Establishing the structural motifs present in small ammonium and aminium bisulfate clusters of relevance to atmospheric new particle formation.

Atmospheric new particle formation is the process by which atmospheric trace gases, typically acids and bases, cluster and grow into potentially climatically relevant particles. Here, we evaluate the structures and structural motifs present in small cationic ammonium and aminium bisulfate clusters that have been studied both experimentally and computationally as seeds for new particles. For several previously studied clusters, multiple different minimum-energy structures have been predicted.

Electrospray Ionization-Based Synthesis and Validation of Amine-Sulfuric Acid Clusters of Relevance to Atmospheric New Particle Formation.

Atmospheric new particle formation (NPF) is the process by which atmospheric trace gases such as sulfuric acid, ammonia, and amines cluster and grow into climatically relevant particles. The mechanism by which these particles form and grow has remained unclear, in large part due to difficulties in obtaining molecular-level information about the clusters as they grow. Mass spectrometry-based methods using electrospray ionization (ESI) as a cluster source have shed light on this process, but the produced cluster distributions have not been rigorously validated against experiments performed in atmospheric conditions.

Chemical Ionization Mass Spectrometry: Applications for the Measurement of Nonvolatile Organics at Ocean Worlds.

A new technique that has applications for the detection of nonvolatile organics on Ocean Worlds has been developed. Here, liquid mixtures of fatty acids (FAs) and/or amino acids (AAs) are introduced directly into a miniature quadrupole ion trap mass spectrometer (QITMS) developed at Jet Propulsion Laboratory and analyzed. Two ionization methods, electron impact and chemical ionization (EI and CI, respectively), are compared and contrasted.

Direct Link between Structure and Hydration in Ammonium and Aminium Bisulfate Clusters Implicated in Atmospheric New Particle Formation.

The acid-base chemistry of amines and sulfuric acid promotes growth in the early stages of atmospheric new particle formation, with more basic amines enhancing growth rates. Hydration of these particles has been proposed to depend on acidity or basicity but is difficult to quantify; therefore, the role of water in this process is not well understood. Using tandem mass spectrometry coupled to a temperature-controlled ion trap, we show that water uptake by aminium bisulfate clusters depends on the total number of free hydrogen bond donors in the cluster and is unaffected by the interchange of amines featuring the same number of substituents but differing gas-phase basicity.

The Interplay Between Hydrogen Bonding and Coulombic Forces in Determining the Structure of Sulfuric Acid-Amine Clusters.

Acid-base cluster chemistry drives atmospheric new particle formation (NPF), but the details of the growth mechanisms are difficult to experimentally probe. Clusters of ammonia, alkylamines, and sulfuric acid, species fundamental to NPF, are probed by infrared spectroscopy. These spectra show that substitution of amines for ammonia, which is linked to accelerated growth, induces profound structural rearrangement in clusters with initial compositions (NH) (HSO) (1 ≤ n ≤ 3).

Effect of Alkyl Group on MxOy(-) + ROH (M = Mo, W; R = Me, Et) Reaction Rates.

A systematic comparison of MxOy(-) + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy(-)·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy(-) + H2O reactivity studies. MxOy(-) + ROH → MxOy+1(-) + RH oxidation reactions are observed for the least oxidized clusters, and MxOy(-) + ROH → MxOyROH(-) addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy(-) + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy(-)·ROH charge-dipole complexes and the Lewis acid-base complexes.

Ce(x)O(y)⁻ (x = 2-3) + D₂O reactions: stoichiometric cluster formation from deuteroxide decomposition and anti-Arrhenius behavior.

Reactions between small cerium oxide cluster anions and deuterated water were monitored as a function of both water concentration and temperature in order to determine the temperature dependence of the rate constants. Sequential oxidation reactions of the Ce(x)O(y)⁻ (x = 2, 3) suboxide cluster anions were found to exhibit anti-Arrhenius behavior, with activation energies ranging from 0 to -18 kJ mol⁻¹. Direct oxidation of species up to y = x was observed, after which, -OD abstraction and D₂O addition reactions were observed.

Comparative study of water reactivity with Mo₂O(y)⁻ and W₂O(y)⁻ clusters: a combined experimental and theoretical investigation.

A computational investigation of the Mo2O(y)(-) + H2O (y = 4, 5) reactions as well as a photoelectron spectroscopic probe of the deuterated Mo2O6D2(-) product have been carried out to understand a puzzling question from a previous study: Why is the rate constant determined for the Mo2O5(-) + H2O/D2O reaction, the terminal reaction in the sequential oxidation of Mo2O(y)(-) by water, higher than the W2O5(-) + H2O/D2O reaction? This disparity was intriguing because W3O(y)(-) clusters were found to be more reactive toward water than their Mo3O(y)(-) analogs. A comparison of molecular structures reveals that the lowest energy structure of Mo2O5(-) provides a less hindered water addition site than the W2O5(-) ground state structure. Several modes of water addition to the most stable molecular and electronic structures of Mo2O4(-) and Mo2O5(-) were explored computationally.

RH and H2 production in reactions between ROH and small molybdenum oxide cluster anions.

To test recent computational studies on the mechanism of metal oxide cluster anion reactions with water [Ramabhadran, R. O.; et al.

New insights on photocatalytic H2 liberation from water using transition-metal oxides: lessons from cluster models of molybdenum and tungsten oxides.

Molecular hydrogen (H2) is an excellent alternative fuel. It can be produced from the abundantly present water on earth. Transition-metal oxides are widely used in the environmentally benign photocatalytic generation of H2 from water, thus actively driving scientific research on the mechanisms for this process.

Simple relationship between oxidation state and electron affinity in gas-phase metal-oxo complexes.

The photoelectron spectra of WO3H(-) and WO2F(-) are presented and analyzed in the context of a series of previous similar measurements on MO(y)(-) (M = Mo, W; y = 0-3), MO4H(-) and AlMOy(-) (y ≤ 4) complexes. The electronic structures of the WO3H and WO2F anion and neutral complexes were investigated using the B3LYP hybrid density functional method. The spectra of WO3H(-), WO2F(-), and previously measured AlWO3(-) photoelectron spectra show that the corresponding neutrals, in which the transition metal centers are all in a +5 oxidation state, have comparable electron affinities.

Asymmetric partitioning of metals among cluster anions and cations generated via laser ablation of mixed aluminum/Group 6 transition metal targets.

While high-power laser ablation of metal alloys indiscriminately produces gas-phase atomic ions in proportion to the abundance of the various metals in the alloy, gas-phase ions produced by moderate-power laser ablation sources coupled with molecular beams are formed by more complicated mechanisms. A mass spectrometric study that directly compares the mass distributions of cluster anions and cations generated from laser ablation of pure aluminum, an aluminum/molybdenum mixed target, and an aluminum/tungsten mixed target is detailed. Mass spectra of anionic species generated from the mixed targets showed that both tungsten and molybdenum were in higher abundance in the negatively charged species than in the target material.

Study of MoNbO(y) (y = 2-5) anion and neutral clusters using photoelectron spectroscopy and density functional theory calculations: impact of spin contamination on single point calculations.

Results of a study combining anion photoelectron spectroscopy and density functional theory calculations on the heteronuclear MoNbO(y)(-) (y = 2-5) transition metal suboxide cluster series are reported and analyzed. The photoelectron spectra, which exhibit broad electronic bands with partially resolved vibrational structure, were compared to spectral simulations generated from calculated spectroscopic parameters for all computationally determined energetically competitive structures. Although computational results on the less oxidized clusters could not be satisfactorily reconciled with experimental spectra, possibly because of heavy spin contamination found in a large portion of the computational results, the results suggest that (1) neutral cluster electron affinity is a strong indicator of whether O-atoms are bound in M-O-M bridge positions or M═O terminal positions, (2) MoNbO(y) anions and neutrals have structures that can be described as intermediate with respect to the unary (homonuclear) Mo(2)O(y) and Nb(2)O(y) clusters, and (3) structures in which O-atoms preferentially bind to the Nb center are slightly more stable than alternative structures.

Electronic structures of WAlO(y) and WAlO(y)(-) (y = 2-4) determined by anion photoelectron spectroscopy and density functional theory calculations.

The anion photoelectron spectra of WAlO(y)(-) (y = 2-4) are presented and assigned based on results of density functional theory calculations. The WAlO(2)(-) and WAlO(3)(-) spectra are both broad, with partially resolved vibrational structure. In contrast, the WAlO(4)(-) spectrum features well-resolved vibrational structure with contributions from three modes.

Electronic structures of AlMoO(y)(-) (y = 1-4) determined by photoelectron spectroscopy and density functional theory calculations.

Vibrationally-resolved photoelectron spectra of AlMoO(y)(-) (y = 1-4) are presented and analyzed in conjunction with density functional theory computational results. The structures determined for the AlMoO(y) anion and neutral clusters suggest ionic bonding between Al(+) and a MoO(y)(-) or MoO(y)(-2) moiety, and point to the relative stability of Mo=O versus Al=O bonds. The highest occupied and partially occupied orbitals in the anions and neutrals can be described as Mo atomic-like orbitals, so while the Mo is in a higher oxidation state than Al, the most energetically accessible electrons are localized on the molybdenum center.

Study of Nb2O(y) (y = 2-5) anion and neutral clusters using anion photoelectron spectroscopy and density functional theory calculations.

A study combining anion photoelectron spectroscopy and density functional theory calculations on the transition metal suboxide series, Nb(2)O(y)(-) (y = 2-5), is described. Photoelectron spectra of the clusters are obtained, and Franck-Condon simulations using calculated anion and neutral structures and frequencies are used to evaluate the calculations and assign transitions observed in the spectra. The spectra, several of which exhibit partially resolved vibrational structure, show an increase in electron affinity with increasing cluster oxidation state.

Structures of trimetallic molybdenum and tungsten suboxide cluster anions.

Anion photoelectron spectra of Mo(3)O(y)(-) and W(3)O(y)(-) (y = 3-6) are reported and analyzed using density functional theory results in an attempt to determine whether electronic and structural trends in the less oxidized clusters (y = 3, 4) could elucidate the disparate chemical properties of the M(3)O(y)(-) (M = Mo, W, y = 5, 6) species. In general, cyclic structures are calculated to be more stable by at least 1 eV than extended structures, and the lowest energy structures calculated for the most reduced species favor M = O terminal bonds. While the numerous low-energy structures found for Mo(3)O(y)(-)/Mo(3)O(y) and W(3)O(y)(-)/W(3)O(y) were, in general, similar, various structures of W(3)O(y)(-)/W(3)O(y) were found to be energetically closer lying than analogous structures of Mo(3)O(y)(-)/Mo(3)O(y).

Study of MoVO(y) (y = 2-5) anion and neutral clusters using anion photoelectron spectroscopy and density functional theory calculations.

The vibrationally resolved anion photoelectron (PE) spectra of MoVO(y)(-) (y = 2 - 5) metal suboxide clusters are presented and analyzed in the context of density functional theory (DFT) calculations. The electronically congested spectra reflect an increase in cluster electron affinity with increasing oxidation state. Ion beam hole-burning results reveal the features in the PE spectra of MoVO(2)(-) and MoVO(4)(-) are a result of only one anion isomer, while at least two isomers contribute to electronic structure observed in the PE spectrum of MoVO(3)(-).