An Approach to Hydration of Model Silica Materials by Exploring Their Multiple Minima Hypersurfaces. The Role of Entropy of Association

Abstract

The influence of molecular water on the structure and formation of silica clusters is modeled with the use of the MMH (multiple minima hypersurfaces) approach. It combines quantum chemical Hamiltonians for the calculation of the internal energy with statistical modeling and formulas for the calculation of thermodynamic functions of association. The structures of the most probable clusters of hydration and some properties of the association with water are proposed. Different simple structures are calculated to consider the entropy of association in place of the simpler approach of a single "global minimum". Ab initio, DFT, and semiempirical calculations of the structures and relevant reactions of silica fragments are also reported, confirming the reliability of the results on very different grounded quantum mechanical methods. Particularly, it has been shown that semiempirical PM3 Hamiltonian is reliable for silica cluster calculations of this kind, in comparison with accurate ab initio SCF and other DFT calculations. Apparently, the well-known systematic failures of this Hamiltonian are absent in this kind of structures and interactions. The increasing hydrophobic character of neutral silica clusters appears as a result of the free energies of association, and therefore, it originates on entropy. It is remarkable that the simple global minimum approach currently used in works of molecular modeling on this kind of compounds, where several hydrogen bridges have a place to exist with several different conformations, must be taken with caution given the remarkable entropy of association of such systems.

Evaluation of the molecular inclusion process of β-hexachlorocyclohexane in cyclodextrins

“…A favorable thermodynamic association implies that the supermolecule will be more stable than the isolated molecules; namely, a greater absolute value of DE assoc corresponds to more energetically favorable associations. The MMH methodology usually works with semiempirical methods [39][40][41][42][43][44] for the evaluation of the energies while using statistical mechanics to obtain thermodynamic properties related to the molecular association. 38 The main procedure of this approach constructs several random non-redundant molecular geometries, starting from the independently optimized structures of the interacting molecules, in order to explore the conguration space of the formed complexes.…”

Density functional modelling of silicate and aluminosilicate dimerisation solution chemistry

“…In several instances during the DFT computations in this investigation the initially trialled cluster configurations needed to be revised and re-optimised (as outlined in the Computational Details section) in order to find lower-energy states rather than being trapped in a local (but not global) minimum on the potential energy surface. This is a similar (but simpler) approach when compared to the study carried out by Mora-Fonz et al, 4 where low-cost molecular dynamics was used to search configurational space, and the study by Montero-Cabrera et al, 36 where a new approach for searching configurational space using DFT and thermodynamics of association of water was employed. Small clusters as studied in this investigation tend to have only a few possible configurations, and therefore the global minimum should ideally be quite straightforward to find.…”

Pyrrolyl–Silicon Compounds as Precursors for Donor–Acceptor Systems Stabilized by Noncovalent Interactions

“…Monomer reactivity was estimated by conceptual DFT through global reactivity descriptors and local (condensed) Fukui functions. Reported studies have indicated good correlation between these functions and experimental electrophilic substitution patterns for a range of aromatic heterocyclic compounds, − including pyrrole derivatives and silicon-based materials. , Multiple minima hypersurface (MMH) methodology, the validity of which for the study of molecular associations and for conformational analysis has been widely demonstrated by some of the coauthors, − was employed for random generation and optimization at semiempirical level of theory of α–α′Py n SiOH ( n = 2, 3). Most stable structures were DFT-refined for conformational analysis.…”