Predissociation Spectra Research Articles

NH4(+) Resides Inside the Water 20-mer Cage As Opposed to H3O(+), Which Resides on the Surface: A First Principles Molecular Dynamics Simulation Study.

Experimental vibrational predissociation spectra of the magic NH4(+)(H2O)20 clusters are close to those of the magic H3O(+)(H2O)20 clusters. It has been assumed that the geometric features of NH4(+)(H2O)20 clusters might be close to those of H3O(+)(H2O)20 clusters, in which H3O(+) resides on the surface. Car-Parrinello molecular dynamics simulations in conjunction with density functional theory calculations are performed to generate the infrared spectra of the magic NH4(+)(H2O)20 clusters. In comparison with the experimental vibrational predissociation spectra of NH4(+)(H2O)20, we find that NH4(+) is inside the cage structure of NH4(+)(H2O)20 as opposed to on the surface structure. This shows a clear distinction between the structures of NH4(+)(H2O)20 and H3O(+)(H2O)20 as well as between the hydration phenomena of NH4(+) and H3O(+).

Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH+ with H2 Predissociation Spectroscopy

We report vibrational predissociation spectra of the four protonated dipeptides derived from glycine and sarcosine, GlyGlyH(+)•(H(2))(1,2), GlySarH(+)•(D(2))(2), SarGlyH(+)•(H(2))(2), and SarSarH(+)•(D(2))(2), generated in a cryogenic ion trap. Sharp bands were recovered by monitoring photoevaporation of the weakly bound H(2) (D(2)) molecules in a linear action regime throughout the 700-4200 cm(-1) range using a table-top laser system. The spectral patterns were analyzed in the context of the low energy structures obtained from electronic structure calculations. These results indicate that all four species are protonated on the N-terminus, and feature an intramolecular H-bond involving the amino group. The large blue-shift in the H-bonded N-H fundamental upon incorporation of a methyl group at the N-terminus indicates that this modification significantly lowers the strength of the intramolecular H-bond. Methylation at the amide nitrogen, on the other hand, induces a significant rotation (~110°) about the peptide backbone.

Infrared spectroscopy of hydrated alkali metal cations: Evidence of multiple photon absorption

Infrared predissociation spectra of M(+)(H(2)O)(4-7), where M = alkali metal, are presented. Hydrogen bonding O-H stretching features are strongly dependent on which fragmentation channel is monitored. Spectra recorded by monitoring the loss of multiple waters show a preference for one absorption feature in the hydrogen-bonded region centered at ∼3430-3500 cm(-1), which is assigned to linear-type hydrogen bonded OH stretches. Cyclic- and bent-type hydrogen bonded OH stretches have diminished photodissociation cross sections in the multiple ligand loss channels. Evidence from Rice-Ramsperger-Kassel-Marcus-evaporative ensemble calculations and laser fluence dependence experiments indicates that the multiple water loss channels are primarily the result of multiple photon absorption which we propose could be due to multiple, independent oscillators within a cluster ion each absorbing a photon during a single, 10 ns laser pulse.

Revisiting Li+(H2O)3–4Ar1 Clusters: Evidence of High-Energy Conformers from Infrared Spectra

New infrared predissociation (IRPD) spectra of Li+(H2O)3–4Ar1 clusters with previously unknown hydrogen-bonding motifs are reported. For Li+(H2O)4Ar1, high-energy conformers were detected by monitoring the [Ar + H2O] fragmentation channel. Hydrogen-bonded O–H stretching features were observed between 3200 and 3450 cm–1, while a band at 3678 cm–1 was assigned to a three-coordinated, or acceptor–acceptor–donor (AAD), H2O free O–H stretching mode. Furthermore, spectroscopic evidence of a high-energy conformer of Li+(H2O)3Ar1 containing a hydrogen-bonded O–H stretching mode was also observed by monitoring the [Ar] fragmentation channel. MP2/aug-cc-pVDZ level calculations were used to identify possible conformers and determine relative energies and simulated vibrational spectra for Li+(H2O)3–4Ar1 cluster ions.

Structure and hydration of the C4H4•+ ion formed by electron impact ionization of acetylene clusters

Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C(4)H(4)(●+) ion in the cluster beam. The measured average collision cross section of the C(4)H(4)(●+) isomers in helium (38.9 ± 1 Å(2)) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C(4)H(4)(●+) ion [methylenecyclopropene (39.9 Å(2)), 1,2,3-butatriene (41.1 Å(2)), cyclobutadiene (38.6 Å(2)), and vinyl acetylene (41.1 Å(2))]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C(4)H(4)(●+) ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C(4)H(4)(●+) ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C(2)H(2))(2)(●+) [R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys. 131, 114305 (2009)]. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C(4)H(4)(●+)(H(2)O)(n) clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C(4)H(4)(●+)H(2)O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene(●+)·H(2)O cluster (41 kJ/mol). The binding energies of the C(4)H(4)(●+)(H(2)O)(n) clusters change little from n = 1 to 5 (39-48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water-C(4)H(4)(●+) and water-water interactions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C(4)H(4)(●+)(H(2)O)(5) cluster. Consequently, a drop in the binding energy of the sixth water molecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C(4)H(4)(●+)(H(2)O)(5) cluster presumably consisting of a cyclobutadiene(●+) cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C(2)H(2))(2)(●+) ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation.

Electron Distribution in Partially Reduced Mixed Metal Oxide Systems: Infrared Spectroscopy of CemVnOo+ Gas-Phase Clusters

Vibrational predissociation spectra of rare-gas-tagged [(CeO(2))(VO(2))(1-2)](+) and [(Ce(2)O(3))(VO(2))](+) clusters are measured in the 400-1200 cm(-1) region. Density functional theory (DFT) is used to determine the geometric and electronic structure of low-energy isomers of the partially reduced clusters. Comparison of experimental and simulated spectra provides evidence for the larger stability of Ce(+3)/V(+5) compared to that of Ce(+4)/V(+4), which confirms that the exceptionally high reducibility of Ce(+4) accounts for the promoting role of ceria in supported vanadium oxide catalysts.

Infrared Predissociation Spectroscopy of M+(C6H6)1−4(H2O)1−2Ar0−1 Cluster Ions, M = Li, Na

Infrared predissociation (IRPD) spectra of Li(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar(0-1) and Na(+)(C(6)H(6))(2-4)(H(2)O)(1-2)Ar(1) are presented along with ab initio calculations. The results indicate that the global minimum energy structure for Li(+)(C(6)H(6))(2)(H(2)O)(2) has each water forming a π-hydrogen bond with the same benzene molecule. This bonding motif is preserved in Li(+)(C(6)H(6))(3-4)(H(2)O)(2)Ar(0-1) with the additional benzene ligands binding to the available free OH groups. Argon tagging allows high-energy Li(+)(C(6)H(6))(2-4)(H(2)O)(2)Ar isomers containing water-water hydrogen bonds to be trapped and detected. The monohydrated, Li(+) containing clusters contain benzene-water interactions with varying strength as indicated by shifts in OH stretching frequencies. The IRPD spectra of M(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar are very different for lithium-bearing versus sodium-bearing cluster ions emphasizing the important role of ion size in determining the most favorable balance of competing noncovalent interactions.

Infrared Spectroscopy of Li+(CH4)1Arn, n = 1−6, Clusters

Infrared predissociation (IRPD) spectra of Li(+)(CH(4))(1)Ar(n), n = 1-6, clusters are reported in the C-H stretching region from 2800 to 3100 cm(-1). The Li(+) electric field perturbs CH(4) lifting its tetrahedral symmetry and gives rise to multiple IR active modes. The observed bands arise from the totally symmetric vibrational mode, v(1), and the triple degenerate vibrational mode, v(3). Each band is shifted to lower frequency relative to the unperturbed CH(4) values. As the number of argon atoms is increased, the C-H red shift becomes less pronounced until the bands are essentially unchanged from n = 5 to n = 6. For n = 6, additional vibrational features were observed which suggested the presence of an additional conformer. By monitoring different photodissociation loss channels (loss of three Ar or loss of CH(4)), one conformer was uniquely associated with the CH(4) loss channel, with two bands at 2914 and 3017 cm(-1), values nearly identical to the neutral CH(4) gas-phase v(1) and v(3) frequencies. With supporting ab initio calculations, the two conformers were identified, both with a first solvent shell size of six. The major conformer had CH(4) in the first shell, while the conformer exclusively present in the CH(4) loss channel had six argons in the first shell and CH(4) in the second shell. This conformer is +11.89 kJ/mol higher in energy than the minimum energy conformer at the MP2/aug-cc-pVDZ level. B3LYP/6-31+G* level vibrational frequencies and MP2/aug-cc-pVDZ level single-point binding energies, D(e) (kJ/mol), are reported to support the interpretation of the experimental data.

Vibrational predissociation spectroscopy and theory of Ar-tagged, protonated Imidazole (Im) Im1–3H+·Ar clusters

We report vibrational predissociation spectra and theoretical analysis of the Ar-tagged cluster ions of imidazole, Im1–3H+·Ar. The frequencies of the external N–H stretches are observed to incrementally blue-shift toward that of neutral imidazole upon addition of the second and third Im molecules, consistent with the calculated behavior of the Im3H+ complex in which the excess charge is symmetrically shared by two internal N–H–N hydrogen bonds. A very strong, symmetrical doublet near 1000cm−1 is observed for the Im2H+ complex and attributed to the parallel bridging proton displacement based on anharmonic frequency calculations.

Vibrational Predissociation Spectrum of the Carbamate Radical Anion, C5H5N-CO2−, Generated by Reaction of Pyridine with (CO2)m−

We report the vibrational predissociation spectrum of C(5)H(5)N-CO(2)(-), a radical anion which is closely related to the key intermediates postulated to control activation of CO(2) in photoelectrocatalysis with pyridine (Py). The anion is prepared by the reaction of Py vapor with (CO(2))(m)(-) clusters carried out in an ionized, supersonic entrainment ion source. Comparison with the results of harmonic frequency calculations establishes that this species is a covalently bound molecular anion derived from the corresponding carbamate, C(5)H(5)N-CO(2)(-) (H(+)). These results confirm the structural assignment inferred in an earlier analysis of the cluster distributions and photoelectron spectra of the mixed Py(m)(CO(2))(n)(-) complexes [J. Chem. Phys. 2000, 113 (2), 596-601]. The spectra of the (CO(2))(m)(-) (m = 5 and 7) clusters are presented for the first time in the lower energy range (1000-2400 cm(-1)), which reveal several of the fundamental modes that had only been characterized previously by their overtones and combination bands. Comparison of these new spectra with those displayed by Py(CO(2))(n)(-) suggests that a small fraction of the Py(CO(2))(n)(-) ions are trapped entrance channel reaction intermediates in which the charge remains localized on the (CO(2))(m)(-) part of the cluster.

Cooperatively Enhanced Ionic Hydrogen Bonds in Cl−(CH3OH)1−3Ar Clusters

Infrared predissociation (IRPD) spectra of Cl−(CH3OH)1-3Ar and Cl-(CH3OD)1-3Ar were obtained in the OH and CH stretching regions. The use of methanol-d1 was necessary to distinguish between CH stretches and hydrogen-bonded OH features. The spectra of Cl-(CH3OH)2-3Ar show intense features at frequencies lower than the CH stretches, indicating structures with very strong hydrogen bonds. These strong hydrogen bonds arise from structures in which a Cl-···methanol ionic hydrogen bond is cooperatively enhanced by the presence of a second shell and, in the case of Cl-(CH3OH)3Ar, a third shell methanol. The strongest hydrogen bond is observed in the Cl-(CH3OH)3Ar spectrum at 2733 cm-1, shifted a remarkable -948 cm-1 from the neutral, gas-phase methanol value. Harmonic, ab initio frequency calculations are not adequate in describing these strong hydrogen bonds. Therefore, we describe a simple computational approach to better approximate the hydrogen bond frequencies. Overall, the results of this study indicate that high-energy isomers are very efficiently trapped using our experimental method of introducing Cl- into neutral, cold methanol-argon clusters.

Ab Initio Studies of Structural and Vibrational Properties of Protonated Water Cluster H7O3(+) and Its Deuterium Isotopologues: An Application of Driven Molecular Dynamics.

In this work, we present infrared (IR) spectra of H7O3(+) and its deuterium isotopomers calculated by direct molecular dynamics (MD) simulations at the B3LYP/6-31+G** computational level. The calculated spectra obtained at 100, 300, and 500 K were compared to available experimental observations, and spectral features were assigned using normal-mode analysis (NMA) and driven molecular dynamics (DMD). Spectral peaks at 2410 and 2540 cm(-1) were assigned to asymmetric and symmetric stretches of the bridging hydrogen (BH) using NMA. The weak spectral features at 2166 and 2275 cm(-1) were assigned to a combination band of BH asymmetric stretch, H2O in phase wagging, OO stretch, and H3O(+) rocking vibrations by DMD simulations. Our observation of BH stretch vibrations as low as 2166 cm(-1) is in good agreement with the assignment of the low-resolution spectrum obtained by Schwarz at 2200-2300 cm(-1) [Schwarz, H. A. J. Chem. Phys. 1977, 67, 5525-5534] and vibrational predissociation spectrum by Lee et al. ∼2300 cm(-1) [Okumura, M.; Yeh, L. I.; Myers, J. D.; Lee, Y. T. J. Chem. Phys. 1990, 94, 3416-3427].

Vibrational predissociation spectra of the Ar-tagged [CH4 · H3O+] binary complex: spectroscopic signature of hydrogen bonding to an alkane

Vibrational predissociation spectra of the Ar-tagged [H3O+ ⋅ X], X = CH4, CD4, N2, and Ar complexes are analysed to explore the hydrogen bonding acceptor properties of an alkane. The observed red shift in the OH stretching transition of the donor is found to be significantly smaller than anticipated by the previously reported trend in this value with the proton affinity of the acceptor [Science 316, 249 (2007)] 1. Specifically, the alkane-induced red shift of the OH stretching frequency is less than that caused by the conventional proton acceptor, N2, even though the latter is a weaker base than methane. The origin of this effect is discussed in the context of the structures of the complexes and the molecular rearrangements required for complete proton transfer to hydrocarbons as opposed to the situation in conventional H-bond acceptors.

Infrared Spectra of Mass-Selected Br−−(NH3)n and I−−NH3 Clusters

Infrared vibrational predissociation spectra are recorded for Br(-)-(NH(3))(n) (n = 1-4) and I(-)-NH(3) clusters in the N-H stretch region (3040-3460 cm(-1)). To aid spectral assignments and clarify structures of the Br(-)-(NH(3))(n) clusters, ab initio calculations are performed at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels of theory. The Br(-)-NH(3) and I(-)-NH(3) dimers are predicted to have structures in which the NH(3) molecule is attached to the halide anion by a single hydrogen-bond. The dominant infrared band for Br(-)-NH(3) at 3171 cm(-1) corresponds to a hydrogen-bonded N-H stretch vibrational mode, whereas two weaker bands are assigned to a symmetric stretch vibration of the nonbonded N-H groups (3347 cm(-1)) and to an ammonia-based bending overtone (3293 cm(-1)) deriving infrared intensity through Fermi interaction with the H-bonded N-H stretch mode. The corresponding I(-)-NH(3) spectrum is dominated by the H-bonded N-H stretch band at 3217 cm(-1), with three weaker bands at 3240, 3305, and 3360 cm(-1) assigned to two bending overtone vibrations and the nonbonded N-H symmetric stretch vibration, respectively. Spectra of the Br(-)-(NH(3))(n), n = 2-4, clusters are similar to the I(-)-NH(3) spectrum, exhibiting evidence for strong Fermi interactions between the H-bonded N-H stretch vibrational mode and ammonia-based bending overtones. On the basis of the infrared spectra and ab initio calculations, the larger Br(-)-(NH(3))(n) clusters are deduced to have structures in which the NH(3) molecules are attached to the Br(-) by single H-bonds, but not necessarily to one other.

Survey of Ar-Tagged Predissociation and Vibrationally Mediated Photodetachment Spectroscopies of the Vinylidene Anion, C2H2−

We report predissociation spectra of Ar-tagged C(2)H(2)(-) and C(2)D(2)(-) anions, and explore vibrationally mediated photodetachment from various vibrational levels of the bare C(2)H(2)(-) ion using velocity-map imaging. Intense photodetachment resonances are observed in the C-H stretching region that are strongly correlated with vibrational hot bands in the anion photoelectron spectra, indicating that one-color, resonant two-photon photodetachment (R2PD) is complicated by excitation of vibrationally excited states with autodetaching upper levels embedded in the continuum. Isolation of the R2PD spectrum was achieved using a two-color, IR-IR scheme in which vibrational excitation and photodetachment were carried out in two separate laser interaction regions.

Anharmonicities and Isotopic Effects in the Vibrational Spectra of X−·H2O, ·HDO, and ·D2O [X = Cl, Br, and I] Binary Complexes

Vibrational predissociation spectra of the argon-tagged halide monohydrates, X(-) .H(2)O.Ar (X = Cl, Br, or I), are recorded from approximately 800 to 3800 cm(-1) by monitoring the loss of the argon atom. We use this set of spectra to investigate how the spectral signatures of the hydrogen-bonding and large-amplitude hindered rotations of the water molecule are affected by incremental substitution of the hydrogen atoms by deuterium. All six vibrational modes of the X(-).H(2)O complexes are assigned through fundamental transitions, overtones, or combination bands. To complement the experimental study, harmonic and reduced-dimensional calculations of the vibrational spectra are performed based on the MP2/aug-cc-pVTZ level of theory and basis set. Comparison of these results with those from the converged six-dimensional calculations of Rheinecker and Bowman [J. Chem. Phys. 2006, 125, 133206.] show good agreement, with differences smaller than 30 cm(-1). The simpler method has the advantage that it can be readily extended to the heavier halides and was found to accurately recover the wide range of behaviors displayed by this series, including the onset of tunneling between equivalent minima arising from the asymmetrical (single ionic hydrogen-bonded) equilibrium structures of the complexes.

Structural characterization of (C2H2)1–6+ cluster ions by vibrational predissociation spectroscopy

Vibrational predissociation spectra are reported for the cationic acetylene clusters, (C(2)H(2))(n) (+), n=1-6, in the region of the C-H stretching fundamentals. For n=1 and 2, predissociation could only be observed for the Ar-tagged clusters. These were prepared by charge-transfer collisions of Ar(k) (+) with C(2)H(2) to create C(2)H(2) (+)Ar(m) clusters, which were then converted into larger members of the (C(2)H(2))(n) (+)Ar series by sequential addition of acetylene molecules. The (C(2)H(2))(2) (+)Ar spectrum indicates that this species is predominantly present as the cyclobutadiene cation. Although mobility measurements on the electron-impact-generated (C(2)H(2))(3) (+) ion indicated that it primarily occurs as the benzene cation, [P. O. Momoh, J. Am. Chem. Soc. 128, 12408 (2006)] photofragmentation of (C(2)H(2))(3) (+)Ar in the C-H stretching region is dominated by the loss of C(2)H(2) in addition to the weakly bound Ar atom. This suggests that the dominant n=3 species formed by sequential addition of C(2)H(2) is based on a covalently bound C(4)H(4) (+) core ion. Interestingly, the spectrum of this core C(4)H(4) (+) species is different from that found for the cyclobutadiene cation, displaying instead a new band pattern that is retained in the higher (C(2)H(2))(3-6) (+) clusters. Multiple isomers are clearly involved, as yet another pattern of bands is recovered when the (C(2)H(2))(3) (+)Ar action spectrum is recorded in the (minor) Ar loss fragmentation channel. One of these features does appear in the location of the single band characteristic of the Ar-tagged benzene cation reported earlier [Phys. Chem. Chem. Phys. 4, 24 (2002)], supporting a scenario where the benzene cation is one of the isomers present. We then compare the Ar predissociation results with (C(2)H(2))(n) (+) spectra obtained when the ions are prepared by electron impact ionization of neutral acetylene clusters. The photofragmentation behavior and vibrational spectra indicate that the dominant species formed in this way also occur with a covalently bound C(4)H(4) (+) core. There are absorptions, however, which are consistent with a minor contribution from (C(2)H(2))(n) (+) clusters based on the benzene cation.

Infrared Spectroscopy of Perdeuterated Protonated Water Clusters in the Vicinity of the Clathrate Cage

We report infrared predissociation spectra of size-selected D(+)(D(2)O)(n) clusters in the size range n = 18-24 for comparison to previous studies of the corresponding H(+)(H(2)O)(n) species (Shin, J.-W.; Hammer, N. I.; Diken, E. G.; Johnson, M. A.; Walters, R. S.; Jaeger, T. D.; Duncan, M. A.; Christie, R. A.; Jordon, K. D. Science 2004, 304, 1137). For n = 18-20, two "free" OD stretch bands are observed and assigned to D(2)O molecules in acceptor-acceptor-donor (AAD) and acceptor-donor (AD) hydrogen bonding arrangements. Only the AAD band is observed for the n = 21 perdeuterated species. This behavior is identical to that observed previously for the corresponding H(+)(H(2)O)(n) clusters. Similar to the all-H protonated species, the AD "free" OD stretch band is also absent for the perdeuterated n = 22 cluster but returns for clusters larger than n = 22. Like the H(+)(H(2)O)(n) systems, the perdeuterated clusters have no spectral band in the lower frequency range where the signature of the hydronium cation is predicted. These observations shed new light on the intriguing spectroscopy and dynamics of large protonated water clusters.

Messenger-Tagging Electrosprayed Ions: Vibrational Spectroscopy of Suberate Dianions

The gas-phase vibrational spectroscopy of bare and monohydrated suberate dianions, (-)OOC-(CH(2))(6)-COO(-) and (-)OOC-(CH(2))(6)-COO(-).H(2)O, is studied by infrared photodissociation aided by electronic structure calculations. To this end, the corresponding ion-Kr atom complexes are formed in a cooled buffer-gas-filled ion trap, and their infrared vibrational predissociation spectra are measured in the range from 660 to 3600 cm(-1). The water molecule binds to one of the two carboxylate groups in a bidentate fashion, characterized by the splitting of the carboxylate stretching bands, a substantially blue-shifted water bending band, and the presence of anomalously broadened bands in the O-H stretching and H(2)O rocking region. The C-C backbone structure remains unperturbed by the addition of a water molecule or a Kr atom. At 63 K, the all-trans isomer is the most abundant species, but evidence for dynamically interconverting conformers is also present from contributions to the absorption cross section on the low-energy tail of the C-H stretching region.

Vibrational predissociation spectra of the [formula omitted], n = 3–10, 12 clusters: Even–odd alternation in the core ion

We report vibrational predissociation spectra of the O n − , n = 3–10, 12 cluster ions in the 700–2400 cm −1 range. The odd numbered clusters are consistent with their identification as O 3 − ⋅ ( O 2 ) n . The even numbered clusters are based on the O 4 − core ion, where the first two O 2 molecules add to O 4 − in locations that individually break the symmetry of the core ion, but together restore this symmetry. Beyond O 4 − ⋅ ( O 2 ) 2 , subsequent O 2 attachment yields bands close to that of neutral O 2, indicating that the special character of the O 4 − ⋅ ( O 2 ) 2 cluster is retained.