Photoelectron Spectroscopy and Ab initio Calculations of Peroxy Form

Apr 22, 2010 - A photoelectron spectrum is reported for gas-phase SO4− formed in an electron-impact ionized free jet of SO2(1%)/O2(10%)/Ar. The vert...
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J. Phys. Chem. A 2010, 114, 5640–5647

Photoelectron Spectroscopy and Ab initio Calculations of Peroxy Form of SO4- Anion Sayuri Zama,† Ryuzo Nakanishi,† Mitsuo Yamamoto,‡ and Takashi Nagata*,† Department of Basic Science, Graduate School of Arts and Sciences, and the Komaba Organization for Educational DeVelopment (KOMED), College of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: October 31, 2009; ReVised Manuscript ReceiVed: April 1, 2010

A photoelectron spectrum is reported for gas-phase SO4- formed in an electron-impact ionized free jet of SO2(1%)/O2(10%)/Ar. The vertical detachment energy (VDE) of the SO4- species is determined to be 3.78 ( 0.02 eV, which is significantly smaller than the VDE value reported for the sulfur-centered form of SO4extracted from an Na2S2O8 solution. With the aid of MP2/6-311+G(2df) calculations, the newly formed species is identified as a peroxy form of SO4-, which is well described as OOSO2-(C1, 2A) having an S-O linkage between the O2 and SO2 units. The electronic properties of OOSO2- are characterized by the singly occupied molecular orbital (SOMO) constructed by an out-of-phase superposition of the 1a2 orbital of SO2 and the half-filled 2pπg* of O2-. On the basis of these findings, a possible formation mechanism of OOSO2- is discussed in terms of a “solvent-mediated” addition reaction, where O2- stabilized by solvation attacks SO2 while retaining the 2pπg* excess electron. Introduction

Process 3.

The sulfate radical anion, SO4-, has often been referred to as an important intermediate in both the condensed-phase and the gas-phase reactions. In the condensed phase, SO4- is produced either by reaction of solvated electrons with peroxodisulfate anion in irradiated aqueous solutions,1 eaq- + S2O82f SO42- + SO4-, or by photolysis of sodium persulfate solutions,2 S2O82- + hν f 2SO4-; these processes have been exploited for extensive investigations of SO4- reactions with various kinds of compounds.3–10 In the gas phase, formation of SO4- has been studied mainly from the viewpoint of atmospheric chemistry. Those studies have revealed that SO4- is formed through negative ion reactions with atmospheric trace constituents; the following three types of reaction processes have been reported in the literature.11–13 Process 1.

O2- + SO2 f SO2- + O2

k ) 1.9 × 10-9 molecule-1 cm3 s-1

(1a)

SO2- + O2 + M f [SO2- · O2]* + M f SO4- + M (1b) Process 2.

CO3- + SO2 f SO3- + CO2

k ) 2.3 × 10-10 molecule-1 cm3 s-1

-

-

SO3 + NO2 f SO4 + NO

(2a)

-11

k ≈ 7 × 10

molecule-1 cm3 s-1

(2b)

O2- · (H2O) + SO2 f SO4- + H2O

k ) 1.8 × 10-9

molecule-1 cm3 s-1

(3)

Interestingly, Ferguson et al. found that SO4- formed in process 1 reacted rapidly with NO2 whereas one formed in process 2 did not; on the basis of this difference in the reactivity, they predicted the existence of two different isomeric forms of SO4- (Scheme 1).11 They assumed a sulfur-centered bonding configuration (A) as the most stable, nonreactive form of SO4-, and a peroxy configuration (B) as the more energetic, reactive isomer. Computational study of the SO4- structures was first made by McKee.14 The B3LYP calculations predicted a sulfur-centered geometry of C2V symmetry for the global minimum structure. The B3LYP results were further confirmed by Zheng et al.;15 they employed an MP2/ CBS method to calculate the adiabatic electron affinities of SO4 isomers. Experimentally, Wang et al. have measured the photoelectron spectrum of SO4- prepared in the gas phase by applying electrospray ionization to an Na2S2O8 solution.16 They observed a large electron detachment energy for the SO4- species: the adiabatic and the vertical detachment energies were determined to be 5.10 ( 0.10 and 5.40 ( 0.10 eV, respectively.16 These large values were in consonance with the theoretical prediction that SO4- had a sulfur-centered bonding configuration of C2V symmetry. Thus, the geometrical and electronic properties of the sulfur-centered SO4- isomer have so far been explored by several research groups. As for the peroxy form of SO4-, however, a theoretical prediction was made about its existence15 but no effort has yet been made to confirm it experimentally. SCHEME 1

* Author to whom correspondence should be addressed. E-mail: nagata@ cluster.c.u-tokyo.ac.jp. † Graduate School of Arts and Sciences. E-mail: S.Z., zama@ cluster.c.u-tokyo.ac.jp; R.N., [email protected]. ‡ College of Arts and Sciences. E-mail: [email protected].

10.1021/jp910403p  2010 American Chemical Society Published on Web 04/22/2010

Peroxy SO4- anion

Figure 1. Mass spectrum of product anions formed in the electronimpact ionized free jet of Ar containing SO2 (1%) and O2 (10%). Each mass peak is accompanied by a tiny peak arising from the 34S isotopomer.

In this article, we report on the formation of a peroxy form of SO4- anion, OOSO2-, by exploiting intracluster reactions that proceed in an SO2/O2 binary cluster accommodating an excess electron. The formation method employed here is obviously inspired by process 1; one advantage of intracluster reactions over the collisional gas-phase reactions is that threebody processes take place more efficiently, and sometimes in more emphatic ways, as “solvent-mediated” processes in clusters. The electronic properties of the product anions are probed by anion photoelectron spectroscopy. In conjunction with the photoelectron measurements, the minimum energy structure of OOSO2- is explored by MP2/6-311+G(2df) calculations. By combining the experimental findings with the theoretical results, we demonstrate the existence of OOSO2- along with its unique electronic properties and discuss the formation mechanism of OOSO2- in terms of “solvent-mediated” addition of superoxide ion to SO2. Experimental Section The experiment was carried out with an apparatus consisting of a cluster-anion source, a time-of-flight (TOF) mass spectrometer and a photoelectron spectrometer. Details of the experimental setup have been described elsewhere.17 The SO4anions were prepared by using an electron-impact ionized free jet. A gas mixture of SO2 (1%), O2 (10%), and Ar was supersonically expanded through a pulsed valve with a stagnation pressure of ≈1 × 105 Pa. The pulsed free jet was then crossed with a continuous beam of 250 eV electrons at the expansion region, which efficiently produced secondary slow electrons. Within the expanding free jet, SO4- anions were formed via cluster reactions involving the slow electrons. The SO4- anions along with the anionic byproduct in the free jet were extracted at ≈15 cm downstream from the nozzle, perpendicularly to the initial beam direction by applying a pulsed electric field. They were further accelerated up to 1.2 keV, massanalyzed by a 2.1 m TOF mass spectrometer, and detected by a microsphere plate. Figure 1 shows a typical mass spectrum of anions obtained under the present experimental conditions. The mass peaks appearing at m/z ) 64, 96, and 128 are assignable to SO2- (and/or O4-), SO4-, and (SO2)2-, respectively. As the mass number of an S atom (A ) 32) is just twice as large as that of an O atom (A ) 16), the m/z values do not provide unambiguous mass assignments. The assignments were confirmed by photoelectron measurements; it turned out that the m/z ) 64 peak contained a tiny contribution of O4-. Photoelectron spectra of the anions were acquired by using a magnetic-bottle type spectrometer equipped at the end of the TOF tube. Mass selection was achieved by a pulsed beam

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5641 deflector prior to photodetachment. At the photodetachment region, the unfocused fourth harmonic (266 nm) of a Q-switched Nd:YAG laser was timed to intersect the mass-selected SO4ion bunch. The laser fluence was kept below 5 mJ pulse-1 cm-2. The photoelectrons were detached in a strong inhomogeneous magnetic field (≈1000 G), guided by a weak magnetic field (≈15 G) to the end of a 1 m flight tube, detected by a microsphere plate with a diameter of 27 mm. Each spectrum presented in this article represents an accumulation of ≈200 000 laser shots with background subtraction. The measured electron kinetic energy is calibrated against the known photoelectron band of (CO2)n-.18 Aiming to mimic the O2- · (H2O)n + SO2 reaction process (eq 3) with the present experimental setup, we exploited another method for preparing SO4-: entrainment of SO2 into a free jet containing O2- · (H2O)n species. The O2- · (H2O)n reactants were produced in the ionized free jet as follows. A gas mixture was prepared by passing Ar containing O2 (10%) at the pressure of 1 × 105 Pa through a reservoir of H2O. The temperature of the reservoir was kept at ≈0 °C. By using the gas mixture with a stagnation pressure below 1 × 105 Pa, we obtained an ionized free jet predominantly containing O2- · (H2O)n species. The target SO2 was introduced into the source chamber through an effusive nozzle. The partial pressure of SO2 was kept in the range (1.5-2.5) × 10-2 Pa. The SO2 molecules in the ambient pressure were entrained into the free jet, where they encountered O2- · (H2O)n while the jet was drifting in the source chamber. The product anions formed in the O2- · (H2O)n + SO2 reaction were extracted, mass-analyzed and detected by the same method as above. Calculation Method Ab initio calculations were performed by using the GAUSSIAN 03 program package19 to obtain the optimized geometries and the total energies for SO4- isomers. We began with geometry optimizations by the second-order Møller-Plesset (MP2) method with the 6-31+G(d) basis set. The geometries obtained at the MP2/6-31+G(d) level were further optimized by using larger basis sets, such as 6-311+G(d) and 6-311+G(2df). Frequency analyses were also made after the optimizations to confirm that all the calculated vibrational frequencies were real so that the optimized structures were indeed located at the local minima of the potential energy surface. The total energies of the SO4- isomers were evaluated by single-point CCSD(T) calculations at the MP2-optimized geometries (i.e., CCSD(T)/ 6-311+G(2df)//MP2/6-311+G(2df)). The vertical detachment energy (VDE) of each isomeric form was also evaluated by subtracting the CCSD(T) energy of the anion from that of the neutral species at the anion equilibrium geometry. Results and Discussion A. Photoelectron Spectrum of SO4-. Figure 2a shows the photoelectron spectrum of SO4- measured at 266 nm (4.66 eV). The photoelectron counts are plotted as a function of the electron binding energy defined as Eb ) hν - Ek, where hν and Ek represent the photon energy and the kinetic energy of the photoelectrons, respectively. The counts start to increase at Eb ≈ 2.6 eV and reach a maximum at ≈3.8 eV. Although the photoelectron band has been recorded only partially in the present study, we assume here that the band consists of a single broad peak and has a symmetrical bell-shaped envelope, as is often the case with anions of molecular clusters. The maximum of the photoelectron band is interpreted as the vertical detachment energy (VDE) of the cluster anion. To determine the VDE,

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Zama et al.

Figure 3. Photofragment mass spectrum of SO4- recorded at the photon energy of 4.66 eV. The upward peaks correspond to the production of photofragment anions while the downward peak to the photodestruction of parent anions. Figure 2. (a) Photoelectron spectrum of SO4- formed in the gas-phase reaction. The spectrum was recorded at the photon energy of 4.66 eV. The dots represent the experimental data. The solid curve is the bestfit Gaussian profile (eq 4). (b) Photoelectron spectrum of SO4- extracted from an Na2S2O8 solution (reprinted from Figure 1 of ref 16). The spectrum was recorded at 6.42 eV.

the observed spectral profile is fitted to a Gaussian function by a nonlinear least-squares method, where the band contour is given as

[

I(Eb) ) C exp

-(Eb - E0)2 δ2

]

(4)

Here E0 corresponds to VDE and δ is related to the spectral width by fwhm ) 2(ln 2)1/2δ. As readily seen in Figure 2a, the observed band profile is well approximated by eq 4. The analysis has provided VDE and fwhm values of 3.78 ( 0.02 and 0.83 ( 0.08 eV, respectively. In Figure 2b also shown for comparison is the photoelectron spectrum of SO4- reported by Wang et al.;16 they employed electrospray ionization (ESI) to produce SO4- anions from an Na2S2O8 solution. It is obvious from the comparison in Figure 2 that the photoelectron band observed in the present study differs remarkably from that reported by Wang et al. The former has its band maximum at 3.78 eV, whereas the latter is at 5.40 eV. This indicates that the gas-phase SO4- species formed in the present study possesses electronic structures quite different from those of SO4- extracted from the solution. To ensure that the observed 3.8 eV photoelectron band arises from the primary photodetachment process of SO4-, and also that there is no contribution from secondary processes such as photodetachment from photofragments and/or photoexcited species, we performed photodestruction measurements. The measurements were carried out by using a function of the equipment as a collinear tandem mass spectrometer. The SO4- anions were spatially and temporally focused at the point 1.10 m downstream from the first acceleration grids, where they were irradiated by 266 nm photons. The laser fluence used was in the range 5-20 mJ pulse-1 cm-2. The photofragments along with the residual parent anions were collinearly reaccelerated by a pulsed electric field (1.5 kV, 150 µs duration) at the second acceleration assembly, and mass-analyzed during the 0.9 m flight in the rest of the TOF tube. The fragment ion signals were averaged over 500 laser shots. Figure 3 represents the observed photofragment mass spectrum of SO4-. The spectrum was obtained by subtracting

Figure 4. Optimized geometries for SO4- isomers calculated at the MP2/6-311+G(2df) level. Bond lengths and bond angles are given in units of angstroms and degrees. The relative energy, ∆E, of each isomeric form with reference to the global minimum structure (isomer I) is calculated at CCSD(T)/6-311+G(2df)//MP2/6-311+G(2df) and given in units of electronvolts.

the photofragment signals measured with laser-off from those with laser-on. The linear dependence of the signal intensities on the irradiation fluence was also confirmed. The downward peak at m/e ) 96 in the spectrum corresponds to the depletion of the parent SO4-, while the upward ones at m/e ) 32 and 64 to the production of fragment anions, O2- and SO2-, respectively. The yield for each photodestruction process is estimated from the peak intensities to be Γ ) 0.92 for the depletion of SO4- due to photodetachment, 0.06 for the SO2- production, and 0.02 for the O2- formation. On the basis of these findings, we conclude that (i) the 3.8 eV photoelectron band is wholly ascribable to the photodetachment from SO4- and that (ii) the small VDE value is a strong evidence for the formation of SO4in an isomeric form other than the sulfur-centered configuration. No information is available on the existence or absence of photoelectron bands in the high-energy region of Eb > 4.3 eV in the present study, as 266 nm radiation is employed for photodetachment. B. Calculated Structures of SO4- Isomers. By referring to the isomeric forms of SO4- predicted in the previous studies,14–16 we considered six configurations of C1, C2V, C3V, D2d, and Td symmetries in the geometry optimization. Within the appropriate point group symmetry, each geometry was fully optimized by using the MP2 method with the 6-31+G(d), 6-311+G(d) and 6-311+G(2df) basis sets. Figure 4 displays the overview of the six optimized structures obtained at the MP2/ 6-311+G(2df) level. The isomeric forms are referred to as I-VI according to the CCSD(T) energy ordering. Isomers I-IV are

Peroxy SO4- anion

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TABLE 1: Ab Initio Energies and Vertical Detachment Energies of SO4- Isomers at CCSD(T)/6-311+G(2df)//MP2/ 6-311+G(2df) isomer I II III IV V VI

a

symmetry C2V C3V D2d Td C1 C2V

term 2

B1 A2 A2 2 F1 2 A 2 A1 2

2

VDE (eV)

total energy (hartrees)

tripletb

singletc

-698.2344652 -698.2295822 -698.2242378 -698.2236869 -698.1550255 -698.0641970

5.33 5.26 5.04 5.10 3.34 4.42

5.75 5.80 5.43 5.52 4.35 1.29

a

Isomers are labeled in ascending order of total energy (see Figure 4). b Vertical detachment energy for the transition to the lowest triplet state of neutral SO4. c Vertical detachment energy for the transition to the lowest singlet state of neutral SO4.

of sulfur-centered type, V has a peroxy form, and VI contains a three-membered O-S-O ring structure. Their CCSD(T) energies are summarized in Table 1. Wang et al. pointed out that the B3LYP/6-31+G(d) optimizations predicted I(C2V, 2B1) as the global minimum structure whereas III(D2d, 2A2) was the lowest in energy at the MP2/6-31+G(d) level.16 The present CCSD(T)/6-311+G(2df)//MP2/6-311+G(2df) calculations provide the energy ordering in that III is higher by 0.28 eV than I, being consistent with the B3LYP results.14 It should be also noted here that the ground electronic state of SO4- in Td symmetry (isomer IV) is triply degenerate and, as a result, Jahn-Teller unstable. The present calculations are consistent with this inference: lower-symmetry structures (isomer I-III), which are all close in energy, are calculated to be more stable than isomer IV. This situation in the sulfur-centered SO4isomers is reminiscent of the C2V, C3V, and D2d Jahn-Teller distortions in CH4+.20 Another point to be stressed here is that the calculated VDE shows a clear configuration-dependent feature; all the calculated values for the sulfur-centered configurations lie above 5 eV, while those for isomers V and VI are in the range 1.29-4.42 eV (Table 1). The main result in the present calculations is a distinct difference in VDE between the sulfur-centered isomers and the peroxy form of SO4-, which can be compared directly with the experimental results. As mentioned in the previous section, the VDE is determined experimentally to be 3.78 ( 0.02 eV. Comparing the experimental value with the ab initio results, we identify V(C1, 2A), for which VDE is calculated to be 3.34 eV, as the spectral carrier of the 3.8 eV photoelectron band. Hereafter, we focus our discussion mainly on V(C1, 2A). The SO4- isomer identified in the present study is henceforth referred to as “OOSO2-”, indicating explicitly its peroxy configuration. The structure parameters of OOSO2-(C1, 2A) are shown pictorially in Figure 5. The OO-SO2 distance is calculated to be 2.03 Å, which is obviously longer than the covalent bonds in ordinary species. In response to this large nuclear distance, the frequency analyses predict a low vibrational frequency of ≈30 cm-1 for the torsion oscillation of the two groups OO and SO2 about the O-S axis. In a related matter, the geometry optimization with the 6-311+G(d) basis set gives a local minimum structure of Cs symmetry, where the terminal O-O lies in the σv plane of SO2 bisecting the angle ∠OSO. The O-O distance is estimated to be 1.29 Å, which lies between the bond length of O2 (1.22 Å at MP2/6-311+G(2df)) and that of O2(1.35 Å at MP2/6-311+G(2df)). This indicates a partial accommodation of the excess electron in the peroxy moiety, being consistent with the calculated net Mulliken charge population (-0.53). Figure 5c depicts the singly occupied molecular orbital

Figure 5. Structure parameters for OOSO2-(C1, 2A) at the MP2/6311+G(2df) level. (a) Bond lengths in units of angstroms. (b) Bond anglesinunitsofdegrees.Thedihedralanglesare∠O(1)-S(3)-O(4)-O(5) ) 145.4° and ∠O(2)-S(3)-O(4)-O(5) ) -98.3°. (c) Singly occupied molecular orbital of OOSO2-.

(SOMO) of OOSO2-, which is constructed by an out-of-phase superposition of the second HOMO (πSO, 1a2) of SO2 and the half-filled π* orbital (2pπg*) of O2-. Interestingly, neither the HOMO nor the LUMO of SO2 take substantial part in the OO-SO2 bond formation. It is even more amazing that the CCSD(T) energy of OOSO2- is higher by 2.16 eV with respect to the global minimum structure, I(C2V, 2B1), while the stabilization energy is calculated to be 0.97 eV with reference to the ˜ 2B1) + O2(X3Σg-) asymptote, and 1.77 eV to SO2(X ˜ 1A1) SO2-(X 2 + O2 (X Πg). A possible formation mechanism of such a highlying SO4- isomer will be discussed in the next section. C. Formation Mechanism. As mentioned above, the halffilled 2pπg* orbital of O2- plays a role as a partial molecular orbital constructing the SOMO of OOSO2-, whereas the LUMO of neutral SO2 makes no substantial contribution to the SOMO construction. This suggests that OOSO2-(2A) correlates elec˜ 1A1) + O2-(X2Πg) dissociation limit tronically to the SO2(X - ˜2 rather than SO2 (X B1) + O2(X3Σg-). To examine the correlation between OOSO2-(2A) and its asymptote, we have calculated the energy profile for the OO-SO2 bond dissociation process. The energy profile was calculated at the MP2/6-311+G(d) level, which consumed less CPU time than MP2/6-311+G(2df) without deteriorating the validity of computational results. In the calculations, the OO-SO2 distance, rO-S, was varied in the range 1.7-5.0 Å at an interval of ∆rO-S ) 0.1-0.5 Å. Starting from the global minimum structure (rO-S ) 2.09 Å at MP2/6311+G(d)) yielded the transient structure at each fixed value of rO-S by optimizing all the parameters other than rO-S under a constraint that OOSO2- retains Cs symmetry. The symmetry constraint arose from the adoption of the MP2/6-311+G(d) level of theory, which provided a Cs configuration as the lowestenergy structure of OOSO2- (see section B). The calculations were made in a predictor-corrector way in that structure parameters optimized at a given rO-S value were used as the initial parameters for the subsequent optimization at rO-S + ∆rO-S; hence, the optimizations at rO-S and rO-S + ∆rO-S were made with the same set of orbitals selected for electron occupation in the Hatree-Fock wave function. Figure 6 depicts the calculated energy profile of OOSO2- plotted against rO-S. ˜ 1A1) + The MP2 energy approaches asymptotically the SO2(X 2 O2 (X Πg) limit without any symptom of inflection or discontinuity in the range rO-S > re. We also found that the energy profile calculated at the UHF/6-311+G(d) level showed the same tendency. This implies that the electron configuration of OOSO2-(2A) can be represented predominantly by a single Slater determinant. The MP2 energy profile is thus indicative of a preferential orbital correlation between OOSO2-(2A) and

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Zama et al. charge transfer from O2- to SO2 would take place, which leads inevitably to the formation of SO2--based species. In fact, the present calculations show that the OOSO2- energy profile ˜ 2 B1 ) + intersects with the potential energy curve to the SO2-(X 3 O2(X Σg ) dissociation products around rO-S ) 3 Å, as depicted ˜ 2B1) + O2(X3Σg-) curve has a minimum in Figure 6. The SO2-(X at rO-S ) 3.4 Å, which corresponds to the formation of an ion-molecule complex SO2- · O2(Cs, 4A′′). Under such circumstances, O2- survives the charge transfer to form OOSO2- on the condition that [SO2 · · · O2-(sol)] is located below [SO2- · · · O2(sol)] throughout the collision process, where O2-(sol) and O2(sol) represent the solvated anion and neutral species, respectively. In the present study, such a favorable condition for the OOSO2- formation is attained eventually in the intracluster reaction system, as discussed below. Within the context of the above discussion, it can be inferred that process 1b is not responsible for the OOSO2-(2A) formation because SO2- retains the excess electron in the reaction. We suggest here a possibility that an ion-molecule complex, SO2- · O2(Cs, 4A′′), is formed in process 1b. Ferguson et al. reported that SO4- prepared in process 1 reacted with NO2 (EA ) 2.273 eV23) to form NO2-. Their finding does not conflict with the present ab initio results: the VDE of SO2- · O2(Cs, 4A′′) is calculated to be 1.28 eV by CCSD(T) single-point energy calculations. Under present experimental conditions, where OOSO2- is formed in an electron-impact ionized free jet containing neutral (SO2)M(O2)N clusters, the initial step of the formation process is slow-electron attachment to (SO2)M(O2)N, which is followed by localization of the excess electron on a smaller moiety in the cluster. As M , N is achieved in the present case, the electron localization possibly proceeds as follows:

(SO2)M(O2)N + e- f [(SO2)M(O2)N]-* f [O4-(2Au) · (SO2)J(O2)K] + (M - J)SO2 + (N-K)O2 Figure 6. MP2 energy profile for the electronic ground state of OOSO2- plotted against the OO-SO2 distance (rO-S) in the range 1.7-5.0 Å (open circles). Also shown is a part of the energy profile for an SO2- · O2 ion-molecule complex (filled squares). The data points ˜ 1A1) + O2-(X2Πg) and at rO-S ) ∞ corresponds to the SO2(X ˜ 2B1) + O2(X3Σg-) asymptotic states. Note the difference in the SO2-(X spin multiplicity between OOSO2- and SO2- · O2: the former is of doublet while the latter of quartet multiplicity. The lower set of illustrations show the SOMO profile of OOSO2- at three different rO-S values: (a) 2.1 Å, (b) 2.3 Å, and (c) 2.9 Å.

˜ 1A1) + O2-(X2Πg): namely, the occupied molecular SO2(X orbitals of OOSO2-(2A) are constructed primarily from those ˜ 1A1) and O2-(X2Πg). The calculations also show that of SO2(X the SOMO of OOSO2- transmutes into the 2pπg* orbital of O2with increasing rO-S (see Figure 6a-c). On the basis of the above considerations, we infer that OOSO2- is formed via a process where O2-approaches SO2 while retaining the excess electron in 2pπg* orbital throughout the reaction. The reaction process can be written nominally as

˜ 1A1) f [O2- · · · SO2]‡ f O2-(X2Πg) + SO2(X OOSO2-(2A)

(5)

As the electron affinity of O2 (EA ) 0.448 eV21) is significantly smaller than that of SO2 (EA ) 1.10 eV22), process 5 occurs only when O2- is stabilized by solvation, which effectively increases the electron binding energy of O2-. Otherwise, simple

(6)

Here we take into account the fact that O4- is preferentially formed as a core ion in (O2)n- clusters.24,25 Considering that the O4-(2Au) core ion, of which the electron binding energy is estimated to be ≈0.9 eV,26 is stabilized due to subsequent solvation by surrounding O2 neutrals in the clusters,24 and that the electronic structure of O4-(2Au) can be viewed as a superposition of the two degenerate valence-bond structures O2-/ O2 and O2/O2-,27 it seems natural to infer that the following reaction proceeds within the cluster system to form OOSO2-:

O4-(2Au) + SO2 f [O2 · · · O2-(X2Πg) · · · SO2]‡ f O2 + OOSO2-(2A)

(7)

Note that the above chemical equation only describes the net reaction with taking no thought of surrounding solvent molecules. In process 7, O4-(2Au) serves as a “synthetic equivalence” to O2-(X2Πg) in process 5, exploiting its electronic nature as a charge-resonance dimer derived from the degenerate O2-/ O2 structures. D. O2- · (H2O)n + SO2 Reaction. In the previous section, we have proposed a possible mechanism for the OOSO2formation, which can be regarded as a “solvent-mediated” addition of a superoxide ion to SO2. To reinforce the plausibility of the proposed mechanism, it seems worthwhile to demonstrate the reactivity of O2- · (H2O)n against SO2. In the O2- · (H2O)n species, the electronic properties of O2-(X2Πg) remain almost

Peroxy SO4- anion

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5645 species produced in the O2- · (H2O)n + SO2 reaction as OOSO2-(2A). The observed spectral shift between the m ) 0 and m ) 1 photoelectron bands is on the order of magnitude expected for the stabilization of OOSO2- by hydration. Hence, the m ) 1 band is assigned to the photodetachment from OOSO2- · (H2O). Consequently, process 8 can be rewritten more explicitly, at least for m ) 0 and 1, as

O2- · (H2O)n + SO2 f [(H2O)n · O2- · · · SO2]† (pick-up process) f [(H2O)n · OOSO2-]†

(9a)

(intracluster reaction)

(9b) f OOSO2- · (H2O)m + (n-m)H2O

Figure 7. Mass spectra of the O2- · (H2O)n reactants (upper trace) and the product anions (lower trace) in the reaction of O2- · (H2O)n with SO2. The product mass peaks are interspersed between the unreacted O2- · (H2O)n peaks. The peak at m/z ) 64 marked with a dot (upper trace) is assignable to O4- while product SO2- contributes to the peak at m/z ) 64 marked with an asterisk (lower trace).

intact.28 The adiabatic detachment energy of O2- · (H2O) is determined to be 1.42 eV,28 suggesting an effective inhibition of simple charge transfer from O2- · (H2O)n to SO2 for n g 1. The upper trace of Figure 7 shows a typical mass spectrum of the O2- · (H2O)n reactants prepared in the present study. The size distribution of O2- · (H2O)n has an onset at n ) 0 and intensity maxima at n ) 3 and 6. These features in the reactant distribution turned out to depend delicately on the beam condition; however, they were reproducible in each batch of experiments. When the SO2 reagent was introduced into the source chamber, mass peaks of the product anions emerged in the spectrum while those of the reactants reduced their intensity by ≈50%, as shown in the lower trace of Figure 7. The substantial decrease in the reactant populations came mainly from the deflection of O2- · (H2O)n out of the beam due to scattering by the ambient gas, and partly from the reactions with SO2. In the mass spectrum with the SO2 sample on, anions with the formulas SO2- and [SO4 · (H2O)m]- (m ) 0 - 8) were detected in the mass region 20 e m/z e 250. As discussed in our previous work,29 these product anions arise primarily from the collisional reactions between preexisting O2- · (H2O)n in the jet and the ambient SO2:

(evaporation) (9c)

As [SO4 · (H2O)m]- (m g 2) species eluded the present photoelectron measurement due to their large electron detachment energy, no information was available on their electronic properties. However, as we see below, there seems to be no compelling reason to assign the [SO4 · (H2O)m]- (m g 2) mass peaks to anions other than OOSO2- · (H2O)m. The product mass distribution shows maxima at m ) 1 and 4, as displayed by the inset in Figure 7. This is reminiscent of the reactant mass distribution with maxima at n ) 3 and 6, indicating that O2- · (H2O)n undergoes the same reaction pathway (process 9), irrespective of size n, with evaporating an average of two H2O molecules. We also estimate the number of H2O lost in process 9c from the overall reaction exothermicity. On the assumption that the hydration energy is almost the same for both reactant and product systems, the heat of reaction is estimated from the stabilization energy of OOSO2-(2A) to be ∆H ≈ -1.77 eV (see section B). As ∆H values for the stepwise hydration to gasphase O2- are in the range from -0.8 to -0.6 eV,30 a maximum of two H2O solvents are lost in process 9c due to the reaction exothermicity. This energetics consideration gives a plausible explanation for the observed shift between the reactant and the product mass distributions, n - m ≈ 2. These considerations lead to the conclusion that the O2- · (H2O)n + SO2 reaction proceeds as eqs 9a-9c at least in the size range (n < 10) studied in the present experiment. Thus, the O2- · (H2O)n + SO2 study also reinforces our central conclusion about the formation

O2- · (H2O)n + SO2 f [O2- · (H2O)n · · · SO2]† f [SO4 · (H2O)m]- + (n-m)H2O

(8)

The electronic properties of the [SO4 · (H2O)m]- products were then probed by photoelectron spectroscopy. Figure 8 displays the photoelectron spectra of [SO4 · (H2O)m]- for m ) 0 and 1. Both spectra consist of a single broad band, from which VDE and fwhm are determined respectively to be 3.82 ( 0.02 and 1.18 ( 0.08 eV for m ) 0, and 4.11 ( 0.02 and 0.80 ( 0.04 eV for m ) 1 by using eq 4 in the band envelope analysis. Judging from the obtained VDE values, we identify the SO4-

Figure 8. Photoelectron spectra of (a) SO4- and (b) SO4- · H2O produced in the reaction of O2- · (H2O)n with SO2. The spectra were recorded at the photon energy of 4.66 eV.

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mechanism that OOSO2- is produced via the “solvent-mediated” addition of O2- to SO2. On the basis of the “loss of two H2O molecules” scenario, we discuss the band widths of the photoelectron spectra of OOSO2- and OOSO2- · (H2O) (Figure 8). As a maximum of two H2O molecules are lost in the reaction, OOSO2- · (H2O)m is produced either from O2- · (H2O)m+2 or from O2- · (H2O)m+1 reacting with SO2. From a thermodynamic point of view, however, it is more likely that the reaction occurs with evaporating as many H2O molecules as possible so that the available excess energy is released as the product kinetic energy (evaporative cooling31). Thus, the reaction (process 9) proceeds predominantly as O2- · (H2O)n + SO2 f OOSO2-(H2O)n-2 + 2H2O. The n ) 1 case, O2- · (H2O) + SO2 f OOSO2- + H2O, is an exception because only one H2O molecule is available for the evaporative cooling. The exothermicity for the n ) 1 reaction is estimated by the equation -∆H ) D[OO-SO2-] D[O2- · (H2O)], where D[OO-SO2-] represents the S-O bond dissociation energy of OOSO2- into SO2 + O2- and D[O2- · (H2O)] is the single-molecule hydration energy of O2-. Substituting D[OO-SO2-] ) 1.77 eV and D[O2- · (H2O)] ) 0.80 eV30 into the equation, -∆H is calculated to be 0.97 eV. As not only O2- · (H2O)2 but also O2- · (H2O) make a contribution to the formation of a bare OOSO2-, a portion of the OOSO2product anions are prepared vibrationally hot because of the large exothermicty (0.97 eV). This possibly gives rise to the broadness of the OOSO2- band (fwhm )1.18 eV) associated with the photodetachment from vibrationally excited states. On the other hand, OOSO2- · (H2O) is produced mainly via the “loss of two H2O molecules” process, O2- · (H2O)3 + SO2 f OOSO2-(H2O) + 2H2O. The excess energy of reaction rapidly dissipates through the evaporation of two H2O molecules, which leads to the formation of a vibrationally less-energized product anion. This provides a possible explanation for the narrower bandwidth (fwhm ) 0.80 eV) for the photoelectron spectrum of OOSO2- · (H2O) shown in Figure 8b. At the end of this section, it seems worthwhile to discuss the similarity and difference between the following reactions and the OOSO2- formation scheme:

O2- · (H2O)n + NO f OONO- · (H2O)n-1 + H2O

(10) O2- · Arn + NO f OONO- · Arm + (n - m)Ar

(11) Processes 1012 and 1132 both result in the production of the peroxy form of OONO- through the O2- addition to NO. As O2 possesses a larger electron affinity than NO (EA ) 0.026 eV33), solvent atoms or molecules in processes 10 and 11 act primarily as a scavenger of the excess energy of reaction in trapping the metastable adducts. In the present scheme, however, the role of solvent molecules is more essential to the OOSO2production: i.e., the solvent molecules act not only as an energy scavenger but also as a protector of O2- against unfavorable charge transfer. Summary We report on the formation of SO4- having a peroxy bonding configuration, which can be well described as OOSO2-. The peroxy form is prepared in an electron-impact ionized free jet of Ar containing SO2(1%) and O2(10%), and detected by anion photoelectron spectroscopy. The vertical detachment energy of

the peroxy form is determined to be 3.78 ( 0.02 eV, which is in good agreement with the theoretical value (3.34 eV) given by CCSD(T)/6-311+G(2df)//MP2/6-311+G(2df) calculations. The total energy of OOSO2- is calculated to be higher by 2.16 eV with reference to the electronic ground state of sulfurcentered SO4-. The calculations also reveal unique features of the OOSO2- electronic structure: (i) the singly occupied molecular orbital is constructed by an out-of-phase superposition of the second HOMO (πSO, 1a2) of SO2 and the half-filled 2pπg* orbital of O2-, and (ii) OOSO2- is electronically correlated to ˜ 1A1) + O2-(X2Πg) asymptote. This indicates that the SO2(X ˜ 2B1) + O2(X3Σg-) nor SO2(X ˜ 1A1) + O2-(X2Πg) neither SO2-(X reach the OOSO2 production under binary collision conditions. The former system is electronically uncorrelated with the peroxy form. The latter leads to simple electron transfer, SO2 + O2f SO2- + O2, due to the larger electron affinity of SO2. We have proposed a “solvent-mediated” mechanism for the OOSO2production, where a solvated O2- attacks SO2 while retaining the excess electron in its 2pπg* orbital; the surrounding solvent molecules stabilize the superoxide ion so that the simple electron transfer from O2- to SO2 is efficiently inhibited. This mechanistic model is further confirmed by demonstrating the OOSO2production in the O2- · (H2O)n + SO2 reaction. Thus, we have experimentally proven the existence of the peroxy form of the SO4- isomer and proposed a possible mechanism for its formation. The mechanism, which can be regarded as a “solvent-mediated” addition reaction of the superoxide ion, suggests a wide range of possible ability of solvated O2- as a reagent for novel reactions in the gas-phase superoxide chemistry. Acknowledgment. We are grateful to Professor K. Takatsuka for the loan of high-performance computers, which enabled us to carry out the ab initio calculations. A part of the calculations was performed by using the computer systems (SGI Altix4700) at the Research Center for Computational Science, Okazaki Research Facilities, National Institutes of Natural Sciences (NINS). This work is partly supported by a Grant-in-Aids for Scientific Research (Grant No. 20038015) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), and partly by the Project to Create Photocatalyst Industry for Recycling-Oriented Society from the New Energy and Industrial Technology Development Organization (NEDO). Supporting Information Available: Structure parameters for the SO4- isomeric forms shown in Figure 4 (MP2/6311+G(2df)). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Roebke, W.; Renz, M.; Henglein, A. Int. J. Radiat. Phys. Chem 1969, 1, 39. (2) Dogliotti, L.; Hayon, E. J. Phys. Chem. 1967, 71, 2511. (3) Chawla, O. P.; Fessenden, R. W. J. Phys. Chem. 1975, 79, 2693. (4) Neta, P.; Madhavan, V.; Zemel, H.; Fessenden, R. W. J. Am. Chem. Soc. 1977, 99, 163. (5) Zemel, H.; Fessenden, R. W. J. Phys. Chem. 1978, 82, 2670. (6) Davies, M. J.; Gilbert, B. C.; Thomas, C. B.; Young, J. J. Chem. Soc., Perkin Trans. 2 1985, 1199. (7) Huie, R. E.; Clifton, C. L. Int. J. Chem. Kinet. 1989, 21, 611. (8) Clifton, C. L.; Huie, R. E. Int. J. Chem. Kinet. 1989, 21, 677. (9) Lomoth, R.; Naumov, S.; Brede, O. J. Phys. Chem. A 1999, 103, 6572. (10) Todres, Z. V. Organic Ion Radicals. Chemistry and Applications; Marcel Dekker: New York, 2003; p 8. (11) Fehsenfeld, F. C.; Ferguson, E. E. J. Chem. Phys. 1974, 61, 3181. (12) Fahey, D. W.; Bo¨hringer, H.; Fehsenfeld, F. C.; Ferguson, E. E. J. Chem. Phys. 1982, 76, 1799.

Peroxy SO4- anion (13) Vacher, J. R.; Jorda, M.; Le Duc, E.; Fitaire, M. Int. J. Mass Spectrom. Ion Processes 1992, 114, 149. (14) McKee, M. L. J. Phys. Chem. 1996, 100, 3473. (15) Zheng, W.; Lau, K.-C.; Wong, N.-B.; Li, W.-K. Chem. Phys. Lett. 2009, 467, 402. (16) Wang, X.-B.; Nicholas, J. B.; Wang, L.-S. J. Phys. Chem. A 2000, 104, 504. (17) Tsukuda, T.; Saeki, M.; Kimura, R.; Nagata, T. J. Chem. Phys. 1999, 110, 7846. (18) Tsukuda, T.; Johnson, M. A.; Nagata, T. Chem. Phys. Lett. 1997, 268, 429. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (20) Frey, R. F.; Davidson, E. R. J. Chem. Phys. 1988, 88, 1775.

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5647 (21) Ervin, K. M.; Anusiewicz, I.; Skurski, P.; Simons, J.; Lineberger, W. C. J. Phys. Chem. A 2003, 107, 8521. (22) Nimlos, M. R.; Ellison, G. B. J. Phys. Chem. 1986, 90, 2574. (23) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Phys. Chem. 1988, 92, 5405. (24) Hiraoka, K. J. Chem. Phys. 1988, 89, 3190. (25) Bopp, J. C.; Alexandrova, A. N.; Elliott, B. M.; Herden, T.; Johnson, M. A. Int. J. Mass Spectrom. 2009, 283, 94. (26) The electron binding energy of O4- is estimated from the O2-O2bond energy (0.46 eV24) together with the electron affinity of O2 (0.448 eV21). Although the estimated value (0.91 eV) corresponds specifically to the ∆H value for O4-(2Au) f 2O2(X3Σg-) + e-, it still gives a good estimate for the electron binding energy of O4- because of the small van der Waals bond energy of (O2)2. The formation energy of (O2)2 was determined to be -0.023 ( 0.003 eV by Ewing et al. (J. Chem. Phys. 1973, 58, 4824). (27) Aquino, A. J. A.; Taylor, P. R.; Walch, S. P. J. Chem. Phys. 2001, 114, 3010. (28) Luong, A. K.; Clements, T. G.; Resat, M. S.; Continetti, R. E. J. Chem. Phys. 2001, 114, 3449. (29) Tsukuda, T.; Nagata, T. J. Phys. Chem. A 2003, 107, 8476. (30) Arshadi, M.; Kebarle, P. J. Phys. Chem. 1970, 74, 1483. (31) Klots, C. E. J. Chem. Phys. 1985, 83, 5854. (32) Relph, R. A.; Bopp, J. C.; Johnson, M. A.; Viggiano, A. A. J. Chem. Phys. 2008, 129, 064305. (33) Travers, M. J.; Cowles, D. C.; Ellison, G. B. Chem. Phys. Lett. 1989, 164, 449.

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