Magic Numbers and Stabilities of Photoionized Water Clusters: Computational and Experimental Characterization of the Nanosolvated Hydronium Ion

Abstract

The stability and distributions of small water clusters generated in a supersonic beam expansion are interrogated by tunable vacuum ultraviolet (VUV) radiation generated at a synchrotron. Time-of-flight mass spectrometry reveals enhanced population of various protonated water clusters (H+(H2O) n ) based upon ionization energy and photoionization distance from source, suggesting there are “magic” numbers below the traditional n = 21 that predominates in the literature. These intensity distributions suggest that VUV threshold photoionization (11.0–11.5 eV) of neutral water clusters close to the nozzle exit leads to a different nonequilibrium state compared to a skimmed molecular beam. This results in the appearance of a new magic number at 14. Metadynamics conformer searches coupled with modern density functional calculations are used to identify the global minimum energy structures of protonated water clusters between n = 2 and 21, as well as the manifold of low-lying metastable minima. New lowest energy structures are reported for the cases of n = 5, 6, 11, 12, 16, and 18, and special stability is identified by several measures. These theoretical results are in agreement with the experiments performed in this work in that n = 14 is shown to exhibit additional stability, based on the computed second-order stabilization energy relative to most cluster sizes, though not to the extent of the well-known n = 21 cluster. Other cluster sizes that show some additional energetic stability are n = 7, 9, 12, 17, and 19. To gain insight into the balance between ion–water and water–water interactions as a function of the cluster size, an analysis of the effective two-body interactions (which sum exactly to the total interaction energy) was performed. This analysis reveals a crossover as a function of cluster size between a water–hydronium-dominated regime for small clusters and a water–water-dominated regime for larger clusters around n = 17.

Exactly Fragment Additive Breakdown of Polarization for Energy Decomposition Analysis Based on the Self-Consistent Field for Molecular Interactions

“…To demonstrate the capability of our polarization decomposition method to deal with clusters with more than two fragments, we show results for a H 3 O + – (H 2 O) 5 cluster, which is a simple model for solvated proton. We used the optimized structure from a previous publication where the geometry is optimized at the ωB97X-V/def2-SVPD level. The EDA calculation was performed at the ωB97X-D/def2-TZVP level.…”

A Vacuum Ultraviolet Photoionization Mass Spectrometry and Density Functional Calculation Study of Formic Acid–Water Clusters

“…The employed methodology includes intensity measurements of various masses at variable photon energies, the appearance energies (AE) of various fragments, and an interpretation of their stability guided by theoretical calculations. This approach, previously used in studying proton transfer in acetaldehyde−water clusters 37 and naphthalene water clusters, 38 also contributed to identifying enhanced intensities of a number of protonated water clusters from VUV photoionization 39 and the identification of the hydrogen bond networks in diol 40 and polyol water clusters. 41 Mass spectra results demonstrate that upon ionization of mixed FA-water clusters, the resulting clusters are protonated.…”