Novel tools for the analysis of non-standard chemical bondsTheoretical insight into the nature of beryllium bonds

Resumen

The chemical bond is among the oldest and most important concepts in chemistry because it allows describing chemical properties of a system, as well as to understand and predict chemical reactions. The main goal of this Ph.D. thesis is to study new types of chemical interactions involving the beryllium atom. Be atom has a rich chemistry due to its low-lying pBe orbitals, but its high toxicity limited the number of experimental studies. Thus, enhancing the importance of theory in the description of Be compounds. This thesis reports the theoretical analysis of three new types of Be bonds using high-level ab-initio and Density Functional Theory, and the applications of the Total Position Spread Tensor (TPS) to molecular systems. First, the non-covalent Beryllium Bonds (BerBs). This type of bond is formed by an interaction between a Be moiety acting as a strong Lewis Acid (LA) and a Lewis Base (LB). The interaction between Be-LA and fluorine derivatives (FR) generates a hole in the fluorine atom, otherwise not possible, opening the possibility to design new materials where fluorine binds through halogen bonds. This same interaction decreases the F-R bond energy, turning the F-R homolytic dissociation in an exothermic process. Therefore, BerBs can be used to produce spontaneously radical species. Intramolecular Beryllium Bonds (IBerBs) were studied in malonaldehyde- and tropolone-like systems. This intramolecular interaction becomes stronger in unsaturated systems, because the LA and the LB are more acidic and basic, respectively. Second, the intramolecular Be-Be bond in disubstituted naphthalene complexes. The anion species of 1,8-diBeX-naphthalene derivatives show a very strong one-electron Be-Be bond. This bond is eight times stronger and 0.5 Å shorter than in the isolated dimer. These systems show high electron affinities, which gives to them an exceptional property: the ability to behave as what we have named anion sponges. It was found that the interaction between anions and Be disubstituted naphthalene is among the highest anion affinities reported in the literature for neutral compounds. This property could lead to a wide range of applications as anions receptors and sensors. Third, the interaction between the Be2 molecule and Lewis bases, which has shown to enhance the strength of the Be-Be bond with respect to the isolated molecule. The non-covalent interaction in complexes of the type L: Be-Be: L decreases the distance and increases the strength of the Be-Be bond. The effect over the Be2 moiety depends on the nature of the Lewis bases. The most dramatic effect occurs when L is a radical species. The Be-Be bond in this type of complexes is among the strongest reported in the literature due to the increase of the oxidation state from Be2 to Be2+. The same effect is found in ligands with L orbitals, which conjugates with the Be orbitals increasing the oxidation state of the Be2 moiety to +1. Therefore, the Be-Be interaction becomes stronger, although remaining weaker than that of complexes with radical species. Lastly, the complexes where the Be2 moiety interacts with the lone pairs of the LB. Be2 in this type of complex remains neutral, and to the best of our knowledge, these complexes show the strongest Be-Be bond reported for the neutral dimer. Finally, the TPS is proposed as a new method describing chemical bonds. The interest in chemical bonding is not only related to the discovery of new interactions, but also to the development of new theoretical tools to characterize it. The TPS is a new quantity that describes the electron and spin fluctuation when a system is perturbed. In this PhD thesis, the TPS is applied to diatomic molecules and to Be-carbonyl derivatives. The tensor shows a different behavior depending on the type of interaction, thus allowing the distinction between a covalent, ionic, charge-shift, and other types of bonds, and at the same time illustrates the nature of the electron correlation of the system.