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Theoretical Studies On The Structures And Properties Of Endohedral Metallofullerenes

Posted on:2020-10-25Degree:MasterType:Thesis
Country:ChinaCandidate:Y LiFull Text:PDF
GTID:2481306464490554Subject:Materials Physics and Chemistry
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Actinide metallofullerenes are hybrid molecules with fullerenes trapping different actinides.They not only inherit the thermal stability of the fullerenes,but also have the attractive physicochemical properties of the actinide elements.Theoretical calculation is an effective method for evaluating the stability and reactivity of metallofullerenes,predicting their chemical and physical properties as well as potential applications.In this thesis,a series of actinide metallofullerenes were calculated by using the density functional theory(DFT)to explore their electronic structures,bonding patterns,dynamic behavior,theoretical spectra,and potential applications.First,DFT calculations characterized the structure of recently synthesized Th@C74 as Th@D3h(14246)-C74.The results showed that the thorium atom adopts an unusual off-axis position inside cage due to small metal ion size and the requirement of large coordination number,which phenomenon was further extended to other Th-based metallofullerenes.In addition,besides the strong metal-cage electrostatic attractions,topological and orbital analysis revealed that all the investigated Th-based metallofullerenes exhibit obvious covalent interactions between metal and cage with substantial contribution from the Th-5f orbitals.The encapsulation by fullerenes is thus proposed as a practical pathway toward the f-orbital covalency for thorium.Interestingly,the anomalous internal position of Th led to a novel three-dimensional metal trajectory at elevated temperatures in the D3h-C74 cavity,as elucidated by the static computations and molecular dynamic simulations.Second,we further studied a series of Th@C2n(2n=64-88)covering different sizes and types by means of DFT calculations.We found that the tetravalent thorium atom mainly coordinates to three pentagonal rings with the metal-pentagon interactions independent on the distribution and distance among these pentagons.This coordination pattern is not only in sharp contrast to that of common organometallic complexes,where four pentagons are indispensable for stabilizing Th(IV),but also different from that of Ti-containing fullerenes,whose valence state highly depends on the pentagon distribution.The specificity of Th-based metallofullerenes was rationalized by the synergetic effect of small metal ion size,low electronegativity,strong metal-cage electrostatic attractions and effective orbital overlap between the metal and cage orbitals.Our work highlights the role of cage pentagons in the Th-cage interactions,and points out the fundamental difference between endohedral metallofullerenes and common organometallic complexes.Finally,based on the U2C@Ih-C80 reported in recent experiments,DFT calculations were performed for a series of U2C@C2n(2n=60-104)analogues.The results showed that the U-C-U bond angles gradually increase from 96.9°in Ih-C60 to 180.0°in D3d-C104,which is caused by not only the cage size but also the cluster-to-cage charge transfer.At the same time,the change of bond angle closely correlates with the charge and hybrid state of internal atom.Significantly,the U2C unit always features two 3c-2e bonds regardless of its size,shape and charge state.
Keywords/Search Tags:Density functional theory, Endohedral metallofullerenes, Actinide elements, Structural characterization, Bonding nature
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