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Interaction of SO2 with the Surface of Water Nanodroplet Jie Zhong, Chongqin Zhu, Lei Li, Geraldine L. Richmond, Joseph S. Francisco, and Xiao Cheng Zeng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09900 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017
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Interaction of SO2 with the Surface of Water Nanodroplet Jie Zhong1, Chongqin Zhu1, Lei Li1, Geraldine L. Richmond2*, Joseph S. Francisco1*, and Xiao Cheng Zeng1* 1Department
of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588, United States
2Department
of Chemistry, University of Oregon, Eugene, Oregon, 97403, United States
ABSTRACT: We present a comprehensive computational study of interaction of a SO2 with water molecules in the gas phase and with the surface of various sized water nanodroplets to investigate the solvation behavior of SO2 in different atmospheric environment. Born-Oppenheimer molecular dynamics (BOMD) simulation shows that in the gas phase and at temperature (300 K), the dominant interaction between SO2 and H2O is (SO2)S··O(H2O), consistent with previous density-functional theory (DFT) computation at 0 K. However, at the surface of water nanodroplet, BOMD simulation shows that the hydrogenbonding interaction of (SO2)O··H(H2O) becomes increasingly important with the increase of droplet size, reflecting a marked effect of the water surface on the SO2 solvation. This conclusion is in good accordance with spectroscopy evidence obtained previously (J. Am. Chem. Soc. 2005, 127, 16806; J. Am. Chem. Soc. 2006, 128, 3256). The prevailing interaction (SO2)O··H(H2O) on large droplet is mainly due to favorable exposure of H atoms of H2O at the air-water interface. Indeed, the conversion of the dominant interaction in the gas phase (SO2)S··O(H2O) to the dominant interaction on the water nanodroplet (SO2)O··H(H2O) may incur effects on the SO2 chemistry in atmospheric aerosols because the solvation of SO2 at the water surface can affect the reactive sites and electrophilicity of SO2. Hence, the solvation of SO2 on aerosol surface may have new implications when studying SO2 chemistry in the aerosol-containing troposphere.
INTRODUCTION Sulfur dioxide (SO2) in the atmosphere largely stems from fossil fuel combustion at power plants1-3, industrial facilities4-6, and anthropogenic activities7,8. Sulfur dioxide from these sources can accumulate in fog water and cloud droplets, thereby playing an important role in many atmospheric processes such as radiation trapping and scattering9-11, and the formation of secondary pollutants including sulfates, organosulfate aerosols, and acid rain12-16. Because of its important atmospheric implication, the interaction between SO2 and H2O, either in gas phase or in aqueous aerosols, has been studied from both experimental17-20 and computational21-23 perspectives. Interestingly, the measured infrared24 and Raman25 spectra of aqueous solutions of SO2 are essentially identical to those of gas-phase SO2, suggesting that “H2SO3” is in fact a loosely aquated SO2 molecule. Density-functional theory (DFT) computations also show high reaction barriers between SO2 and H2O (higher than 30 kcal/mol)26,27. Recently, the surface complexes have been identified as the initial step in a number of surface reactions27-29. However, there is an ongoing debate about the existence and composition of SO2 surface complexes, ranging from (SO2)(H2O)0 to (SO2)(H2O)3 27-29. Spectroscopy experiments designed to address issues of SO2 complexes on surface of water droplets have provided valuable information, but still cannot fully characterize microscopic events and behavior30, such as orientation of SO2, hydration geometry, and dynamic behavior on surface of water droplets. The latter information would assist understanding details of various reactions of SO2 in atmos-
phere. Computational studies, when coupled with spectroscopy experiments, can provide much more complete picture of SO2 adsorption to aqueous surfaces. Many previous computational studies have determined microscopic configurations for SO2/H2O complexes, ranging from (SO2)(H2O)1 to (SO2)(H2O)3 at 0 K. It is established that the global-minimum complex of (SO2)(H2O)1 has the Cs symmetry with H2O lying above the plane of SO2 and the complex forms the “sandwich” structure 26-28,31. Such a structure stems mainly from the interaction between S atom in SO2 and O atom in H2O. This gas-phase configuration seems very different from the complex configuration on water surface, determined from vibrational sumfrequency measurement by Richmond and co-workers.32 On water surface, the hydrogen bond formed between O atom of SO2 with H atom of water appears to be the dominant interaction whereas the “sandwich” structure is less likely to form. To gain more insight into the SO 2/H2O complex on water surface, two groups have employed BornOppenheimer molecular dynamical simulations to investigate the hydration structure of SO230,33. Both studies reported variability of the SO2 hydrate structures. However, to our knowledge, few studies concerning the effects of water droplet sizes on SO2 solvation have been reported, which could have important atmospheric implications because different sizes of droplet are existed during the aerosol nucleation and growth process in atmosphere. Moreover, the water droplet effects on chemical properties of SO2, such as potential reactive sites and electronic properties that are relevant to the reactivity are seldom discussed previously.
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In this article, we present a comprehensive computational study of interaction of a SO2 with water molecules in the gas phase and with the surface of water nanodroplet with various sizes to examine hydration behavior of SO2 in different atmospheric environment. Towards this end, the Born-Oppenheimer molecular dynamics (BOMD) simulations at gradient-corrected density-functional theory (DFT) level with semi-empirical dispersion correction are carried out.
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systems indicate that the BLYP-D functional tends to overestimate the freezing point of bulk water, and the computed freezing point of water is about 360 K at 1 bar. In all BOMD simulations, we set the temperature of the water droplet system being at 300 K, about 60 K below the computed freezing point of the BLYP-D water. RESULTS AND DISCUSSION Benchmark Test. The hydration behavior of SO2 at the air-water interface was previously investigated by Richmond and co-workers30, using the BOMD simulation (based on B3LYP hybrid functional) with water layer containing 24 water molecules. Here, the same system is employed to reproduce previous simulation results. In the previous BOMD simulation, the SO2 has the highest probability (45.8%) in the state such that it completely unbounds from the surface water. Such a state is also dominant in our BOMD simulation (49.3%). Furthermore, the complexes probability of “O” type (interact with water via O atom in SO2), “S” type (interact with water via S atom in SO2), “SO” type (interact with water via both O atom and S atom in SO2) are also found similar for these two simulations. Their differences are 7%, 5.2% and 5.2%, respectively, likely due to the different choice of functional (B3LYP vs. BLYP-D). Recently, the Grimme’s dispersion correction was suggested to give an overall improvement of the properties of liquid water45. Hence, in our following works, the Grimme’s dispersion correction is considered, and its reliability is confirmed by comparison with previous DFT calculations28. For the (H2O)1-SO2 system, the most stable complex exhibits Cs symmetry with H2O lying above the plane of SO2. The interaction energy of this complex is -16.16 kJ/mol and -17.32 kJ/mol at the level theories of MP2/augcc-pVTZ and CCSD(T)/aug-cc-pVTZ, respectively. In our calculation, this interaction energy is -15.02 kJ/mol. Furthermore, the interaction energy of the second minimum (SO2 approaching the hydrogen of water by one of its O atoms) is calculated to be 7.8 kJ/mol larger than that of the most stable complex, which is consistent with the result of MP2/aug-cc-pVTZ (6.68 kJ/mol) and CCSD(T)/aug-ccpVTZ (7.42 kJ/mol). Moreover, based on our BOMD simulation, the S-O bond distance of SO2 is around 1.45 Å (Figure S1A). The radial distribution function of S··Hw (S atom in SO2 with H atoms in water, Figure S1B) exhibits a broad first peak starting at 2.5 Å. These features are consistent with previous results based on Car–Parrinello molecular dynamics simulations48. The radial distribution function profile of S··Ow (S atom in SO2 with O atoms in water) shows the hydration radius of S atom is around 2.75 Å (Figure S2). This result is consistent with previous DFT calculations and the microwave spectra (equilibrium distance between S··Ow ranges from 2.74 Å to 2.84 Å)26,28,29. Hydration structure in the gas phase. The SO2 interactions with water molecules in the gas phase are analyzed based on the BOMD trajectories. Figure 2 (upper panel) shows the probabilities of three different interaction con-
COMPUTATION METHODS All BOMD simulations are performed using the QUICKSTEP module implemented in the CP2K package 34,35. Specifically, for the BOMD simulations, the exchange and correlation interactions of electrons are treated within the generalized gradient approximation in the Becke-LeeYang-Parr (BLYP) functional form.36,37 The BLYP functional has been often selected in BOMD simulations of water droplets 36-39. Here, we choose the Gaussian DZVP basis set40 and auxiliary plane waves for expanding electron density, and the Goedecker−Teter−Hutter (GTH) normconserved pseudopotentials41,42 for treating core electrons. The Grimme’s dispersion correction method43,44 is used to couple with the BLYP functional (BLYP-D), which can give more accurate properties of liquid water.45 The simulation systems for SO2 in the gas phase ranges from (SO2)(H2O)1 to (SO2)(H2O)3. The dimension of the simulation cubic box is x = 30 Å, y = 30 Å, z = 30 Å as depicted in Figure 1A. The simulation system for SO2 on water droplet is composed of one SO2 and different number of water molecules ranging from 24 (Figure 1B) to 191 (Figure 1C). The SO2 is initially placed near the surface region. The dimension of the simulation box is x=35 Å, y=35 Å, z=35 Å. All the systems are large enough to avoid interactions between adjacent periodic images of water droplet.
Figure 1. Simulation systems for SO2 (A) in the gas phase and at the surface of a water nanodroplet with (B) 24 water molecules and (C) 191 water molecules.
Prior to the BOMD simulation, each system is fully relaxed first at DFT level. All the BOMD trajectories are generated in the constant volume and constant temperature (NVT) ensemble with using the Nose-Hoover chain method for controlling the temperature of system. The time step of 0.2 fs is used for the three systems of (SO2) (H2O)1-3. For the system of SO2 on water surface, the time step is set as 1.0 fs, which has been proven to yield good energy conservation in BOMD simulations of water systems. Previous BOMD simulations46,47 of coexisting two-phase ice/water
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figurations between SO2 and water (interacting via O, S or both atoms in SO2) in SO2(H2O)1-3 systems. As depicted in Figure 2 (upper panel), in all gas-phase systems, the dominant complexes are contributed by the connection via S atom in SO2. It indicates that the interaction of (SO2)S··O(H2O) is stronger than that of hydrogen bond in (SO2)O··H(H2O). This result is consistent with previous DFT calculations26,27,49.
primarily because the extra water molecule in the system of SO2(H2O)3 might interact strongly with other water molecules. Such a strong self-interaction between water molecules may screen their connections with SO2. As shown in Figure 2 (blue bar in upper panel), the number of complexes “SO” type decreases as the number of water molecules increases from 2 to 3. Even in the gas phase, the SO2water interaction is found largely affected by the water numbers. As more water molecules are added into the system, which can happen during aerosol nucleation process, the water nanodroplet could be formed with different sizes. How SO2-water interaction would be influenced by the size of water nanodroplet? By comparing the hydration behavior in the gas phase and at the surface of various sizes of water droplet, the air-water surface effects on SO2 hydration can be carefully addressed. Droplet size influence. The SO2 adsorption behavior on droplet with 24, 48, 96 and 191 water molecules are investigated. Figure 3A shows four typical hydration configurations formed between SO2 and water molecules. The definitions for these four configurations are given in the caption below, and the corresponding probabilities are depicted in Figure 3B.
Figure 2. (Upper Panel) The SO2 bonding configuration with different probabilities in SO2(H2O)1-3 systems. Black, red and blue bar represent the connection with water molecules via O atoms, S atom and both S and O atoms in SO2, respectively. (Lower Panel) Cyclic hydrate structures form during the BOMD simulations, involving the SO2 and one or more water molecules in SO2(H2O)1-3 systems. Black, red and blue bar represent the single, double and triple water cycles formed by SO2 and H2O, respectively. The values are the percentage of MD time steps spent in the given configuration.
Figure 3. (A) Four typical configurations between SO2-H2O. Config1 corresponds to non-interaction with water molecules. Config2 corresponds to the H-bond interaction of (SO2)O··H(H2O). Config3 corresponds to (SO2)S··O(H2O) interaction. Config4 corresponds to (SO2)S··O(H2O) interaction and an H-bond interaction of (SO2)O··H(H2O). (B) The probabilities of different interaction configurations (right panel).
As shown in Figure 3B, the probability of Config1 decreases with the increasing size of the water nanodroplet. It indicates that for small droplet, the SO2 has more chance to fully unbound with the surface water. This phenomenon is likely related with the interfacial water behavior on droplet. Firstly, the radial distribution function profiles (RDF in Figure S3) between interfacial water molecules are calculated. Although the first peaks of RDF in the largest droplet seems to be slightly stronger, no significant differences are found for various sizes of water droplet, suggesting that the self-interaction between interfacial water is not responsible for the probability changing of Config1 in different droplets. Next, the deformation ability for various sizes of water droplets are calculated along with the simulation time. Here, the deformation ability (D) is evaluated by the following equation.
For the complex connected via both S atom and O atom in SO2, it has the chance to form the cyclic structures as shown in Figure 2 (lower panel). Such loop structures have been confirmed by both experimental29,50 and computational28 studies, which might be helpful to the proton transfer reactions and form H2SO3, HSO3-, SO32-. The plot of Figure 2 (lower panel) shows the probability distribution on describing how often various types of cyclic hydrates are encountered in the SO2(H2O)1-3 systems. For SO2(H2O)2, the percentage of single water cycles is only about 1%, far less than that of double water cycle. Previous DFT study also indicates the double water cycle is the most stable configuration for SO2(H2O)2 system28. Similarly, for system of SO2(H2O)3, the percentage of triple water cycle is much higher than that of single and double water cycles. By comparing the percentage of cycles formed in systems of SO2(H2O)2 and SO2(H2O)3, we notice that SO2(H2O)2 system has higher chance to form cycle structures. This is
∑𝑁 〈𝑅𝑖 − 𝑅̅𝑖 〉2 〈𝐷〉 = 〈√ 𝑖=1 〉 /𝑟 𝑁
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where 𝑅𝑖 is the distance between the center of mass of water droplet and the interfacial water molecule, 𝑅̅𝑖 is the average of 𝑅𝑖 , N is the number of interfacial water molecules, r is the radius of the water droplet. Based on this definition, a small D indicates the subtle deformation from spherical shape. As shown in Figure 4A, the D value of small droplets exhibits either a large value (48 water) or strong vibration (24 water) during the MD run. These results indicate that the smaller droplet is subjected to large deformation during the system evolution. This behavior can be also recognized in Figure 4B which depicts the snapshots for various sizes of water droplets at their largest D value. Such strong deformation of smaller droplet would decrease the dynamical stability of the interaction between SO2 and interfacial water. Here, the dynamic stability of configurations (SO2)S··O(H2O) and (SO2)O··H(H2O) is investigated by computing the correlation function 𝐶(𝑡), as shown in Figure 4C and 4D. Our model defines an (SO2)S··O(H2O) interaction if (SO2)S−O(H2O) distance is
