On the dissolution processes of Na2I+ and Na3I2+ with the association of water molecules: mechanistic and energetic details

Solvation of magnesium chloride dimer in water: The case of anionic and neutral clusters

The structures of magnesium chloride dimer-water clusters, (MgCl2)2(H2O)n-/0, were investigated with size-selected anion photoelectron spectroscopy and theoretical calculations to understand the dissolution of magnesium chloride in water. The most stable structures were confirmed by comparing vertical detachment energies (VDEs) with the experimental measurements. A dramatic drop of VDE at n = 3 has been observed in the experiment, which is in accordance with the structural change of (MgCl2)2(H2O)n-. Compared to the neutral clusters, the excess electron induces two significant phenomena in (MgCl2)2(H2O)n-. First, the planar D2h geometry can be converted into a C3v structure at n = 0, making the Mg-Cl bonds easier to be broken by water molecules. More importantly, a negative charge-transfer-to-solvent process occurs after adding three water molecules (i.e., at n = 3), which leads to an obvious deviation in the evolution of the clusters. Such electron transfer behavior was noticed at n = 1 in monomer MgCl2(H2O)n-, indicating that the dimerization between two MgCl2 molecules can make the cluster more capable of binding electron. In neutral (MgCl2)2(H2O)n, this dimerization provides more sites for the added water molecules, which can stabilize the entire cluster and maintain its initial structure. Specifically, filling the coordination number to be 6 for Mg atoms can be seen as a link between structural preferences in the dissolution of the monomers, dimers, and extended bulk-state of MgCl2. This work represents an important step forward into fully understanding the solvation of MgCl2 crystals and other multivalent salt oligomers.

Comparison of the Microsolvation of CaX2 (X = F, Cl, Br, I) in Water: Size-Selected Anion Photoelectron Spectroscopy and Theoretical Calculations

To understand the microsolvation of alkaline-earth dihalides in water and provide information about the dependence of solvation processes on different halides, we investigated CaBr₂(H₂O)_n^-, CaI₂(H₂O)_n^-, and CaF₂(H₂O)_n^- (n = 0-6) clusters using size-selected anion photoelectron spectroscopy and conducted theoretical calculations on these clusters and their neutrals. The results are compared with those of CaCl₂(H₂O)_n^-/0 clusters reported previously. It is found that the vertical detachment energies (VDEs) of CaCl₂(H₂O)_n^-, CaBr₂(H₂O)_n^-, and CaI₂(H₂O)_n^- show a similar trend with increasing cluster size, while the VDEs of CaF₂(H₂O)_n^- show a different trend. The VDEs of CaF₂(H₂O)_n^- are much lower than those of CaCl₂(H₂O)_n^-, CaBr₂(H₂O)_n^-, and CaI₂(H₂O)_n^-. A detailed probing of the structures shows that a significant increase of the Ca-X distance (separation of Ca²⁺-X^- ion pair) in CaCl₂(H₂O)_n^-/0, CaBr₂(H₂O)_n^-/0, and CaI₂(H₂O)_n^-/0 clusters occurred at about n = 5. However, for CaF₂(H₂O)_n^-/0, no abrupt change of the Ca-F distance with the increasing cluster size has been observed. In CaCl₂(H₂O)₆^-/0, CaBr₂(H₂O)₆^-/0, and CaI₂(H₂O)₆^-/0, the Ca atom coordinates directly with 5 H₂O molecules. However, in CaF₂(H₂O)_n^-/0, the Ca atom coordinates directly with only 2 or 3 H₂O molecules. The similarity or differences in the structures and coordination numbers are consistent with the fact that CaCl₂, CaBr₂, and CaI₂ have similar solubility, while CaF₂ has much lower solubility.

Hydration processes of barium chloride: Size-selected anion photoelectron spectroscopy and theoretical calculations of BaCl2-water clusters

In order to understand the hydration processes of BaCl₂, we investigated BaCl₂(H₂O)_n ^- (n = 0-5) clusters using size-selected anion photoelectron spectroscopy and theoretical calculations. The structures of neutral BaCl₂(H₂O)_n clusters up to n = 8 were also investigated by theoretical calculations. It is found that in BaCl₂(H₂O)_n ^-/0, the Ba-Cl distances increase very slowly with the cluster size. The hydration process is not able to induce the breaking of a Ba-Cl bond in the cluster size range (n = 0-8) studied in this work. In small BaCl₂(H₂O)_n clusters with n ≤ 5, the Ba atom has a coordination number of n + 2; however, in BaCl₂(H₂O)_6-8 clusters, the Ba atom coordinates with two Cl atoms and (n - 1) water molecules, and it has a coordination number of n + 1. Unlike the previously studied MgCl₂(H₂O)_n ^- and CaCl₂(H₂O)_n ^-, negative charge-transfer-to-solvent behavior has not been observed for BaCl₂(H₂O)_n ^-, and the excess electron of BaCl₂(H₂O)_n ^- is mainly localized on the Ba atom rather on the water molecules. No observation of Ba²⁺-Cl^- separation in current work is consistent with the lower solubility of BaCl₂ compared to MgCl₂ and CaCl₂. Considering the BaCl₂/H₂O mole ratio in the saturated solution, one would expect that about 20-30 H₂O molecules are needed to break the first Ba-Cl bond in BaCl₂.

Infrared multiple photon dissociation of cesium iodide clusters doped with mono-, di- and triglycine

Charged cesium iodide clusters doped with mono-, di- and triglycine serve as a model system for sea salt aerosols containing biological molecules. Here, we investigate reactions of these complexes under infrared irradiation, with spectra obtained by infrared multiple photon dissociation. The cluster ions are generated via electrospray ionization and analyzed in the cell of a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Depending on the cluster size and peptide length, loss of HI or loss of a glycine unit is observed. The experimental measurements are supported by quantum chemical calculations. We show that N-H and O-H stretching modes dominate the spectrum, with large shifts depending on local interactions, namely due to interaction with iodide anions or intramolecular hydrogen bonding. Both experiment and theory indicate that several isomers are present in the experimental mixture, with different infrared fingerprints as well as dissociation pathways.

Photodissociation of Sodium Iodide Clusters Doped with Small Hydrocarbons

Marine aerosols consist of a variety of compounds and play an important role in many atmospheric processes. In the present study, sodium iodide clusters with their simple isotope pattern serve as model systems for laboratory studies to investigate the role of iodide in the photochemical processing of sea-salt aerosols. Salt clusters doped with camphor, formate and pyruvate are studied in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) coupled to a tunable laser system in both UV and IR range. The analysis is supported by ab initio calculations of absorption spectra and energetics of dissociative channels. We provide quantitative analysis of IRMPD measurements by reconstructing one-photon spectra and comparing them with the calculated ones. While neutral camphor is adsorbed on the cluster surface, the formate and pyruvate ions replace an iodide ion. The photodissociation spectra revealed several wavelength-specific fragmentation pathways, including the carbon dioxide radical anion formed by photolysis of pyruvate. Camphor and pyruvate doped clusters absorb in the spectral region above 290 nm, which is relevant for tropospheric photochemistry, leading to internal conversion followed by intramolecular vibrational redistribution, which leads to decomposition of the cluster. Potential photodissociation products of pyruvate in the actinic region may be formed with a cross section of <2×10-20  cm2 , determined by the experimental noise 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.

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.

Initial hydration behavior of sodium iodide dimer: photoelectron spectroscopy and ab initio calculations

We investigated (NaI)₂⁻(H₂O)_n (n = 0–6) clusters to examine the initial solvation process of (NaI)₂ in water, using negative ion photoelectron spectroscopy and theoretical calculations.

Microsolvation of LiBO2 in water: anion photoelectron spectroscopy and ab initio calculations

The microsolvation of LiBO2 in water was investigated by conducting anion photoelectron spectroscopy and ab initio studies on the LiBO2(H2O)n(-) (n = 0-5) clusters. By comparing calculations with experiments, the structures of these clusters and their corresponding neutrals were assigned, and their structural evolutions were revealed. During the anionic structural evolution with n increasing to 5, hydroxyborate and metaborate channels were identified and the metaborate channel is more favorable. For the hydroxyborate structures, the anionic Li(+)-BO2(-) ion pair reacts with a water molecule to produce the LiBO(OH)2(-) moiety and three water molecules tend to dissolve this moiety. In the metaborate channel, two types of solvent-separated ion pair (SSIP) geometries were determined as the ring-type and linear-type. The transition from the contact ion pair (CIP) to the ring-type of SSIP starts at n = 3, while that to the linear-type of SSIP occurs at n = 4. In neutral LiBO2(H2O)n clusters, the first water molecule prefers to react with the Li(+)-BO2(-) ion pair to generate the LiBO(OH)2 moiety, analogous to the bulk crystal phase of α-LiBO2 with two O atoms substituted by two OH groups. The Li-O distance in the LiBO(OH)2 moiety increases with the increasing number of water molecules and elongates abruptly at n = 4. Our studies provide new insight into the initial dissolution of LiBO2 salt in water at the molecular level and may be correlated to the bulk state.

On the dissolution of lithium sulfate in water: anion photoelectron spectroscopy and density functional theory calculations

The initial dissolution steps of lithium sulfate (Li2SO4) in water were investigated by performing anion photoelectron spectroscopy and density functional theory calculations on the Li2SO4(H2O)n(-) (n = 0-5) clusters. The plausible structures of these clusters and the corresponding neutral clusters were obtained using LC-ωPBE/6-311++G(d,p) calculations by comparing the experimental and theoretical vertical electron detachment energies. Two types of structures for bare Li2SO4(-/0) were found: a turtle-shaped structure and a propeller-shaped structure. For Li2SO4(H2O)n(-) cluster anions with n = 1-3, two kinds of isomers derived from the turtle-shaped and propeller-shaped structures of bare Li2SO4(-) were identified. For n = 4-5, these two kinds of isomers present similar structural and energetic features and thus are not distinguishable. For the anionic clusters the water molecules prefer to firstly interact with one Li atom until fully coordinating it. While for the neutral clusters, the water molecules interact with the two Li atoms alternately, therefore, showing a pairwise solvation behavior. The Li-S distance increases smoothly upon addition of water molecules one by one. Addition of five water molecules to Li2SO4 cannot induce the dissociation of one Li(+) ion because the water molecules are shared by two Li(+) ions.

First steps towards dissolution of NaSO4? by water

NaSO(4)(-)(H(2)O)(n) (n = 0-4) clusters have been generated in the gas phase as model systems to simulate the first dissolution steps of sulfate salts in water; photoelectron spectroscopy and theoretical calculations indicate that the first three water molecules strongly interact with both Na(+) and SO(4)(2-), forming a three-water solvation ring to start to pry apart the Na(+)SO(4)(2-) contact ion pair.

Size-dependent structures of NanIn−1+ cluster ions with a methanol adsorbate: A combined study by photodissociation spectroscopy and density-functional theory calculation

Methanol adsorption sites on NanI+n-1 ions were investigated. Photoexcitation to charge-transfer states of NanI+n-1 (methanol) predominantly produces two fragment ions: Nan-1I+n-2 (methanol) (neutral NaI loss) and Nan-1I+n-2(neutral NaI and methanol loss), without forming NanI+n-1 (methanol loss). The relative intensities of these fragments are correlated with the geometries and binding energies.