Why Does Argon Bind to Deuterium? Isotope Effects and Structures of Ar·H 5O 2 + Complexes
Recently, we reported the spectrum of Ar•D 4HO 2 + [McCunn; et, al. J. Phys. Chem. B 2008, 112, 321], and here, we extend that work to include the Ar•H 4DO 2 + isotopologue in order to explore why the Ar atom has a much greater propensity for attachment to a dangling OD group than it does for OH, even when many more of the latter binding sites are available. Calculated (MP2/6-311+G(d,p) level of theory/basis) harmonic frequencies reproduce the observed multiplet patterns of OH and OD stretches and confirm the presence of various isomers arising from the different Ar binding sites. The preferential bonding of Ar to OD is traced to changes in the frequencies of the wag and rock modes of the H 5O 2 + moiety rather than to shifts in the oscillator that directly binds the Ar atom.
Probing isomer interconversion in anionic water clusters using an Ar-mediated pump-probe approach: Combining vibrational predissociation and velocity-map photoelectron imaging spectroscopies
We present the first results from an experiment designed to explore barriers for interconversion between isomers of cluster anions using an Ar-cluster mediated pump-probe technique. In this approach, anions are generated with many Ar atoms attached, and one of the isomers present is selectively excited by tuning an infrared laser to one of the isomer’s characteristic vibrational resonances. The excited cluster is then cooled by evaporation of Ar atoms, and the isomer distribution in the lighter daughter ions is measured after secondary mass selection by recording their photoelectron spectra using velocity-map imaging. We apply the method to the water hexamer anion, (H2O)6−, which is known to occur in two isomeric forms with different electron-binding energies. We find that conversion of the high-binding (type I) form to the low-binding (type II) isomer is not efficiently driven in (H2O)6− with excitation energies in the 0.4 eV range even though it is possible to create both isomers in abundance in the ion source. This observation is discussed in the context of the competition between isomerization and electron autodetachment, which depends on the relative positions of the neutral and ionic potential surfaces along the isomerization pathway. Application of the method to the more complex heptamer ion, however, does reveal that interconversion is available among the highest binding isomer classes (I and I′).
To clarify the nature of the motions contributing to the observed multiplet structures in the low-energy (900−1800 cm-1) vibrational spectrum of the H5O2+ “Zundel” ion, we report the evolution of its vibrational fingerprint with sequential H/D isotopic substitution in a predissociation study of the Ar complexes. Of particular interest is the D4HO2+ complex, which displays a single intense band in the vicinity of the asymmetric OHO stretch of the bridging proton, in contrast to the more complex multiplet observed for both H5O2+ and D5O2+ isotopologues. These intensity patterns are consistent with the recent assignment of the bridging proton band's doublet in the H5O2+•Ne spectrum to a 2 × 2 Fermi resonance interaction between the shared proton stretch and a complex background level primarily derived from the O−O stretch together with two quanta of the wagging vibration involving the pyramidal deformations of the flanking H2O groups (Vendrell, O.; Gatti, F.; Meyer, H.-D.Angew. Chem., Int. Ed. 2007, 46, 6918). In addition, the observed trends rule out assignment of the 1800 cm-1 feature in H5O2+ to a combination band of the bridging proton vibration with the O−O stretch, providing a secure foundation for the previously reported scheme that attributes this band to the out-of-phase intramolecular bending fundamental. The observed feature occurs at an unusually high energy for typical HOH bends, however, and we explore the participation of the bridging proton in these eigenstates by following how the calculated harmonic spectrum evolves when artificially large masses are assigned to the proton. The empirical assignments are supported by anharmonic estimates of the isotope shifts evaluated by the diffusion Monte Carlo method.
Site-specific addition of D2O to the (H2O)6− “hydrated electron”cluster: isomer interconversion and substitution at the double H-bond acceptor (AA) electron-binding site
We report the results of an experimental study designed to establish whether, once formed, one of the isomer classes of the hydrated electron clusters, (H2O)n−, can interconvert with others when a water molecule is added by condensation. This is accomplished in an Ar-cluster mediated approach where a single intact D2O molecule is collisionally incorporated into argon-solvated water hexamer anions, creating the isotopically labeled D2O•(H2O)6−•Arn heptamer anion. Photoelectron and infrared predissociation spectroscopies are employed both to characterize the isomers generated in the condensation event and to track the position that the D2O label adopts within these isomeric structures. Despite the fact that the water hexamer anion precursor clusters initially exist in the isomer I form, incorporation of D2O produces mostly isomers I′ and II in the labeled heptamer, which bind the electron more (I′) or less (II) strongly than does the isomer I class. Isomers I and I′ are known to feature electron binding primarily onto a single water molecule that resides in an AA (A = H-bond acceptor) site in the network. Surprisingly, the D2O molecule can displace this special electron-binding H2O molecule such that the anionic cluster retains the high binding arrangement. In the more weakly binding isomer II clusters, the D2O molecule fractionates preferentially to sites that give rise to the vibrational signature of isomer II.
Theoretical and infrared spectroscopic investigation of the O2−∙benzene and O4−∙benzene complexes
The infrared spectra of the O2−∙benzene and O4−∙benzene complexes are determined by means of Ar predissociation spectroscopy. Several transitions due to CH stretch fundamentals and various combination bands are observed in the 2700–3100 cm−1 region. The experimental results are interpreted with the aid of electronic structure calculations. A comparison of the calculated and experimental spectra reveals that the spectrum of O2−∙benzene most likely arises from an isomer where the superoxide molecule binds preferentially to one CH group of benzene. In contrast, the spectrum of O4−∙benzene yields a CH pattern remarkably similar to that displayed by the C2ν X−∙benzene (X = halogen) complexes, consistent with a structure with two CH groups equally involved in the bonding. The lower energy vibrational fundamental transitions of the O4− anion are recovered with a slight redshift in the O4−∙benzene spectrum, establishing that this charge-delocalized dimer ion retains its identity upon complexation.
Kinetics for the Reactions of O- and O2- with O2(a1Δg) Measured in a Selected Ion Flow Tube at 300 K
The kinetics of the reactions of O- and O2- with O2(a1Δg) have been studied at 300 K in a selected ion flow tube (SIFT). The O2(a1Δg) concentrations have been determined using emission at 1270 nm from the O2(a1Δg, v = 0 → X3Σg-, v = 0) transition measured with an InGaAs detector calibrated against absolute spectrally dispersed emission measurements. The rate constants measured for O- and O2- are 1.1 × 10-10 and 6.6 × 10-10 cm3 s-1, respectively, with uncertainties of ±35%. The O2- reaction only produces electrons and can be described as Penning detachment, while the O- reaction has been found to produce both O2- and e-. The O2- branching fraction has a lower limit of 0.30. Comparison of the present results to previous measurements found in the literature provides a resolution to a previous discrepancy in the rate constant values.
Vibrational spectroscopy of hydrated electron clusters (H2O)15–50− via infrared multiple photon dissociation
Infrared multiple photon dissociation spectra for size-selected water cluster anions(H2O)n−, n = 15–50, are presented covering the frequency range of 560–1820 cm−1. The cluster ions are trapped and cooled by collisions with ambient He gas at K, with the goal of defining the cluster temperature better than in previous investigations of these species. Signal is seen in two frequency regions centered around 700 and 1500–1650 cm−1, corresponding to water librational and bending motions, respectively. The bending feature associated with a double-acceptor water molecule binding to the excess electron is clearly seen up to em class="emphitalic">n = 35, but above n = 25; this feature begins to blueshift and broadens, suggesting a more delocalized electron binding motif for the larger clusters in which the excess electron interacts with multiple water molecules.
Quantum Structure of the Intermolecular Proton Bond
A proton shared between two closed-shell molecules, [A•H+•B], constitutes a ubiquitous soft binding motif in biological processes. The vibrational transitions associated with the shared proton, which provide a direct probe of this interaction, have been extensively studied in the condensed phase but have yielded only limited detailed information because of their diffuse character. We exploited recent advances in gas-phase ion spectroscopy to identify sharp spectral features that can be assigned to both the shared proton and the two tethered molecules in a survey of 18 cold, isolated [A•H+•B] ions. These data yield a picture of the intermolecular proton bond at a microscopic scale, facilitating analysis of its properties within the context of a floppy polyatomic molecule.
Spectroscopic Characterization of the Isolated SF6- and C4F8- Anions: Observation of Very Long Harmonic Progressions in Symmetric Deformation Modes upon Photodetachment
Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S−F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S−F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-•Ar complex. The infrared predissociation spectrum of SF6-•Ar consists of a strong band at 683(5) cm-1, which we assign to the ν3 (t1u) fundamental, the same mode that yields the strong 948 cm-1 infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.
Isomer-specific spectroscopy of the (H2O)8− cluster anion in the intramolecular bending region by selective photodepletion of the more weakly electron binding species (isomer II)
The vibrational predissociation spectra of the two more strongly electron binding forms of the (H2O)8− anion are obtained in the HOH intramolecular bending region. This is accomplished by deconvoluting the overlapping spectra obtained from a mixed ensemble using a population modulation scheme in which the low electron binding isomer (II) is removed from the ion packet prior to spectroscopic analysis. By choosing the energy of the photodepletion laser to lie between the vertical detachment energies of the two isomers, the contribution from isomer II can be quantitatively eliminated, leaving the population of I largely unaffected. The low binding energies involved in the application of the method to the water cluster anions necessitate that this should be carried out in the midinfrared, thus requiring two tunable ir laser systems for implementation. The isolated spectrum of isomer 1 displays a strong, redshifted feature associated with a double H-bond acceptor (AA) water molecule in direct contact with the excess electron and a large gap before higher energy features appear that are typically associated with (acceptor/donor) AD and ADD binding sites in the network. The more weakly binding isomer II does not display the AA feature and instead contributes broad structure at intermediate redshifts that merges with the region associated with neutral water cluster networks.
Gas-Phase Infrared Spectroscopy and Multidimensional Quantum Calculations of the Protonated Ammonia Dimer N2H7+
The challenge of understanding the unusually high proton conductivity in water and in ice as well as water-mediated proton transfer across biomembranes has triggered considerable work on protonated water networks...
Infrared Spectroscopy of Water Cluster Anions, (H2O)n=3-24- in the HOH Bending Region: Persistence of the Double H-Bond Acceptor (AA) Water Molecule in the Excess Electron Binding Site of the Class I Isomers
We report vibrational predissociation spectra of water cluster anions, (H2O)n=3-24- in the HOH bending region to explore whether the characteristic red-shifted feature associated with electron binding onto a double H-bond acceptor (AA) water molecule survives into the intermediate cluster size regime. The spectra of the “tagged” (H2O)n-•Ar clusters indeed exhibit the signature AA band, but assignment of this motif to a particular isomer is complicated by the fact that argon attachment produces significant population of three isomeric forms (as evidenced by their photoelectron spectra). We therefore also investigated the bare clusters since they can be prepared exclusively in the high binding (isomer class I) form. Because the energy required to dissociate a water molecule from the bare complexes is much larger than the transition energies in the bending region, the resulting (linear) action spectroscopy selectively explores the properties of clusters with most internal energy content. The (H2O)15- predissociation spectrum obtained under these conditions displays a more intense AA feature than was found in the spectra of the Ar tagged species. This observation implies that not only is the AA motif present in the class I isomer, but also that it persists when the clusters contain considerable internal energy.
Determination of the CO3− bond strength via the resonant two-photon photodissociation threshold: Electronic and vibrational spectroscopy of CO3−∙Arn
We use a two-laser pump-probe technique coupled with messenger atom tagging to determine the bond energy of O− to CO2 in the CO3− ion, a prevalent species in the upper atmosphere. In this technique, the argon-tagged ion is first electronically excited using a visible laser, then irradiated with a tunable near-infrared beam across the CO2⋯O−dissociation threshold while O− products are monitored. This method yields a bond energy of 2.79±0.05 eV, which is about 0.5 eV higher than previously reported. Combining this with the well-known heats of formation of O− and CO2, 105.6 and −393.1 kJ/mol, respectively [ Thermodynamic Properties of Individual Substances, edited by L. V. Gurvich, I. V. Veyts, and C. B. Alcock (Hemisphere, New York, 1989), Vol. 1 and CODATA Thermodynamic Tables, edited by O. Garvin, V. B. Parker, and J. H. J. White (Hemisphere, New York, 1987) ], yields the CO3 heat of formation: ΔH00 = −556.7±4.8 kJ/mol. The one-photon (i.e., linear) infrared and electronic spectra of CO3− are also presented and compared to those obtained previously. The one-photon electronic spectrum is nearly identical to two-photon spectra, implying that argon does not significantly perturb the ion or its symmetry. The infrared spectrum is drastically different than that obtained in an argon matrix, however, indicating that the ion is likely distorted in the matrix environment.
Prying Apart a Water Molecule with Anionic H-Bonding: A Comparative Spectroscopic Study of the X-·H2O (X = OH, O, F, Cl, and Br) Binary Complexes in the 600−3800 cm-1 Region
A detailed picture of the structural distortions suffered by a water molecule in direct contact with small inorganic anions (e.g., X = halide) is emerging from a series of recent vibrational spectroscopy studies of the gas-phase X-•H2O binary complexes. The extended spectral coverage (600−3800 cm-1) presently available with tabletop laser systems, when combined with versatile argon “messenger” techniques for acquiring action spectra of cold complexes, now provides a comprehensive survey of how the interaction evolves from an ion−solvent configuration into a three-center, two-electron covalent bond as the proton affinity of the anion increases. We focus on the behavior of H2O in the X-•H2O (X = Br, Cl, F, O, and OH) complexes, which all adopt asymmetric structures where one hydrogen atom is H-bonded to the ion while the other is free. The positions and intensities of the bands clearly reveal the mechanical consequences of both (zero-point) vibrationally averaged and infrared photoinduced excess charge delocalization mediated by intracluster proton transfer (X-•H2O → HX•OH-). The fundamentals of the shared proton stretch become quite intense, for example, and exhibit extreme red-shifts as the intracluster proton-transfer process becomes available, first in the vibrationally excited states (F-•H2O) and then finally at the zero-point level (OH-•H2O). In the latter case, the loss of the water molecule's independent character is confirmed through the disappearance of the 1600 cm-1 HOH intramolecular bending transition and the dramatic (>3000 cm-1) red-shift of the shared proton stretch. An unexpected manifestation of vibrationally mediated charge transfer is also observed in the low frequency region, where the 2 ← 0 overtones of the out-of-plane frustrated rotation of the water are remarkably intense in the Cl-•H2O and Br-•H2O spectra. This effect is traced to changes in the charge distribution along the X-•O axis as the shared proton is displaced perpendicular to it, reducing the charge transfer character of the H-bonding interaction and giving rise to a large quadratic contribution to the dipole moment component that is parallel to the bond axis. Thus, all of these systems are found to exhibit distinct spectral characteristics that can be directly traced to the crucial role of vibrationally mediated charge redistribution within the complex.
Vibrational predissociation spectroscopy of the (H2O)6–21− clusters in the OH stretching region: Evolution of the excess electron-binding signature into the intermediate cluster size regime
We report vibrational predissociation spectra of the (H2O)n− cluster ions in the OH stretching region to determine whether the spectral signature of the electron-binding motif identified in the smaller clusters [ Hammer et al. Science 306, 675 (2004) ] continues to be important in the intermediate size regime (n = 7−21). This signature consists of a redshifted doublet that dominates the OH stretching region, and has been traced primarily to the excitation of a single water molecule residing in a double H-bond acceptor (AA) binding site, oriented with both of its H atoms pointing toward the excess electron cloud. Strong absorption near the characteristic AA doublet is found to persist in the spectra of the larger clusters, but the pattern evolves into a broadened triplet around n= 11. A single free OH feature associated with dangling hydrogen atoms on the cluster surface is observed to emerge for n ≥ 15, in sharp contrast to the multiplet pattern of unbonded OH stretches displayed by the H+∙(H2O)n clusters throughout the n = 2−29 range. We also explore the vibration-electronic coupling associated with normal-mode displacements of the AA molecule that most strongly interact with the excess electron. Specifically, electronic structure calculations on the hexamer anion indicate that displacement along the −OH2 symmetric stretching mode dramatically distorts the excess electron cloud, thus accounting for the anomalously large oscillator strength of the AA water stretching vibrations. We also discuss these vibronic interactions in the context of a possible relaxation mechanism for the excited electronic states involving the excess electron.