Unimolecular and hydrolysis channels for the detachment of water from microsolvated alkaline earth dication (Mg2+, Ca2+, Sr2+, Ba2+) clusters

Infrared spectroscopy of RG-Co+(H2O) complexes (RG = Ar, Ne, He): The role of rare gas "tag" atoms

RG_n-Co⁺(H₂O) cation complexes (RG = Ar, Ne, He) are generated in a supersonic expansion by pulsed laser vaporization. Complexes are mass-selected using a time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy, measuring the respective mass channels corresponding to the elimination of the rare gas "tag" atom. Spectral patterns and theory indicate that the structures of the ions with a single rare gas atom have this bound to the cobalt cation opposite the water moiety in a near-C_2v arrangement. The O-H stretch vibrations of the complex are shifted compared to those of water because of the metal cation charge-transfer interaction; these frequencies also vary systematically with the rare gas atom attached. The efficiencies of photodissociation also vary with the rare gas atoms because of their widely different binding energies to the cobalt cation. The spectrum of the argon complex could only be measured when at least three argon atoms were attached. In the case of the helium complex, the low binding energy allows the spectra to be measured for the low-frequency H-O-H scissors bending mode and for the O-D stretches of the deuterated analog. The partially resolved rotational structure for the antisymmetric O-H and O-D stretches reveals the temperature of these complexes (6 K) and establishes the electronic ground state. The helium complex has the same ³B₁ ground state as the tag-free complex studied previously by Metz and co-workers ["Dissociation energy and electronic and vibrational spectroscopy of Co⁺(H₂O) and its isotopomers," J. Phys. Chem. A 117, 1254 (2013)], but the A rotational constant is contaminated by vibrational averaging from the bending motion of the helium.

Microsolvation in V+(H2O)n Clusters Studied with Selected-Ion Infrared Spectroscopy

Gas-phase ion-molecule clusters of the form V⁺(H₂O)_n (n = 1-30) are produced by laser vaporization in a supersonic expansion. These ions are analyzed and mass-selected with a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy. The small clusters (n ≤ 7) are studied with argon tagging, while the larger clusters are studied via the elimination of water molecules. The vibrational spectra for the small clusters include only free O-H stretching vibrations, while larger clusters exhibit redshifted hydrogen bonding vibrations. The spectral patterns reveal that the coordination around V⁺ ions is completed with four water molecules. A symmetric square-planar structure forms for the n = 4 ion, and this becomes the core ion in larger structures. Clusters up to n = 8 have mostly two-dimensional structures, but hydrogen bonding networks evolve to three-dimensional structures in larger clusters. The free O-H vibration of acceptor-acceptor-donor (AAD)-coordinated surface molecules converges to a frequency near that of bulk water by the cluster size of n = 30. However, the splitting of this vibration for AAD- versus AD-coordinated molecules is still different compared to other singly charged or doubly charged cation-water clusters. This indicates that cation identity and charge-site location in the cluster can produce discernable spectral differences for clusters in this size range.

The Solvation of Ca2+ with Gas Phase Clusters of Alcohol Molecules

A comprehensive examination of how the identity of an alcohol molecule can change the behavior of a solvated, alkaline earth dication has been undertaken. The metal dication of Ca²⁺ has been clustered with a range of different alcohols to form [Ca(ROH)_n]²⁺ complexes, where n lies in the range 2-20. Following collisional activation via electron capture from nitrogen gas, complexes for n in the range 2-6 exhibit a switch in reaction product as a function of n. For low values, solvated CaOH⁺ is the dominant fragment, but as n increases beyond 4, this is displaced by the appearance of solvated CaOR⁺. A separate study of unimolecular metastable decay by [Ca(ROH)_n]²⁺ complexes found evidence of charge separation to form CaOH⁺(ROH)_n-1 + R⁺. For two isomers of butanol, the n = 3 complexes were found to follow parallel, but different metastable pathways: one leading to the appearance of CaOH⁺ and another that resulted in proton abstraction to form ROH₂⁺. These differences have been attributed to the precursor complexes adopting geometries where one ROH molecule occupies a secondary solvation shell. Comparisons were made with a previous study of magnesium complexes; [Mg(ROH)_n]²⁺ show that the difference in second ionization energy Mg⁺ (15.09 eV) as opposed to Ca⁺ (11.88 eV) influences behavior. A complex between Ca²⁺ and 1-chloroethanol is shown to favor the formation of CaCl⁺ as opposed to CaOH⁺ as a unimolecular charge separation product, which is attributed to differences in bond energy in the precursor molecule.

Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20-70: spontaneous formation of a hydrated electron?

Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)n-1]+ to [Mg(H2O)n]+ around n = 15-22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H2O)n]+ in the range of n = 20-70 measured for photon energies of 1.0-5.0 eV. The spectra exhibit a broad absorption from 1.4 to 3.2 eV, with two local maxima around 1.7-1.8 eV and 2.1-2.5 eV, depending on cluster size. The spectra shift slowly from n = 20 to n = 50, but no significant change is observed for n = 50-70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including five- and six-fold coordinated Mg2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg2+/e- pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.

Aqueous solvation of Mg(ii) and Ca(ii): A Born-Oppenheimer molecular dynamics study of microhydrated gas phase clusters

The hydration features of [Mg(H₂O)_n]²⁺ and [Ca(H₂O)_n]²⁺ clusters with n = 3-6, 8, 18, and 27 were studied by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/6-31+G** level of theory. For both ions, it is energetically more favorable to have all water molecules in the first hydration shell when n ≤ 6, but stable lower coordination average structures with one water molecule not directly interacting with the ion were found for Mg²⁺ at room temperature, showing signatures of proton transfer events for the smaller cation but not for the larger one. A more rigid octahedral-type structure for Mg²⁺ than for Ca²⁺ was observed in all simulations, with no exchange of water molecules to the second hydration shell. Significant thermal effects on the average structure of clusters were found: while static optimizations lead to compact, spherically symmetric hydration geometries, the effects introduced by finite-temperature dynamics yield more prolate configurations. The calculated vibrational spectra are in agreement with infrared spectroscopy results. Previous studies proposed an increase in the coordination number (CN) from six to eight water molecules for [Ca(H₂O)_n]²⁺ clusters when n ≥ 12; however, in agreement with recent measurements of binding energies, no transition to a larger CN was found when n > 8. Moreover, the excellent agreement found between the calculated extended X-ray absorption fine structure spectroscopy spectra for the larger cluster and the experimental data of the aqueous solution supports a CN of six for Ca²⁺.

Coordination of Be and Mg Centres by HCN Ligands - Be…N and Mg…N Interactions

ωB97XD/aug-cc-pVTZ calculations were performed for clusters of Z2+ cations (Z=Be and Mg) and HCN molecules (up to six molecules). The clusters of Be(CH3 )2 and Mg(CH3 )2 with HCN species were also calculated to analyse the influence of the Be/Mg-C formally covalent bonds on interactions of Be or Mg centre with ligands. The beryllium and magnesium centres possess different areas of a positive electrostatic potential that depend on a number of HCN ligands in the cluster considered. Numerous correlations between geometrical, energetic and topological parameters of the clusters considered are discussed since various theoretical approaches are applied; Quantum Theory of 'Atoms in Molecules', Natural Bond Orbital method and decomposition of the energy of interaction. The Be/Mg…N interactions classified as beryllium and magnesium bonds possess numerous characteristics which are known for the hydrogen bonds. Different types of coordination of Be and Mg centres analysed here exist also in crystal structures.