Theoretical Exploration And Stability Research Of Novel 16 Valence Electrons Planar Carbon Structures

As a new anti-traditional structure,planar carbon molecules have attracted a lot of researchers'attention for their unique electronic structure and potential chemical properties.Since 1968,Monkhorst put forward the first hypothetical example of plane methane,namely plane tetracoordinated carbon?ptC?.The simplest penta-atom ptC,hexa-atom plane pentacoordinated carbon?ppC?and plane hypercoordinated carbon?phC?structures have been designed by various means and strategies,and the structures and properties of these novel molecules have been explored and researched deeply.Among them,some simple planar carbon structures are used as basic units,which are designed as metallocene framework,clusters or two-dimensional materials through polymerization or periodic conditions,and are used in new applications of excellent electronic,magnetic and optical properties.Among the strategies for designing planar carbon molecules,“Wade-Mingos rule”,4N+2/4N Hückel aromaticity/anti-aromaticity,“isolobal relationship”and 18valence electrons?ve?rule are included.In the applications of these strategies,“valence-isoelectronic replacement”strategy is essentially related to them.This strategy introduces one or more isoelectronic or isovalent elements,which can not only keep the similar topological structure but also obtain new clusters before and after replacement.The necessary conditions for designing and obtaining new planar carbon molecules are:1)there is such a planar carbon structure;2)these planar carbon molecules are global minimum?GM?structure,or at least local minimum?LM?structures.It is very important for experimental verification of an unconventional molecule by negative ion photoelectron.We have noticed that for the plane carbon molecule of the smallest cluster,take the penta-atom CAl₄²?as the prototype of stable ptC structure,researchers designed the most 17/18ve ptC molecules,some of which are not only obtained in theoretical calculation but also detected in experiments,confirming that such novel molecules can really exist;the second is the research of 12/14/15ve ptC.The acquisition of these novel molecules not only increases the number of members of ptC family,but also undoubtedly promotes the study of non-traditional molecular structures.In the meantime,some researchers also developed new strategies for designing planar molecules.However,the penta-atomic 16ve ptC has not been reported,which is due to the fact that the the methane-like CAl₄ follows the van't Hoff/Le bel rule and is generally recognized as having a tetrahedral carbon?thC?atom structure.Inspired by this issue,we wonder whether the the first GM penta-atomic ptC cane be obtained by means of theoretical calculation without changing the number of valence electrons.Therefore,this thesis has carried out the following research around this topic:1)CAl₃X?X=B/Al/Ga/In/Tl?with 16 valence electrons:can planar tetracoordinate carbon be stable?As a perpetual chemical curiosity,planar tetracoordinate carbon?ptC?that violates the traditional tetrahedral carbon?thC?has made enormous achievements.In particular,the 18-valence-electron?18ve?counting rule has been found to be very effective in predicting ptC structures,as in CX₄²??X=Al/Ga/In/Tl?.By contrast,the corresponding neutral CX₄ with 16ve each takes the thC form like methane.In the third chapter of this thesis,we report a mono-substituted neutral 16ve-CAl₃X?X=Al/Ga/In/Tl?.Our theoretical results showed that the competition between thC and ptC can be well tuned upon variation of X,and for X=In and Tl,the ptC structure becomes isoenergetic to and even more stable than thC,respectively.Thus,a low-lying ptC can be achieved in the 16ve-CAl₃X set without acquiring additional electrons.This unintuitive result can be ascribed to the increased energetic preference of the ionic sub-structure[CAl₃?]X⁺from X=Al to Tl.We thus predict the first penta-atomic ptC species with 16ve,and the ionic strategy presented in this work is expected to promote novel designs of ptC molecules.2)A sixteen-valence-electron carbon-group 13 family with global penta-atomic planar tetracoordinate carbon:ionic strategy.The design of planar tetracoordinate carbon?ptC?has always been a challenge due to its unique bonding mode that necessitates the perfect balancing between the carbon center and surrounded ligands both electronically and mechanically.A unique type of 18-valence-electron?18ve?template,i.e.,CAl₄²?,has been found to be very effective in designing various novel18ve-species upon skeletal substitution.In the fourth chapter of this thesis,we showed that though ptC is not the global structure for the parent 16ve-CAl₄,suitable skeletal substitution can allow for a series of global minimum ptC species.Theoretical calculations at the level of CCSD?T?/def2-QZVP//B3LYP/def2-QZVP for 35carbon-group 13 systems with 16ve,i.e.,CX_aY_bZ_cK_d?X,Y,Z,K=Al/Ga/In/Tl;0?a,b,c,d?4,a+b+c+d=4?,showed that 9 systems?CAl₃Tl,CGa₃Tl,CGa₂Tl₂,CAl₂GaTl,CAl₂InTl,CGa₂InTl,CAlGa₂Tl,CGa₂InTl and CAlGaInTl?possess global ptC and 2 systems?CAl₃In and CAl₂Tl₂?have quasi-GM ptC.Except for CAl₃Tl and CAl₃In,all the ptCs were predicted for the first time.All these stable ptC structures have the same skeleton and can be described as the same ionic sub-structure,i.e.,[A?]B⁺.This study not only enriches 16ve-ptC,but also directly demonstrates that utilizing an ionic strategy,non-ptC CAl₄ also can be used as a template to extend the ptC family.3)Mono-silicon Isoelectronic Replacement in CAl4:van't Hoff/Le Bel Carbon or Not?In cluster studies,the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology.The well-known penta-atomic 18 valence electron?ve?species CAl₄²?and its Al?/Si or Al/Si⁺isoelectronically replaced clusters CAl₃Si?,CAl₂Si₂,CAlSi₃?,and CSi₄²⁺,all possess the same anti-van't Hoff/Le Bel skeletons,that is,nontraditional planar tetracoordinate carbon?ptC?structure.In the fifth chapter of this thesis,however,we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl₄ with the traditional van't Hoff/Le Bel tetrahedral carbon?thC?and its isoelectronic derivatives CAl₃X?X=Ga/In/Tl?.At the level of CCSD?T?/def2-QZVP//B3LYP/def2-QZVP,none of the global minima of the 16ve mono-Si-containing clusters CAl₂SiX⁺?X=Al/Ga/In/Tl?maintains thC as the parent CAl₄ does.Instead,X=Al/Ga globally favors an unusual ptC structure that has one long C?X distance yet with significant bond index value,and X=In/Tl prefers the planar tricoordinate carbon.The frustrated formation of thC in these clusters is ascribed to the C?Si bonding that prefers a planar fashion.Inclusion of chloride ion would further stabilize the ptC of CAl₂SiAl⁺and CAl₂SiGa⁺.The unexpectedly disclosed CAl₂SiAl⁺and CAl₂SiGa⁺represent the first type of 16ve-cationic ptCs with multiple bonds.