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Title: The boron conundrum : the viability of large hollow boron cages
Other Titles: Het raadsel van de boor allotropen: de haalbaarheid van grote kooimoleculen van boor
Authors: Tshishimbi Muya, Jules-Christophe; S0187315
Issue Date: 20-Dec-2012
Abstract: The boron conundrum: the viability of large hollow boron cages<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">In this doctoral thesis we investigated withquantumchemical methods the viability and properties of new inorganicfullerenes based on boron as alternative to carbon fullerenes starting from thearchetypal molecule boron buckyball, with n=80, up to 160. Two proposalschallenge the current research of large boron nanoclusters, namely the hollowand stuffed boron cages. Large boron clusters are only observed in silico,and the expected synthesis of boron fullerenes is a remote possibility. We havefocussed our study on large hollow boron cages using density functional theoryand group theory.<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">From molecular symmetrical analysis, the boronbuckyball B80 is seen as a spherical network of 80 boron atoms,which has a shape similar to the celebrated C60. The 80 borons spantwo orbits: while the first contains 60 atoms localised on the vertices of atruncated icosahedron like C60, the second includes 20 extra boronatoms capping the hexagons of the frame. Its geometry is slightly distorted fromIh to Th symmetry. There is notyet a benchmark study on more reliable functional for large boron clusters. Wehave used the most popular functional B3LYP at SVP and 6-31G(d) level and haveanalyzed the chemical bonding in B80, the symmetry of B80and B80+, the possibility of methyne substitution and thestability of endohedral boron buckyball complexes. Further we have studied theregioselectivity of B80 towards nucleophilic or electrophilicreactants, investigated the ability of this molecule in the transport ofcurrent, and have proposed some principles that underly the formation of largeboron fullerenes.<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">The chemical bonding analysis has revealed a perfectmatch between the occupied molecular orbitals representations in leapfrog B80and C60. The cap atoms transfer in part their electrons to thetruncated icosahedral frame, and contribute essentially to the formation of &#963;bonds. The frontier molecular orbitals have &#960; character and are localised onthe B60 truncated icosahedral frame. The replacement of boron capatoms by methyne (CH) groups in T and Th symmetries has yielded twostable endo methyne boron buckyballs, namely endo-B80-x(CH)x,with x= 4,8. The stability of these compounds was found to originate from theformation of six boron 4-centre bonding motifs in between the substitutedhexagons. These localized bonding motifs appear to be at the basis of theobserved symmetry lowering, via a pseudo-Jahn-Teller effect. The carbon andhydrogen atoms in the two endohedral fullerenes were subsequently replaced byother atoms, which are able to form cubane or tetrahedral endohedral boron fullerenes.(Chapter3)<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">A detailed analysis of the second-order Jahn-Tellerdistortion in leapfrog B80 offers some comprehensive understandingwhy the symmetry of B80 falls from icosahedral to tetrahedralsymmetry. Low-lying occupied orbitals are found interacting more strongly thanfrontier molecular orbitals. The weak PJT energy calculated is associated withthe large energy gaps between active occupied and virtual molecularorbitals.(Chapter 4)<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">A removal of one electron in the HOMO of hurepresentation yields partially filled fivefold degenerate-orbitals. Themonocation boron buckyball formed is subject to Jahn-Teller, its icosahedralstructure undergoes a symmetry breaking towards S6 point group. Asuperposition of Jahn-Teller and pseudo-Jahn-Teller effects in this ion isidentified as responsible for this distortion. The imaginary mode of ggrepresentation in the neutral boron buckyball is identified as the main modebeing at the basis for this unusual distortion mechanism outside the epikernelsspace. (Chapter 5)<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">Theoretical study on the regioselectivity of B80and on the encapsulation of small bases molecules, tetrahedral and cubane likeclusters of Group V atoms in B80 shows that the boron buckyball is ahard acid and prefers hard bases like NH3 or N2H4,to form stables off-centred complexes with B80. Tetrahedral andcubane like clusters of this family are usually metastable in the encapsulatedstate, due to steric strain. The most favorable clusters are mixed tetrahedraland cubane clusters formed by nitrogen and phosphorus atoms such as P2N2@B80,P3N@B80 and P4N4@B80.The boron buckyball has an amphoteric character with the boron cap and theboron frame atoms acting as electrophilic and nucleophilic centres,respectively. (Chapter 3, Chapter 6)<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">The ring current calculations in B80 andin C60 have shown an extension in analogy between leapfrog carbonfullerenes with leapfrog boron fullerenes. The magnetic induced currentsproperties identified in C60 and B80 have revealed thatin both compounds the currents arise from the same orbitals patterns. However,by comparison B80 was suggested to conduct current better than C60.(Chapter 7)<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">A detailed analysis on chemical bonding in large setof boron fullerenes with partially and totally capped pentagons have yieldedfollowing key factors that are stabilizing boron large clusters: omnicapping ofpentagons, homogenous distribution of caps, formation of B10 motifspreferably fused to B16, isolation of empty hexagons and caps, andflakes formation of alfa and gamma-sheet. (Chapter 8-9)<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">A deep analysis of all isomers with pentagons<br>totally and partially capped of the boron buckyball B80 extended tolarge hollow B-fullerenes combined with a careful inspection of multiple-shellsboron type clusters with an outer shell completely filled are needed for abetter understanding of the chemical bonding principle of large boron clusters.This may also highlight the way boron tends to compensate its electrondeficiency in large bare boron nanoclusters.<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US">The core-shell clustersconstructed on basis of the icosahedral B12 are on a differentgrowth path presumably leading to the solid state structures, and leave no roomfor alternative bonding principles, that are analogous to the fullerenes in thecase of elemental carbon. In the course of some geometry optimization migrationof cap atoms to endohedral sites with additional bonding was observed, whichmay point to a pathway of collapse of cages to solid clusters.<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt">From the methodological point of view, in the searchof the most stable multiple shells boron clusters, one way may consist of <span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US">filling systematically the outer shell of the most stable B80core-shell with extra boron atoms to get a complete shell. The hollow dimensionof fullerene B80 cage is sufficient to encapsulate an icosahedron B12and may also be used as outer shell of the core Ih B12.Several envelops or outer shells with Bn fullerene-like geometry canbe used to encapsulate the icosahedron B12 differing in shape by theway caps are distributed on their spherical surface. To take advantage of highsymmetry, the vertices of the icosahedron B12 can be orientedtowards the centre of the twelve caps located on the centre of the pentagons ofthe truncated icosahedral frame. This will keep the Ih symmetry, andcomplies with our observation that capping pentagons yield more stablestructures than capping hexagons. The remaining caps can be distributed onhexagons following our revealed key factors of stability for B-fullerenes suchas homogeneous distribution of caps, isolation of empty hexagons, formation ofB10 or B16 units. The holes density of 1/9 criterionobserved in hollow boron cages appears to be not convenient for multiple-shellclusters. One should also investigate the geometries of the remaining boronclusters gap ranging in between B23 and B80 cluster inorder to understand the boron clusters growth mechanism.<span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt"><span style="font-size:10.0pt;mso-bidi-font-size:12.0pt;mso-ansi-language:EN-US" lang="EN-US"><span 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Publication status: published
KU Leuven publication type: TH
Appears in Collections:Quantum Chemistry and Physical Chemistry Section

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