Infrared Spectroscopic Characterization of the Symmetrical Hydration Motif in the SO2- ·H2O Complex
We report for the first time the vibrational spectrum of a molecular anion−water complex in which the water molecule adopts a symmetrical binding motif. In the specific complex considered, S O2- •H2O, each hydrogen atom is attached to one of the oxygen atoms of the anion. The vibrational spectrum is rather simple and is analyzed with the aid of isotopic substitution and ab initio calculations of the complex structure and harmonic vibrational frequencies. The symmetric and asymmetric OH stretching modes in the S O2- •H2O complex are much closer in energy than in the isolated water molecule, an effect that is traced to the reduction in the HOH angle of the water molecule upon complexation.
Infrared Characterization of the Icosahedral Shell Closing in Cl-·H2O·Arn(1 ≤ n ≤ 13) Clusters
The mid-infrared predissociation spectra of Cl-•H2O•Arn clusters are dominated by an intense band that is due to excitation of the ionic H-bonded OH stretching vibration. The shape of this band is found to be quite sensitive to the number of attached argon atoms, becoming incrementally narrower until n = 11, the size at which a strong “magic number” appears in the cluster intensity profile. This observation is consistent with the formation of a capped icosahedral structure where the water molecule replaces one argon atom in the closed shell Cl-•Ar12 structure. This conjecture is supported by a theoretical study of the Cl-•H2O•Arnstructures using a simulated annealing procedure. These calculations find only one isomeric form for n = 11 but identify several distinct local minima for n ≠ 11. We recover the qualitative features of the band envelopes by calculating the electrostatic effects of the various isomeric forms on the potential energy surface describing the motion of a hydrogen atom toward the chloride ion.
Infrared predissociation spectroscopy of I−⋅(CH3OH)n, n = 1,2: Cooperativity in asymmetric solvation
Infrared spectra of I−⋅(CH3OH)n⋅Arm, n = 1,2 clusters, obtained via argon and methanol predissociation, are interpreted with the aid of ab initio calculations of the OH stretching fundamentals. The spectra of the cold, argon-solvated clusters establish the coexistence of two isomeric forms of the n = 2 cluster, with the asymmetric isomer displaying a dramatic (∼150 cm−1) OH red-shift relative to both the symmetric isomer and the n = 1complex. We trace this red-shift to cooperative H-bonding which is only operative in the asymmetric form. At the higher internal energies afforded by the bare (i.e., Ar-free) complexes, the spectra are radically changed. The strongly red-shifted band is suppressed, reflecting the loss of the cooperative effect as the methanol molecules are separated, while the bands assigned to the more open form are enhanced.
Solvation of the Cl-·H2O Complex in CCl4 Clusters: The Effect of Solvent-Mediated Charge Redistribution on the Ionic H-Bond
The OH stretching bands associated with the ionic H-bond in the Cl-•H2O complex are found to dramatically shift toward higher energy upon complexation with carbon tetrachloride molecules in a size selected photofragmentation study of the Cl-•H2O•(CCl4)n, 1 ≤ n ≤ 5, clusters. The large sequential shifts (80−100 cm-1/CCl4) associated with addition of the first few molecules effectively “tune” the OH stretch v = 1 level through a Fermi resonance with the v = 2 level of the intramolecular bending vibration. By the addition of the fifth molecule, the large red-shift displayed by the bare complex (577 cm-1 relative to isolated water) is reduced by a factor of 2. We discuss the origin of these effects in the context of charge redistrubition from chloride to the surrounding solvent molecules.
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.
Argon predissociation and electron autodetachment spectroscopy of size-selected CH3NO2−⋅Arn clusters
Photodetachment spectra of CH3NO2−⋅Arn clusters in the mid-IR are dominated by three strong resonances. These are assigned to autodetaching (AD) C–H stretching vibrational transitions in the valence (as opposed to dipole-bound) form of the molecular anion on the basis of a H/D isotopic substitution study and their solvation dependence. The AD resonances disappear promptly upon addition of the third argon atom, while the resonant structure appears in the action spectrum for formation of CH3NO2− photoproducts for n ≥ 2. The strong argon solvation dependence of the photoproducts is traced to the rapidly changing endoergicity of the electron loss channel due to the differential solvation behavior of the valence anion relative to the neutral. We discuss a statistical limit for this competition, and introduce an intramolecular vibrational energy redistribution mediated AD mechanism unique to polyatomic anions.
Observation of resonant two-photon photodetachment of water cluster anions via femtosecond photoelectron spectroscopy
Photoexcitation of the (H2O)−n (n=20–100) clusters with 100 fs pulses at 800 nm results in an increasing propensity for two-photon electron photoejection with increasing cluster size. This increase correlates with the size range (n≈30) where the first excited electronic state drops below the electron continuum, and the electronic absorption band approaches the energy of the 800 nm pump photon. No above-threshold, two-photon detachment is observed for n=20. Differences in the shape of the resonant two-photon photoelectron spectrum compared to that arising from direct (high energy) photodetachment are interpreted in terms of the vibrational state selection created in the resonant step.
Hydration of a structured excess charge distribution: Infrared spectroscopy of the O2−⋅(H2O)n, (1 ≤ n ≤ 5)clusters
To explore how a structured excess charge distribution affects the hydration of an anion, we report mid-IR, argon predissociation spectra for the hydrated superoxide cluster anions, O2−⋅(H2O)n, 1≤ n ≤5. This size range was chosen to establish the evolution of the structures through the putative shell closing [Weber et al., Science 287, 2461 (2000)] for superoxide hydration at the tetrahydrate. Whereas the observed bonding motifs for n ≤4 are those of single water molecules and dimeric subclusters bound to the ion, the pentahydrate spectrum displays strong bands in the region typically associated with ring modes of the water trimer. The present results reinforce the conclusion that the tetrahydrate adopts an especially robust structure in which each water molecule forms a single ionic H bond to one of the lobes of the π∗ highest occupied molecular orbital in superoxide.
Dipole bound and valence state coupling in argon-solvated nitromethane anions
The coupling between the dipole bound and valence electronic states of the nitromethane anion has been investigated via Rydberg electron transfer spectroscopy and field detachment spectroscopy of the bare and argon-solvated anions. The unique aspects of the nitromethane system are highlighted by comparing the solvation behavior of nitromethane anion with that of acetonitrile, a system in which the dipole bound state is well-isolated from the resonance arising from excess electron occupation of its high lying lowest unoccupied molecular orbital.
Double-contact ion-molecule binding: Infrared characterization of the ionic H bonds to formic acid in the I−⋅HCOOH complex
We report mid-IR predissociation spectra of the I−⋅HCOOH⋅Arm(m = 1–4) ion-acid complexes. The spectra are consistent with a planar structure where both hydrogens are engaged in ionic H bonds. Upon binding to the ion, the OH stretching fundamental displays a much more dramatic redshift (792 cm−1) than that of the CH stretch (99 cm−1), giving rise to a complex series of bands in the 2750–2950 cm−1 region. The contributions of the CH and OH stretches to the spectrum are isolated by recording spectra of theI−⋅DCOOH and I−⋅HCOOD species, which reveal that the OH stretching vibration is accompanied by combination bands involving soft modes while the CH stretch spectrum is dominated by a single feature. Some of the complexity in the I−⋅HCOOH spectrum arises from a strong Fermi resonance interaction between the v = 1 level of the OH stretch and an overtone or combination band involving CH motion. We compare this behavior to that of the previously reported I−⋅CH3OH and I−⋅H2O complexes.
Isolating the Spectroscopic Signature of a Hydration Shell With the Use of Clusters: Superoxide Tetrahydrate
Cluster spectroscopy, aided by ab initio theory, was used to determine the detailed structure of a complete hydration shell around an anion. Infrared spectra of size-selected O2 −• (H2O)n(n = 1 to 4) cluster ions were obtained by photoevaporation of an argon nanomatrix. Four water molecules are required to complete the coordination shell. The simple spectrum of the tetrahydrate reveals a structure in which each water molecule is engaged in a single hydrogen bond to one of the four lobes of the π* orbital of the superoxide, whereas the water molecules bind together in pairs. This illustrates how water networks deform upon accommodating a solute ion to create a distinct supramolecular species.
The infrared predissociation spectra of Cl−·H2O·Arn (n=1–5): experimental determination of the influence of Ar solvent atoms
We establish the argon solvent size dependence of the Cl−•H2O•Arn predissociation spectra, and discuss the discrepancies between previously reported predissociation spectra of the Cl−•H2O and Cl−•H2O•Ar3complexes [Choi et al., J. Phys. Chem. A 102 (1998) 503; Ayotte et al., J. Am. Chem. Soc. 120 (1998) 12361]. The argon-induced shift in the ∼3130 cm−1 ionic H-bonded OH stretching band, calculated to be large (>30 cm−1/Ar red-shift) by Satoh and Iwata [Chem. Phys. Lett. 312 (1999) 522], is found to be quite small (IHB band center=3128±3 cm−1 for 1≤n≤5). We compare this result with similar behavior displayed by the bare versus argon-solvated bromide monohydrate.
Vibrational spectroscopy of the F−·H2O complex via argon predissociation: photoinduced, intracluster proton transfer?
The mid-IR vibrational spectrum of the strongly bound F−•H2O complex is reported via predissociation of the size-selected F−•H2O•Arm (m=1–3) clusters. A weak, sharp band at 3690 cm−1 confirms that this species adopts the asymmetric arrangement typical of the heavier halides, while the band arising from the ionic H-bond (IHB) is shifted very far to the red of the free H2O bands (shift ∼800 cm−1). The observed band position is actually found in the region of the predicted `overtone' of the H-bonded oscillator, where the OH stretching vibration occurs in a very strongly anharmonic O–H⋯F− potential surface in which the first excited vibrational level samples the HF–OH− proton transfer configuration.
Spectroscopic observation of vibrational Feshbach resonances in near-threshold photoexcitation of X−·CH3NO2 (X−=I− and Br−)
We report the observation of resonance structure in the absorption and X−/NO2− photofragment action spectra of the X−•CH3NO2 (X−=I− and Br−) complexes in the region above the electron detachment threshold. The resonance structure corresponds to peaks which appear at the onsets for vibrational excitation of the –NO2 wag, scissors, and stretch modes of neutral CH3NO2, the modes which most strongly distort upon electron capture into its π* lowest unoccupied molecular orbital. We attribute the peaks to excitation of vibrational Feshbach resonances of the CH3NO2− transient negative ion, where near-threshold excitation of X−•CH3NO2 spectroscopically accesses states of the free electron–CH3NO2 system.
Spectroscopic Observation of Ion-Induced Water Dimer Dissociation in the X-·(H2O)2 (X = F, Cl, Br, I) Clusters
We elucidate the interplay between the ion−water and water−water interactions in determining the structures of halide ion−water clusters using infrared spectroscopy, interpreted with ab initio theory. Vibrational predissociation spectra of the X-•(H2O)2•Arm (X = F, Cl, Br, I) clusters in the OH stretching region (2300−3800 cm-1) reveal a strongly halide-dependent pattern of bands. These spectra encode the incremental weakening of the interaction between the water molecules with the lighter halides, finally leading to their complete dissociation in the fluoride complex. A consequence of this is that the F-•(H2O)2cluster is likely to be a floppy system with high amplitude zero point motion, in contrast to the pseudo-rigid behavior of the other halide hydrates.