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Potential Energy Surface of O2-(H2O) and Factors Controlling Water-to-O2- Binding Motifs Wen-Shyan Sheu* and Mong-Feng Chiou Department of Chemistry, Fu-Jen Catholic UniVersity, Taipei, Taiwan 242, Republic of China ReceiVed: October 27, 2010; ReVised Manuscript ReceiVed: NoVember 26, 2010
The potential energy surface (PES) of O2-(H2O) is investigated by varying the interoxygen distance of O2via ab initio calculations with a large basis set. Although two stationary points, Cs and C2V conformers, are found along the interoxygen-distance coordinate, only the Cs conformer is identified as a minimum-energy species. We find a critical distance, rc, separating these two conformers in the PES. The Cs conformer prevails at interoxygen distances of O2- that are less than rc, while the C2V conformer dominates at the distances larger than rc. The structural features of these two conformers are also discussed. Although the water deformation energy is shown to be the stabilization source responsible for the prevalence of the Cs cluster conformer at the interoxygen distances of O2- less than rc, the ionic hydrogen bonding is the major driving force for transformation of the water binding motif from Cs to C2V when the interoxygen distance of O2increases. I. Introduction While most chemical reactions occur in solution, many degrees of freedom involved in solution phases impede a clear understanding of the reactions. Studies of the interactions between a solute molecule and a small number of solvent molecules are, therefore, important, because these interactions are crucial in order to understand how the properties of bulk solvated species evolve from those of their cluster counterparts. One of particularly interesting cluster systems is hydrated superoxide clusters, O2-(H2O)n, because O2-(H2O)n plays a critical role as an anionic charge carrier in biological systems1-4 and in the atmosphere.5-7 In aerobic life, the superoxide radical O2- is one of the continuous byproducts of respiration, and the overproduction of this superoxide radical has been implicated in diseases such as cancer, diabetes, Alzheimer’s disease, and neurodegenerative disease.1-3 Hydrated superoxide clusters play a significant role in physiological processes, as they can disproportionately dissociate to form radicals such as HOO and OH, which play a central role in aging and inflammation.1-4 In addition, O2- is a major anionic charge carrier in the atmosphere.5-7 In the gas phase, the superoxide anion is involved in many charge-transfer processes5-9 such as NO3- production, the chain reaction of O3, and clustering with water molecules. Because the superoxide anion plays important roles in many fields, studies of hydrated superoxide clusters offer an opportunity to examine the properties of these processes from a molecular perspective. A particularly interesting area of investigation is the interaction of the superoxide anion with water molecules. Theoretical studies10-15 and experimental studies8,9,16-21 have attempted to elucidate the properties of the hydration shell around the superoxide and the interaction between those water molecules and O2-. Very recent investigations have focused on the binding motifs of the small hydrated superoxide clusters, O2-(H2O)n, where 1 e n e 5. By probing the O-H stretch region of the infrared spectrum, Johnson and his co-workers revealed that four water molecules are required to complete the first solvated shell.17-19,22 Unlike the binding motifs in hydrated halide ions, which adopt cyclic water networks,22,23 water * Corresponding author. E-mail:
[email protected].
SCHEME 1: Geometric Structures of Two Stationary Conformers of O2-(H2O) a
a
Parts a and b represent the Cs and C2V forms, respectively. The symbols for atoms are also defined.
molecules attaching to the superoxide form a planar structure with two dimeric subclusters. It has already been established that superoxide suppresses the formation of cyclic water networks because of the (π*)3 electron configuration of the superoxide.17,18 The superoxide monohydrate anion was also intensively studied in order to focus on the interactions between O2- and H2O without the interference of interwater hydrogen bonding. One of the main controversies regarding the O2-(H2O) cluster is the binding motif of the water molecule to O2-. Johnson et al. showed from their IR spectra analyses that the O2-(H2O) cluster adopted an asymmetrical Cs configuration (cf. Scheme 1a) by forming a single ionic hydrogen bond (SIHB) between O2- and H2O.17,18 Apart from spectral analyses, many theoretical calculations have also been performed on O2-(H2O).10-13,15 Depending on the basis sets and theoretical methods used, the lowest energy conformer of O2-(H2O) has been identified in a Cs configuration by some approaches, but in a C2V configuration by others.10-13 However, if a more symmetric C2V binding motif (cf. Scheme 1b, double ionic hydrogen bond (DIHB) formed
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between O2- and H2O) was found to be more stable,10,11 adding the zero-point-energy (ZPE) correction reversed the energy ordering and made the Cs binding motif the most stable configuration.10-13 Therefore, the theoretical findings were consistent with the experimental results.17,18 Studies have also indicated that the potential energy surface (PES) around the minima is very flat, and the basis sets adopted should be large enough to obtain the correct binding motif.10,11,18 The Cs binding motif of O2-(H2O) should contrast with the C2V configuration adopted by Cl2-(H2O).23 Motivated by the suggestion of Johnson et al.23 that the water binding motif depends on the interatomic distance of X2-, this study investigates the PES of O2-(H2O) as a function of the interoxygen distance of O2-. This PES was previously addressed by Robinson et al. using CASSCF(13,8)/6-31++G**, and they found only one minimum (the Cs conformer) in the PES.12 However, they did not address the detailed geometric features of the cluster along the PES or discuss the implications of these features for the water-to-O2- binding motifs. Here, we will use a large basis set in the MP4(SDQ) calculation and discuss the geometric variation of the cluster along the obtained PES and the factors affecting the binding configuration. The present study will be helpful in understanding the interactions of molecular solvent anions with water molecules, the factors controlling the water-binding motifs of O2-(H2O), and the reaction dynamics involving O2-(H2O)n. II. Computational Methods Identifying the optimized structures of O2-(H2O) was not an objective of this study, but the O2-(H2O) geometries were optimized first to validate our methods and to determine the initial structures to be used in finding the PES by varying the interoxygen distance. Because of the diffuse nature of an excess electron in O2-(H2O), appropriate diffuse basis sets must be added to the standard basis sets for atoms.24-26 In all O2-(H2O) calculations, the aug-cc-pVDZ + diffs(2s2p, 2s2p) basis set was used for all atoms. The first and second exponents of the extra diffuse basis sets were used with one-third and one-ninth of the outermost exponent values of each atomic basis set. In addition, we added one d-type diffuse basis set with the exponent of one-third of the outermost exponent to the oxygen atom of water. This additional d-type diffuse basis set was necessary to accurately describe the excess electron, but it had never been previously considered.10,13 Our optimization results showed that the molecular orbital occupied by the excess electron in both the C2V and Cs forms of O2-(H2O) clusters had a larger contribution from this extra d function than any other d functions in the aug-cc-pVDZ basis set. Because the excess electron mainly occupied the antibonding π* molecular orbital of the O2, this d-type diffuse basis set may have overlapped better with the π* orbital and stabilized the singly occupied molecular orbitals (SOMOs; cf. Figure 1). The optimized geometry of O2-(H2O) was obtained by employing the Møller-Plesset fourth-order perturbation theory, which incorporates single, double, and quadruple substitutions (MP4(SDQ)). The optimized geometries found with this method were checked for stability using frequency calculations. To confirm that the theoretical method and the basis sets adopted were accurate enough in obtaining the geometries and energies of the O2-(H2O) anionic complex, geometry optimization was also performed employing the MP4(SDQ), QCISD, and CCSD levels of theory with aug-cc-pVDZ + diffs(2s2p,2s2p) and aug-cc-pVTZ basis sets, respectively. In addition, to verify the necessity of the extra diffuse functions in accurately
Sheu and Chiou
Figure 1. Contour plots of the SOMOs for O2-(H2O) in the Cs and C2V conformers.
describing the distribution of the excess electron, single point calculations at the QCISD/aug-cc-pVTZ + diffs(2s2p,2s2p)// QCISD/aug-cc-pVTZ level were also performed. The geometrical and energetic data obtained are shown in Table 1. Our results for both optimized C2V and Cs conformers calculated at the MP4(SDQ)/aug-cc-pVTZ and QCISD/aug-cc-pVTZ levels were essentially identical with the results obtained by Bell and Wright.11 However, the stability checks of the optimized geometries by vibrational frequencies analysis were unsuccessful with the MP4(SDQ)/aug-cc-pVTZ and QCISD/aug-cc-pVTZ calculations due to the excessive CPU time required, as reported by Bell and Wright.11 At each level of theory, the optimized geometries for both conformers obtained using the aug-cc-pVDZ + diffs(2s2p,2s2p) basis sets were akin to the geometries obtained using the aug-cc-pVTZ basis set (cf. Table 1). While the energies were not the same, these two basis sets produced similar energy differences between the optimized C2V and Cs conformers at each theoretical level, indicating the ability of the aug-cc-pVDZ + diffs(2s2p,2s2p) basis sets to distinguish between these two conformers. Moreover, the SOMOs were found to have negligible coefficients from the f functions but large coefficients from the extra diffuse functions in the QCISD/ aug-cc-pVTZ + diffs(2s2p,2s2p)//QCISD/aug-cc-pVTZ calculations. These observations led to the conclusion that the augcc-pVDZ + diffs(2s2p,2s2p) basis set was suitable for these calculations. In addition, because the MP4(SDQ) calculations gave results similar to the more demanding QCISD and CCSD calculations, the MP4(SDQ)/aug-cc-pVDZ + diffs(2s2p,2s2p) calculations were also used to find the PES of O2-(H2O). All calculations were carried out using the GAUSSIAN 03 package.27 In addition to the geometry optimization calculations, a relaxed potential energy surface (RPES) scan of O2-(H2O) was also performed. The interoxygen distance of O2- was changed systematically to obtain the relaxed potential energy of O2-(H2O) clusters at various interoxygen distances. In the scanning calculation, no constraints were imposed on any degrees of freedom other than the interoxygen distance of O2-. The scanning region of the interoxygen distance was extended from 1.27 to 1.43 Å to include the two found stationary structures in the RPES. The step size along the scanning coordinate was 0.01 Å. The 〈S2〉 values for scanned geometries were less than 0.8, close to the expected value of 0.75. This indicates that the spin contamination was small.11 Finally, the geometric features and other factors of the cluster along the RPES were analyzed to understand the factors controlling the binding motif of the O2-(H2O) cluster.
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TABLE 1: Geometric Parameters and Energies for the Optimized Structures of O2-(H2O) Calculated at the MP4(SDQ), QCISD, and CCSD Levels of Theorya aug-cc-pVDZ + diffs(2s2p,2s2p) MP4(SDQ)
QCISD
aug-cc-pVTZ CCSD
MP4(SDQ)
QCISD
CCSD
1.341 0.973 2.006 93.7 132.1 91.1 -226.499 49
1.339 0.973 1.998 93.8 132.2 91.3 -226.500 03
1.336 0.973 1.995 93.8 132.2 91.3 -226.497 37
1.330 0.995 0.959 1.692 98.7 165.7 94.7 -226.500 02 -0.333
1.342 1.002 0.959 1.653 99.5 168.1 96.3 -226.501 34 -0.822
1.335 1.000 0.959 1.657 99.5 168.1 96.2 -226.498 50 -0.709
C2V Conformers geometry r(O1-O2) r(Ow-H1) r(O1-H1) ∠H1-Ow-H2 ∠Ow-H1-O1 ∠H1-O1-O2 energy/hartrees
1.351 0.978 1.997 93.4 132.3 91.0 -226.319 13
1.349 0.978 2.006 93.6 132.5 91.0 -226.321 16
1.348 0.978 1.997 93.5 132.6 91.1 -226.318 22 Cs Conformers
geometry r(O1-O2) r(Ow-H1) r(Ow-H2) r(O1-H1) ∠H1-Ow-H2 ∠Ow-H1-O1 ∠H1-O1-O2 energy/hartrees ∆E/(kcal mol-1) a
1.339 0.999 0.965 1.701 98.5 166.0 94.1 -226.319 78 -0.408
1.356 1.006 0.964 1.659 99.4 168.8 95.9 -226.322 73 -0.986
1.347 1.004 0.964 1.665 99.4 168.8 95.8 -226.319 53 -0.822
Bond lengths are in angstroms, and angles are in degrees. ∆E is defined as ∆E ≡ ECs - EC2V.
III. Results and Discussion A. RPES and Geometry Features. Two stationary points of O2-(H2O) were obtained by geometry optimization at the MP4(SDQ)/aug-cc-pVDZ + diffs(2s2p,2s2p) level of theory. As shown in Table 1, the optimized structures of O2-(H2O) are in the Cs and C2V forms with superoxide interoxygen distances of 1.339 and 1.351 Å, respectively. In addition, the optimized Cs conformer is found to be 0.408 kcal/mol lower in energy than the C2V conformer. While the vibrational frequencies for the optimized Cs conformer are all real, there is an imaginary vibrational frequency, corresponding to the in-plane rocking motion of water, in the C2V conformer. Therefore, there is only one stable conformer of the Cs form for O2-(H2O). The contour plots of the SOMOs for the Cs and C2V conformers are shown in Figure 1. The excess electron is diffuse and spreads all over the O2-(H2O) cluster with a narrower distribution in the O-O direction of O2-. In addition, the nodal planes cutting through the hydrogen and oxygen atoms are visible for both Cs and C2V conformers. Figure 2 shows the RPES. There is only one minimum-energy conformer of the Cs form in the RPES, consistent with the CASSCF results of Robinson et al.12 The curve is very flat around the minimum-energy region, as indicated by previous researchers.10,11,18 A change of the curvature is visible near 1.37 Å, implying that certain conformation transition occurs in this region. The water-to-O2- binding motif is revealed in Figure 3. The asymmetric factor, χ, is defined as the difference between ∠Ow-O1-O2 and ∠Ow-O2-O1. Therefore, χ should be “0” for the C2V conformer because the cluster is symmetrical along the C2 axis of the water molecule. It is apparent from the figure that, because of χ ∼ 0, the water molecule is bound to O2with the DIHB, i.e., the C2V motif, at large O1-O2 distances. However, a transition in the water binding motif is observed at the critical distance rc ∼ 1.37 Å (cf. Figure 3). At interoxygen distances smaller than this critical distance, the cluster adopts a Cs conformation, confirming that the interoxygen distance of O2- is the critical factor in determining the water binding motif of the cluster, as suggested by Price et al.23 However, χ is not a constant for the Cs conformers with respect to the interoxygen
Figure 2. Relaxed potential energy surface of an O2-(H2O) cluster. All energies are plotted relative to the lowest energy of this potential surface. The open squares in the small interoxygen distance range are the energies obtained with the restricted geometries of the C2V structures. The geometry changed from Cs to C2V at r(O1-O2) greater than the critical distance rc ∼ 1.37 Å. Lines are drawn to provide a visual aid.
distances. The angle difference continuously increases when the O1-O2 distance decreases, indicating that the water binding motif becomes more asymmetric at smaller interoxygen distances in the Cs binding region. Other geometry features of O2-(H2O) are also examined. Figure 4 plots various O-H distances. The IHB distance of O1-H1 in the C2V conformer is nearly a constant of ∼1.99 Å, which is slightly larger than the hydrogen binding distance of ∼1.94 Å for a water dimer.28,29 The greater distance indicates that the IHB is slightly weaker than the hydrogen bond of a water dimer. Nevertheless, the IHB distance significantly decreases when the O1-O2 distance approaches rc. Even after the Cs minimum point is passed, this decreasing trend continues in the studied distance range. This result indicates a continuous increase in the interaction strength between O2- and H2O when the O1-O2 distance is shortened. The IHB distance in the Cs conformer is less than 1.7 Å, a distance much smaller than 1.94 Å, indicating a strong IHB interaction for the Cs conformer. In fact, this IHB distance is so small that the proton should be
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Figure 3. Dependence of the asymmetric factor, χ, on the interoxygen distance of O2-. Lines are drawn to provide a visual aid.
Figure 4. Dependence of various O-H distances on the interoxygen distance of O2-. Lines are drawn to provide a visual aid.
considered to be, at least partially, transferred to O2- for Cs conformers.30 This observation is consistent with the expected formation of HOO for the O2-/H2O reaction.4 A similar result was also found in the closely related S2-(H2O) complex.31 In comparison, the O-H distances of H2O do not change as much during the transition from the C2V to Cs binding motif. They only vary by (0.02 Å, depending on whether or not the O-H bond under consideration is pointing toward O2-. This variation is due to the response of O-H bond distances to the changes in IHB strength during the transition stages of the binding motif. Various angle variations with r(O1-O2) are also plotted in Figure 5. In the C2V binding motif, the Ow-H1-O1 binding angle is approximately 133°, which is much smaller than the expected bonding angle of about 172° for the hydrogen bond of a water dimer,28 implying a weaker IHB for the C2V conformers than the hydrogen bond of a water dimer. However, when the O1-O2 distance decreases, the Ow-H1-O1 binding angle increases, reaching about 167° at r(O1-O2) ) ∼1.27 Å, which signifies a strong IHB interaction between O2- and H2O in the Cs motif. These observations are in line with the findings derived from the IHB distances discussed above. With regard to the H1-O1-O2 and the H1-Ow-H2 angles, they are about the same for the C2V conformers, but these angles slightly increase in response to the decrease in the O1-O2 distance in the Cs motif, reflecting the increasing one-sided OH interaction strength of H2O with O2-.
Sheu and Chiou
Figure 5. Dependence of various angles on the interoxygen distance of O2-. Lines are drawn to provide a visual aid.
Johnson and his co-workers found that the O-H stretching spectrum of O2-(H2O) in the mid-IR region featured a small sharp band in the vicinity of the free OH transition, denoted as the F-band, and diffuse peaks red-shifted with respect to the F-band. This observation led them to conclude that O2-(H2O) adopted a Cs binding motif,17,18 whereas a corresponding sharp peak observed for Cl2-(H2O) was attributed to the C2V binding motif.23 However, the questions of why the diffuse peaks of O2-(H2O) were red-shifted to such a significant degree, even by comparison with those of Cl2-(H2O), and why these diffuse features still persisted at a temperature so low that three Ar atoms could attach to the cluster remain to be answered.18 The present findings can shed some light on these questions. As addressed previously, at a low temperature, the cluster is in the minimum-energy configuration of the Cs binding motif with a fluctuating interoxygen distance of O2- because of the ZPE. As a result, the asymmetric factor χ constantly changes in the Cs region, which corresponds to a continuous variation in the IHB strength between H2O and O2- molecules, as supported by the sensitive dependence of the IHB bond length on r(O1-O2) in the Cs conformation shown in Figure 4. In addition, the strong IHB strength in the Cs motif weakens the O2--pointing O-H bond of H2O, which triggers a considerable red shift of the O-H stretching frequency. These reasons account for the diffuse and significantly red-shifted feature observed for the O2-(H2O) spectrum in the mid-IR region.17,18 By contrast, the sharp and less red-shifted peak of Cl2-(H2O) can be explained by the small variation in the IHB distance and weaker IHB strength in the C2V conformation. In fact, when Cl2-(H2O) is in the Cs conformation at a warmer temperature, diffuse peaks appear,23 just as in the case of O2-(H2O). B. Factors Controlling the Binding Motifs of O2-(H2O). As addressed above, C2V conformers are preferred at larger interoxygen distances, but Cs conformers are predominant at smaller distances. Because a cluster binding motif is determined by its binding energy, in order to understand the conformation transition, it is necessary to explore various energy components involved in determining the binding energy between H2O and O2-, symbolized by EB, which is calculated as follows:
EB ≡ E(O2-(H2O)) - EF(H2O) - EF(O2-)
(1)
where EF(H2O) and EF(O2-) are the energies of free H2O and O2- molecules, respectively. With the introduction of the H2O
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Figure 6. Dependence of various energy differences on the interoxygen distance of O2- in the range of r(O1-O2) less than the critical distance rc of 1.37 Å. Lines are drawn to provide a visual aid.
and O2- energies constrained at the geometries of a conformation of O2-(H2O), denoted as EC(H2O) and EC(O2-), the binding energy can be further divided into several parts as EB ) {E(O2-(H2O)) - EC(H2O) - EC(O2-)} + {EC(H2O) - EF(H2O)} + {EC(O2-) - EF(O2-)} ) EH + ED(H2O) + ED(O2-)
(2) where ED(X) ≡ EC(X) - EF(X) is the deformation energy of X ()H2O and O2-) on the formation of an O2-(H2O) cluster. In addition, EH ≡ E(O2-(H2O)) - EC(H2O) - EC(O2-) is the interaction energy between H2O and O2-, while the H2O and O2- geometries remain fixed at the cluster conformation. Hence, EH is mainly due to the IHB energy between these two species. To draw a meaningful comparison between the two conformers with respect to relative stability, it is desirable to determine both C2V and Cs conformers at the same O1-O2 distance. This determination can be achieved for r(O1-O2) < rc, because, at these distances, the Cs conformers have lower energies and the C2V conformers can be obtained by geometry optimization with a C2V symmetry constraint. The RPES for the C2V conformers obtained through this approach is also plotted in Figure 2. As expected, the obtained RPES forms a smooth curve with that in the region of r(O1-O2) > rc. Therefore, the binding energy difference between these two conformers at the same interoxygen distance, ∆EB, can be expressed as follows:
∆EB ≡ EBCs - EBC2V ) ∆EH + ∆ED(H2O) + ∆ED(O2)
(3)
where EBCs and EBC2V are the binding energies for the Cs and C2V conformers, respectively. In addition, ∆ED(X) is the deformation energy difference between the Cs and C2V conformers at the same interoxygen distance. ∆ED(O2-) is zero because EC(O2-) is the same for the Cs and C2V conformers. Therefore, the binding energy difference for these two conformers arises from the ionic hydrogen binding energy between H2O and O2- as well as the deformation energy of the water molecule upon binding, as anticipated.
Figure 6 shows the calculation results. The counterpoise method was employed to correct the basis set superposition error.32 Noted from the definition, a negative energy difference for ∆EB, ∆EH, or ∆ED indicates that a Cs conformer is preferred at that particular interoxygen distance; otherwise a C2V conformer is preferred. The figure shows that ∆EB is negative, as expected, because a Cs conformer is more favorable at smaller O1-O2 distances. However, this energy difference is small (less than 1.0 kcal/mol). In addition, this difference is reduced when the O1-O2 distance increases, indicating that the Cs binding motif becomes less stable when the interoxygen distance increases and eventually transforms into a C2V binding motif, as observed in Figure 3. The driving force for the transformation can be discerned from the trends of ∆EH and ∆ED(H2O) in Figure 6. This figure clearly indicates that ∆ED(H2O) is negative and ∆EH is positive in the range of an interoxygen distance of less than rc, suggesting that the deformation energy of H2O is the major stabilization source for the prevailing Cs binding motifs in the region. The positive ∆EH can be attributed to two IHBs in the C2V conformers, although the individual IHB strength is stronger for the Cs conformers than the C2V conformers. When the O1-O2 distances increase, ∆ED(H2O) tends to decrease only slightly, signifying that the water deformation energy difference is not very sensitive to the distance variation under consideration. This observation is at odds with the trend of ∆EB. On the other hand, ∆EH increases as the interoxygen distance increases, a trend similar to that of the binding energy difference. Therefore, it can be concluded that when the O1-O2 distance increases, the driving force for the Cs-to-C2V binding motif transformation in the O2-(H2O) cluster is the IHB energy difference between these two conformers. The IHB energy difference acts as the driving force for the conformation transition because of the relatively sensitive dependence of the IHB strength for the Cs conformers on the interoxygen distance, as revealed in the IHB bond distance variation discussed in Figure 4. IV. Conclusions We have investigated the potential energy surface of O2-(H2O) by varying the interoxygen distance of O2- via ab initio calculations with a large basis set. Two stationary conformers, Cs and C2V, are found along the r(O1-O2) coordinate. However, only the optimized Cs conformer is identified as a minimum-energy species. The optimized C2V conformer is discovered to have an imaginary vibrational frequency, corresponding to the in-plane rocking motion of water. In addition, we find that a critical distance rc of about 1.37 Å separates these two conformers in the PES: the Cs conformer prevails at r(O1-O2) < rc while the C2V conformer dominates at r(O1-O2) > rc. The structural features of these two conformers are also discussed. After exploring the ionic hydrogen binding O1-H1 distances and Ow-H1-O1 angles, we conclude that the IHB strength of the C2V conformer is weaker than that of the Cs conformer and that, at least, a partial proton transfer from H2O to O2- occurs in the Cs conformer. In addition, although the water deformation energy difference, ∆ED(H2O), is the stabilization source for the prevalent Cs cluster conformer at O1-O2 distances less than rc, the ionic hydrogen bonding strength difference, ∆EH, is the major driving force for the Cs-to-C2V transformation of the water binding motif when the interoxygen distance of O2- increases. We believe that the present study will be beneficial in understanding the interactions of molecular solvent anions with water molecules, the factors controlling the water binding motifs of O2-(H2O), and the reaction dynamics involving O2-(H2O)n.
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Acknowledgment. The work was supported by the National Science Council, Taiwan under Contract No. NSC 98-2113M-030-005-MY2. We are also grateful to the National Center for High-Performance Computing for providing computing resources. References and Notes (1) Prabhakar, R.; Siegbahn, P. E. M.; Minaev, B. F.; Agren, H. J. Phys. Chem. B 2002, 106, 3742. (2) Salvemini, D.; Wang, Z.-Q.; Zweier, J. L.; Samouilov, A.; Macarthur, H.; Misko, T. P.; Currie, M. G.; Cuzzocrea, S.; Sikorski, J. A.; Riley, D. P. Science 1999, 286, 304. (3) Hearn, A. S.; Cabelli, D. E.; Nick, H. S.; Tainer, J. A.; Silverman, D. N. Biochemistry 2009, 48, 3417. (4) Schoneich, C. Exp. Gerontol. 1999, 34, 19. (5) Ferguson, E. E.; Arnold, F. Acc. Chem. Res. 1981, 14, 327. (6) Yang, X.; Castleman, A. W. J. Am. Chem. Soc. 1991, 113, 6766. (7) Wayne, R. P. Chemistry of Atmospheres, 3rd ed.; Oxford University Press: Oxford, U.K., 2002. (8) Buntine, M. A.; Lavrich, D. J.; Dessent, C. E.; Scarton, M. G.; Johnson, M. A. Chem. Phys. Lett. 1993, 216, 471. (9) Arnold, S. T.; Morris, R. A.; Viggiano, A. A.; Johnson, M. A. J. Phys. Chem. 1996, 100, 2900. (10) Lee, H. M.; Kim, K. S. Mol. Phys. 2002, 100, 875. (11) Bell, A. J.; Wright, T. G. Phys. Chem. Chem. Phys. 2004, 6, 4385. (12) Robinson, E. M. C.; Holstein, W. L.; Stewart, G. M.; Buntine, M. A. Phys. Chem. Chem. Phys. 1999, 1, 3961. (13) Antonchenko, V. Y.; Kryachko, E. S. J. Phys. Chem. A 2005, 109, 3052. (14) Antonchenko, V. Y.; Kryachko, E. S. Chem. Phys. 2006, 327, 485. (15) Seta, T.; Yamamoto, M.; Nishioka, M.; Sadakata, M. J. Phys. Chem. A 2003, 107, 962. (16) Lavrich, D. J.; Buntine, M. A.; Serxner, D.; Johnson, M. A. J. Phys. Chem. 1995, 99, 8453. (17) Weber, J. M.; Kelley, J. A.; Nielsen, S. B.; Ayotte, P.; Johnson, M. A. Science 2000, 287, 2461. (18) Weber, J. M.; Kelley, J. A.; Robertson, W. H.; Johnson, M. A. J. Chem. Phys. 2001, 114, 2698.
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