Infrared Signature of Structures Associated with the H+(H2O)n (n = 6 to 27) Clusters
We report the OH stretching vibrational spectra of size-selected H+(H2O)n clusters through the region of the pronounced “magic number” at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6 ≤ n ≤ 27. A multiplet of such bands appears at small cluster sizes. This pattern simplifies to a doublet at n = 11, with the doublet persisting up to n = 20, but then collapsing to a single line in the n = 21 and n = 22 clusters and reemerging at n = 23. This spectral simplification provides direct evidence that, for the magic number cluster, all the dangling OH groups arise from water molecules in similar binding sites.
A Cluster Study of Cl2- Microhydration: Size-Dependent Competition between Symmetrical H-Bonding to the Anion and the Formation of Cyclic Water Networks in the Cl2-·1−5(H2O) Series
We report argon predissociation spectra of the Cl2-•nH2O (n = 1−5) clusters in the OH stretching region. The spectra of the mono- and dihydrates consist of only a single sharp feature, indicating that the water molecules bind symmetrically to the ion in both cases. The spectra become much more complex in the n = 3−5 clusters, displaying band positions consistent with the formation of cyclic networks similar to those found in the hydration of other small diatomic anions. We contrast this behavior with that of the related Cl-•(H2O)1-3clusters, where the primary difference arises from delocalization of the excess electron density over two atomic centers in the diatomic anion.
Isotopic fractionation and zero-point effects in anionic H-bonded complexes: a comparison of the I− · HDO and F− · HDO ion–molecule clusters
Mid-infrared spectra of the argon-solvated X− • H2O, X− • HDO, and X− • D2O complexes (X=I, F) are reported in both the OH and OD stretching regions. Due to the asymmetric H-bonding arrangement adopted by the monohydrates, two isomers are possible for the mixed isotopomer (X− • DOH and X− • HOD). The spectra establish that the isomeric form with ionic H-bonding to the OD group is preferentially formed for the iodide complex (I− • DOH), while the fluoride ion binds preferentially to OH (F− • HOD). The behavior of the iodide monohydrate is traced to the zero-point contribution from the ion–molecule, out-of-plane bending vibrational mode, while the very strongly red-shifted, anharmonic OH stretches appear to overcome this effect in the F− • HOD complex.
The Vibrational Spectrum of the Neutral (H2O)6 Precursor to the “Magic” (H2O)6- Cluster Anion by Argon-Mediated, Population-Modulated Electron Attachment Spectroscopy
A new technique is introduced for the acquisition of size-selected neutral cluster spectra on the basis of argon-mediated, population-modulated electron attachment. This method is demonstrated and used to obtain the vibrational spectrum of the em>neutral water hexamer precursor to the (H2O)6- cluster ion. The mid-infrared spectrum of the neutral species is dominated by four intense features above 3400 cm-1, clearly indicating that significant structural rearrangements occur upon slow electron attachment to form the “magic” hexamer cluster anion. Comparison with previous spectroscopic reports and theoretical predictions indicates that the low-energy “book” isomer is most consistent with the observed band pattern and is suggested to be the species that captures a low-energy electron to form the hexamer anion.
Large anharmonic effects in the infrared spectra of the symmetrical CH3NO2−⋅(H2O) and CH3CO2−⋅(H2O)complexes
The observed IR spectra of the CH3NO2−⋅(H2O) and CH3CO2−⋅(H2O) complexes display sequences of up to seven transitions in the region of the OH stretch fundamentals. This is indicative of strong anharmonic coupling between the OH stretch modes and one or more low-frequency modes. Cubic force fields have been calculated for these two complexes using the MP2 method, and these have been used to calculate the vibrational spectra and to identify the key couplings responsible for the “extra” lines in the observed spectra. In addition, a simple adiabatic model involving the OH stretch and intermolecular rock vibrations is introduced and shown to account in an near quantitative manner for the structure in the OH stretch region of the spectra.
Strong Similarities in the Local Hydration Environments of the Bromide Ion and the Cl-·CCl3• Ion−Radical Complex: Factors Contributing to Intramolecular Distortions in the Primary Hydration Shell
We report the infrared photofragmentation spectra of the Cl-•CCl3•nH2O, n = 1−3 complexes and compare the observed pattern of OH stretching vibrations with those displayed by the X-•nH2O, [X = Cl, Br] clusters. The predissociation spectra reveal hydrogen bonded networks in the di- and trihydrate clusters. Addition of the CCl3• radical to the Cl-•nH2O complexes causes a significant blue shift in the OH stretching vibrations involving the hydrogen atoms bound to the ion. As a result, the Cl-•CCl3•nH2O spectra appear remarkably similar to those observed previously for the bromide hydrates. We trace this effect to partial charge delocalization onto the CCl3• radical through an analysis of the electronic structure carried out with density functional calculations. Two important factors contributing to the ion-bound OH stretch frequencies are the radius, (ri), and excess charge, (qi), associated with the halide. The red shifts displayed by the water molecule in a number of halide−water complexes are found to be strongly correlated with the parameter qi/ri2, which accounts for both effects through their contributions to the electric field at the surface of the ion.
We present mid-IR argon predissociation spectra for a series of complexes, M-•H2O (M = CS2-, OCS-, SO2-, CH3NO2-, CH3CO2-, and NO2-), chosen to explore how changes in the triatomic binding site affect the H-bonding configuration of the attached water molecule. With the exception of NO2-, the calculated global minima on the potential surfaces of all of the complexes occur in a configuration where both OH groups are attached to the anion. The observed spectra, on the other hand, fall into three distinct categories. Simple spectra characteristic of the double ionic H-bonding arrangement are observed for the monohydrates of SO2-, OCS-, and CS2-, whereas the CH3NO2-•H2O and CH3CO2-•H2O spectra are complicated, displaying a progression of closely spaced bands with a broad, bell-shaped envelope beginning several hundred wavenumbers below the calculated fundamentals. Although the spectrum of the NO2-•H2O complex is the most red-shifted, it is again simple, reflecting the expected asymmetric (single ionic H-bonded) motif. These data indicate that the transition from single to double ionic H-bonding occurs at a critical domain length of about 2.2 Å. We explore the potential surfaces governing the interconversion between the two forms with density functional calculations and construct vibrationally adiabatic potential surfaces to assess the cause of the spectral complexity displayed by the methylated anion hydrates.
Snapshots of water at work
The behavior of ions in water--an essential aspect of many chemical processes--is usually deduced from highly averaged measurements of viscosity and reaction rate. In their Perspective, Robertson et al . highlight two reports that elucidate the molecular details of such processes. Omta et al . show that a solvated anion only affects the structure of water in its immediate vicinity. And Rini et al . demonstrate how water in the hydration shell of two anions participated in the chemical reaction between the anions. The results should help to test models of aqueous chemistry.
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−⋅(H2O)3[X=Cl,Br,I] clusters and the open structure displayed by the O2−⋅(H2O)3 system. The structural differences between the complexes of water with O2− and NO− are discussed in the context of the relative proton affinities and the electronic structures of their ground states (i.e., 3∑−NO− versus 2∏gO2−). Spectra of the NO−⋅(H2O)4,5 clusters are also presented together with a qualitative discussion of the likely morphologies at play in these higher hydrates.
Spectroscopic Determination of the OH− Solvation Shell in the OH−·(H2O)nClusters
There has been long-standing uncertainty about the number of water molecules in the primary coordination environment of the OH− and F− ions in aqueous chemistry. We report the vibrational spectra of the OH−•(H2O)nand F−•(H2O)n clusters and interpret the pattern of OH stretching fundamentals with ab initio calculations. The spectra of the cold complexes are obtained by first attaching weakly bound argon atoms to the clusters and then monitoring the photoinduced evaporation of these atoms when an infrared laser is tuned to a vibrational resonance. The small clusters (n ≤ 3) display an isolated, sharp feature near the free OH stretching vibration, the signature of open solvation morphologies where each water molecule binds independently to the ion. Pronounced changes in the spectra are observed at n = 4 in the hydroxide ion and at n = 5 in the fluoride ion. In both cases, new features appear in the region typically associated with interwater hydrogen bonding. This behavior establishes that the primary hydration shells occur at n = 3 and 4 in hydroxide and fluoride, respectively.
MOLECULAR ASPECTS OF HALIDE ION HYDRATION: The Cluster Approach
This review provides a historical context for our understanding of the hydration shell surrounding halide ions and illustrates how the cluster systems can be used, in combination with theory, to elucidate the behavior of water molecules in direct contact with the anion. We discuss how vibrational predissociation spectroscopy, carried out with weakly bound argon atoms, has been employed to deduce the morphology of the small water networks attached to anions in the primary steps of hydration. We emphasize the importance of charge-transfer in the binary interaction, and discuss how this process affects the structures of the larger networks. Finally, we survey how the negatively charged water clusters (H2O)n−are providing a molecular-level perspective on how diffuse excess electrons interact with the water networks.
Argon predissociation infrared spectroscopy of the hydroxide–water complex (OH−·H2O)
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.
Isolating the Charge-Transfer Component of the Anionic H Bond Via Spin Suppression of the Intracluster Proton Transfer Reaction in the NO-·H2O Entrance Channel Complex
We report the vibrational predissociation spectrum of the NO-•H2O complex in the OH stretching region. The ionic H-bonded OH stretch is much less red-shifted and less structured in the vibrational spectrum of NO-•H2O than in that of the O2-•H2O cluster, despite the larger proton affinity of NO- relative to O2-. It is argued that the differences between the spectra of these two species are primarily a consequence of the greater charge-transfer character in the O2-•H2O cluster and that the charge-transfer component is much less important in NO-•H2O because of its triplet spin character.
Caught in the Act of Dissolution
The ionic dissociation of acids such as HCl and HBr in water is driven by a substantial energy release when the ions are formed in an extended medium. But how many water molecules does it take to trigger this dissociation? In their Perspective, Robertson and Johnson highlight the report by Hurley et al., who have answered this question for the case of HBr. Their experiments on small water clusters indicate that just five water molecules are enough to ionize HBr.
Observation of sharp vibronic bands in the O4− `core ion' by mid infrared predissociation spectroscopy of O4−·Arn clusters
We report the first observation of an infrared electronic band system arising from excitation of the ground state O4− ion, which we discuss in the context of the expected transition (J. Chem. Phys. 114 (2001) 3010) between the two low lying isomeric forms of this species. Surprisingly, the band displays sharp vibrational fine structure, opening the way for a detailed spectroscopic characterization of a charge–resonance stabilized dimer ion.