Time-sliced ion-velocity imaging study of the reaction Y + O2 → YO + O

The oxidation reaction dynamics of the gas-phase yttrium atoms by oxygen molecules was studied under crossed-beam conditions. The product YO was detected using a time-of-flight mass spectrometer combined with laser single-photon ionization. An acceleration lens system designed for the ion-velocity mapping conditions, a two-dimensional (2-D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products. Two ionization wavelengths were used for the internal (vibrational and/or electronic) energy selective detection of YO. The single photon of the shorter wavelength (202.0 nm) can ionize all states of YO(X?(2)Σ, A'?(2)Δ, and A?(2)Π), while electronically excited YO(A' and A) are dominantly ionized at a longer wavelength (285.0 nm). Time-sliced images were converted to the velocity and angular distributions in the center-of-mass frame. The general features of the velocity distributions of YO, determined at two wavelengths, were well represented by those expected from the statistical energy disposal model. The forward-backward symmetry was also observed for two images. These results suggest that the reaction proceeds via long-lived intermediates, and that this mechanism is consistent with previous chemiluminescence/LIF studies.     

Quasi-classical trajectory studies of the stereodynamics of the reaction O + HCl → ClO + H

The stereodynamics of the O + HCl → ClO + H reaction are investigated by quasi-classical trajectory (QCT) method. The calculations are carried out on the ground 1 ¹ A′ potential energy surface (PES). The orientation and alignments of the product rotational angular momentum for the title reaction are reported. The influence of collision energy on the product vector properties is also studied in the present work. Four (2π/σ)(dσ₀₀/dω _t ), (2π/σ)(dσ₂₀/dω _t ), (2π/σ)(dσ₂₂₊/dω _t ), and (2π / σ)(dσ₂₁₋/dω _t ), and have been calculated in the center of mass frame.     

Lack of salt effect on the kinetics of the reaction SO2−4 + H3O+ → HSO−4 + H2O

It is found that the broadening of the 1100-cm⁻¹ line of SO⁻²₄, caused by increasing [H₃O⁺], is unaffected by addition of 4 M LiCl, NaBr, KCl and NH₄Cl. This finding is in line with the lack of influence of NaCl reported earlier. The significance of these findings, in terms of the reaction mechanism, is discussed.     

17O excess transfer during the NO2 + O3 → NO3 + O2 reaction

The ozone molecule possesses a unique and distinctive (17)O excess (Δ(17)O), which can be transferred to some of the atmospheric molecules via oxidation. This isotopic signal can be used to trace oxidation reactions in the atmosphere. However, such an approach depends on a robust and quantitative understanding of the oxygen transfer mechanism, which is currently lacking for the gas-phase NO(2) + O(3) reaction, an important step in the nocturnal production of atmospheric nitrate. In the present study, the transfer of Δ(17)O from ozone to nitrate radical (NO(3)) during the gas-phase NO(2) + O(3) → NO(3) + O(2) reaction was investigated in a series of laboratory experiments. The isotopic composition (δ(17)O, δ(18)O) of the bulk ozone and the oxygen gas produced in the reaction was determined via isotope ratio mass spectrometry. The Δ(17)O transfer function for the NO(2) + O(3) reaction was determined to be: Δ(17)O(O(3)?) = (1.23 ± 0.19) × Δ(17)O(O(3))(bulk) + (9.02 ± 0.99). The intramolecular oxygen isotope distribution of ozone was evaluated and results suggest that the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The results obtained in this study will be useful in the interpretation of high Δ(17)O values measured for atmospheric nitrate, thus leading to a better understanding of the natural cycling of atmospheric reactive nitrogen.     

Theoretical determination of the rate coefficient for the HO2 + HO2 → H2O2+O2 reaction: adiabatic treatment of anharmonic torsional effects

The HO(2) + HO(2) → H(2)O(2) + O(2) chemical reaction is studied using statistical rate theory in conjunction with high level ab initio electronic structure calculations. A new theoretical rate coefficient is generated that is appropriate for both high and low temperature regimes. The transition state region for the ground triplet potential energy surface is characterized using the CASPT2/CBS/aug-cc-pVTZ method with 14 active electrons and 10 active orbitals. The reaction is found to proceed through an intermediate complex bound by approximately 9.79 kcal/mol. There is no potential barrier in the entrance channel, although the free energy barrier was determined using a large Monte Carlo sampling of the HO(2) orientations. The inner (tight) transition state lies below the entrance threshold. It is found that this inner transition state exhibits two saddle points corresponding to torsional conformations of the complex. A unified treatment based on vibrational adiabatic theory is presented that permits the reaction to occur on an equal footing for any value of the torsional angle. The quantum tunneling is also reformulated based on this new approach. The rate coefficient obtained is in good agreement with low temperature experimental results but is significantly lower than the results of shock tube experiments for high temperatures.     

Comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In a recent paper (Radiation Physics and Chemistry, 2005, vol. 74, pp. 210) it was suggested that the anomalous increase of molecular hydrogen radiolysis yields observed in high-temperature water is explained by a high activation energy for the reaction H+H₂O→H₂+OH. In this comment we present thermodynamic arguments to demonstrate that this reaction cannot be as fast as suggested. A best estimate for the rate constant is 2.2×10³ M⁻¹ s⁻¹ at 300 °C. Central to this argument is an estimate of the OH radical hydration free energy vs. temperature, ΔG_hyd(OH)=0.0278t−18.4 kJ/mole (t in °C, equidensity standard states), which is based on analogy with the hydration free energy of water and of hydrogen peroxide.     

Quasi-classical trajectory approach to the stereo-dynamics of the reaction F+HO→HF+O

Quasi-classical trajectory (QCT) calculations are employed for the reaction F + HO(0,0)→HF + O based on the adiabatic potential energy surface (PES) of the ground 3A″triplet state. The average rotational alignment factor P2(j′·k) as a function of collision energy and the four polarization dependent generalized differential cross sections have been calculated in the center-of-mass (CM) frame, separately. The distribution P(θr) of the angle between k and j′, the distribution P(θr) of dihedral angle denoting k-k′-j′ correlation, and the angular distribution P(θr, Φr) of product rotational vectors in the form of polar plots are calculated as well. The effect of Heavy-Light-Heavy (HLH) mass combination and atom F's relatively strong absorbability to charges on the alignment and the orientation of product molecule HF rotational angular momentum vectors j′ is revealed.     

Wavelength dependence of the BaO* product chemiluminescence on the N2O reactant orientation in the Ba + N2O → BaO* + N2 reaction

Coarse wavelength analysis of chemiluminescence from the crossed-beam reaction of Ba with oriented N₂O reveals a distinct dependence of the BaO final product internal energy distribution on the initial collision geometry. Favorable (Ba approaches the “O” end of N₂O) and unfavorable (Ba approaches the “N” end of N₂O) orientations are compared at two collision energies. Reactions taking place in the unfavorable orientation produce BaO with relatively higher internal energy than reactions via the favorable orientation. The steric effect depends strongly on the wavelength and it is more pronounced at a higher collision energy.     

A theoretical study of the H2SO4+H2O → HSO4 −+H3O+ reaction at the surface of aqueous aerosols

The surface region of sulfate aerosols (supercooled aqueous concentrated sulfuric acid solutions) is the likely site of a number of important heterogeneous reactions in various locations in the atmosphere, but the surface region ionic composition is not known. As a first step in exploring this issue, the first acid ionization reaction for sulfuric acid, H₂SO₄ + H₂O HSO₄^– + H₃O⁺, is studied via electronic structure calculations at the Hartree–Fock level on an H₂SO₄ molecule embedded in the surface region of a cluster containing 33 water molecules. An initial H₂SO₄ configuration is selected which could produce H₃O⁺ readily available for heterogeneous reactions, but which involves reduced solvation and is consistent with no dangling OH bonds for H₂SO₄. It is found that at 0 K and with zero-point energy included, the proton transfer is endothermic by 3.4 kcal/mol. This result is discussed in the context of reactions on sulfate aerosol surfaces and, further, more complex calculations.Contribution to the Jacopo Tomasi Honorary Issue     

Ab initio study on the dynamical properties of the hydrogen abstraction reaction NH2 + OH → NH + H2O

 The geometry of the transition state of the title reaction was optimized at the unrestricted Hartree–Fock, the spin-unrestricted second-order M?ller–Plesset, and the spin-unrestricted quadratic configuration interaction with all single and double substitutions levels of theory. The changes in the geometry, the bound vibrational modes, and the potential energy along the minimum energy path are discussed. Variational transition-state theory rate constants calculated with the tunneling and curvature effect correction agree very well with the experimental values. Received: 23 April 1999 / Accepted: 9 June 1999 / Published online: 15 December 1999     

Reply to comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In reply to “Comment on the possible role of reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures” (Bartels, 2009 Comment on the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 78, 191–194) we present an alternative thermodynamic estimation of the reaction rate constant k. Based on the non-symmetric standard state convention we have calculated that the Gibbs energy of reaction Δ_rG=57.26 kJ mol^?1 and the reaction rate constant k=7.23×10^?5 M^?1 s^?1 at ambient temperature. Re-analysis of the thermodynamic estimation (Bartels, 2009 Comment on the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 78, 191–194) showed that the upper limit for the rate constant at 573 K is k=1.75×10⁴ M^?1 s^?1 compared to the value predicted by the diffusion-kinetic modelling (3.18±1.25)×10⁴ M^?1 s^?1 (Swiatla-Wojcik, D., Buxton, G.V., 2005. On the possible role of the reaction H+H₂O→H₂+OH in the radiolysis of water at high temperatures. Radiat. Phys. Chem. 74(3–4), 210–219). The presented thermodynamic evaluation of k(573) is based on the assumption that k can be calculated from Δ_rG and the rate constant of the reverse reaction which, as discussed, are both uncertain at high temperatures.     

A priori rate constant for the reaction NH2+NO → N2+H2O

Capture rates for a dipole-dipole+Morse potential, from approximate semiclassical trajectory calculations, have been combined with RRKM rates for passing through entropy bottlenecks on the potential surface for NH₂ + NO → N₂ + H₂O. The resulting overall rate constants are in good agreement with experiment. The effective lifetime of the NH₂NO collision complex is found to be of the order of 10⁻¹¹s at room temperature, which accounts for the observed lack of pressure dependence of the rate constant. Calculated rate constants increase markedly at low temperatures, though not as rapidly as some of the experimental data would indicate.     

Rotational analysis and deperturbation of the A2Π → X2Σ+ and B'2Σ+ → X2Σ+ emission spectra of MgH

Deperturbation analysis of the A(2)Π → X(2)Σ(+) and B(')(2)Σ(+) → X(2)Σ(+) emission spectra of (24)MgH is reported. Spectroscopic data for the v = 0 to 3 levels of the A (2)Π state and the v = 0 to 4 levels of the B'(2)Σ(+) state were fitted together using a single Hamiltonian matrix that includes (2)Π and (2)Σ(+) matrix elements, as well as off-diagonal elements coupling several vibrational levels of the two states. A Dunham-type fit was performed and the resulting Y(l,0) and Y(l,1) coefficients were used to generate Rydberg-Klein-Rees (RKR) potential curves for the A (2)Π and the B'(2)Σ(+) states. Vibrational overlap integrals were computed from the RKR potentials, and the off-diagonal matrix elements coupling the electronic wavefunctions (a(+) and b) were determined. Zero point dissociation energies (D(0)) of the A(2)Π and B'(2)Σ(+) states of (24)MgH were determined to be 12,957.5 ± 0.5 and 10,133.6 ± 0.5 cm(-1), respectively. Using the Y(0,1) coefficients, the equilibrium internuclear distances (r(e)) of the A(2)Π and B'(2)Σ(+) states were determined to be 1.67827(1) ? and 2.59404(4) A?, respectively.     

Theoretical studies on the reaction path and dynamics of the reaction CH_3+H_2O→CH_4+OH

The reaction path, the dynamical properties along the reaction path and CVT rate constants are computed by the ab initio MO method, the reaction path Hamiltonian theory and the variational transition state theory. The results show that the effect of the electron correlation energy on activation barrier is large, the recrossing and tunneling effects exist in the reaction.     

电子转移反应O2+O2·-→O2·-+O2的从头算研究

利用abinitio方法,在UHF,UMP2及不同基组3-21G,6-31G^*,6-311+G^*和UMP2(full)/6-311+G^*水平上,研究了O~2/O~2^.^-自交换电子转移反应。优化了电子转移前后反应物和产物的结构,研究了体系能量的变化,计算了自交换电子转移反应的内重组能。对UHF方法和UMP2方法的计算结果进行了比较,并与实验结果进行了对照。结果表明UHF方法由于没有考虑组态相互作用,计算结果存在较大偏差,UMP2(full)/6-311+G^*水平上的计算结果与实验值吻合较好。在UMP2(full)/6-311+G^*水平上计算了气相自交换电子转移反应速率常数。在优化了电子转移复合物结构的基础上考虑了溶剂效应的影响,计算了水溶液中的溶剂重组能。研究结果表明O~2/O~2^.^-体系电子转移反应的活化能主要来源于溶剂重组能的贡献。最后计算了该反应在水溶液中的反应速率常数。理论计算结果与实验值吻合得很好。     

HNCO+OH→H2O+NCO的反应机理

采用从头算分子轨道法(UHF/6-31G**水平,并用MP4加以相关能校正)研究了HNCO+OH→H2O+NCO反应机理.同时用Morokuma数值法获得了反应途径即内禀反应坐标(IRC).沿着IRC,运用反应途径哈密顿理论,获得反应途径动态学信息.在此基础上,根据过渡态理论和相应隧道效应校正,计算了在不同温度下的反应速率常数,得到了和实验相一致的结果.计算结果表明,此反应是一步直接型的抽提H反应.     

Mean potential statistical theory of the H+ + D2 → HD + D+ reaction

The reactive collision process H(+) + D(2)(ν = 0, j = 0) → HD + D(+) is theoretically analyzed for collision energies ranging from threshold up to 1.3 eV. It is assumed that the reaction takes place via formation of a collision complex. In calculations, a statistical theory is used, based on a mean isotropic potential deduced from a full potential energy surface. Calculated integral cross sections, opacity functions, and rotational distributions of the HD products are compared with recent statistical and quantum mechanical calculations performed using a full potential energy surface. Satisfactory agreement between the results obtained using the two statistical methods is found, both of which however overestimate the existing quantum mechanical predictions. The effects due to the presence of identical particles are also discussed.     

Nonadiabatic collision model calculations of ion-pair formation cross sections of Cs+O2→Cs++O2-

This paper has improved Hickman's nonadiabatic collision model by substituting Hickman's constant velocity classical straight line trajectory approximation with the solution of motion equation mR=-dV(R)/dR, and has calculated the cross sections of ion-pair formation Cs+O2 -Cs++O2- with the improved nonadiabatic collision model (INCM). A comparison of our results with other theoretical and experimental results has been made.     

NH+O3→ONH+O2反应热力学和动力学研究

在量子化学对 NH自由基与臭氧 O3反应计算的基础上,应用统计热力学方法研究了 100～ 1600 K温度范围内 NH和 O3反应过程的各热力学量的变化及平衡常数,用经 Wigner校正的 Eyring过渡态理论计算了不同温度下该反应两不同反应通道的活化热力学量、反应速率常数及频率因子.计算表明,相对于反应通道 II,反应通道 I不仅有很强的反应自发性,而且在动力学上也是较容易实现的反应.     

Kinetics of the Thermal Desorption of Atomic Oxygen during Transformations BiO2 – x→ β-Bi2O3→ α-Bi2O3

The thermal desorption of atomic oxygen during the transformations BiO_{2 – x} -Bi₂O₃ -Bi₂O₃is shown to be due to the removal of overstoichiometric oxygen from the bulk of -Bi₂O₃. Oxygen formed at the first stage is desorbed in a molecular form. The maximum desorption rate of atomic oxygen is found before the phase transformation -Bi₂O₃ -Bi₂O₃. The activation energy of the diffusion of excess oxygen in the -Bi₂O₃lattice is 30 kcal/mol.