Infrared Spectrum and Structural Assignment of the Water Trimer Anion
The bending vibrational spectrum of the perdeutero isotopomer of the water trimer anion has been measured and compared with spectra calculated using the MP2, CCSD, and Becke3LYP electronic structure methods. Due to its low electron binding energy (≈150 meV), only the OD bending region of the IR spectrum of (D2O)3- is accessible experimentally, with electron ejection dominating at higher photon energies. The calculated spectrum of the isomer having three water molecules arranged in a chain agrees best with the experimental spectrum. In the chain isomer, the excess electron is bound to the terminal water monomer with two dangling OH groups. This is consistent with the electron binding mechanism established previously for the (H2O)n- (n = 2, 4−6) anions.
Mid-infrared characterization of the NH4+∙(H2O)n clusters in the neighborhood of the n = 20 “magic” number
Vibrational predissociation spectra are reported for size-selected NH4+∙(H2O)n clusters (n = 5–22) in the 2500–3900 cm−1 region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+∙(H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n = 9, and then to a single peak at the n = 20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+∙(H2O)n clusters. We characterize the likely structures at play for the n = 19 and 20 clusters with electronic structure calculations. The most stable form of the n = 20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.
Identification of Two Distinct Electron Binding Motifs in the Anionic Water Clusters: A Vibrational Spectroscopic Study of the (H2O)6- Isomers
Photoelectron spectroscopy of the water cluster anions, (H2O)n-, has revealed that several isomeric forms are present for most sizes, and here, we use vibrational spectroscopy to address the structure of the (H2O)6- isomer that more weakly binds the extra electron. To overcome the severe line broadening that occurs in the OH stretching region of this isomer caused by fast electron autodetachment, we concentrate on the low-energy bending modes of the perdeutero isotopomer. Sharp spectroscopic signatures are recovered for two isomers using argon predissociation spectroscopy, and the resulting bands are heavily overlapped. To extract their independent contributions to the observed spectra, we exploit the substantial dependence of their relative populations on the number of attached argon atoms in the (D2O)6-•Arm clusters, determined by photoelectron spectroscopy. The vibrational spectra of each isomer can then be isolated by spectral subtraction, which is implemented with a covariance mapping approach. The resulting band patterns establish that the more weakly binding isomer does not display the characteristic electron-binding motif common to the more strongly bound isomer class. Whereas the strongly binding isomer features a single water molecule pointing toward the excess electron cloud with both of its hydrogen atoms, the spectrum of the more weakly binding isomer suggests a structure where the electron is bound by a number of dangling OH groups corresponding to water molecules in acceptor−donor binding sites.
The vibrational predissociation spectra of the H 5O2+∙RGn(RG = Ar,Ne) clusters: Correlation of the solvent perturbations in the free OH and shared proton transitions of the Zundel ion
Predissociation spectra of the H5O2+∙RGn(RG = Ar, Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350–3850 cm−1) and shared proton (850–1950 cm−1) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H5O2+ [ L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989) ], indicating that the symmetrical H5O2+ “Zundel” ion remains largely intact in H5O2+∙Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H5O2+∙Ar, and is dominated by two sharp transitions at 928 and 1047 cm−1, with a weaker feature at 1763 cm−1. The H5O2+∙Arn,n = 1–5 spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of H5O2+ rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on H5O2+. These results are consistent with assignment of the strong low-energy bands in the H5O2+∙Ne spectrum to the vibration of the shared proton mostly along the O–O axis, with the 1763 cm−1 band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules.
Spectral signatures of hydrated proton vibrations in water clusters
The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H3O+) and Zundel (H5O2+) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.
Photoelectron spectroscopy of the [glycine∙(H2O)1,2]−clusters: Sequential hydration shifts and observation of isomers
The electron binding energies of the small hydrated amino acid anions,[glycine∙(H2O)1,2]−, are determined using photoelectron spectroscopy. The vertical electron detachment energies (VDEs) are found to increase by approximately 0.12 eV with each additional water molecule such that the higher electron binding isomer of the dihydrate is rather robust, with a VDE value of 0.33 eV. A weak binding isomer of the dihydrate is also recovered, however, with a VDE value (0.14 eV) lower than that of the monohydrate. Unlike the situation in the smaller (n ≤ 13) water cluster anions, the[Gly∙(H2O)n ≥ 6]− clusters are observed to photodissociate via water monomer evaporation upon photoexcitation in the O–H stretching region. We discuss this observation in the context of the mechanism responsible for the previously observed [ S. Xu, M. Nilles, and K. H. Bowen, Jr., J. Chem. Phys. 119, 10696 (2003) ] sudden onset in the cluster formation at [Gly∙(H2O)5]−.
An Infrared Investigation of the (CO2)n- Clusters: Core Ion Switching from Both the Ion and Solvent Perspectives
The (CO2)n- clusters are thought to accommodate the excess electron by forming a localized molecular anion, or “core ion”, solvated by the remaining, largely neutral CO2 molecules. Earlier studies interpreted discontinuities in the (CO2)n- photoelectron spectra to indicate that both the CO2- and C2O4- species were present in a size-dependent fashion. Here we use vibrational predissociation spectroscopy to unambiguously establish the molecular structures of the core ions in the 2 ≤ n ≤ 17 size range. Spectra are reported in the 2300−3800 cm-1region, which allows us to independently monitor the contribution of each ion through its characteristic overtone and combination bands. These signature bands are observed to be essentially intact in the larger clusters, establishing that the CO2- and C2O4- molecular ions are indeed the only electron accommodation modes at play. The size dependence of the core ion suggested in earlier analyses of the photoelectron spectra is largely confirmed, although both species are present over a range of clusters near the expected critical cluster sizes, as opposed to the prompt changes inferred earlier. Perturbations in the bands associated with the nominally neutral CO2 “solvent” molecules are correlated with the changes in the molecular structure of the core ion. These observations are discussed in the context of a diabatic model for electron delocalization over the CO2 dimer. In this picture, the driving force leading to the transient formation of the monomer ion is traced to the solvent asymmetry inherent in an incomplete coordination shell.
Fundamental Excitations of the Shared Proton in the H3O2- and H5O2+Complexes
We exploit recent advances in argon predissociation spectroscopy to record the spectroscopic signature of the shared proton oscillations in the H3O2- system and compare the resulting spectrum with that of the H5O2+ ion taken under similar conditions. Very intense 1 ← 0 transitions are observed below 1100 cm-1 in both cases and are surprisingly sharp, with the 697 cm-1 transition in H3O2- being among the lowest in energy of any shared proton system measured to date. The assignments of the three fundamental transitions associated with the three-dimensional confinement of the shared proton in H3O2- are carried out with full-dimensional (DMC) calculations to treat this strongly anharmonic vibrational problem.
Argon Predissociation Spectroscopy of the OH-·H2O and Cl-·H2O Complexes in the 1000−1900 cm-1 Region: Intramolecular Bending Transitions and the Search for the Shared-Proton Fundamental in the Hydroxide Monohydrate
We present argon predissociation vibrational spectra of the OH-•H2O and Cl-•H2O complexes in the 1000−1900 cm-1 energy range, far below the OH stretching region reported in previous studies. This extension allows us to explore the fundamental transitions of the intramolecular bending vibrations associated with the water molecule, as well as that of the shared proton inferred from previous assignments of overtones in the higher energy region. Although the water bending fundamental in the Cl-•H2O spectrum is in very good agreement with expectations, the OH-•H2O spectrum is quite different than anticipated, being dominated by a strong feature at 1090 cm-1. New full-dimensionality calculations of the OH-•H2O vibrational level structure using diffusion Monte Carlo and the VSCF/CI methods indicate this band arises from excitation of the shared proton.
Role of Water in Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances
An understanding of electron-initiated processes in aqueous systems and the subsequent radical chemistry these processes induce is significant in such diverse fields as waste remediation and environmental cleanup, radiation processing, nuclear reactors, and medical diagnosis and therapy. We review the state of the art in the physical chemistry and chemical physics of electron-initiated processes in aqueous systems and raise critical research issues and fundamental questions that remain unanswered.
Predissociation spectroscopy of the argon-solvated H5O2+ nbsp;“zundel” cation in the 1000–1900 cm−1 region
Predissociation spectra of the H5O2+⋅Ar1,2 cluster ions are reported in the 1000–1900 cm−1 region. The weakly bound argon atoms enable investigation of the complex in a linear action mode, and the resulting spectra are much simpler than those reported previously in this region [Asmis et al., Science 299, 1375 (2003) and Fridgen et al., J. Phys. Chem. A 108, 9008 (2004)], which were obtained using infrared multiphoton dissociation of the bare complex. The observed spectrum consists of two relatively narrow bands at 1080 and 1770 cm−1 that are likely due to excitation of the shared proton and intramolecular bending vibrations of the two water molecules, respectively. The narrow linewidths and relatively small (60 cm−1) perturbation introduced by the addition of a second argon atom indicate that the basic “zundel” character of the H5O2+ ion survives upon complexation.
Preparation and photoelectron spectrum of the ‘missing’ (H2O)4- cluster
Argon Cluster-Mediated Trapping and Vibrational Spectroscopic Characterization of an OH-·HCH2• Intermediate in the O•- + CH4 Reaction
We isolate an [O•CH4]•- intermediate in the reactive O•- + CH4 encounter using an argon cluster-mediated trapping technique and characterize it using vibrational predissociation spectroscopy. The spectra of the argon-solvated complexes establish that only the OH-•CH3•ion−radical adduct is prepared. Its formation is firmly established by the appearance of the signature OH- stretching band close to that of the free hydroxide ion. The band origin locations and partially resolved rotational spacings indicate that hydroxide binds onto one of the methyl hydrogen atoms, much like the motif observed previously in the I-•HCH2• ion−radical complex. This OH-•CH3• species is best regarded as an entrance-channel complex in the secondary (endothermic) OH- + CH3• → H2O + CH2- proton transfer reaction. These observations indicate that the initial H-atom abstraction step (O•- + CH4 → OH- + CH3•) occurs too quickly to enable capture of the intermediates directly associated with this process.
How Do Small Water Clusters Bind an Excess Electron?
The arrangement of water molecules around a hydrated electron has eluded explanation for more than 40 years. Here we report sharp vibrational bands for small gas-phase water cluster anions, (H2O)4-6– and (D2O)4-6–. Analysis of these bands reveals a detailed picture of the diffuse electron-binding site. The electron is closely associated with a single water molecule attached to the supporting network through a double H-bond acceptor motif. The local OH stretching bands of this molecule are dramatically distorted in the pentamer and smaller clusters because the excited vibrational levels are strongly coupled to the electron continuum. The vibration–to–electronic energy transfer rates, as revealed by line shape analysis, are mode-specific and remarkably fast, with the symmetric stretching mode surviving for less than 10 vibrational periods [50 fs in (H2O)4–].
Preparation and photoelectron spectrum of the glycine molecular anion: Assignment to a dipole-bound electron species with a high-dipole moment, non-zwitterionic form of the neutral core
We report the gas-phase preparation of negatively charged glycine as well as theGly⋅(H2O)1,2− complexes by entrainment of the neutral precursor into an ionized supersonic expansion tuned to optimize the (H2O)n−⋅Arm clusters. The photoelectron spectrum of Gly− displays the signature of a dipole-bound species, with sufficient vibrational fine structure to characterize the core neutral as a higher energy, non-zwitterionic isomer of the amino acid. © 2004 American Institute of Physics.