Daniel J. Goebbert

Infrared Spectroscopy of the Microhydrated Nitrate Ions NO3−(H2O)1−6†

We present infrared photodissociation spectra of the microhydrated nitrate ions NO(3)(-)(H(2)O)(1-6), measured from 600 to 1800 cm(-1). The assignment of the spectra is aided by comparison with calculated B3LYP/aug-cc-pVDZ harmonic frequencies, as well as with higher-level calculations. The IR spectra are dominated by the antisymmetric stretching mode of NO(3)(-), which is doubly degenerate in the bare ion but splits into its two components for most microhydrated ions studied here due to asymmetric solvation of the nitrate core. However, for NO(3)(-)(H(2)O)(3), the spectrum reveals no lifting of this degeneracy, indicating an ion with a highly symmetric solvation shell. The first three water molecules bind in a bidentate fashion to the terminal oxygen atoms of the nitrate ion, keeping the planar symmetry. The onset of extensive water-water hydrogen bonding is observed starting with four water molecules and persists in the larger clusters.

Messenger-tagging electrosprayed ions: Vibrational spectroscopy of suberate dianions

The gas-phase vibrational spectroscopy of bare and monohydrated suberate dianions, (-)OOC-(CH(2))(6)-COO(-) and (-)OOC-(CH(2))(6)-COO(-).H(2)O, is studied by infrared photodissociation aided by electronic structure calculations. To this end, the corresponding ion-Kr atom complexes are formed in a cooled buffer-gas-filled ion trap, and their infrared vibrational predissociation spectra are measured in the range from 660 to 3600 cm(-1). The water molecule binds to one of the two carboxylate groups in a bidentate fashion, characterized by the splitting of the carboxylate stretching bands, a substantially blue-shifted water bending band, and the presence of anomalously broadened bands in the O-H stretching and H(2)O rocking region. The C-C backbone structure remains unperturbed by the addition of a water molecule or a Kr atom. At 63 K, the all-trans isomer is the most abundant species, but evidence for dynamically interconverting conformers is also present from contributions to the absorption cross section on the low-energy tail of the C-H stretching region.

Infrared Spectroscopy of Hydrated Bicarbonate Anion Clusters: HCO3−(H2O)1−10

Infrared multiple photon dissociation spectra are reported for HCO(3)(-)(H(2)O)(1-10) clusters in the spectral range of 600-1800 cm(-1). In addition, electronic structure calculations at the MP2/6-311+G(d,p) level have been performed on the n = 1-8 clusters to identify the structure of the low-lying isomers and to assign the observed spectral features. General trends in the stepwise solvation motifs of the bicarbonate anion can be deduced from the overall agreement between the calculated and experimental spectra. The most important of these is the strong preference of the water molecules to bind to the negatively charged CO(2) moiety of the HCO(3)(-) anion. However, a maximum of four water molecules interact directly with this site. The binding motif in the most stable isomer of the n = 4 cluster, a four-membered ring with each water forming a single H-bond with the CO(2) moiety, is retained in all of the lowest-energy isomers of the larger clusters. Starting at n = 6, additional solvent molecules are found to form a second hydration layer, resulting in a water-water network bound to the CO(2) moiety of the bicarbonate anion. Binding of a water to the hydroxyl group of HCO(3)(-) is particularly disfavored and apparently does not occur in any of the clusters investigated here. Similarities and differences with the infrared spectrum of aqueous bicarbonate are discussed in light of these trends.

The lowest singlet and triplet states of the oxyallyl diradical

Small S-T splitting : The photoelectron spectrum of the oxyallyl radical anion (see picture) reveals that the electronic ground state of oxyallyl is singlet, and the lowest triplet state is separated from the singlet state by only (55 ± 2) meV in adiabatic energy.

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 20 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 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.

10 K Ring Electrode Trap - Tandem Mass Spectrometer for Infrared Spectroscopy of Mass Selected Ions

A novel instrumental setup for measuring infrared photodissociation spectra of buffer gas cooled, mass‐selected ions is described and tested. It combines a cryogenically cooled, linear radio frequency ion trap with a tandem mass spectrometer, optimally coupling continuous ion sources to pulsed laser experiments. The use of six independently adjustable DC potentials superimposed over the trapping radio frequency field provides control over the ion distribution within, as well as the kinetic energy distribution of the ions extracted from the ion trap. The scheme allows focusing the ions in space and time, such that they can be optimally irradiated by a pulsed, widely tunable infrared photodissociation laser. Ion intensities are monitored with a time‐of‐flight mass spectrometer mounted orthogonally to the ion trap axis.

Vibrational signatures of hydrogen bonding in the protonated ammonia clusters NH4+(NH3)1−4

The gas phase vibrational spectroscopy of the protonated ammonia dimer N(2)H(7)(+), a prototypical system for strong hydrogen bonding, is studied in the spectral region from 330 to 1650 cm(-1) by combining infrared multiple photon dissociation and multidimensional quantum mechanical simulations. The fundamental transition of the antisymmetric proton stretching vibration is observed at 374 cm(-1) and assigned on the basis of a six-dimensional model Hamiltonian, which predicts this transition at 471 cm(-1). Photodissociation spectra of the larger protonated ammonia clusters NH(4)(+)(NH(3))(n) with n=2-4 are also reported for the range from 1050 to 1575 cm(-1). The main absorption features can be assigned within the harmonic approximation, supporting earlier evidence that hydrogen bonding in these clusters is considerably weaker than for n=1.

Electronic Structure and Spectroscopy of Oxyallyl: A Theoretical Study

Electronic structure of the oxyallyl diradical and the anion is investigated using high-level ab initio methods. Converged theoretical estimates of the energy differences between low-lying electronic states of oxyallyl (OXA) as well as detachment energies of the anion are reported. Our best estimates of the adiabatic energy differences between the anion (2)A(2) and the neutral (3)B(2) and (3)B(1) states are 1.94 and 2.73 eV, respectively. The (1)A(1) state lies above (3)B(2) vertically, but geometric relaxation brings it below the triplet. The two-dimensional scan of the singlet (1)A(1) potential energy surface (PES) reveals that there is no minimum corresponding to a singlet diradical structure. Thus, singlet OXA undergoes prompt barrierless ring closure. However, a flat shape of the PES results in the resonance trapping in the Franck-Condon region, giving rise to the experimentally observable features in the photoelectron spectrum. By performing reduced-dimensionality wave packet calculations, we estimated that the wave packet lingers in the Franck-Condon region for about 170 fs, which corresponds to the spectral line broadening of about 200 cm(-1). We also present calculations of the photodetachment spectrum and compare it with experimental data. Our calculations lend strong support to the assignment of the photoelectron spectrum of the OXA anion reported in Ichino et al. (Angew. Chem., Int. Ed. Engl. 2009, 48, 8509).

Photodetachment anisotropy for mixed s-p states: 8/3 and other fractions

An approximate model for analytical prediction of photoelectron angular distributions in anion photodetachment from mixed s-p states is presented. Considering the dipole-allowed s, p, and d free-electron partial waves, the model describes photodetachment anisotropy in terms of the fractional p character of the initial orbital and the A and B coefficients describing the relative intensities of the p → d to p → s and s → p to p → s channels, respectively. The model represents an extension of the central-potential model to an intermediate regime encompassing varying degrees of s and p contributions to the initial bound orbital. This description is applicable to a broad class of hybrid molecular orbitals, particularly those localized predominantly on a single atom. Under the additional assumption of hydrogenic or Slater-type orbitals, the B/A ratio in photodetachment from a mixed 2s-2p state is shown to equal 8/3. Corresponding fractions are derived for other ns-np mixing cases. The predictions of the model are tested on several anion systems, including NH(2)(-) and CCl(2)(-). The quantitative discrepancies in the latter case are attributed to the breakdown of the central-atom approximation and a mechanism for corresponding corrections is indicated.

Photodetachment, photofragmentation, and fragment autodetachment of [O2n(H2O)m]− clusters: Core-anion structures and fragment energy partitioning

Building on the past studies of the O2n− and O2−(H2O)m cluster anion series, we assess the effect of the strong hydration interactions on the oxygen-core clusters using photoelectron imaging and photofragment mass spectroscopy of [O2n(H2O)m]− (n=1–4, m=0–3) at 355 nm. The results show that both pure-oxygen and hydrated clusters with n≥2 form an O4− core anion, indicated in the past work on the pure-oxygen clusters. All clusters studied can be therefore described in terms of O4−(H2O)m(O2)n−2 structures, although the O4− core may be strongly perturbed by hydration in some of these clusters. Fragmentation of these clusters yields predominantly O2− and O2−(H2O)l (l<m) anionic products. The low-electron kinetic energy O2− autodetachment features, prominent in the photoelectron images, signal that the fragments are vibrationally excited. The relative intensity of photoelectrons arising from O2− fragment autodetachment is used to shed light on the varying degree of fragment excitation resulting from the cluster ...