Theoretical Studies On The Design And Stability For Novel Lanar Molecules

Planar chemistry is one of hottest research fields, and planar molecules haveattracted numerous attentions in both theory and experiment due to their novelstructural and electronic properties. Numerous planar molecules have been designed,isolated and characterized both experimentally and theoretically ever since the proposalof the "ptC" concept by Hoffmann et al. in1970s. The species of planar family wereenlarging constantly, including coordination number from tetra-coordination tohyper-coordination and central atom from carbon to the others. The continuousrealization of planar molecules not only greatly broadened human being's imaginationin molecular architecture, but also promoted bonding capability of the central atoms. Inthe case of planar carbon molecules, planar carbon structure have been realized byintrinsic electronic properties, generally speaking, such kind of planar-carbonmolecules is the cluster type, which possesses the features of simple and novelelectronic structure. In addition, as the effective "building block", planar carbonmolecules can organize macro-size novel cluster-assembled materials. Clearly, theexperimental detection in gas phase of any planar carbon molecule predicted in silico isnot easy and it could be successful only if the proposed form is one thermodynamicallyfavored. Based on the described above, the theoretical design and stability of planarcarbon molecules can provide important technique support to experimental exploration.The planarity of cluster-type ptC could only benefit from their intrinsic and uniqueelectronic properties, which makes the design and isolate of the class of ptC verydifficult and quite challenging. Bearing no bulky substitutions, the smallest ptC speciescannot benefit from steric effects for stabilization. Thus, an effective electronicapproach is the overwhelming driving force for achieving a ptC structure. Traditionally, the ptC structure has been devised by choosing suitable ligand atoms (usually of thesecond and third-row elements), which have similar atomic radii and similarelectronegativity. Such ''balanced'' bonding environments lead to the effectivedelocalization of the2pzlone pair at the central carbon and ligand–ligand interaction.The formed ptC in such a design strategy usually possesses appreciable Ï€-aromaticity,(quasi-) averaged structures in both the peripheral bonds and the central-atom-ligandbonds, and effective ligand–ligand interaction. By contrast, in our work, the general"localization" concept in organic chemistry has been effectively transplanted to exoticptC chemistry. Recently, based on the "localization" stabilization concept, i.e., thecentral carbon atom and one ligand atom X effectively form a highly localized multiplebond, we have been successfully designed a set of novel pent-atomic ptC species withC-X multiple bonds. Moreover, we constantly broadened "localization" electronicstrategy and enriched the kinds of planar centers and ligands. In addition, planar carbonmolecules formed by the binary carbon-boron systems were subsequently found to beunstable because the higher electronegativity of carbon compared to boron clearlydisfavors planar multiple-coordinate center form. We expect to promote thethermodynamic stability of such planar carbon molecules by the modification ofmolecular charge and heteroatom substitution and counterions. By means of oursystematic investigation on structural design and stability of planar molecules, wepresented important information for achieving more planar molecules, enriched planarchemistry and provided impressive theoretical evidence for further isolation andcharacterization in experiment. The main results are summarized as follows:(1) Based on "localization" electronic strategy, we first computationally designed aset of planar tetra-coordinate carbon (ptC) species, i.e.,[XCAl₃]q;[(X,q)=(B,-2),(C,-1),(N,0)] and XCAl₃(X=P, As, Sb, Bi). At the aug-cc-pVTZ-B3LYP, MP2and CCSD(T)levels, the designed18-valence-electron penta-atomic species are found to each possessa stable ptC structure, in which the central carbon atom and one electronegative ligandatom X effectively form a highly localized C X multiple bonding, converting the lonepair at the central carbon to a two-center two-electron Ï€-bond, indicated by thestructural, molecular orbital, Wiberg bonding, potential energy surface and Born-Oppenheimer molecular dynamics (BOMD) analysis. Moreover, our OVGFcalculations showed that the presently disclosed (yet previously unconsidered) ptCstructure of [C2Al₃] could well account for the observed photoelectron spectrum(previously only ascribed to a close-energy fan-like structure). Therefore,[C2Al₃] could be the first ptC that bears the highly localized C X double bond and has beenexperimentally generated. Notably, the pptC structure is the respective global minimumpoint for [BCAl₃]2,[NCAl₃] and XCAl₃(X=P, As, Sb, Bi), and the counterion(s)would further stabilize [BCAl₃]2and [C2Al₃]. Thus, these newly designed ptC specieswith interesting bonding structure should be viable for future experimentalcharacterization. The presently proposed "localization" approach well complements theprevious "delocalization" one, indicating that the general "localization vsdelocalization" concept in organic chemistry can be effectively transplanted to theexotic ptC chemistry.(2) Contrasting to the theoretically/experimentally characterized examples of pt/hC,the species containing pt/hN are much rarer. One possible reason is that the highlyelectronegative nitrogen generally prefers to form the "localized" bonding rather thanthe "delocalized" bonding. Through the extensive isomeric search of a series of groupV-based systems NXAl₃+(X=N, P and As) in both singlet and triplet electronic states atthe B3LYP/6-311+G(d) level, based on the "localization" stabilization concept, wereport a class of novel pptN with unique chemical bonding, i.e., the central nitrogen andthe connected ligand X (X=N, P and As) effectively form a highly "localized" N-Xmultiple bonding, as confirmed by the aug-cc-pVTZ-B3LYP and MP2calculations.The high-level CCSD(T)/aug-cc-pVTZ energetic calculations show that the three pptNspecies each have appreciable kinetic stability against structural transformation andfragmentation, which is confirmed by the Born-Oppenheimer molecular dynamicscalculations. In particular, the pptN isomer with X=P, i.e., NPAl₃+. is the correspondingglobal minimum. Thus, we propose that the three pptN isomers can be realized via themass spectroscopic techniques. Possible formation pathways of the three pptNs arediscussed. The present work demonstrates that the frequently used concept"localization vs delocalization" in organic chemistry can also be transplanted to the exotic planar chemistry like pptN.(3) Higher electronegativity of carbon compared to boron prefers to participate inlocalized2c-2e σ-bonding and clearly disfavors the planar multi-coordinate centralposition in the boron-carbon species. Furthermore, this factor makes the design andexperimental characterization of stable boron-based phC very difficult and quitechallenging. The only known exception is the simplest penta-atomic CB₄, for which thesinglet ptC structure is the global minimum CCSD(T)/cc-pVTZ//MP2/cc-pVTZ level.The result is contradictory with previous studies. Thus, by means of the extensivecalculations including the B3LYP, MP2and CCSD(T) methods with aug-cc-pVTZ andaug-cc-pVQZ basis sets as well as zero-point energy corrections, we reveal a quitedifferent picture. The singlet ptC structure does not constitute the global minimum ofCB₄. Instead, it lies1.4kcal/mol higher than a previously unrecognized tricoordinatecarbon singlet structure at the CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVTZ+ZPVE level. Kinetically, the ptC isomer possesses good stability both againstconversion and fragmentation. Moreover, our first theoretical investigation on theionized CB₄+unexpectedly reveals that the ptC structure is the global minimum,making CB₄+the first carbon-boron species with the global ptC structure. Strikingly, theCB₄+cluster ion has been detected in an early mass spectroscopic study.On the other hand, we show that by means of the cooperative Al-doping andcounterions, the stability of ptC/phC in C&B systems can be significantly promoted toeven be the global minima. A series of free and salt-stabilized CBxAly2(x+y=4;y≤2)species with18valence electrons were investigated for the first time. In the freeCBxAly2species, the ptB instead of ptC is the global minimum, in agreement with theresults of Boldyrev and Wang. Yet, the thermodynamic preference of the planar centralcoordination of carbon increases along with the doping of Al atoms and thealkali/alkaline-earth counterions. Moreover, the effective interaction of counterionswith the central carbon gestated the planar penta-coordinate carbon (ppC) structures.Noticeably, the ppC structure is the global minima in the [CB3Al2]Ca2+and[CB2Al22]M2+(M=Be, Mg, Ca) species. The present work demonstrates that theAl-doping and counterions can cooperatively stabilize the planar central coordination of carbon in C&B systems. The new salt-stabilized ppCs as global minima arepromising candidates for future experimental detection.(4) To date, the thermodynamically stable phC is still scarce with reference to thenumerous ptCs. We present in the work a systematical theoretical investigation on thepossibility of ppC centers in the complexes of the decorated Al4C cluster by group4transition metal difluorides MF2(M=Ti, Zr, Hf). By extensive potential energy surfaces(PES) search and high-level single-point calculations of the (Al4C)MF2(M=Ti, Zr, Hf)species reveal that the most stable structure has a C2v-symmetry ppC center at theCCSD(T)//B3LYP, CCSD(T)//M06and CCSD(T)//PBE0levels with the def2-TZVPbasis set. Born-Oppenheimer molecular dynamics (BOMD) simulations indicate thatthe novel global ppC species have reasonable kinetic stability. The complexes(Al4C)MF2with global ppC waits for future experimental detection.