Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO4Mg(H2O)n=4–11]2+ Clusters

Isomer-Specific Vibrational Spectroscopy of Microhydrated Lithium Dichloride Anions: Spectral Fingerprint of Solvent-Shared Ion Pairs

The vibrational spectroscopy of lithium dichloride anions microhydrated with one to three water molecules, [LiCl2 (H2 O)1-3 ]- , is studied in the OH stretching region (3800-2800 cm-1 ) using isomer-specific IR/IR double-resonance population labelling experiments. The spectroscopic fingerprints of individual isomers can only be unambiguously assigned after anharmonic effects are considered, but then yield molecular level insight into the onset of salt dissolution in these gas phase model systems. Based on the extent of the observed frequency shifts ΔνOH of the hydrogen-bonded OH stretching oscillators solvent-shared ion pair motifs (<3200 cm-1 ) can be distinguished from intact-core structures (>3200 cm-1 ). The characteristic fingerprint of a water molecule trapped directly in-between two ions of opposite charge provides an alternative route to evaluate the extent of ion pairing in aqueous electrolyte solutions.

Photoelectron Spectroscopy and Theoretical Study on Monosolvated Cyanate Analogue Clusters ECX-·Sol (ECX- = NCSe-, AsCSe-, and AsCS-; Sol = H2O, CH3CN)

Six monosolvated cyanate analogue clusters ECX^-·Sol (ECX^- = NCSe^-, AsCSe^-, and AsCS^-; Sol = H₂O and CH₃CN) were investigated using negative ion photoelectron spectroscopy (NIPES). NIPES experiments show that these clusters possess similar spectra overall compared to their respective isolated ECX^- anions but shift to higher electron binding energy with CH₃CN solvent, stabilizing the excess electrons slightly more than H₂O. For the ECX^-·H₂O series, vertical detachment energies and their increments relative to the bare species are measured to be 3.700/0.370, 3.085/0.415, and 3.085/0.430 eV for NCSe^-, AsCSe^- and AsCS^-, respectively, while the corresponding values in the ECX^-·CH₃CN series are 3.835/0.505, 3.145/0.475, and 3.135/0.480 eV. Ab initio electronic structure calculations indicate that the excess charges were located at the terminal N and Se atoms in NCSe^- and migrated to the central C atom in AsCSe^- and AsCS^-. For NCSe^-, the solvation is driven by the interactions with the two negatively charged terminal ends, while for AsCSe^- and AsCS^-, the solvation revolves around the interactions with the central C atom, where all the excess negative charge is concentrated. Two nearly degenerate isomers for NCSe^-·H₂O are identified, one forming a single strong N···H-O hydrogen bond (HB) and the other featuring a bidentate HB with two hydroxyl H atoms pointing to N and Se ends. In contrast, the negative central C atom in AsCSe^-/AsCS^- allows the formation of a bifurcated HB with H₂O. Similar effects are observed for the acetonitrile case, in which the three H atoms of the methyl group interact with the two negatively charged terminal ends in NCSe^-, while preferring to bind to the central negative carbon atom in AsCSe^-/AsCS^-. The different binding motifs derived in this work may suggest different solvation properties in NCSe^- versus AsCSe^-/AsCS^- with the former anion leading to asymmetric solvation at the N end of the solute, while the latter species creates more "isotropic" solvation around the central C equatorial plane.

Hydration motifs of ammonium bisulfate clusters show complex temperature dependence

The role of water in the formation of particles from atmospheric trace gases is not well understood, in large part due to difficulties in detecting its presence under atmospheric conditions and the variety of possible structures that must be screened computationally. Here, we use infrared spectroscopy and variable-temperature ion trap mass spectrometry to investigate the structural motifs adopted by water bound to ammonium bisulfate clusters and their temperature dependence. For clusters featuring only acid-base linkages, water adopts a bridging arrangement spanning an adjacent ammonium and bisulfate. For larger clusters, water can also insert into a bisulfate-bisulfate hydrogen bond, yielding hydration isomers with very similar binding energies. The population of these isomers shows a complex temperature evolution, as an apparent third isomer appears with a temperature dependence that is difficult to explain using simple thermodynamic arguments. These observations suggest that the thermodynamics of water binding to atmospheric clusters such as these may not be straightforward.

Hydration motifs of ammonium bisulfate clusters of relevance to atmospheric new particle formation

We have analyzed the binding motifs of water bound to a prototypical cluster containing three ammonium cations and two bisulfate anions using mass-selective vibrational spectroscopy and quantum chemical calculations.

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. Analyzing this trend reveals site-specific propensities for hydration. These results indicate that hydration is determined by structural factors and that reported dependences on acidity or basicity arise from the weaker correlation between the number of hydrogen bond donors of amines and their gas-phase basicity.

Insights into water-mediated ion clustering in aqueous CaSO4 solutions: pre-nucleation cluster characteristics studied by ab initio calculations and molecular dynamics simulations

The molecular structure of growth units building crystals is a fundamental issue in the crystallization processes from aqueous solutions. In this work, a systematic investigation of pre-nucleation clusters and their hydration characteristics in aqueous CaSO₄ solutions was performed using ab initio calculations and molecular dynamics (MD) simulations. The results of ab initio calculations and MD simulations indicate that the dominant species in aqueous CaSO₄ solutions are monodentate ion-associated structures. Compared with charged ion clusters, neutral clusters are more likely to be present in an aqueous CaSO₄ solution. Neutral (CaSO₄)_m clusters are probably the growth units involved in the pre-nucleation or crystallization processes. Meanwhile, hydration behavior around ion associated species in aqueous CaSO₄ solutions plays an important role in related phase/polymorphism selections. Upon ion clustering, the residence of some water molecules around Ca²⁺ in ion-associated species is weakened while that of some bridging waters is enhanced due to dual interaction by Ca²⁺ and SO₄^2-. Some phase/polymorphism selections can be achieved in aqueous CaSO₄ solutions by controlling the hydration around pre-nucleation clusters. Moreover, the association trend between calcium and sulfate is found to be relatively strong, which hints at the low solubility of calcium sulfate in water.

Coordination-Dependent Spectroscopic Signatures of Divalent Metal Ion Binding to Carboxylate Head Groups: H2- and He-Tagged Vibrational Spectra of M2+·RCO2¯ (M = Mg and Ca, R = -CD3, -CD2CD3) Complexes

We explore the intramolecular distortions present in divalent metal ion-carboxylate ion pairs using vibrational spectroscopy of the cryogenically cooled, mass-selected species isolated in the gas phase. The spectral signatures of the C-O stretching modes are identified using the perdeutero isotopologues of the acetate and propionate anions to avoid congestion arising from the CH₂ fundamentals. Both Ca²⁺ and Mg²⁺ are observed to bind in a symmetrical, so-called "bidentate" arrangement to the -CO₂¯ group. The very strong deformations of the head groups displayed by the binary complexes dramatically relax when either neutral water molecules or counterions are attached to the Mg²⁺RCO₂¯ cation. These results emphasize the critical role that local coordination plays when using the RCO₂¯ bands to deduce the metal ion complexation motif in condensed media.

Emergence of Solvent-Separated Na+-Cl- Ion Pair in Salt Water: Photoelectron Spectroscopy and Theoretical Calculations

Solvation of salts in water is a fundamental physical chemical process, but the underlying mechanism remains unclear. We investigated the contact ion pair (CIP) to solvent-separated ion pair (SSIP) transition in NaCl(H₂O)_n clusters with anion photoelectron spectroscopy and ab initio calculations. It is found that the SSIP type of structures show up at n = 2 for NaCl^-(H₂O)_n anions. For neutral NaCl(H₂O)_n, the CIP structures are dominant at n < 9. At n = 9-12, the CIP structures and SSIP structures of NaCl(H₂O)_n are nearly degenerate in energy, coincident to the H₂O:NaCl molar ratio of NaCl saturated solution and implying that the CIP and SSIP structures can coexist in concentrated solutions. These results are useful for understanding the solvation of salts at the molecular level.

Initial hydration processes of magnesium chloride: size-selected anion photoelectron spectroscopy and ab initio calculations

Separation of Cl⁻–Mg²⁺ ion pairs starts at n = 4 in MgCl₂(H₂O)_n⁻ anions and at n = 7 in neutral MgCl₂(H₂O)_n.

Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H2SO4+MgSO4) based on sulfate and bisulfate concentrations determined from νs(SO4(2-)) and νs(HSO4(-)), the acid dissociation constant, and activity coefficients from extended Debye-Hückel calculations. Shifts in pH and peak positions of νs(SO4(2-)) and νs(HSO4(-)) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.

Probing the microsolvation of a quaternary ion complex: gas phase vibrational spectroscopy of (NaSO4−)2(H2O)n=0–6, 8

We report infrared multiple photon dissociation spectra of cryogenically-cooled (NaSO₄⁻)₂(H₂O)_n dianions (n = 0–6, 8) in the fingerprint spectral region, which provide evidence for a remarkable stability of the quaternary ion complex upon microhydration.