Introduction to Cluster Chemistry and Magnesium Clusters
Clusters, consisting of a defined number of atoms, form the bridge between atomic-level entities and macroscopic materials. Due to their distinct properties, clusters are essential for various chemical processes and the development of new materials. In particular, metal nanoclusters, such as gold and silver clusters, have been crucial for catalysis and nanotechnology applications. Among these, the study of magnesium-based clusters has been growing due to their hydrogen storage, antioxidant properties, and their structural diversity.
Magnesium clusters, specifically, exhibit varying characteristics when doped with atoms like Si, Pd, Ga, or F. This work, however, primarily focuses on the fluorine-doped magnesium clusters FMgn. The doping of fluorine atoms onto magnesium clusters is particularly interesting because magnesium and fluorine form a stable chemical compound MgF2, commonly used as a flux in metal preparations. However, despite the well-known interaction between magnesium and fluorine, the behavior of fluorine atoms in small magnesium clusters has not been studied extensively. This study aims to fill this gap by exploring the structural, electronic, and chemical properties of FMgn clusters.
Computational Approach: CALYPSO Software and DFT Calculations
The CALYPSO software was used to predict the initial structures of FMgn clusters. This particle swarm optimization-based algorithm is effective in generating a variety of cluster structures by searching for possible stable configurations. Following this, DFT calculations were performed to optimize the lowest energy isomers of FMgn clusters. The B3LYP functional and 6-311g(d) basis set were employed for structural optimization, and vibrational frequency calculations were performed to ensure the structures corresponded to the lowest energy states i.e., not excited states.
To gain a deeper understanding of the electronic structure of FMgn clusters, Charge Transfer and Natural Bond Orbital analysis were conducted. Additionally, the UV-Vis spectra were computed using Time-Dependent Density Functional Theory to offer insights into the potential experimental spectroscopic signatures of FMgn clusters. Chemical bonding analysis was performed using Atoms-in-Molecules theory, focusing on Laplacian of Electron Density Δρ, Electron Localization Function, and Interaction Region Indicator (IRI) to study the F–Mg and Mg–Mg bond characteristics.
Structural Diversity and Stability of FMgn Clusters
One of the most striking findings in this study is the structural diversity of FMgn clusters. For clusters with sizes greater than FMg4, most structures exhibit three-dimensional geometries, where the fluorine atom is located on the outermost layer, surrounded by two or three magnesium atoms. For example, the structure of FMg6–FMg20 clusters is characterized by low symmetry C1, suggesting that the system prefers to adopt lower symmetry to minimize energy and maximize stability.
The FMg18 cluster, in particular, was identified as having high relative stability among all FMgn clusters. To evaluate the stability of these clusters quantitatively, key parameters such as binding energy Eb, second-order energy difference Δ2E, and HOMO-LUMO energy gap Egap were analyzed. These metrics are essential for determining the magic number of clusters and understanding the relative stability of clusters as their size increases.
Bonding Analysis: F–Mg and Mg–Mg Interactions
Chemical bonding in FMgn clusters was investigated by examining the nature of the F–Mg and Mg–Mg interactions. Laplacian of electron density Δρ and Electron Localization Function were used to explore the bonding characteristics at critical points. The analysis confirmed that the F atom is primarily stabilized at the outer layer of the Mg cluster, where it interacts weakly with a few Mg atoms, two or three. The Mg-Mg bonds, on the other hand, are more robust, forming a framework that stabilizes the cluster's overall structure.
The bonding topology of FMgn clusters is unique because, unlike other doping atoms such as Si or Pd, the F atom does not get trapped inside the magnesium cluster but remains at the periphery. This gives rise to a distinctive bonding environment that requires further investigation for understanding the mechanisms of stability in these clusters.
Spectroscopic Features and Future Experimental Work
To complement the theoretical insights, IR, Raman, and UV-Vis spectra of the FMgn clusters were computed. These spectra serve as guidelines for future experimental studies, where the spectral data can be used to verify the predicted electronic and structural properties of FMgn clusters. UV-Vis absorption spectra, in particular, can help identify the unique optical properties of these clusters and provide insights into their potential applications in areas such as nanotechnology, sensing, and catalysis.
Conclusion
The study of fluorine-doped magnesium clusters (FMgn, n = 2–20) provides valuable insights into their structural, electronic, and chemical bonding properties. The results suggest that these clusters exhibit a rare structural diversity, with the F atom positioned at the outer layer, surrounded by a small number of Mg atoms. The clusters are highly stable, especially FMg18, which shows the highest stability. Theoretical calculations of spectra and bonding provide key insights into the potential experimental verification of these clusters. Further experimental work will be essential to confirm these findings and to explore the practical applications of FMgn clusters in various fields.
