Tunable reducibility of alkaline earth metal clusters for carbon dioxide and nitrogen molecule activation: a QM-QSPR study

Abstract

A hybrid approach combining ab initio computational techniques of quantum chemistry with a machine learning strategy was used to design and investigate BAe₃ (Ae = Be, Mg, Ca, Sr, and Ba) molecular clusters with strong reducing abilities. In these systems, the type of electropositive alkaline earth metal atoms was varied to tune the physicochemical properties of the resulting BAe₃ system. Both basic systems (built of three identical substituents, such as BSr₃) and mixed systems (containing various substituents, such as BCaMg₂) were considered. The BAe₃ clusters feature low ionization energies (IEs) and a highly delocalized singly occupied molecular orbital (SOMO). Among them, the BBa₃ cluster was identified as having the lowest IE here (3.82 eV), exhibiting the superalkali characteristic, which is smaller than that of any alkali [3.89 eV (cesium atom)]. The BAe₃⁺ thermodynamically stable closed-shell cations were shown to accommodate two electrons into their Rydberg orbitals, forming the double-Rydberg anions with electron binding energies in the 0.434–1.988 eV range. A mathematical model describing the dependence of IEs of BAe₃ clusters on their composition was developed. Different from the conventional formulation-assisted methodology, the quantitative structure–property relationship (QSPR) strategy predicts the reducing ability of a BAe₃ superalkali, where a suitable alkaline earth metal decreases the IE of the resulting BAe₃ cluster via the B–Ae and Ae–Ae electrostatic effects. Finally, the potential application of BAe₃ electron donors in the reduction of counterpart systems with low electron affinity (such as carbon dioxide or nitrogen molecules) was demonstrated. From the analysis of the binding energy between BAe₃ and the Y (Y = CO₂ and N₂) counterparts, as well as the charge transfer, and the geometry of BAe₃/Y systems, it follows that the resulting structures can be considered as either the [BAe₃][Y] complexes or the BAe₃Y compounds. It is shown that the IE and the dipole moment of BAe₃ determine the stability and geometry of the resulting BAe₃/Y species. The lower IE and larger dipole moment promote the reactivity of BAe₃ and result in the formation of stable, strongly bound compounds. These findings highlight how the structure and stability of the BAe₃/Y systems can be tuned upon single atom substitution and can be used to bond and remove toxic molecules from the environment.