock calculations indicate that there should be some strong charge transfer to solvent transitions at higher energies.     

Six-dimensional quantum dynamics of H2 dissociative adsorption on the Pt(211) stepped surface

Results of experimental studies, and theoretical calculations utilizing classical trajectories, have shown that dissociation of H2 on the Pt(211) stepped surface is enhanced at low energies by a molecular trapping mechanism. Because quantum effects can play a large role at the low energies and long lifetimes that characterize molecular trapping, we have undertaken quantum dynamics calculations for this system, the first to treat all molecular degrees of freedom of a gas molecule reacting on a stepped metallic surface. The calculations show that molecular trapping persists in the quantum system, but only at much lower energies than experimentally seen, pointing to possible deficiencies in the potential energy surface. Classical and quasiclassical trajectory calculations on the same potential provide a reasonable picture of reaction overall, but many of the finer details are inaccurate, and certain classical reaction mechanisms are entirely invalid. We conclude that some skepticism should be shown toward any classical study for which long-lived trapping states play a role.     

A theoretical study of the effects of transition metal dopants on the adsorption and dissociation of hydrogen on nickel clusters

The structure, stability, adsorption, and dissociation of H₂ on nickel clusters doped with late transition metals were investigated using density functional theory with the BP86 functional. Molecular hydrogen physisorption occurred at a vertex atom with a low coordination number. Charge transfer between clusters and the H₂ molecule stabilized the physisorption. The chemisorption of H₂ occurred at the bridge sites, without any structural or spin change of the clusters. Among the pentamer clusters, Cd, Zn, and Au had the lowest chemisorption energies, while Ir and Pt had higher chemisorption energies for hydrogen. The computed reaction energies and activation barriers for the dissociation mechanism showed that dopants such as Rh, Pd, Pt, and Au have endothermic reaction energies and low activation barriers. This facilitates the reversible adsorption/dissociation of the H₂ molecule on these metal‐doped clusters. The dopant atoms play a major role in modulating the physisorption, chemisorption, and dissociation mechanism of H₂ on nickel clusters. © 2013 Wiley Periodicals, Inc.