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.     

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.     

Theoretical studies of the radical reaction H2CSiH2 + H

Ab initio molecular orbital calculations on the transition states and barrier heights for the addition of atomic hydrogen to silaethylene are carried out. The activation energy for the addition to the silicon site is lower than that to the carbon site, while the exothermicity is smaller.     

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     

O?+?C2H4 potential energy surface: lowest-lying singlet at the multireference level

Extensive density functional theory (DFT) calculations have been performed to develop a force field for the classical molecular dynamics (MD) simulations of various azobenzene derivatives. Besides azobenzene, we focused on a thiolated azobenzene’s molecular rod (4′-{[(1,1′-biphenyl)-4-yl]diazenyl}-(1,1′-biphenyl)-4-thiol) that has been previously demonstrated to photoisomerize from trans to cis with high yields on surfaces. The developed force field is an extension of OPLS All Atoms, and key bonding parameters are parameterized to reproduce the potential energy profiles calculated by DFT. For each of the parameterized molecule, we propose three sets of parameters: one best suited for the trans configuration, one for the cis configuration, and finally, a set able to describe both at a satisfactory degree. The quality of the derived parameters is evaluated by comparing with structural and vibrational experimental data. The developed force field opens the way to the classical MD simulations of self-assembled monolayers (SAMs) of azobenzene’s molecular rods, as well as to the quantum mechanics/molecular mechanics study of photoisomerization in SAMs.     

Infinite order sudden approximation treatment of the H + D2 → HD + D reaction

Results of a study of the H + D₂ → HD + D reaction within the quantum infinite order sudden approximation (RIOS) are reported here for the total energy range 1.0317 ⩽ E_t ⩽ 2.1417 eV. We present the vibrationally selected integral and total integral cross sections, and the latter are compared with both classical and experimental results. The RIOS results are in excellent agreement with the experimental results at 1.0317 eV and in reasonable agreement (≈ 20% high) at 2.1417 eV.     

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.     

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.     

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.     

Rate constant for the reaction C2H5 + HBr → C2H6 + Br

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ?mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ～4 kJ?mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.     

Mechanism of HCS + O2 reaction: hydrogen- or oxygen-transfer?

In spite of the potential importance of the HCS radical in both combustion and interstellar processes, its chemical reactivity has not been tackled previously. In the present paper, the oxidation reaction of the HCS radical is theoretically investigated for the first time at the CCSD(T)/6-311++G(3df,2p)//BH&HLYP/6-311++G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels. It is shown that the most feasible pathway is the O2 addition to the HCS radical forming the intermediate SC(H)OO which can undergo a subsequent O-extrusion leading to SC(H)O + 3O. This features an indirect O-transfer mechanism with the overall barrier of 4.4 and 3.5 kcal mol(-1), respectively, at the two levels. However, formation of the H-transfer product CS + HO2 is kinetically much less feasible, i.e., the direct mechanism has barriers of 14.3 and 8.7 kcal mol(-1), whereas the indirect mechanism has barriers of 12.6 and 10.7 kcal mol(-1), respectively. This result is in sharp contrast to the analogous HCO + O2 reaction, where the direct (with a barrier of 2.98 kcal mol(-1)) and indirect (2.26 kcal mol(-1)) H-transfer processes are highly competitive over the indirect O-transfer process (the least endothermicity is 19.9 kcal mol(-1)). The possible explanations and implications of the present results are provided.     

Quantum instanton calculation of rate constants for the C2H6 + H → C2H5 + H2 reaction: anharmonicity and kinetic isotope effects

Thermal rate constants and kinetic isotope effects for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The obtained results are in good agreement with experimental measurements at high temperatures. The detailed investigation reveals that the anharmonicity of the hindered internal rotation motion does not influence the rate too much compared to its harmonic oscillator approximation. However, the motion of the nonreactive methyl group in C(2)H(6) significantly enhances the rates compared to its rigid case, which makes conventional reduced-dimensionality calculations a challenge. In addition, the temperature dependence of kinetic isotope effects is also revealed.     

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.     

Dynamics of the O(3P) + CH4 → OH + CH3 reaction is similar to that of a triatomic reaction

The O((3)P) + CH(4) reaction has been investigated using the quasi-classical trajectory (QCT) method and an ab initio pseudotriatomic potential energy surface (PES). This has been mainly motivated by very recent experiments which support the reliability of the triatomic modeling even at high collision energy ( = 64 kcal mol(-1)). The QCT results agree rather well with the experiments (translational and angular distributions of products); i.e., the ab initio pseudotriatomic modeling "captures" the essence of the reaction dynamics, although the PES was not optimized for high E(col). Furthermore, similar experiments on the O((3)P) + CD(4) reaction at moderate E(col) (12.49 kcal mol(-1)) have also been of a large interest here and, under these softer reaction conditions, the QCT method leads to results which are almost in quantitative agreement with experiments. The utility of the ab initio pseudotriatomic modeling has also been recognized for other analogous systems (X + CH(4)) but with very different PESs.     

The gas-phase and solution mechanism of the isotope exchange reaction OH- + D2 = OD- + HD: Beam study of the solvated-ion reaction OH-. H2O + D2 in the collision energy range 0–2 eV

In a tandem mass spectrometer we have measured the excitation functions (reaction cross section as a function of collision energy) for the following solvated-ion reactant pairs: OH^-.(H₂O) + H₂; OD^-.(D₂O) + D₂; and OH^-.(H₂O) + D₂—in the collision energy range 0–2 eV. Product channels include H₃O^--type production, collision-induced dissociation of reactants and products (OH^- and H^- types) and isotopic mixing. These solvated-ion reactions are used to correlate the reactivity of the isotope exchange reaction OH^- + D₂→OD^- + HD occuring in the gas phase and solution, identifying a proton-transfer mechanism occuring within an H₃O^- intermediate.     

The System HCl + NdCl3 + H2O from 5 to 55 ∘C: A Study of Harned's Rule

The activity coefficients of HCl (γ_A) in aqueous mixtures of HCl and NdCl₃ were determined by the electromotive-force (emf) measurement of cells without liquid junctions of the type:

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.     

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.