Theoretical assignments of the photo-dissociation excitation spectra of Mg+ ion complexes with water clusters: Multi-reference CI studies

The silicoaluminophosphate zeotype ECR-40, which has the MEI topology, contains linkages of AlO4 tetrahedra via a common oxygen atom, thereby violating the famous “Löwenstein’s rule”. Due to the proven existence of Al-O-Al linkages in this material, it constitutes an ideal model system to study the acidity and mobility of protons associated with such unusual linkages. In addition, their properties can be directly compared to those of protons associated with more common Si-O-Al linkages, which are also present in ECR-40. In this work, static density functional theory (DFT) calculations including a dispersion correction were employed to study the preferred proton sites as well as the Brønsted acidity of the framework protons, followed by DFT-based ab-initio molecular dynamics (AIMD) to investigate the proton mobility in guest-free and hydrated ECR-40. Initially, two different proton arrangements were compared, one containing both H[O6] protons associated with Al-O-Al linkages and H[O10] protons at Si-O-Al linkages, the other one containing only H[O10] protons. The former model was found to be thermodynamically favoured, as a removal of protons from the Al-O-Al linkages causes a local accumulation of negative charge. Calculations of the deprotonation energy showed a moderately higher Brønsted acidity of the H[O10] protons, at variance with previous empirical explanations, which attributed the exceptional performance of ECR-40 as acid catalyst to the presence of Al‑O‑Al linkages. The AIMD simulations (T = 298 K) delivered no appreciable proton mobility for guest-free ECR-40 and for low levels of hydration (one H2O per framework proton). Under saturation conditions, framework deprotonation occurred, leading to the formation of protonated water clusters in the pores. Pronounced differences between the two types of framework protons were observed: While the H[O10] protons were always removed from the Si-O-Al linkages, the Al-O-Al linkages remained mostly protonated, but deprotonation did occur to a minor extent. The observation of a degree of framework deprotonation of Al-O-Al linkages differs from the findings reported in a recent computational study of hydrated aluminosilicate zeolites with such linkages (Heard et al., Chem. Sci. 2019, 10, 5705), pointing to an influence of the overall framework composition. Further inspection of the AIMD results showed that a coordination of water molecules to framework Al atoms occurred in many cases, especially in the vicinity of the Al-O-Al linkages, sometimes resulting in a pronounced modification of the linkages through additional bridging oxygen atoms. Given the changes in the local structure, it can be expected that such modified linkages are especially prone to break upon dehydration. Thus, in addition to elucidating the deprotonation behaviour of protons associated with different types of linkages, the calculations also provide insights into possible reasons for the instability of Al-O-Al linkages, clarifying why Löwenstein’s rule is mostly obeyed in materials that are formed via a hydrothermal route.

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Reciprocal Polarizable Embedding with a Transferable H2O Potential Function II: Application to (H2O)n Clusters & Liquid Water

10.26434/chemrxiv.9224483 ◽

2019 ◽

Author(s):

Asmus Ougaard Dohn ◽

Elvar Jónsson ◽

Hannes Jonsson

Keyword(s):

Density Functional Theory ◽

Liquid Water ◽

Density Functional ◽

Water Clusters ◽

Theory Model ◽

Coupling Scheme ◽

Functional Theory ◽

Total Polarization ◽

Electrostatic Embedding ◽

Polarizable Embedding

The manuscript analyzes the accuracy of our recently developed reciprocal polarizable embedding scheme, where a density functional theory model of the QM region is coupled to a dipole- and quadrupole polarizable water potential of the MM region. We present calculations of water clusters and liquid water where we analyze the energy, atomic forces and total polarization to demonstrate that artifacts in energy and polarization introduced by the QM/MM coupling are small and well-behaved. Furthermore, our methodology improves the consistency of the structure of optimized water hexamer geometries when compared to results obtained with models that neglect polarization. Additionally, the manuscript provides evidence that our coupling scheme eliminates artifacts in the structure of liquid water obtained with simpler electrostatic embedding models.

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Faculty Opinions recommendation of Excitation spectra and brightness optimization of two-photon excited probes.

Faculty Opinions – Post-Publication Peer Review of the Biomedical Literature ◽

10.3410/f.13968961.15426066 ◽

2012 ◽

Author(s):

Michael Hausser ◽

Christian Wilms

Keyword(s):

Excitation Spectra ◽

Two Photon

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Interaction of Gold Atom with Clusters of Water: Few Computational Mise-En-Scènes with Hydrogen Bonding Motif

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc20081457 ◽

2008 ◽

Vol 73 (11) ◽

pp. 1457-1474 ◽

Cited By ~ 3

Author(s):

Eugene S. Kryachko

Keyword(s):

Hydrogen Bond ◽

Hydrogen Bonding ◽

Computational Model ◽

Photoelectron Spectroscopy ◽

Water Clusters ◽

Gold Atom ◽

Proton Acceptor ◽

Anion Photoelectron Spectroscopy ◽

Relationship Of ◽

First Time

The present work outlines the fair relationship of the computational model with the experiments on anion photoelectron spectroscopy for the gold-water complexes [Au(H2O)1≤n≤2]- that is established between the auride anion Au- and water monomer and dimer thanks to the nonconventional hydrogen bond where Au- casts as the nonconventional proton acceptor. This work also extends the computational model to the larger complexes [Au(H2O)3≤n≤5]- where gold considerably thwarts the shape of water clusters and even particularly breaks their conventional hydrogen bonding patterns. The fascinating phenomenon of the lavish proton acceptor character of Au- to form at least six hydrogen bonds with molecules of water is computationally unveiled in the present work for the first time.

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