Excess charge driven dissociative hydrogen adsorption on Ti2O4.

The mechanism of dissociative D2 adsorption on Ti2O4-, which serves as a model for an oxygen vacancy on a titania surface, is studied using infrared photodissociation spectroscopy in combination with density functional theory calculations and a recently developed single-component artificial force induced reaction method. Ti2O4- readily reacts with D2 under multiple collision conditions in a gas-filled ion trap held at 16 K forming a global minimum-energy structure (DO-Ti-(O)2-Ti(D)-O)-. The highly exergonic reaction proceeds quasi barrier-free via several intermediate species, involving heterolytic D2-bond cleavage followed by D-atom migration. We show that, compared to neutral Ti2O4, the excess negative charge in Ti2O4- is responsible for the substantial lowering of the D2 dissociation barrier, but does not affect the molecular D2 adsorption energy in the initial physisorption step.

Dissociative Water Adsorption by Al3O4+ in the Gas Phase.

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory (DFT) to study the adsorption of up to four water molecules on Al3O4+. The infrared photodissociation spectra of [Al3O4(D2O)1-4]+ are measured in the O-D stretching (3000-2000 cm-1) as well as the fingerprint spectral region (1300-400 cm-1) and are assigned based on a comparison with simulated harmonic infrared spectra for global minimum-energy structures obtained with DFT. We find that dissociative water adsorption is favored in all cases. The unambiguous assignment of the vibrational spectra of these gas phase model systems allows identifying characteristic spectral regions for O-D and O-H stretching modes of terminal (µ1) and bridging (µ2) hydroxyl groups in aluminum oxide/water systems, which sheds new light on controversial assignments for solid Al2O3 phases.

Gas-Phase Vibrational Spectroscopy of the Aluminum Oxide Anions (Al2 O3 )1-6 AlO2.

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory to probe how the structural variability of alumina manifests itself in the structures of the gas-phase clusters (Al2 O3 )n AlO2- with n=1-6. The infrared photodissociation spectra of the D2 -tagged complexes, measured in the fingerprint spectral range (400-1200 cm-1 ), are rich in spectral features and start approaching the vibrational spectrum of amorphous alumina particles for n>4. Aided by a genetic algorithm, we find a trend towards the formation of irregular structures for larger n, with the exception of n=4, which exhibits a C3v ground-state structure. Locating the global minima of the larger systems proves challenging.

Structure and Fluxionality of B13+ Probed by Infrared Photodissociation Spectroscopy.

We use cryogenic ion vibrational spectroscopy to characterize the structure and fluxionality of the magic number boron cluster B13+ . The infrared photodissociation (IRPD) spectrum of the D2 -tagged all-11 B isotopologue of B13+ is reported in the spectral range from 435 to 1790 cm-1 and unambiguously assigned to a planar boron double wheel structure based on a comparison to simulated IR spectra of low energy isomers from density-functional-theory (DFT) computations. Born-Oppenheimer DFT molecular dynamics simulations show that B13+ exhibits internal quasi-rotation already at 100 K. Vibrational spectra derived from these simulations allow extracting the first spectroscopic evidence from the IRPD spectrum for the exceptional fluxionality of B13+ .

Spectroscopic snapshots of the proton-transfer mechanism in water.

The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded (H-bonded) network. However, the vibrational spectroscopic signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D+(D2O)n, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton-transfer event. By complexing the solvated hydronium "Eigen" cluster [D3O+(D2O)3] with increasingly strong H-bond acceptor molecules (D2, N2, CO, and D2O), we are able to track the frequency of every O-D stretch vibration in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule.

Gas phase vibrational spectroscopy of the protonated water pentamer: the role of isomers and nuclear quantum effects.

We use cryogenic ion trap vibrational spectroscopy to study the structure of the protonated water pentamer, H+(H2O)5, and its fully deuterated isotopologue, D+(D2O)5, over nearly the complete infrared spectral range (220-4000 cm-1) in combination with harmonic and anharmonic electronic structure calculations as well as RRKM modelling. Isomer-selective IR-IR double-resonance measurements on the H+(H2O)5 isotopologue establish that the spectrum is due to a single constitutional isomer, thus discounting the recent analysis of the band pattern in the context of two isomers based on AIMD simulations 〈W. Kulig and N. Agmon, Phys. Chem. Chem. Phys., 2014, 16, 4933-4941〉. The evolution of the persistent bands in the D+(D2O)5 cluster allows the assignment of the fundamentals in the spectra of both isotopologues, and the simpler pattern displayed by the heavier isotopologue is consistent with the calculated spectrum for the branched, Eigen-based structure originally proposed 〈J.-C. Jiang, et al., J. Am. Chem. Soc., 2000, 122, 1398-1410〉. This pattern persists in the vibrational spectra of H+(H2O)5 in the temperature range from 13 K up to 250 K. The present study also underscores the importance of considering nuclear quantum effects in predicting the kinetic stability of these isomers at low temperatures.

Gas phase structures and charge localization in small aluminum oxide anions: Infrared photodissociation spectroscopy and electronic structure calculations.

We use cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations to study the structure of mono- and dialuminum oxide anions. The infrared photodissociation spectra of D2-tagged AlO1-4 (-) and Al2O3-6 (-) are measured in the region from 400 to 1200 cm(-1). Structures are assigned based on a comparison to simulated harmonic and anharmonic IR spectra derived from electronic structure calculations. The monoaluminum anions contain an even number of electrons and exhibit an electronic closed-shell ground state. The Al2O3-6 (-) anions are oxygen-centered radicals. As a result of a delicate balance between localization and delocalization of the unpaired electron, only the BHLYP functional is able to qualitatively describe the observed IR spectra of all species with the exception of AlO3 (-). Terminal Al-O stretching modes are found between 1140 and 960 cm(-1). Superoxo and peroxo stretching modes are found at higher (1120-1010 cm(-1)) and lower energies (850-570 cm(-1)), respectively. Four modes in-between 910 and 530 cm(-1) represent the IR fingerprint of the common structural motif of dialuminum oxide anions, an asymmetric four-member Al-(O)2-Al ring.

Infrared Photodissociation Spectroscopy of C4 N- , C6 N- and C8 N.

The gas-phase vibrational spectroscopy of cold C2n N- (n=2-4) anions is investigated in the CC and CN multiple bond stretching region (1700-2250 cm-1 ) by means of infrared photodissociation (IRPD) spectroscopy in a cryogenically cooled ion trap of the corresponding messenger-tagged complexes. The IRPD spectra are assigned to N-terminated linear structures with triplet ground states (3 Σ- ) based on a comparison with harmonic vibrational frequencies and intensities from density functional theory computations. In contrast to the polyacetylenic C2n+1 N- anions, the linear C-C chains investigated here exhibit cumulenic character, which is most pronounced in C4 N- and decreases with chain length. Additional intense transitions are observed for C6 N- above 3000 cm-1 and are attributed to overtone and combination bands involving the CC stretching modes, based on anharmonic computations. The influence of a D2 tag on the vibrational features of C2n N- anions is shown to be small.

Gas phase vibrational spectroscopy of cold (TiO2)n(-) (n = 3-8) clusters.

We report infrared photodissociation (IRPD) spectra for the D2-tagged titanium oxide cluster anions (TiO2)n(-) with n = 3-8 in the spectral region from 450 to 1200 cm(-1). The IRPD spectra are interpreted with the aid of harmonic spectra from BP86/6-311+G* density functional theory calculations of energetically low-lying isomers. We conclusively assign the IRPD spectra of the n = 3 and n = 6 clusters to global minimum energy structures with Cs and C2 symmetry, respectively. The vibrational spectra of the n = 4 and n = 7 clusters can be attributed to contributions of at most two low-lying structures. While our calculations indicate that the n = 5 and n = 8 clusters have many more low-lying isomers than the other clusters, the dominant contributions to their spectra can be assigned to the lowest energy structures. Through comparison between the calculated and experimental spectra, we can draw conclusions about the size-dependent evolution of the properties of (TiO2)n(-) clusters, and on their potential utility as model systems for catalysis on a bulk TiO2 surface.

Disentangling the Contribution of Multiple Isomers to the Infrared Spectrum of the Protonated Water Heptamer.

We use infrared/infrared double-resonance population labeling (IR(2)MS(2)) spectroscopy in the spectral region of the free and hydrogen-bonded OH stretching fundamentals (2880-3850 cm(-1)) to identify the number and to isolate the vibrational signatures of individual isomers contributing to the gas-phase IR spectra of the cryogenically cooled protonated water clusters H(+)(H2O)n·H2/D2 with n = 7-10. For n = 7, four isomers are identified and assigned. Surprisingly, the IR(2)MS(2) spectra of the protonated water octa-, nona-, and decamer show no evidence for multiple isomers. The present spectra support the prediction that the quasi-2D to 3D structural transition occurs in between n = 8 and 9 in the cold cluster regime. However, the same models have difficulty explaining the remarkable size dependence of the isomer population reported here.

Site-specific vibrational spectral signatures of water molecules in the magic H3O+ (H2O)20 and Cs+ (H2O)20 clusters.

Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, along with isotopic substitution, to quench water clusters attached to the H3O(+) and Cs(+) ions into structures that yield well-resolved vibrational bands over the entire 215- to 3,800-cm(-1) range. The magic H3O(+)(H2O)20 cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface.

Isomer-selective detection of hydrogen-bond vibrations in the protonated water hexamer.

The properties of hydrogen ions in aqueous solution are governed by the ability of water to incorporate ions in a dynamical hydrogen bond network, characterized by a structural variability that has complicated the development of a consistent molecular level description of H(+)(aq). Isolated protonated water clusters, H(+)(H2O)n, serve as finite model systems for H(+)(aq), which are amenable to highly sensitive and selective gas phase spectroscopic techniques. Here, we isolate and assign the infrared (IR) signatures of the Zundel-type and Eigen-type isomers of H(+)(H2O)6, the smallest protonated water cluster for which both of these characteristic binding motifs coexist, down into the terahertz spectral region. We use isomer-selective double-resonance population labeling spectroscopy on messenger-tagged H(+)(H2O)6·H2 complexes from 260 to 3900 cm(-1). Ab initio molecular dynamics calculations qualitatively recover the IR spectra of the two isomers and allow attributing the increased width of IR bands associated with H-bonded moieties to anharmonicities rather than excited state lifetime broadening. Characteristic hydrogen-bond stretching bands are observed below 400 cm(-1).