Theoretical And Experimental Studies On Elemental Boron Clusters And Cationic Aluminum Carbonyl Complexes

Owing to the electron-deficiency of boron,elemental boron clusters and boron compounds possess unique structures and exotic chemical bonding.In the past 30 years,systematic gas-phase spectroscopic experiments in combination with computational chemistry have been carried out to elucidate the structures and chemical bonding of elemental boron clusters,uncovering a whole flatland of planar or quasi-planar（2D）clusters.It is now known that boron clusters and low-dimensional nanostructures are composed of boron double chains and a triangular B₃ ring serves as their basic structural unit.A B₃ ring is normally considered to be aromatic,which is associated to delocalized bonding,in line with boron’s electron-deficiency.Triangular B₃ sites are physically（that is,geometrically）indistinguishable in a boron cluster.Can such sites be distinctly different in term of aromaticity,antiaromaticity,and non-aromaticity?Or,are all boron clusters aromatic?This remains a fundamental issue in physical chemistry.Boron clusters are composed of an outer periphery and one or more interior atoms.Chemical bonding analyses have shown that all peripheral boron atoms are bounded by classical2-center 2-electron（2c-2e）Lewis bonds,whereas the interior atoms are held together by delocalized bonds,due to boron’s electron deficiency.There are both delocalizedπandσbonds in 2D boron clusters,giving rise to the concepts of multipleπ/σaromaticity or conflicting aromaticity.For certain 2D boron clusters,an analogy to polycyclic aromatic hydrocarbons（PAHs）may be established.However,it remains unclear if all2D boron clusters are really aromatic.In this thesis,selected 2D boron clusters(D_2h B₆^2-,C_2h B₂₈^2-,and C_2v B^-₂₉)are chosen to address a number of fundamental issues in chemical bonding,aromaticity,and boron chemistry.The change in Wiberg bond index（WBI）from global-minimum（GM）to transition-state（TS）structures for radial B-B links in the prototypical Wankel moter B^-₁₉ cluster is developed as a semi-quantitative measure of dynamic structural fluxionality,which helps rationalize the low energy barrier for rotation movement.Comprehensive chemical bonding analyses are performed and chemical analogy to PAHs established for bowl-like B₃₆ and B₅₀ clusters,offering new or updated bonding models for such complicated chemical systems.Infrared photodissociation spectroscopy is applied to produce and characterize cationic aluminum carbonyl complex clusters.The main contents of this thesis are as follows:1.Wankel motor B^-₁₉ cluster:inner 2π/6σand outer 10π/14σconcentric four-fold aromaticity and an insight into its dynamic structural fluxionality.We offer herein an updated chemical bonding model for Wankel motor C_2v B^-₁₉ GM cluster on the bases of canonical molecular orbital（CMO）analysis and adaptive natural density partitioning（Ad NDP）,further aided by natural bond orbital（NBO）analysis and orbital composition calculations.The computational data indicate that B^-₁₉ cluster has concentric four-foldπ/σaromaticity.Being spatially isolated from each other,the inner B₆ disk supports 2πand 6σsubsystems,whereas the outer double-ring ribbon has 10πand 14σsubsystems.All fourπ/σsubsystems are intrinsically delocalized and conform to the（4n+2）Hückel rule for aromaticity.The structural evolution of B^-₁₉ cluster for in-plane rotation is shown to be extremely delicate.The change of Wiberg bond index（△WBI）from GM to TS for radial B-B links is minimal and uniform,which offers a semi-quantitative measure of structural dynamics and underlies the low energy barrier.2.Counter examples of aromatic boron clusters:island or globalπantiaromaticity.The flatland of 2D boron clusters is believed to possess aromaticity for all members,which remains a fundamental issue in debate in boron chemistry.Using a selected set of D_2h B₆^2-,C_2h B₂₈^2-,and C_2v B^-₂₉ clusters as counter examples,we present computational evidence for global or island?antiaromaticity in 2D boron clusters.Chemical bonding in the clusters is elucidated collectively on the bases of CMO analysis,Ad NDP,electron localization functions（ELFs）,and localized molecular orbital（LMO）analysis.These analyses are complementary to each other and yet highly coherent.As a quantitative indicator,nucleus-independent chemical shifts（NICSs）are calculated at selected specific points in the clusters,which differentiate?aromaticity,nonaromaticity,and antiaromaticity.This phenomenon can be understood in analogy to hydrocarbons and PAHs.Indeed,even perfect sheet-like boron clusters are readily convertible to polycyclic systems,suggesting that a notion such as“all boron clusters are aromatic”is moot.3.Bowl-like B₃₆ cluster:inner 6?and outer 18?double aromaticity and reason for the preference of hexagonal hole in central location.Bowl-shaped C_6vB₃₆ cluster with a central hexagon hole is considered an ideal molecular model for low-dimensional boron-based nanosystems.Herein we offer an in-depth bonding analysis via CMOs and Ad NDP,further aided with NBO analysis and orbital composition calculations.The concerted computational data establish the idea of concentric double?aromaticity for B₃₆ cluster,with inner 6?and outer 18?electron-counting,which both conform to the（4n+2）H（?）ckel rule.The updated bonding picture differs from existing knowledge of the system.A refined bonding model is also proposed for coronene C₂₄H₁₂,of which B₃₆ cluster is an inorganic analogue.It is further shown that concentric double?aromaticity in B₃₆ cluster is retained and spatially fixed,irrespective of migration of the hexagonal hole;the latter process varies the system energetically.Hexagonal hole is found to be a destabilization factor forπ/σCMOs.The central hexagon hole affects substantially fewer CMOs,thus making bowl-shaped C_6v B₃₆cluster the GM structure.4.Concentric（10π+22π）dual aromaticity in bowl-like B₅₀ cluster and its analogy to ovalene C₃₂H₁₄.Owing to the electron-deficiency of boron,chemical bonding in bowl-like B₅₀ cluster is intriguing and yet complicated.We have carried out chemical bonding analyses via CMO and Ad NDP,which are further aided with NBO analysis,orbital composition analysis and NICS calculations.The computational data help establish the scheme of concentric dual?aromaticity for B₅₀ cluster,with inner10?and outer 22?electron-counting,which both conform to the（4n+2）Hückel rule.A bonding model for ovalene C₃₂H₁₄ is also proposed,of which B₅₀ cluster is an inorganic analogue.This bonding pattern is closely similar to that of bowl-like B₃₆cluster（see above）,suggesting that concentric dual?aromaticity is probably a rather general bonding concept in 2D boron clusters.We anticipate that this concept may be extended to further examples of 2D boron systems.5.Infrared photodissociation spectroscopy of cationic aluminum carbonyl Al（CO）⁺_n（n≤5）clusters.Aluminum carbonyl cation complexes,Al（CO）⁺_n（n≤5）,are generated in the gas phase using the laser vaporization technique.They are characterized via infrared photodissociation（IRPD）spectroscopy and density functional theory（DFT）calculations.The mass spectrum shows that Al（CO）⁺₃is the strongest in intensity,suggesting relatively high stability.The Al（CO）⁺₄and Al（CO）⁺₅cations are confirmed experimentally to possess very weakly bound CO ligands and involve an Al（CO）₃⁺core.The Al（CO）⁺₃cluster is predicted to assume a quasi-planar C_3v structure,whose Al center maintains a favorable 8-electron configuration.Chemical bonding is elucidated collectively via CMOs and Ad NDP,further aided with NBO analysis and orbital composition calculations.The computational data indicate thatσ-donation andπback-donation play an important role in stabilizing this complex system,similar to transition metal carbonyls.