Gary H. Weddle

Mass-selected “matrix isolation” infrared spectroscopy of the I−·(H2O)2 complex: making and breaking the inter-water hydrogen-bond

Abstract Infrared spectra of the cold I − ·W and I − ·W 2 clusters are reported via vibrational predissociation spectroscopy of the argon solvated species, I − ·W n ·Ar m , in the OH stretching region. Several argon atoms serve to significantly simplify the spectra by collapsing complex band contours into sharp features at the vibrational origins. This effect, in addition to the substantial cooling afforded by argon solvation, dramatically change the appearance of the bare dihydrate spectrum reported earlier [P. Ayotte, C.G. Bailey, G.H. Weddle, M.A. Johnson, J. Phys. Chem. A 102 (1998) 3067]. The cold spectrum consists of a simple four line pattern anticipated by ab initio calculations for the asymmetric structure where the two waters are bound together on one side of the ion. The dramatic changes in the spectrum of the argon complex relative to that of bare I − ·W 2 are readily interpreted to be a consequence of internal energy in the latter leading to rupture of the inter-solvent H-bond.

AN INFRARED STUDY OF THE COMPETITION BETWEEN HYDROGEN-BOND NETWORKING AND IONIC SOLVATION : HALIDE-DEPENDENT DISTORTIONS OF THE WATER TRIMER IN THE X- .(H2O)3, (X=CL, BR, I) SYSTEMS

Vibrational spectra of the water trimers solvating the halide anions (Cl−, Br−, I−) have been acquired in the OH stretching region by predissociation spectroscopy of the X−⋅(H2O)3⋅Ar3 complexes. These “wet” ions display two groups of bands assigned to normal modes of the (C3) pyramidal structure. We interpret the evolution of the spectra down the halogens in the context of the rings closing up toward the structure of the bare (H2O)3 neutral. This trend is discussed in terms of the disruptive effect of the ionic H bonds on the water network.

Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.

Vibrational spectral signature of the proton defect in the three-dimensional H+(H2O)21 cluster

Blackjack water cluster detected Spectroscopy of protonated water clusters has played a pivotal role in elucidating the molecular arrangement of acid solutions. Whereas bulk liquids manifest broad spectral features, the cluster bands tend to be sharper. The 21-membered water cluster has for decades inspired particular interest on account of its stability and its place in the transition from two-dimensional to three-dimensional hydrogen-bonding network motifs, but the spectral signature of its bound proton has proved elusive. Fournier et al. have now detected this long-sought vibrational feature by applying an innovative ion cooling technique. Science, this issue p. 1009 An ion-cooling technique enables detection of a long-sought motif in the study of acid structure. The way in which a three-dimensional network of water molecules accommodates an excess proton is hard to discern from the broad vibrational spectra of dilute acids. The sharper bands displayed by cold gas-phase clusters, H+(H2O)n, are therefore useful because they encode the network-dependent speciation of the proton defect and yet are small enough to be accurately treated with electronic structure theory. We identified the previously elusive spectral signature of the proton defect in the three-dimensional cage structure adopted by the particularly stable H+(H2O)21 cluster. Cryogenically cooling the ion and tagging it with loosely bound deuterium (D2) enabled detection of its vibrational spectrum over the 600 to 4000 cm−1 range. The excess charge is consistent with a tricoordinated H3O+ moiety embedded on the surface of a clathrate-like cage.

Argon predissociation infrared spectroscopy of the hydroxide-water complex (OH-.H2O)

Abstract We report the first vibrational spectrum of the degenerate proton transfer system OH−·H2O. The complex is cooled by attachment of argon atoms and the spectrum is observed by argon predissociation spectroscopy in the OH stretching region. A strong, sharp transition is observed just below the region usually associated with the free OH stretch, while broader bands appear lower in energy and are weaker than the dominant free OH peak. The latter are assigned with the aid of ab initio calculations to the first overtone of the coupled intramolecular bend and strongly red-shifted H-bonded OH stretching modes.

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.

Dominant structural motifs of NO-.(H2O)n complexes: Infrared spectroscopic and ab initio studies

Argon predissociation spectroscopy is used together with ab initio electronic structure calculations to characterize the NO−⋅(H2O)n=1–3 clusters. In all cases, the water molecules bind to the ion through single ionic H bonds. Two isomeric forms are assigned for the n=1 species that differ according to whether the H bond occurs to the N or O atom of the core ion. While the spectra of the dihydrate indicate formation of an H-bonded water dimer subcluster consistent with all four predicted isomers, their calculated vibrational spectra are too similar to establish which of these forms is created in the ion source. Three classes of isomers are predicted for the NO−⋅(H2O)3 clusters, and in this case a comparison of the experimental and theoretical infrared spectra indicates the formation of a bridging arrangement in which two of the water molecules are attached to one atom and the third to the other atom of NO−. This distorted water trimer motif is intermediate between the symmetrical trimer found in the X−⋅(H2...

Linking the photoelectron and infrared spectroscopies of the (H2O)6− isomers

We report a novel photoelectron spectroscopy variation of population labeling spectroscopy and apply it to assign the isomeric carrier of the strong autodetaching OH stretching vibrational resonances reported previously [J. Phys. Chem. 100, 16782 (1996) and J. Chem. Phys. 108, 444 (1998)] for a mixed ensemble of (H2O)6− isomers. The vibrational bands are traced to the isomer with the higher vertical electron detachment energy (VDE). This result indicates that resonances are most readily observed for vibrational bands which lie below the VDE of the parent species.

Preparation and photoelectron spectrum of the CH3I− anion: rare gas cluster mediated synthesis of an ion–radical complex

We report the preparation of the bare and argon-solvated anion of CH3I, and characterize this species using negative ion photoelectron spectroscopy at 3.495 eV. The photoelectron spectrum consists of a narrow band appearing 0.11±0.02 eV above the binding energy of isolated iodide. Such behavior is similar to that displayed by iodide-(closed shell) solvent molecule complexes, indicating that photodetachment does not access the bound region of the CH3I potential. These observations suggest that CH3I− rearranges (after electron capture) to an ion-radical complex. We advance the hypothesis that this complex adopts a C2v structure where the ion is hydrogen bonded to the methyl radical.

Infrared spectra of hydrogen-bonded ion–radical complexes: I−⋅HCH2 and Br−⋅HCHBr

We report the preparation and infrared spectra of the CH3I− and CH2Br2− anions formed by argon cluster-mediated electron attachment to the neutral molecular precursors. Infrared predissociation spectra were acquired for both the bare and argon-solvated species in the C–H stretching region. Partial rotational structure was recovered in the CH3I− system, consistent with the hydrogen-bonded, C2v structure suggested in an earlier analysis of its photoelectron spectrum [J. Kim et al., J. Am. Soc. Mass Spectrom. 10, 810 (1999)]. The spectrum and photofragmentation pattern confirm that this species is trapped in a very weakly bound ion–methyl radical form (I−⋅HCH2) involving a single ionic H bond. The CH2Br2− anion displays a similar spectrum, where one CH stretch is significantly redshifted, again signaling the single H-bonding motif.