The Search for Protonated Dihydrogen Trioxide (HOOOH) - American

Jul 9, 2010 - Glasgow, G1 1XL, United Kingdom, and Department of Chemistry, Faculty of Chemistry and Chemical. Technology ... can occur at either the ...
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J. Phys. Chem. A 2010, 114, 8003–8008

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The Search for Protonated Dihydrogen Trioxide (HOOOH): Insights from Theory and Experiment Tell Tuttle,*,† Janez Cerkovnik,*,‡ Jozˇe Koller,‡ and Bozˇo Plesnicˇar‡ WestCHEM, Department of Pure and Applied Chemistry, UniVersity of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, United Kingdom, and Department of Chemistry, Faculty of Chemistry and Chemical Technology, UniVersity of Ljubljana, 1000 Ljubljana, SloVenia ReceiVed: April 29, 2010; ReVised Manuscript ReceiVed: June 11, 2010

Protonated dihydrogen trioxide (HOOOH) has been postulated in various forms for many years. Protonation can occur at either the terminal (HOOO(H)H+) or central (HOOH(OH)+) oxygen atom. However, to date there has been no definitive evidence provided for either of these species. In the current work we have employed ab initio methods, CCSD(T) and MP2, with a large basis set (6-311++G(3df,3pd)) to determine the relative stabilities of these species. It is shown that the terminally protonated species is strongly favored relative to the centrally protonated species (∆E ) 15.8 kcal/mol, CCSD(T)//MP2). The mechanism of formation of HOOO(H)H+ was determined to occur with a low barrier with the H3O+ occurring in a thermoneutral reaction (∆E ) -0.3 kcal/mol, CCSD(T)//MP2). Although HOOO(H)H+ exists as a stable intermediate, it is extremely short-lived and rapidly decomposes (∆E* ) 8.6 kcal/mol, MP2) to H3O+ and O2(1∆g). The decomposition reaction is stabilized by solvent water molecules. The short-lived nature of the intermediate implies that the intermediate species can not be observed in 17O NMR spectra, which has been demonstrated experimentally. Introduction The existence of dihydrogen trioxide (HOOOH) has by now been unambiguously established by NMR (1H, 17O)1-8 and IR9 spectroscopy. This polyoxide is believed to be the key intermediate involved in the low-temperature ozonation of various saturated organic compounds2 as well as in oxidation processes that span atmospheric,10 environmental,3,11,12 and biological13-17 systems. Several quantum chemical studies on monomeric and dimeric (oligomeric) forms of this simplest of polyoxides were reported.12,13,18-26 More recently, the structure of HOOOH has also been determined by microwave spectroscopy.27 However, very little is known about the existence of the protonated HOOOH species ((HOOOH)(H+)). One of the possible entities, that is, HOOH(OH)+, has already been suggested as a possible hydroxylating agent in superacid media,28 and an attempt has been reported to calculate the structure of the protonated HOOOH.21 Also, dihydrogen trioxide is known to decompose in water media to form singlet oxygen (1∆g O2) and water.3,7,12 We suspect that this type of decomposition might be facilitated by Brønsted acid catalysis. In the present work, we have undertaken a computational study of the protonated HOOOH species in order to establish the most likely protonation site and the stability of the protonated species. Attempts to detect the protonated HOOOH species by 17 O NMR spectroscopy using a variety of proton sources have also been made. Computational Methods All species were optimized at the MP2 level of theory29,30 employing the 6-311++G(3df,3pd) basis set.31-33 Optimization of the HOOO(H)H+ and HOOH(OH)+ species was additionally * To whom correspondence should be addressed. E-mail: tell.tuttle@ strath.ac.uk; [email protected]. † University of Strathclyde. ‡ University of Ljubljana.

carried out at the CCSD(T)34,35 level of theory with the same basis set. Single point CCSD(T) calculations were performed on the MP2, gas-phase optimized structures. The polarizing effect of the solvent was also investigated through the reoptimization of the gas phase structures using the polarizable conductor continuum model36,37 (CPCM) with the dielectric constant of water ( ) 78.4) at the MP2 level of theory. All geometry optimizations were performed with the Gaussian 03 program.38 NMR chemical shifts have been calculated at the CPCM/ MP2 level of theory using gauge-including atomic orbitals (GIAO).39-42 The QZVPP basis set43 was employed for the NMR calculations as this has previously been shown to provide good results in conjunction with the GIAO MP2 approach.8 17O NMR chemical shifts are reported with respect to H217O, which has been obtained at the same level of theory, using the equation δ ) σH2O - σ. GIAO/MP2 NMR chemical shift calculations were performed with the TurboMole suite of programs.44-48 Results and Discussion Stability and Structure of Protonated HOOOH. The possible protonation sites of HOOOH were investigated through the optimization of the protonated species in two forms, namely, protonation at the terminal O atom (Scheme 1a) or protonation at the central O atom (Scheme 1b). Both species have previously been postulated in the literature,21,28 and a computational study on the terminally protonated species has predicted a long O · · · O bond between the protonated and central O atoms. Therefore, we initially sought to verify the ability of the MP2 method to describe the electronic structure of the protonated HOOOH species through the optimization of the structures at the CCSD(T) and MP2 levels of theory. The resulting internal coordinate values at both the MP2 and CCSD(T) levels of theory are shown in Scheme 1a. The MP2 method predicts a slightly longer distance for the extended

10.1021/jp103882e  2010 American Chemical Society Published on Web 07/09/2010

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SCHEME 1: CCSD(T) Optimized Internal Coordinates of the (a) HOOO(H)H+ and (b) HOOH(OH)+ Complexa

a

Distances in Å, angles in °. Values in parentheses correspond to the MP2/6-311++G(3df,3pd) optimized structure.

TABLE 1:

17

O NMR Chemical Shifts Relative to H217Oa

molecule HOHH+ HOOH HOO(H)H+ HOOOH HOOO(H)H+ HOOH(OH)+

nucleusb

δBPW91(gas)c

δBPW91(solv)d

∆solve

δMP2(gas)f

δMP2(solv)g

δexp

O O O(H) O(H2) O(t) O(c) O1 O2 O3 O1 O2 O3

23.6 216.0 401.4 114.8 326.0 490.5 743.6 1577.8 201.5 445.6 295.0 472.4

29.7 217.2 358.8 129.4 334.6 491.2 427.2 845.1 225.7 447.4 335.4 450.0

6.1 1.2 -42.6 14.6 8.6 0.7 -316.4 -732.7 24.2 1.8 40.4 -22.4

23.5 191.6 357.0 99.1 306.5 432.0 479.2 -308.1 274.5 419.6 240.7 438.0

29.6 192.8 314.4 113.7 315.1 432.7 162.8 -1040.8 298.7 421.4 281.1 415.6

28h 187i,j 151i 305j 421j

a All chemical shifts are in ppm. b See Scheme 1 for atom numbering. c Relative to H217O shielding in gas phase at the BPW91//MP2 level of theory (328.2 ppm). d Relative to H217O shielding in the solvent phase at the CPCM/BPW91//CPCM/MP2 level of theory (336.7 ppm). e ∆solv is the shift due to the electrostatic influence of the environment (i.e., ∆solv ) δBPW91(solv) - δBPW91(gas)). f Relative to H217O shielding in gas phase at the MP2 level of theory (345.3 ppm). g δMP2(solv) ) δMP2(gas) + ∆solv. h See the Supporting Information. i Reference 50. j Reference 5.

O2sO3 bond in HOOO(H)H+ (MP2: 1.800 Å, CCSD(T): 1.776 Å; see Scheme 1a for atom numbering) relative to the CCSD(T) optimized structure. The longer bond in the MP2 structure results in a greater localization of the OOH fragment in the MP2 structure, yielding a shortened O1sO2 bond and a slight increase in the O1sH1 bond length (Scheme 1a). However, in both the MP2 and CCSD(T) optimized structure the OH2 fragment of the complex retains a very similar internal geometry. A single point energy calculation at the CCSD(T) level of theory was carried out on the MP2 optimized structure and was found to be only 0.48 kcal/mol higher in energy than the CCSD(T) optimized structure. These results differ somewhat from the previously optimized geometry of HOOO(H)H+ that was performed at the MP2 level of theory using the DZ+P basis set.21 The smaller basis set used in this earlier work results in a shorter O2sO3 bond (1.739 Å)21 and a longer O1 · · · O2 bond (1.310 Å).21 Although qualitatively similar, the earlier work suggests a structure of HOOO(H)H+ that is closer to the equilibrium structure of HOOOH49 and as such overstabilizes the protonated species. However, given the use of a larger basis set in this work and the good agreement between the MP2 and CCSD(T) optimized structures, the values reported in this work are considered to be more reliable. The geometry of the HOOH(OH)+ structure was also confirmed by optimization at both the CCSD(T) and MP2 levels of theory (Scheme 1b). As in the case of HOOO(H)H+, the MP2 geometry agrees very well with that obtained at the CCSD(T) level of theory. At both levels of theory the central O3 unit remains intact for the centrally protonated structure. The equilibrium bond lengths of 1.433 Å (O1-O2, Scheme 1b) and 1.430 Å (O2-O3, Scheme 1b) are very close to the value

expected for the unprotonated HOOOH.49 A single point energy calculation at the CCSD(T) level of theory was carried out on the MP2 optimized structure and was found to be only 0.22 kcal/mol higher in energy than the CCSD(T) optimized structure. Given this excellent agreement between the energetics of both structures at the two levels of theory, we are satisfied with the use of MP2 optimized structures with subsequent CCSD(T) single point calculations in order to explore the reaction mechanisms that may lead to the generation of these species and their subsequent decomposition (vida infra). The ability to locate the protonated HOOOH species on their respective potential energy surfaces was encouraging and we therefore carried out a number of 17O NMR spectroscopic studies, using a variety of proton sources, in an attempt to detect these peculiar, and seemingly stable species. 17 O NMR Spectroscopy of Protonated HOOOH. To determine the expected peak location for the protonated species, we have carried out 17O NMR chemical shift calculations of the HOOO(H)H+ and HOOH(OH)+ optimized structures, along with a series of reference molecules for comparison.5,50 The NMR calculations were carried out within the CPCM model for implicit solvation as solvent effects have been shown previously to play an important role in affecting the calculated NMR chemical shift.8 The comparison between the calculated 17O NMR chemical shifts (δMP2(solv), Table 1) and the experimental values is quite reasonable for the reference molecules, HOHH+, HOOH and HOOOH. For the neutral species, the error is only 3% of the chemical shift magnitude and well within the expected limitations of the method. Experimentally, it is known that the 17O NMR chemical shifts for HOHH+ are strongly dependent on

The Search for Protonated Dihydrogen Trioxide the solvent,51 the kind of the acid and its concentration employed, and to a lesser extent on the temperature (in our hands, the range was from 5 to 32 ppm; see Supporting Information). The experimental chemical shift around 28 ppm listed in Table 1, was observed in tert-butyl methyl ether and methyl acetate in which water was always present (Figures S1 and S2 in Supporting Information). Thus, the solvation of HOHH+ with a cloud of more “polar” water molecules is almost certain. The alternative experimental value of ∼14 ppm was found, when using SO2 as the solvent (Figure S3 in Supporting Information). As such, the calculated value of 29.6 in water (as solvent) is also well within the error associated with these calculations and measurements. The excellent agreement between the calculated and experimental values for the reference molecules is a good indication of the ability of the computational method to accurately predict the 17O chemical shift. Therefore, the observed difference between the calculated and experimentally attributed chemical shift of the protonated oxygen in HOOHH+ is concerning (Table 1). The difference between the calculated and experimental values is larger than that observed for the reference molecules, and a simple averaging of the two calculated shifts for the O atoms decreases this error. Given the good performance of the method in the case of the reference molecules this result brings into question the experimental assignment of the observed chemical shift to the HOO(H)H+ species.50 All attempts to observe the protonated HOOOH experimentally by low-temperature 17O NMR spectroscopy52 failed. Namely, the intense absorption at 305 ppm in 17O NMR spectra for terminal oxygen atoms in HOOOH, prepared by the lowtemperature ozonation of diphenylhydrazine with 17O-enriched ozone in methyl acetate and tert-butyl methyl ether,7 disappeared after gradual addition of magic acid (and also HF-SbF5) above -30 °C (Figures S1 and S2 in Supporting Information). No other additional signals, which could be attributed to the protonated HOOOH, were observed. At the same time, the protonated HOH (HOHH+) was observed at 28 ppm.53 It is interesting to note that the 17O NMR absorption for HOOH at 187 ppm, which is always present in the reaction mixture, gradually disappeared from the spectrum at room temperature, and a new absorption at 164 ppm appeared (see Figure S2 in Supporting Information). A similar observation was made by Olah and co-workers,50 when a mixture of equal volumes of 90% H2O2 and 30 atom % enriched H217O was added to an SO2 solution of HF-SbF5 at -25 °C. The authors observed two absorptions in the 17O NMR spectrum, and they attributed the major peak at 10 ppm to the oxonium ion (H3O+), and the second peak, with a chemical shift of 151 ppm, to the peroxonium ion (HOOH2+). However, we found that the mixture of equal volumes of 90% H2O2 and 40 atom % enriched H217O, when dissolved in SO2, produced two new 17O-enriched signals in the 17O NMR spectrum at 505 ppm (SO2) and 156 ppm, respectively, together with the signal for protonated HOH (14 ppm)53 (Figure S3 in Supporting Information). In addition, when a small amount of SO2 was added to the solution of equal volumes of 90% HOOH and 40 atom % enriched H217O in various solvents, two new 17O-enriched signals in 17O NMR spectrum at 500 ppm (SO2) and 168 ppm immediately appeared (Figure S4 in Supporting Information). Similar observations were made when SO2 was added to the solutions of HOOOH in methyl acetate and tert-butyl methyl ether at low-temperatures. Two new 17O-enriched signals appeared in 17O NMR spectrum at 500 ppm (SO2) and 167 ppm, respectively, together with the signal for protonated HOH (Figures S5 and S6 in

J. Phys. Chem. A, Vol. 114, No. 30, 2010 8005 TABLE 2: Reaction Energetics for the Protonation of HOOOH by H3O+ a HOOOH + H3O+ f 1 1 f TS(1-2) 1f2 2 f HOOO(H)H+ + H2O

∆E(CCSD(T)/MP2)b

∆E(MP2)c

-27.4 0.5 -0.3 29.0

-27.3 0.6 -0.3 30.3

a All energies given in kcal/mol. b Relative energies calculated at the CCSD(T) level of theory on the MP2 optimized structures. c Relative energies calculated at the MP2 level of theory on the MP2 optimized structures.

Supporting Information). Because of the rapid simultaneous 17Oenrichment of the SO2 absorption, when a small amount of SO2 was added to the mixture of HOOH and 17O-enriched HOH, we believe that 17O NMR signal at 167 ppm (151 ppm in the previous study)50 more likely belongs to the species containing the SO2 unit rather than HOOH2+.54 The inability to detect the presence of either protonated HOOOH molecules suggests that: (a) neither species is formed under the reaction conditions investigated; or (b) that the protonated species is formed as a very short-lived intermediate that rapidly decomposes. The possibility of generating the protonated HOOOH species can be evaluated by determining the activation energy along the reaction path to its formation. That is, if the activation energy is within the accessible range, under the experimental conditions, we assume that the protonation of HOOOH is possible. Protonation of HOOOH. The protonation of HOOOH by H3O+ can occur at either the terminal O atoms (O1 or O3, Scheme 1), or at the central O atom (O2, Scheme 1). The reaction of HOOOH + H3O+ f HOOO(H)H+ + H2O, is endothermic in the gas phase by 1.3 kcal/mol (CCSD(T)//MP2) and as such protonation at the terminal O atom is slightly disfavored from a thermodynamic perspective, although the energetic difference between the two states is small. However, at the CCSD(T)//MP2 level of theory the O2 (central O atom) protonated system is 15.8 kcal/mol higher in energy than the terminally protonated system. Protonation at the central O atom is therefore unlikely given the relative thermodynamic instability of the resulting structure. Thus, the mechanistic study focuses on the protonation at the terminal O atom. The formation of a reaction complex is strongly favored, with the reaction complex (1) stabilized by 27.4 kcal/mol (∆E(CCSD(T)//MP2), Table 2), relative to the separated reactants. However, in the complex form the proton is shared between the HOOOH and H2O fragments of the complex, rather than being localized on either fragment. The product complex (2, Scheme 2) is marginally different from 1, with the proton shifted slightly closer to the HOOOH unit, but still strongly attached to the H2O unit as well. This slight proton shift is essentially thermoneutral (∆E ) -0.3 kcal/mol) and the barrier to the reaction is also minimal (∆E* ) 0.5 kcal/mol). The activation and reaction energies calculated for the proton shift from 1 to 2 are negligible and within the limits of the accuracy of the methodology employed. Thus, while two distinct minima were located on the potential energy surface in the gas phase reaction the complexes 1 and 2 are essentially equivalent. The dissociation of 2 to the final products is strongly disfavored, relative to the product complex, with a dissociation energy of 29.0 kcal/mol. The energetic gain upon association and the proton sharing between the closed shell fragments results from the instability of the isolated charged species. Therefore, as a first approxima-

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SCHEME 2: Protonation of HOOOH by H3O+ a

a

Distances in Å.

SCHEME 3: CPCM/MP2 Optimized Structures and Energetics for the Protonation of HOOOHa

a

Distances in Å. Energies in kcal/mol are relative to the separated HOOOH and H3O+

tion, we considered the polarizing effect of the solvent through the continuum solvent model. The structures were reoptimized at the CPCM-MP2/6-311++G(3df,3pd) level of theory. Given the good agreement between the gas-phase relative energies at the MP2 and CCSD(T) levels of theory, the continuum calculations have only been performed at the MP2 level of theory. At the CPCM-MP2 level of theory the separated products of HOOO(H)H+ + H2O are 6.8 kcal/mol (∆Esolv, Scheme 3) higher in energy than the separate reacting species (HOOOH + H3O+). The structure of HOOO(H)H+ is strongly altered by inclusion of the solvent medium. The O2sO3 bond is decreased to 1.546 Å in the solvent optimized structure (Scheme 3), relative to 1.800 Å in the gas-phase (Scheme 2). Whereas the O1sO2 bond is increased, relative to the gas phase (Scheme 2), to 1.326 Å (Scheme 3). These geometric changes suggest that a HOOO(H)H+ molecule is more stable in the solvent phase, as opposed to the more loosely associated fragments obtained through optimization in the gas phase. However, the instability of the products relative to the reactants indicate that in the solvent phase the protonation of HOOOH, to form HOOO(H)H+, is thermodynamically unfavored and therefore the product must be very short-lived species. Optimization of the reactant complex (1, Scheme 2) and product complex (2, Scheme 2) was carried out at the CPCMMP2 level of theory, and in both cases the optimization resulted in a reactant-like complex (Scheme 3). That is, in the solvent phase the proton is stabilized on the water fragment to create H3O+, which forms a strong H-bond to the HOOOH fragment. The H-bond length is 1.506 Å and the H-OH2 bond length is reduced to 1.027 Å (Scheme 3), relative to 1.122 Å in the gas phase (Scheme 2). All attempts to locate the protonated HOOO(H)H+ fragment complexed to HOH resulted in the transfer of the proton back to the water fragment. Similarly, attempts to locate a TS structure that could connect the two potential minima were unsuccessful, and a manual scan of the reaction coordinate, corresponding to the transfer of the proton between the two fragments, indicates that the PES is very flat and continuously uphill for the proton transfer to HOOOH. The reactant complex (1) is stabilized by 1.7 kcal/mol (∆Esolv, Scheme 3) relative to the separated species in the solvent medium. This indicates that in addition to the polarizing effect of the solvent, specific H-bonding interactions between the polyoxide and the water molecules help to stabilize the positive charge in the system. Therefore, the inclusion of additional water molecules around the polyoxide should also help to stabilize

the transfer of the proton from the HOHH+ to HOOOH. As mentioned above, the separated products are 6.8 kcal/mol higher in energy than the separated reactants, and this energy difference clearly favors the reactants from a thermodynamic perspective; however, given that there is no additional barrier to the proton transfer, the HOOO(H)H+ is clearly accessible under the experimental conditions. Therefore, if HOOO(H)H+ is formed, the question remains: is there a decomposition product that will favor the formation of the unstable intermediate? Decomposition of HOOO(H)H+. The retro reaction for the formation of HOOO(H)H+ is possible and will form the reacting HOOOH and HOHH+ in an exothermic reaction. However, experimentally there is a clear decomposition of HOOOH during the reaction.55 The barrier for the water-catalyzed decomposition of HOOOH has been previously calculated by us, and others, to be ca. 20 kcal/mol.12,56 However, in the proton-rich environment an alternative, lower-energy decomposition pathway exists via the protonated HOOOH species. The HOOO(H)H+ molecule is stabilized in the gas phase when solvated explicitly by three water molecules, as is evidenced by the contracted O-O bond distance at the protonated O atom (1.507 Å, Scheme 4) relative to the analogous bond in the gas phase structure (1.800 Å, Scheme 1). This solvent cluster used in these calculations is too large for CCSD(T) single-point calculations using our resources; therefore, in the following we rely on the MP2 gas phase energies only as these have been shown above to provide accurate energies with respect to the CCSD(T) values (e.g., see Table 2). The dissociation of the HOOO(H)H+ occurs with a low barrier of 8.6 kcal/mol (∆E*(MP2), Figure 1), relative to the reactant complex (3, Scheme 4). The transition state for the decomposition involves the dissociation of the OH2 unit of HOOO(H)H+si.e., the O2sO3 bond (see Scheme 1 for atom numbering) is cleaved during the TS as the bond length increases from 1.507 Å in 3 to 1.976 Å in TS(3-4) (Scheme 4). Concomitant with the O2sO3 bond cleavage is the transfer of the proton (red, Scheme 4) from the OOH unit to a stabilizing water molecule (distance between the O atom of the water and the proton decreases from 1.651 Å in 3 to 1.214 Å in TS(3-4), Scheme 4) and the formation of the double bond between the O1sO2 atoms (O1sO2 bond length decreases from 1.354 Å in 3 to 1.245 Å in TS(3-4), Scheme 4). Finally, there is a second, subsequent transfer of the (red) proton that occurs after the transition state has passed in a barrierless reaction (see below).

The Search for Protonated Dihydrogen Trioxide SCHEME 4: Decomposition of HOOO(H)H+

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Distances in Å.

Conclusions

Figure 1. Energetics for the water-catalyzed decomposition of HOOO(H)H+. All relative energies are in kcal/mol and calculated in the gas phase at the MP2/6-311++G(3df,3pd) level of theory.

The reaction results in the rearrangement of the complex, from TS(3-4) to 4, to provide maximum hydration of the HOHH+ molecule. The large rearrangement of the product cluster, relative to the transition state, meant that following the reaction through to completion via an IRC calculation was not tractable. Instead, we performed a short IRC calculation on the transition state structure, which confirms that the single imaginary frequency for this structure corresponds to the bond-making and bond-breaking steps described above. To further ensure the connectivity of the transition state and the product, we utilized the optimized transition state structure (TS(3-4)) as the input geometry for the optimization of the product cluster (4). During the optimization of 4, the singlet O2 that is formed in the reaction is expelled from the solvent cluster and the protonated water molecule is drawn toward the water cluster. The proton (shown in red in Scheme 4) is transferred, in a barrierless step, to the central water molecule that was generated from HOOO(H)H+ (shown in black in Scheme 4) to provide greater screening for H3O+ (4, Scheme 4). MP2 is unable to correctly describe the multiconfigurational electronic structure of O2(1∆g). As such, we have calculated 4 in the triplet state and subsequently corrected the energy of the complex by the experimental O2(3Σg) - O2(1∆g) splitting energy of 22.6 kcal/mol.56 Given the lack of interaction between the O2 unit and the solvent cluster in 4, the use of the free O2 triplet-singlet splitting energy is justified. Using this correction, the reaction is strongly exothermic, with a reaction energy of -29.7 kcal/mol (∆E(MP2), Figure 1.)

CCSD(T) and MP2 calculations have been carried out to determine the relative stabilities of the protonation sites in HOOOH. Although both the centrally and terminally protonated species are found to be minima on the potential energy surface, we have shown that the terminally protonated species, HOOO(H)H+, is thermodynamically favored. Moreover, the structure of the terminally protonated species is stabilized by water solvent molecules that are always present in the reaction mixture. The protonated species is formed in a thermoneutral reaction, relative to HOOOH + H3O+, with a minimal barrier. However, the formation of the protonated species provides a low-energy decomposition pathway for HOOOH. The exothermicity of the decomposition reaction is the driving force that leads to the formation of the HOOO(H)H+ intermediate. Although this intermediate is higher in energy than the reactant species (HOOOH + H3O+), the activation energy that is required for the decomposition of HOOO(H)H+ is significantly lower than that of the analogous decomposition of HOOOH. Attempts to detect the protonated intermediate by 17O NMR spectroscopy were unsuccessful, which also implies a rapid decomposition of this species. Thus, HOOO(H)H+ is most likely formed as a very short-lived intermediate in the acidcatalyzed decomposition of HOOOH in the solvents under investigation to form the thermodynamically favored reaction products of O2 and water. Acknowledgment. T.T. thanks the Glasgow Centre for Physical Organic Chemistry for Support. J.C. and B.P. acknowledge the financial support by the Slovenian Research Agency (J1-9410) and Dr. J. Plavec (Slovenian NMR Centre, National Institute of Chemistry, Ljubljana) for running the 17O NMR spectra. Supporting Information Available: Experimental details and the accompanying 17O NMR spectra are available in the Supporting Information, along with the Cartesian coordinates for the gas phase optimized structures mentioned in the text. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Bergant, A.; Cerkovnik, J.; Plesnicˇar, B.; Tuttle, T. J. Am. Chem. Soc. 2008, 130, 14086. (2) Cerkovnik, J.; Tuttle, T.; Kraka, E.; Lendero, N.; Plesnicˇar, B.; Cremer, D. J. Am. Chem. Soc. 2006, 128, 4090. (3) Nyffeler, P. T.; Boyle, N. A.; Eltepu, L.; Wong, C.-H.; Eschenmoser, A.; Lerner, R. A.; Wentworth, P. Angew. Chem., Int. Ed. 2004, 43, 4656.

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