Ab initio investigations on the HOSO[sub 2]+O[sub 2]→SO[sub 3]+HO[sub 2] reaction (original) (raw)
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Atmospheric Environment, 2011
The formation of HOSO 2 from OH and SO 2 has been thoroughly investigated using several different methods (MP2¼Full, MP2¼FC, B3LYP, HF and composite G* methods) and basis sets (6e31G(d,p), 6e31þ þG(d,p), 6e31þþG(2d,2p), 6e31þþG(2df,2p) and aug-cc-pVnZ). We have found two different possible transition state structures, one of which is a true transition state since it has a higher energy than the reactants and products (MP2¼Full, MP2¼FC and HF), while the other is not a true transition state since it has an energy which lies between that of the reactants and products (B3LYP and B3LYP based methods). The transition state structure (from MP2) has a twist angle of the OH fragment relative to the SO bond of the SO 2 fragment of À50.0 , whereas this angle is 26.7 in the product molecule. Examination of the displacement vectors confirms that this is a true transition state structure. The MP2¼Full method with a larger basis set (MP2¼Full/6e31þþG(2df,2p)) predicts the enthalpy of reaction to be À112.8 kJ mol À1 which is close to the experimental value of À113.3 AE 6 kJ mol À1 , and predicts a rather high barrier of 20.0 kJ mol À1. When the TS structure obtained by the MP2 method is used as the input for calculating the energetics using the QCISD/6e31þþG(2df,2p) method, a barrier of 4.1 kJ mol À1 is obtained (ZPE corrected). The rate constant calculated from this barrier is 1.3 Â 10 À13 cm 3 molecule À1 s À1. We conclude that while the MP2 methods correctly predict the TS from a structural point of view, higher level energy corrections are needed for estimation of exact barrier height.
Hydroxyl radical reduction and peroxide bond breaking in hydrogen peroxide are reactions involved in various processes such as the Fenton reaction, which has applications as e.g. groundwater remediation. Here, we study these two reactions from a thermodynamical point of view through the bond dissociation energy (BDE) of the O-O bond in hydrogen peroxide and the electron affinity (EA) of the hydroxyl radical. High-level ab-initio calculations at the complete basis set (CBS) limit were carried out, and the performance of different DFT-based methods was addressed by following a specific classification on the basis of the Jacob's ladder in combination with various Pople's basis sets. The ab-initio calculations at the CBS limit are in agreement with experimental reference data and identify a significant contribution of the electron correlation energy to the BDE and EA. The studied DFT-based methods were able to reproduce the ab-initio reference values, although no functional was particularly detected as the best for both reactions. The inclusion of certain percentage of Hartree-Fock (HF) exchange in DFT functionals leads in most cases to smaller BDE and EA values, which might be related to the poor description of the two reactions by the HF method. Considering the computational cost, DFT methods provide better BDE and EA values than HF methods with an accuracy comparable to the MP2 or CCSD level of theory. Additionally, the quality of the hydrogen peroxide, hydroxyl radical and hydroxyl anion structures obtained from these functionals was compared to experimental reference data. In general, bond lengths were well reproduced and the errors in the angles were between one and two degrees with some systematic trend with respect to the basis set's size. From our results we conclude that DFT methods present a computationally less expensive alternative to describe these two reactions that play a role in the Fenton reaction. The benchmark that is carried out in this study provides a systematic validation of various approximated E xc [ ] functionals combined with different basis sets, which could serve as a stepping-stone for future research on the Fenton reaction.
Journal of Theoretical and Computational Chemistry, 2017
Using computational calculations, we have revisited the potential energy surface (PES) of the reaction between OH and SO2, which is believed as the rate-limiting step in the atmospheric formation of H2SO4. In this work, we report for the first time the presence of a pre-reaction hydrogen-bonded complex between OH and SO2 in the reaction PES. Based on this finding, it has been shown that the reaction can be considered as a two-step process in which the first step is the formation of the pre-reaction complex and the second step is the transformation of this complex to the product. It was observed that due to the presence of this pre-reaction complex as a potential well in the reaction PES, the barrier height got increased by around two-fold for the second step. Based on this observation, it has been proposed that the kinetics of the reaction is going to be affected. Also based on the analysis of the geometries of this pre-reaction complex and the transition state, it has been argued t...
The Journal of Physical Chemistry A, 2009
The possible reactions of HO 2 radical with the intermediates of the Cl 2 SO photolysis (ClSO and SO) were studied using G3MP2//B3LYP/cc-pVTZ+d level of theory and Martin's W1U method. For the reaction between HO 2 and ClSO radicals, the following mechanisms are supposed to be the main reaction pathways On the basis of G3MP2//B3LYP/cc-pVTZ+d and highly accurate W1U calculations, the reaction of HOO with 3 SO species has also been explored, and the following dominant consecutive reactions may describe the fast oxygen transfer
Physical Chemistry Chemical Physics, 2010
The influence of a single water molecule on the gas-phase reactivity of the HO 2 radical has been investigated by studying the reactions of SO 3 with the HO 2 radical and with the H 2 OÁ Á ÁHO 2 radical complex. The naked reaction leads to the formation of the HSO 5 radical, with a computed binding energy of 13.81 kcal mol À1. The reaction with the H 2 OÁ Á ÁHO 2 radical complex can give two different products, namely (a) HSO 5 + H 2 O, which has a binding energy that is computed to be 4.76 kcal mol À1 more stable than the SO 3 + H 2 OÁ Á ÁHO 2 reactants (D(E + ZPE) at 0K) and an estimated branching ratio of about 34% at 298K and (b) sulfuric acid and the hydroperoxyl radical, which is computed to be 10.51 kcal mol À1 below the energy of the reactants (D(E + ZPE) at 0K), with an estimated branching ratio of about 66% at 298K. The fact that one of the products is H 2 SO 4 may have relevance in the chemistry of the atmosphere. Interestingly, the water molecule acts as a catalyst, [as it occurs in (a)] or as a reactant [as it occurs in (b)]. For a sake of completeness we have also calculated the anharmonic vibrational frequencies for HO 2 , HSO 5 , the HSO 5 Á Á ÁH 2 O hydrogen bonded complex, H 2 SO 4 , and two H 2 SO 4 Á Á ÁH 2 O complexes, in order to help with the possible experimental identification of some of these species.
The HOO-SO3 radical complex: ab initio and density-functional study
Chemical physics letters, 2004
The electronic structure and thermochemical stability of the HOO-SO 3 complex is studied using both second-order Møller-Plesset perturbation theory (MP2) and the B3LYP density-functional theory (DFT) method. The calculated dissociation energies of the complex are 10.25 and 11.51 kcal mol À1 at the G3(MP2) and G3 levels, respectively. Anharmonic OH stretching frequencies of the HO 2 moiety along with the frequency shifts upon complex formation are calculated at the MP2/6-311++G(2df,2p) and B3LYP/ 6-311++G(2df,2p) levels, and also AIM analyses of the MP2 and Kohn-Sham densities were performed. Theoretical data strongly encourage performing of matrix-isolation studies of the title complex and its spectroscopic identification.
Journal of Molecular Structure-theochem, 2006
A theoretical kinetic study of the elementary chemical reaction involving hydrogen atom abstraction from hydrogen peroxide by hydrogen atom leading to H2 molecule and HO2 radical was reported. Rate constants were calculated using transition state theory. The values of the kinetic parameters, the classical barrier height and the pre-exponential factor, were derived from ab initio (MP2//CASSCF) and density functional theory calculations of the electronic structure of each chemical species implied in the title reaction using the aug-cc-pVTZ basis set. Two hybrid BHHLYP and B3LYP functionals were used. The ZPE and BSSE corrected classical barrier height was predicted to be 9.31 and 2.34 kcal mol−1, respectively, for the two used DFT methods and 8.08 kcal mol−1 for the ab initio calculation. The experimental value derived from fitted Arrhenius expressions ranges from 3.75 to 9.44 kcal mol−1. The rate constants based on the ab initio and on the BHHLYP calculations were found to be in reasonable agreement with the available observed ones. However, those based on the B3LYP calculation were found to be far from accurately predicted.
The Journal of Physical Chemistry A, 2002
The variational transition state theory (VTST) is used to calculate thermal rate constants for the reactions H + O 3 f OH + O 2 (R1) and O + HO 2 f OH + O 2 (R2). Both reactions are studied using a double manybody expansion (DMBE) potential energy surface for ground state HO 3. The VTST results are compared with quasiclassical trajectory calculations (QCT) and experiment. Reaction R1 shows a planar transition state which, including the zero-point energy, is 0.16 kcal mol-1 above the reactants. This reaction presents two maxima in the vibrational adiabatic potential, and hence, unified statistical theory in its canonical (CUS) and microcanonical (US) versions has been employed in addition to the canonical (CVT) and microcanonical (µVT) variational transition state theories. The results obtained by the CUS and US methods compare well with QCT and experiment. The DMBE potential energy surface predicts that reaction R2 occurs via oxygen abstraction. Two possible reaction paths were found for this reaction. One path has no transition state with an oxygen angle of attack close to 155°, and the other path presents a transition state with an oxygen angle of attack of about 80°. Because the potential energy surface for this reaction is quite flat, the CVT and µVT methods were used together with an algorithm that reorients the dividing surface to maximize the Gibbs free energy. The VTST results are found to agree reasonably well with experiment and with QCT calculations.
On the Dissociation of Ground State trans -HOOO Radical: A Theoretical Study
Journal of Chemical Theory and Computation, 2010
The hydrotrioxyl radical (HOOO •) plays a crucial role in atmospheric processes involving the hydroxyl radical (HO •) and molecular oxygen (O 2). The equilibrium geometry of the electronic ground state (X 2 A′′) of the trans conformer of HOOO • and its unimolecular dissociation into HO • (X 2 Π) and O 2 (X 3 Σ g-) have been studied theoretically using CASSCF and CASPT2 methodologies with the aug-cc-pVTZ basis set. On the one hand, CASSCF(19,15) calculations predict for trans-HOOO • (X 2 A′′) an equilibrium structure showing a central O-O bond length of 1.674 Å and give a classical dissociation energy D e) 1.1 kcal/mol. At this level of theory, it is found that the dissociation proceeds through a transition structure involving a low energy barrier of 1.5 kcal/mol. On the other hand, CASPT2(19,15) calculations predict for trans-HOOO • (X 2 A′′) a central O-O bond length of 1.682 Å, which is in excellent agreement with the experimental value of 1.688 Å, and give D e) 5.8 kcal/mol. Inclusion of the zero-point energy correction (determined from CASSCF(19,15)/aug-cc-pVTZ harmonic vibrational frequencies) in this D e leads to a dissociation energy at 0 K of D 0) 3.0 kcal/mol. This value of D 0 is in excellent agreement with the recent experimentally determined D 0) 2.9 (0.1 kcal/mol of Le Picard et al. (Science 2010, 328, 1258-1262). At the CASPT2 level of theory, we did not find for the dissociation of trans-HOOO • (X 2 A′′) an energetic barrier other than that imposed by the endoergicity of the reaction. This prediction is in accordance with the experimental findings of Le Picard et al., indicating that the reaction of HO • with O 2 yielding HOOO • is a barrierless association process.