Una perspectiva mecánica sobre la estabilidad del enlace químico (a mechanical insight into the stability of the chemical bond)

Resumen

This PhD Thesis was devoted to demonstrating that the nature and intrinsic features of a given pairwise interaction (covalent, electrostatic or van der Waals) defines a suitable framework to relate both molecular and bulk properties. To achieve this goal, we have considered classical, well-established concepts from the field of condensed matter and translated them into the molecular realm, and vice versa. Among the several concepts studied, special attention deserves the applicability of the spinodal or mechanical stability limit to chemical bonds making possible to predict the bond rupture distance of a chemical bond from experimentally accessible equilibrium properties: the dissociation energy and the stretching force constant at the equilibrium distance. Our model has been corroborated using theoretical methodologies based on the topological analysis of electron density and related scalar fields, we have demonstrated how the mechanical stability limit of a bond is also reflected as bonding electronic changes. The chemical implications of this idea have strong impact in diverse fields, including the nature of the chemical bond, reactivity, mechanochemistry, or synthesis of novel materials by predicting which compounds can be synthesized or not, and the conditions under they would be (meta)stable. As an example, in this PhD Thesis we show how C-C covalent bond cannot be elongated far beyond 2 Å, explaining the constancy of the transition state distances of typical C-C forming reactions found in the literature. Likewise, based on the same argument we have postulated the chained-interactions conjecture. Essentially, it establishes that the mechanical and electronic stability conditions of the successive interactions are overlapped giving rise to a covalent¿electrostatic¿van der Waals sequence of interactions, similar to that described by the electronic density. Under this view, we have classified and determined the optimal distances to stabilize the different interactions demonstrating which are the maximum and minimum length at which O-H hydrogen bonds can be extended. All the results summarized above and based on the spinodal hypothesis, have underlaying evidenced that the effects exerted by a chemical or mechanical interaction are essentially analogous and, therefore, molecules and solids can be understood in terms of positive and negative pressure regions. Using a recent methodology called DFT-Chemical Pressure, we have shown how these pressures regions define areas totally consistent with the molecular bond and lone pairs, in clear analogy with the valence shell electron pair repulsion (VSEPR) theory. Indeed, considering that a positive pressure is like a repulsion and a negative pressure as an attraction, we have quantified the activity of the lone pairs, and related them to the electronegativity of the atom to which they are attached and to their corresponding potential reactivity. Finally, we have analyzed under this perspective, several inorganic structures and we have proposed a generalized stress¿redox equivalence that is able to account for the two well¿established phenomena observed in Solid State Chemistry: (i) the expansion or contraction experienced by the metal structure after hosting the nonmetallic element while its topology is maintained and (ii) the increasing or decreasing of the effective charge associated with the anions in inorganic compounds with respect to the charge already present in the interstices of the metal network. Both are intrinsic mechanisms to the metal sublattice and can be understood as an equalization of the electronegativity between the metal sublattice and the anion (guest).