Understanding how protons are hydrated remains an important and challenging research area. The anomalously high proton mobility of water, for example, can be explained by a periodic isomerization between the Eigen and Zundel binding motifs, H₃O⁺(aq) and H₅O₂⁺(aq), respectively, even though the detailed mechanism is considerably more complex and not completely understood. These rapidly interconverting structures from the condensed phase can be stabilized, isolated and studied in the gas phase in the form of protonated water clusters. Infrared photodissociation (IRPD) spectroscopy serves as a powerful tool for studying the structure of gas phase clusters. However, the contribution of multiple isomers to the IRPD spectrum can complicate the assignment. Here, results on the isomer-specific IR/IR double resonance (IR²MS²) spectroscopy of the protonated water clusters H⁺(H₂O)_n·H₂ with n = 5-10 are reported. IR²MS² spectra are measured in the spectral region of the free and hydrogen-bonded OH-stretching vibrations (2880-3850 cm⁻¹) and assigned on the basis of a comparison to the results of electronic structure calculations. For the protonated water hexamer, it is demonstrated that combining the radiation from an IR free electron laser with that from a widely tunable table-top IR laser allows extending this technique across nearly the complete IR region (260-3900 cm⁻¹). Ab initio molecular dynamics calculations qualitatively recover the IR spectra of the two isomers for n = 6 and allow identifying characteristic hydrogen-bond stretching bands below 400 cm⁻¹.

Based on similairity between vibrational spectra of H⁺(H₂O)_n and (H₂O)_n⁺, it has been suggestted that the structure of (H₂O)_n⁺can be understood as H⁺(H₂O)_n−1OH.¹ Dominating isomers for n ≤ 5 have been analyzed in detail based on free OH stretching modes. In this work, we perform extensive search for local minima structures for n=5~8 with first-principles methods. More than 600 stable isomers were located for n=8. Based on these structures, we analyze the structural evolution, assign the OH stretching regions and the solvation of OH radical in (H₂O)_n⁺.
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¹K. Mizuse, J.-L Kuo, A. Fujii, Chem. Sci. 2011, 2, 868, K. Mizuse, A. Fujii, J. Phys. Chem. A 2013, 117, 929

Recent experimental observations of (H₂O)₃⁺, (H₂O)₃⁺Ar, H⁺(H₂O)₃, and H⁺(H₂O)₃Ar clusters in the region 1400-3800 cm⁻¹show that the OH stretching vibration has distinct characteristics.¹²³ Multidimensional normal mode calculations were carried out for OH stretching vibrations in the 1200-4000 cm⁻¹photon energy range. The potential energy and dipole surfaces were evaluated by using first-principles methods. By comparing the calculated frequencies and intensities of OH stretching vibration with experimental spectra, we found that the assignment of OH strecthing of H₃O⁺ moiety and free OH strectching vibration have resonable agreement with experimental data.
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¹Jeffrey M. Headrick, Eric G. Diken, Richard S. Walters, Nathan I. Hammer, Richard A. Christie, Jun Cui, Evgeniy M. Myshakin, Michael A. Duncan, Mark A. Johnson, Kenneth D. Jordan, Science, 2005, 17, 1765. ²Kenta Mizuse, Jer-Lai Kuo and Asuka Fujii, Chem. Sci., 2011, 2, 868. ³Kenta Mizuse and Asuka Fujii, J. Phys. Chem. A, 2013, 117, 929.

There are two types of bands in the OH stretch region of the vibrational spectra of hydrogen-bonded complexes; narrow peaks due to isolated OH stretches and a broadened feature reflecting the OH stretches involved in strong hydrogen bonding. This second region can be as wide as several hundred wavenumbers and is shifted to the red of the narrow peaks. In this work we focus on (CaOH)⁺·(H₂O)_n and (MgOH)⁺·(H₂O)_n systems.¹ When n < 4, the spectra are characterized by only the narrow peaks near 3700 cm⁻¹. When n ≥ 4, there is an additional band that is several hundred cm⁻¹ wide, which is attributed to hydrogen bonding. This breadth arises from coupling between the OH stretches in the water molecules and the low frequency modes of the complex. To understand the broadening observed in the spectra, we have developed a computational framework in which we sample displacement geometries from the equilibrium structure based on the ground state harmonic wavefunction. Then we combine the harmonic spectra in the OH stretch region for each computed geometry to generate the spectrum for this complex. As the calculated spectra agree well with the experimental spectra, we then investigated which geometric parameters in the system are correlated to the size of the red-shift of the frequencies. The hydrogen-bonded OH stretches were found to be very sensitive to how the water molecules were arranged around the hydroxide group. 1. Leavitt, C.M.; Johnson, C.J.; Johnson, M.A. Unpublished work.

Over the past decade, an intensive integrated effort involving theory and experiment have revealed the structures at play in the first hydration shell of simple ions. The water molecules generally adopt configurations in which one OH group is directed toward the ion while the other is integrated into a water network. One of the reasons this endeavor was difficult is that the three-body repulsion in this regime acts to significantly lower the effective inter-water binding energies, making the equilibrium structures much more fragile than their neutral counterparts. Here we exploit very recent advances in the temperature control of size-selected ionic clusters using cryogenic ion traps to monitor how the spectroscopic signatures of the water networks evolve as the temperature of the I⁻(H₂O)_n clusters is varied over the range 10 to 200 K. The breaking of the hydrogen bond interactions is observed around 150 K in the dimer. Qualitative assignments of the free, bound and ring hydrogens in the OH stretching region clarify the evolution from closed to linear hydrogen bond networks as the temperature changes. The success of this temperature programmed ion vibration predissociation (TPIVP) spectroscopy opens the way to sample large amplitude exploration of potential energy landscapes of such systems.

Alkali metal cations play vital roles in chemical and biochemical systems. Lithium is widely used in psychiatric treatment of manic states and bipolar disorder; Sodium and potassium are essential elements, having major biological roles as electrolytes, balancing osmotic pressure on body cells and assisting the electroneurographic signal transmission; Rubidium has seen increasing usage as a supplementation for manic depression and depression treatment; Cesium doped compounds are used as essential catalysts in chemical production and organic synthesis. Since hydrated alkali metal cations are ubiquitous and the basic form of the alkali metal cations in chemical and biochemical systems, their structural and thermodynamic properties serve as the foundation for modeling more complex chemical and biochemical processes, such as ion transport and ion size-selectivity of ionophores and protein channels. By combining mass spectrometry and infrared photodissociation spectroscopy, we have characterized the structures and thermodynamic properties of the hydrated alkali metal cations, i.e. M⁺(H₂O)_nAr, (M = Li, Na, K, Rb and Cs, n = 3-5). Ab initio calculations and RRKM-EE (evaporative ensemble) calculations were used to assist in the spectral assignments and thermodynamic analysis. Results showed that the structures of hydrated alkali metal cations were determined predominantly by the competition between non-covalent interactions, i.e. the water-water hydrogen bonding interactions and the water-cation electrostatic interactions. This balance, however, is very delicate and small changes, i.e. different cations, different levels of hydration and different effective temperatures clearly impact the balance.

The importance of hydrogen bonding in imidazolium-based ionic liquids (IL) has become a topic of vigorous debate. Red shifted features in the ring CH stretching region observed in bulk FTIR of several IL have been identified as the hydrogen bonded C₍₂₎H stretch. However, recent theoretical analysis suggests that the complexity of the ring CH stretching region is a result of Fermi resonance interactions between the overtones and combination bands of the ring stretching modes with the ring CH stretches. To help clarify the role of the C₍₂₎H group and the nature of the intermolecular cation-anion interactions, we report the vibrational spectra of cryogenically cooled, composition-selected ionic clusters of the prototypical IL [EMIM][BF₄]. We have confirmed that the CH stretching region is indeed plagued by strong Fermi resonance interactions and, therefore, have turned to isotopic and chemical substitution to determine the position and role of the C₍₂₎H oscillator. The spectra are consistent with electrostatics being the dominant interaction while hydrogen bonding is not critical in this IL. Time permitting, recent spectra on temperature controlled protonated water clusters will be discussed.

The Air Force has a pressing need to find new means of spacecraft propulsion, enabling cheaper, safer, more efficient maneuvering on orbit. Ionic liquids are a potential replacement for hydrazine in hypergolic combustion propellant systems and for xenon in electric propulsion systems. However, both applications require considerable further development, leading us to study the fundamental structural and optical properties of candidate systems. Our benchmark measurements will provide validation of theoretical models of all types, from ab initio methods up to codes describing full thruster plumes. Using standard supersonic jet time-of-flight spectroscopy techniques, we have measured the ultraviolet and infrared spectra of ion pairs of the only space-qualified ionic liquid, [emim][Tf₂N]. The ultraviolet photodissociation spectrum, though broad and essentially featureless, reveals rich underlying photodynamics involving both single- and multi-photon excitations and a wealth of interacting excited states. The infrared spectrum and MP2 calculations establish the structure as one in which the cation and anion are stacked on top of one another rather than sitting in the same plane, answering a long-standing question in this field. The complexity of the infrared spectrum and its behavior under varying jet temperatures indicates the presence of multiple conformations and likely contributions from Fermi resonance.

We present infrared photodissociation spectra of Co(CO₂)_n⁻ ions (n = 3 – 11) in the wavenumber region 1000 – 2400 cm⁻¹, interpreted with the aid of density functional theory calculations. The spectra show signatures of several structural motifs for the interaction of a Co atom and CO₂ ligands. The spectra are dominated by a core ion showing bidentate interaction of two CO₂ ligands forming C-Co and O-Co bonds. The prevalence of triplet vs singlet states and the charge distribution in the Co(CO₂)₂⁻ core ion will also be discussed.

Vibrational spectra are measured for Cu⁺(CH₄)_n and Ag⁺(CH₄)_n (n = 1 - 6) in the C-H stretching region (2500-3100 cm⁻¹) using photofragment spectroscopy. Spectra are obtained by monitoring CH₄ fragment loss following absorption of one photon (for n = 3 - 6), sequential absorption of multiple photons for Ag⁺(CH₄)_n(n = 1 - 2) or one photon absorption by Ar-tagged Cu⁺(CH₄)_n (for n = 1-2). Determination of the structures of the complexes was done by comparing calculated vibrational spectra of low-lying candidate structures to the observed IR photodissociation spectra. Calculations were carried out using both the B3LYP and CAM-B3LYP hybrid density functionals with the 6-311++G(3df,3pd) basis set for Cu, C and H and the aug-cc-pVTZ basis with an effective core potential (ECP) for Ag. Calculations predict that the positions and intensities of bands in the C-H stretching region depend strongly on the coordination of the CH₄ to the metal (η² or η³) and on the M-C bond length, and thus is sensitive to the number of ligands in the first and second shell. For clusters with n ≤ 4 these ions adopt symmetrical structures with η² methane coordination. For copper, the larger clusters are primarily formed by addition of second-shell CH₄ to a tetrahedral core; silver primarily coordinates the 5th and 6th ligands in the first shell.

Anion photoelectron spectroscopy has been a major tool in understanding the vibronic structure of metal oxide clusters, due to its universality and sensitivity. However, high ion temperatures and modest photoelectron energy resolutions have hampered the observation of vibrational structure. We have recently coupled our high-resolution slow photoelectron velocity-map imaging (SEVI) spectrometer to a cryogenic ion trap and a laser ablation ion source, allowing for the acquisition of photoelectron spectra of vibrationally cold metal oxide anions with a resolution down to  ∼ 4 cm⁻¹, limited by unresolved rotational structure. A test study of the simple d⁰ group 4 MO₂ triatomic metal oxides yielded fully vibrationally-resolved spectra, allowing for reassignments of electron affinities, new measurements of vibrational fundamentals, and estimates of the anion geometries based on the observed FC structure. Studies of the corresponding Ti₂O₄ and Zr₂O₄ systems revealed vibrational progressions that allows for an unambiguous assignment of the anion isomers; previous photoelectron spectra could not distinguish the isomers based on detachment energies alone. Spectra of the VO₂⁻ anion identified the first three electronic states of the d¹ neutral as well as ν₁ and ν₂ vibrations in each state.

Iron is an important element in the formation of solids in space. Spectroscopic observations of interstellar iron shows that its atomic gas-phase abundance is strongly depleted with respect to that of hydrogen. In contrast, sulfur is mostly found in the gas phase in low-density regions of interstellar space, but is highly depleted in regions of star- and planet formation. Furthermore, the dominant source of sulfur in our solar system is solid FeS, as found in primitive meteorites, implying an efficient chemical pathway to convert sulphur or sulphur containing compounds into solid FeS during the (early phases of) the star formation process. We address the evolution of iron and sulfur in space on a molecular level by studying metal nanoclusters and their interaction with ligands using IR action spectroscopy. Clusters are formed through laser ablation of solid precursor materials and brought into a molecular beam environment. Complexes with ligands are obtained by directing the beam through a reaction channel containing low-pressure reactant gas. Mass-selected IR action spectra are recorded by irradiating the clusters using the Free Electron Laser for Infrared eXperiments (FELIX). Experimental spectra are then compared with DFT predictions which enables us to determine the structure of the selected cluster and its binding interactions with ligands. As part of this project, we here present IR action spectra of size-selected Fe clusters and the chemically closely related Co clusters, and their complexes with relevant ligands.

Strongly bound clusters are often used as convenient models for bulk material. Silicon clusters are particularly interesting due to their importance in the electronics industry. We perform experimental IR multiple photon dissociation spectroscopy in the gas-phase, which makes use of a free electron laser, and compare the results with that predicted by density functional and MP2 theory calculations. Comparison of the vibrational spectra with that predicted by theoretical calculations for several structural isomers for each cluster size leads to accurate structural assignments. Here, we present our results for silicon clusters,¹ and compare the structures with those of select transition metal doped Si_nM clusters.² Of particular interest is the transition from exohedral to endoheral metal doped silicon clusters and how the transition size changes for different metal dopant atoms.
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¹Journal of Chemical Physics 2012, 136, 064301. ²e.g., ChemPhysChem 2014, 15, 328.

It has been a long-sought goal in cluster science to discover stable atomic clusters as building blocks for cluster-assembled nanomaterials, as exemplified by the fullerenes and their subsequent bulk syntheses.^[1,2] Clusters have also been considered as models to understand bulk properties, providing a bridge between molecular and solid-state chemistry.^[3] Herein we report a joint photoelectron spectroscopy and theoretical study on the [Ta₂B₆]⁻ and [Ta₂B₆] clusters.^[4] The photoelectron spectrum of [Ta₂B₆]⁻ displays a simple spectral pattern and a large HOMO-LUMO gap, suggesting its high symmetry. Theoretical calculations show that both the neutral and anion are D_6h pyramidal. The chemical bonding analyses for [Ta₂B₆] revealed the nature of the B₆ and Ta interactions and uncovered strong covalent bonding between B₆ and Ta. The D_6h-[TaB₆Ta] gaseous cluster is reminiscent of the structural pattern in the ReB₆X₆Re core in the [(Cp*Re)₂B₆H₄Cl₂] and the TiB₆Ti motif in the newly synthesized Ti₇Rh₄Ir₂B₈ solid-state compound.^[5,6] The current work provides an intrinsic link between a gaseous cluster and motifs for solid materials. Continued investigations of the transition-metal boron clusters may lead to the discovery of new structural motifs involving pure boron clusters for the design of novel boride materials. Reference [1] H.W. Kroto, J. R. Heath, S. C. OBrien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162 – 163. [2] W. Krtschmer, L. D. Lamb, K. Fostiropoulos, D. R. Huffman, Nature 1990, 347, 354 – 358. [3] T. P. Fehlner, J.-F. Halet, J.-Y. Saillard, Molecular Clusters: A Bridge to Solid-State Chemitry, Cambridge University Press, UK, 2007. [4] W. L. Li, L. Xie, T. Jian, C. Romanescu, X. Huang, L.-S. Wang, Angew. Chem. Int. Ed. 2014, 126, 1312 – 1316. [5] B. Le Guennic, H. Jiao, S. Kahlal, J.-Y. Saillard, J.-F. Halet, S. Ghosh, M. Shang, A. M. Beatty, A. L. Rheingold, T. P. Fehlner, J. Am. Chem. Soc. 2004, 126, 3203 – 3217. [6] B. P. T. Fokwa, M. Hermus, Angew. Chem. 2012, 124, 1734 – 1737; Angew. Chem. Int. Ed. 2012, 51, 1702 – 1705.