| Summary: | 碩士 === 中國文化大學 === 化學系應用化學碩士班 === 103 === In this study, we investigated the structures and bonding properties of Si13 and Si14 based early alkaline earth metal (AEM) binary clusters (AEM = Be, Mg, Ca), including their exohedral and endohedral isomers. BeSi14, MgSi13,14 and CaSi13,14 were found not to have stable caged structures (AEM@Si13,14). Therefore, we also only searched for the conversion pathways from the most stable BeSi13 adsorptive precursor to the Be@Si13 at the global minimum of potential energy surface. Geometric relaxations//single point energies were carried out at the B3LYP/6-311G*//B3LYP/6-311++G(3df) level, which is a common strategy to speed up optimizations while allow us to obtain finer cluster energies; the same method was applied to transition-state search. Hirshfeld charges (HCs) were to show atomic charge partition in clusters; Mayer bond orders (MBOs) were employed to analyze bond strength and Mayer total valences (MTVs) were to represent the bonding capacity of an atom.Our results showed that Be is commonly hypervalent with MTV > 3, higher than the expected value 2, especially in those endohedrons. HCs and molecular orbitals indicated that back-donation from hosts facilitated such hypervalency.
MgSi13,14 and CaSi13,14 have less covalent characteristic, while Ca-Si bonds are more ionic than Mg-Si. The consequence was that BeSi13,14 have the highest binding energies and MgSi13,14 have the lowest, CaSi13,14 are in between when the total numbers of atoms are same. The ionicity in CaSi13,14 must also contributes to cluster stability as the covalency in BeSi13,14, and sometime can give a binding energy pretty close to that of BeSi13,14. Finally, we also used the QST3 and intrinsic reacrion coordinate methods to locate and confirm the transition state between the most stable adsorptive BeSi13 precursor and the most stable encaged Be@Si13 product.
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