Bonding Study in All-Metal Clusters Containing Al₄ Units

Abstract

The nature of the bonding of a series of gas-phase all-metal clusters containing the Al4 unit attached to an alkaline, alkaline earth, or transition metal is investigated at the DFT level using Mulliken, quantum theory of atoms in molecules (QTAIM), and Hirshfeld iterative (Hirshfeld-I) atomic partitionings. The characterization of ionic, covalent, and metallic bonds is done by means of charge polarization and multicenter electron delocalization. This Article uses for the first time Hirshfeld-I multicenter indices as well as Hirshfeld-I based atomic energy calculations. The QTAIM charges are in line with the electronegativity scale, whereas Hirshfeld-I calculations display deviations for transition metal clusters. The Mulliken charges fail to represent the charge polarization in alkaline metal clusters. The large ionic character of Li-Al and Na-Al bonds results in weak covalent bonds. On the contrary, scarcely ionic bonds (Be-Al, Cu-Al and Zn-Al) display stronger covalent bonds. These findings are in line with the topology of the electron density. The metallic character of these clusters is reflected in large 3-, 4- and 5-center electron delocalization, which is found for all the molecular fragments using the three atomic definitions. The previously reported magnetic inactivity (based on means of magnetic ring currents) of the pi system in the Al42- cluster contrasts with its large pi electron delocalization. However, it is shown that the different results not necessary contradict each other.

A Critical Assessment of the Performance of Magnetic and Electronic Indices of Aromaticity

“…For inorganic clusters, we have shown that in some cases the trends provided by NICS and MCI are divergent. Furthermore, the fact that the π-component of the MCI in Al 4 2-is almost the same as that of C 4 H 4 2+ seems to indicate an apparent good behavior of MCI for allmetal clusters [50]. Moreover, we showed that electronic indices such as multicenter indices are superior to NICS to quantify aromaticity in classical organic aromatic systems [73,117].…”

“…For the moment, the most widely used methods to discuss aromaticity in inorganic clusters are the basic electron counting based on the 4n + 2 Hückel's rule [41][42][43][44] and the magnetic-based indicators of aromaticity, in particular, the nucleus-independent chemical shifts (NICS) [45]. In some cases, electronic multicenter indices (MCI) [37,[46][47][48][49] have also been used [50][51][52].…”

Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules

“…Of particular importance is the excellent linear correlation of the NICS zz (0) values of the four-membered rings of 13 group atoms with the R av values of the rings (Figure 7). [73] and subsequently justified by the use of different criteria of aromaticity, such as the induced magnetic field analysis [74], the aromatic ring current shielding (ARCS) [39,75], the ring current maps [76][77][78][79], NICS [80], resonance energy (RE) estimations [39,81], valence bond (VB) calculations [82], bifurcation analysis of electron localization function (ELF) [83], conceptual DFT descriptors [84], and quantum theory of atoms in molecules (QTAIM) calculations [85]. It should also be noticed that Sundholm et al 4-cluster exhibits two maxima at a distance of 0.9 Å above and below the molecular plane (Fig.…”

Structural stabilities and electronic properties of Mg28-nAln clusters: A first-principles study

“…We analyze the structural evolution of Mg 28-n Al n (n=1 4, 24 27) and conduct over 20 different isomers as starting geometries for Mg Al clusters with n=5 13, 15, 17, 19, 21, 23. The CALYPSO method is also used to conduct a global structure search for n=2 5,7,10,11,14,20,22,24,26.…”