Water-Accelerated OH Addition to Sulfur Dioxide SO2: Direct Ab Initio Molecular Dynamics (AIMD) Study Hiroto Tachikawa* Division of Materials Chemistry, Graduate School of Engineering Hokkaido University, Sapporo 060-8628, Japan S Supporting Information *

ABSTRACT: Ionization dynamics of water microsolvated sulfur dioxide SO2(H2O)n (n = 1−3 and 6) have been investigated by means of direct ab initio molecular dynamics (AIMD) method to elucidate the hydration effects of OH addition reaction to SO2 following the ionization. The calculations showed that the neutral 1:1 complex SO2−H2O has a Cs symmetry and the sulfur of SO2 interacts with the oxygen of H2O with an eclipsed form. In the case of ionization of SO2−H2O 1:1 complex (n = 1), the cation complex composed of [H2O−SO2]+ with a face-to-face form was obtained as the product. The OH addition reactions to SO2 were found in larger systems (n = 2, 3, and 6) following the ionization. The reaction was expressed as SO2+(H2O)n → SO2(OH)···H+(H2O)n−1 (n = 2, 3, and 6). The proton generated as (SO2−H2O)+ → (HSO3) + H+ was stabilized by the second water molecule as the reaction: H+ + H2O → H3O+. These processes occurred and were completed within the cluster. The OH addition mechanism in SO2+(H2O)n cluster was discussed on the basis of the present results. SO2 (H 2O)n + hv → SO2+(H 2O)n + e−

1. INTRODUCTION Tropospheric oxidation of SO2 to form sulfuric acid (H2SO4) is significant in the atmospheric the oxidation process.1−10 A candidate of the initial oxidation reaction of SO2 is a hydroxyl radical (OH) addition to the SO2: SO2 + OH → HSO3

and electron capture reaction SO2 (H 2O)n + e− → SO2−(H 2O)n

Dong et al.15 investigated the ionization of (SO2)n(H2O)m clusters’ n gas phase by using single photon ionization of soft Xray laser. They found that (SO2)n+ cluster ion dominates the mass spectrum, and the unprotonated mixed cluster ions [(SO2)nH2O]+ (n = 1−5) were also observed. Under the condition of low SO2 concentration, the protonated water clusters H+(H2O)m were obtained as the product ion. A few experiments and theoretical works have been carried out to elucidate the electronic properties of hydrated SO2 and ionic clusters.16−20 Microwave spectroscopy of SO2−H2O 1:1 complex shows the non-hydrogen-bonded structure, and the water and sulfur are tilted ∼45° from a parallel orientation.16 Ab initio calculation indicates that a high barrier exists for the reaction, SO2 + H2O → H2SO3.17 Thus, information on the static properties of SO2(H2O) and SO2+(H2O) has been accumulated. However, the relaxation process and reaction of SO2+(H2O)n and SO2−(H2O)n are not clearly understood. In the present study, the ionization dynamics of microhydrated sulfur dioxide SO2(H2O)n (n = 1−3 and 6) have been


The origin of OH radical is considered as a photodissociation of ozone molecule (O3), which forms O(1D) and reacts with H2O: O(1D) + H2O → 2OH. Using the chemical ionization mass spectrometer (CIMS), Benson et al. measured H2SO4−H2O nucleation rates at atmospheric pressure, 288 K, and 10−55% relative humidity. They showed that H2SO4 is produced from the SO2 + OH → HSO3 reaction as an initial oxidation process.11 HSO3 adds O2 to give HSO5, which decomposes to SO3 + H2O. SO3 then adds water in a water-catalyzed reaction to give H2SO4. Thus, the initially formed hydroxysulfonyl radical HSO3 is further oxidized to H2SO4. The SO2 molecule has the specific properties that can interact easily with several molecules. In the gas phase, SO2 can form a microhydrated cluster with water molecules as SO2(H2O)n, although the water concentrations required for this are far too high for the clusters to form in appreciable amounts in our atmosphere.12−14 Upon UV and UVU irradiations to SO2(H2O)n, several photoreactions take place: one of the photoreactions is the ionization-induced reaction of SO2(H2O)n, which is expressed by © 2014 American Chemical Society

Received: February 9, 2014 Revised: April 15, 2014 Published: April 16, 2014 3230 | J. Phys. Chem. A 2014, 118, 3230−3236

The Journal of Physical Chemistry A


investigated by means of direct ab initio molecular dynamics (AIMD) method to elucidate the effect of number of water molecule on the reaction mechanism of relaxation process of SO2+(H2O)n. Time scale of hydrogen bond breaking and reformation processes between SO2+ and H2O were estimated.

2. METHOD OF CALCULATION We calculated the structures of neutral clusters SO2(H2O)n (n = 1−3 and 6) at the second-order Møller−Plesset perturbation (MP2) with a standard 6-311++G(d,p) basis set. For n = 1, coupled clusters with single and double excitations (CCSD) and quadratic configuration interaction with single- and doubleexcitation (QCISD) methods with the aug-cc-pVDZ, aug-ccpVTZ basis sets were also used. The restricted and unrestricted Hartree−Fock orbitals were used for neutral and cationic systems, respectively. We used the usual convention of the computational methods, MP2 and UMP2, for the closed- and open-shell systems, respectively. The frozen core approximation was employed for both cationic and neutral states. The structures and electronic states were calculated using Q-Chem and Gaussian 09 programs.21,22 In the direct AIMD calculations,23−27 first, the structures of the neutral clusters SO2(H2O)n (n = 1−3 and 6) were fully optimized at the MP2/6-311++G(d,p) level. Next, trajectories for the cationic systems were then propagated from the vertical ionization point of the parent neutral cluster. In addition to the trajectory from the equilibrium point, we generated geometries around the equilibrium point randomly and selected 10 geometries with the energy difference lower than 1.0 kcal/ mol from the equilibrium point of SO2(H2O)n. A total of 10 geometrical configurations for each system were generated around the optimized geometry of neutral state using the sampling method. The potential energy and its gradient of the cationic system were calculated at the UMP2/6-311G(d,p) level. The equation of motion was solved by the velocity Verlet algorithm with a time step of 0.25 fs. The drifts of total energies were

Water-accelerated OH addition to sulfur dioxide SO₂: direct ab initio molecular dynamics (AIMD) study.

Ionization dynamics of water microsolvated sulfur dioxide SO2(H2O)n (n = 1-3 and 6) have been investigated by means of direct ab initio molecular dyna...
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