Trapping and Structural Characterization of the XNO2·NO3- (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated X- + N2O5 Reactions with Cryogenic Vibrational Spectroscopy.

The heterogeneous reaction of N2O5 with sea spray aerosols yields the ClNO2 molecule, which is postulated to occur through water-mediated charge separation into NO3- and NO2+ ions followed by association with Cl-. Here we address an alternative mechanism where the attack by a halide ion can yield XNO2 by direct insertion in the presence of water. This was accomplished by reacting X-(D2O)n (X = Cl, Br, I) cluster ions with N2O5 to produce ions with stoichiometry [XN2O5]-. These species were cooled in a 20 K ion trap and structurally characterized by vibrational spectroscopy using the D2 messenger tagging technique. Analysis of the resulting band patterns with DFT calculations indicates that they all correspond to exit channel ion-molecule complexes based on the association of NO3- with XNO2, with the NO3- constituent increasingly perturbed in the order I > Br > Cl. These results establish that XNO2 can be generated even when more exoergic reaction pathways involving hydrolysis are available and demonstrate the role of the intermediate [XN2O5]- in the formation of XNO2.

Water network-mediated, electron-induced proton transfer in [C5H5N ⋠(H2O)n](-) clusters.

The role of proton-assisted charge accommodation in electron capture by a heterocyclic electron scavenger is investigated through theoretical analysis of the vibrational spectra of cold, gas phase [Py ⋅ (H2O)n=3-5](-) clusters. These radical anions are formed when an excess electron is attached to water clusters containing a single pyridine (Py) molecule in a supersonic jet ion source. Under these conditions, the cluster ion distribution starts promptly at n = 3, and the photoelectron spectra, combined with vibrational predissociation spectra of the Ar-tagged anions, establish that for n > 3, these species are best described as hydrated hydroxide ions with the neutral pyridinium radical, PyH((0)), occupying one of the primary solvation sites of the OH(-). The n = 3 cluster appears to be a special case where charge localization on Py and hydroxide is nearly isoenergetic, and the nature of this species is explored with ab initio molecular dynamics calculations of the trajectories that start from metastable arrangements of the anion based on a diffuse, essentially dipole-bound electron. These calculations indicate that the reaction proceeds via a relatively slow rearrangement of the water network to create a favorable hydration configuration around the water molecule that eventually donates a proton to the Py nitrogen atom to yield the product hydroxide ion. The correlation between the degree of excess charge localization and the evolving shape of the water network revealed by this approach thus provides a microscopic picture of the "solvent coordinate" at the heart of a prototypical proton-coupled electron transfer reaction.

Thermodynamics of water dimer dissociation in the primary hydration shell of the iodide ion with temperature-dependent vibrational predissociation spectroscopy.

The strong temperature dependence of the I(-)·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process is determined using van't Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I(-)·(H2O)2 system sample the extended potential energy surface with increasing temperature.

Communication: He-tagged vibrational spectra of the SarGlyH⁺ and H⁺(H2O)(2,3) ions: quantifying tag effects in cryogenic ion vibrational predissociation (CIVP) spectroscopy.

To assess the degree to which more perturbative, but widely used "tag" species (Ar, H2, Ne) affect the intrinsic band patterns of the isolated ions, we describe the extension of mass-selective, cryogenic ion vibrational spectroscopy to the very weakly interacting helium complexes of three archetypal ions: the dipeptide SarGlyH(+) and the small protonated water clusters: H(+)(H2O)(2,3), including the H5O2(+) "Zundel" ion. He adducts were generated in a 4.5 K octopole ion trap interfaced to a double-focusing, tandem time-of-flight photofragmentation mass spectrometer to record mass-selected vibrational predissociation spectra. The H2 tag-induced shift (relative to that by He) on the tag-bound NH stretch of the SarGlyH(+) spectrum is quite small (12 cm(-1)), while the effect on the floppy H5O2(+) ion is more dramatic (125 cm(-1)) in going from Ar (or H2) to Ne. The shifts from Ne to He, on the other hand, while quantitatively significant (maximum of 10 cm(-1)), display the same basic H5O2(+) band structure, indicating that the He-tagged H5O2(+) spectrum accurately represents the delocalized nature of the vibrational zero-point level. Interestingly, the He-tagged spectrum of H(+)(H2O)3 reveals the location of the non-bonded OH group on the central H3O(+) ion to fall between the collective non-bonded OH stretches on the flanking water molecules in a position typically associated with a neutral OH group.

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

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.

Survey of Ar-tagged predissociation and vibrationally mediated photodetachment spectroscopies of the vinylidene anion, C2H2-.

We report predissociation spectra of Ar-tagged C(2)H(2)(-) and C(2)D(2)(-) anions, and explore vibrationally mediated photodetachment from various vibrational levels of the bare C(2)H(2)(-) ion using velocity-map imaging. Intense photodetachment resonances are observed in the C-H stretching region that are strongly correlated with vibrational hot bands in the anion photoelectron spectra, indicating that one-color, resonant two-photon photodetachment (R2PD) is complicated by excitation of vibrationally excited states with autodetaching upper levels embedded in the continuum. Isolation of the R2PD spectrum was achieved using a two-color, IR-IR scheme in which vibrational excitation and photodetachment were carried out in two separate laser interaction regions.

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.

Vibrationally induced interconversion of H-bonded NO2(-) x H2O isomers within NO2(-) x H2O x Ar(m) clusters using IR-IR pump-probe through the OH and NO stretching vibrations.

We introduce a method based on sequential application of vibrational predissociation spectroscopy to explore the high-amplitude rearrangements available in a small H-bonded complex that is vibrationally excited within a larger Ar cluster. The weakly bound Ar atoms play the role of a solvent in mediating the energy content of the embedded system, ultimately quenching it into local minima through evaporation. We demonstrate the approach on the NO(2)(-) x H(2)O binary hydrate, which is known to occur in two nearly isoenergetic isomeric forms. The scheme involves three stages of mass separation to select a particular NO(2)(-) x H(2)O x Ar(m) parent ion cluster prior to vibrational excitation and then isolate the NO(2)(-) x H(2)O x Ar fragment ions for interrogation using resonant vibrational predissociation with a second infrared laser. The initial vibrational excitation selectively energizes one of the isomers through one of its characteristic resonances while the predissociation spectrum of the NO(2)(-) x H(2)O x Ar fragment encodes the distribution of isomers present after Ar evaporation. Isomerization from the front- to backside form is found to occur upon excitation of the NO stretch near 1200 cm(-1); although the reverse reaction is not observed upon excitation of the NO stretch, it is observed upon excitation of the higher-energy OH stretching fundamental near 3000 cm(-1). We discuss these observations in the context of the calculated isomerization energetics, which focus on the minimum energy structures for the isomers as well as the transition states for their interconversion.

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-].