On The Structures,chemical Bonding,and Molecular Dynamics Of Boron-Based Nanoclusters

Boron,carbon's lighter neighbor in the periodic table,is a typical electron-deficient element.The small radius,high ionization energy,and large coordination number of boron govern the unique geometries,chemical bonding,and dynamic fluxionality of boron-based clusters.Over the last 20years,a series of experiments and quantum chemical calculations have shown that boron clusters favor unique planar or quasi-planar structures over a wide range of sizes,which help establish a solid theoretical foundation for fabricating novel boron-based nanomachines.In this thesis,we have systematically studied the geonetries,chemical bonding,and dynamic fluxionality of elongated B^-₁₁,B₁₁,and B⁺₁₅ clusters at the density functional theory?DFT?and coupled-cluster with single,double,and perturbative triple excitations?CCSD?T??levels,and proposed for the first time the concept of“subnanoscale tank treads”.Using B₁₀C and B₁₀Ca clusters as models,we have elucidated the mechanism of dynamic fluxionality in boron nanomachines.We have further realized the halting of B^-₁₁ tank tread,as well as tuned/controlled the rotation barrier of a non-fluxional B₁₀ cluster.Our analyses have offered a quasi-quantitative measure of intramolecular rotation barrier in strongly covalent nanosystems.We have designed two three-dimesional boron-based nanomachines,that is,nanocompass Mg₂B₈ and nanoumbrella Li₄B₇^-,on the basis of B₈ and B₇molecular wheels.Finally,we have discovered and characterized the smallest borospherenes of B^-₂₈ and B₂₈,which are free-standing,seashell-like all-boron cages.Borospherenes have been studied herein via joint photoelectron spectroscopy?PES?and quantum chemistry calculations.The main contents of the thesis are as follows:1.Boron-based subnanoscale tank treads:B^-₁₁ and B₁₁We present a concept that an elongated,planar boron cluster can serve as a“tank tread”at the subnanometer scale,a novel propulsion system for potential nanomachines.DFT calculations at the PBE0/6-311+G*level for the global minimum?GM?structures of B^-₁₁(C_2v,¹A₁)and B₁₁(C_2v,²B₂)clusters along the soft in-plane rotational mode allow identification of their corresponding B^-₁₁ C_2v and B₁₁ C_2v transition state?TS?geometries,with small rotational energy barriers of 0.35 and 0.60 kcal mol^-1,respectively,at single-point CCSD?T?level,suggesting that the clusters are structurally fluxional at room temperature.Born-Oppenheimer molecular dynamics?BOMD?simulations show that B^-₁₁ and B₁₁ behave exactly like a tank tread,in which the peripheral B₉ ring rotates almost freely around the B₂ core.A full turn of rotation may be accomplished in around 2 ps.In contrast to molecular wheels or Wankel motors,the peripheral boron atoms in subnanoscale tank treads behave as a flexible chain gliding around,rather than as rigid wheel rotation.This finding is beyond imagination,which expands the concepts of molecular wheels and Wankel motors.2.Subnanoscale Tank Tread with Double Engines:B⁺₁₅A planar,elongated B⁺₁₅ cationic cluster is shown to be structurally fluxional and functions as a nanoscale tank tread on the basis of electronic structure calculations,bonding analyses,and BOMD simulations.The outer B₁₁ peripheral ring glides around an inner B₄ rhombus core almost freely at500 K.The rotational energy barrier is only 1.37 kcal mol^-1?0.06 eV?at the PBE0/6-311+G*level,further refined to 1.66 kcal mol^-1?0.07 eV?at single-point CCSD?T?level.Two soft vibrational modes of 166.3 and 258.3cm^-1 are associated to the rotation,serving as double engines for the system.Bonding analysis suggests that the“island”electron clouds,both?and?,between peripheral ring and inner core flow and shift continuously during the intramolecular rotation,facilitating dynamic fluxionality of the system with a small barrier.The B⁺₁₅ cluster is the first nanoscale tank tread equipped with two“engines”.3.On the Mechanism of Subnanoscale Tank Treads:Model B₁₀C ClusterFor an in-depth mechanistic understanding of boron-based rotors?including tank treads?,we investigate a doped boron cluster,B₁₀C,in which a C atom isovalently substitutes B^-in the B^-₁₁ tank tread.Two critical structures are reached:C_s?¹A??GM with C positioned in the peripheral ring and C_2v?¹A₁?local minimum?LM?with C in diatomic core.In GM the C atom completely halts the rotation of B₁₀C,whereas in LM dynamic fluxionality retains.Energy barriers for in-plane rotation differ markedly:12.93/18.31 kcal mol^-1 for GM versus 1.84 kcal mol^-1 for LM at single-point CCSD?T?level.The GM rotates via two transition states,compared to one for the LM.Chemical bonding analyses suggest that electron delocalization is essential for structural fluxionality.In particular,the variation of WBIs from the GM or LM geometries to their TS structures correlates positively to the energy barrier,which offers a quasi-quantitative measure of the barrier height and hence controls the dynamics.This study helps rationalize why a strongly covalent bound system has dynamical fluxionality.4.Tuning the Rotation Barrier of the Smallest Elongated Boron Cluster:B₁₀CaThe geometry,bonding,and dynamic fluxionality of boron-based binary cluster B₁₀Ca is studied at the DFT level.The C₂?¹A?GM of B₁₀Ca cluster is shown to adopt a half-sandwich structure via computational global searches.BOMD simulation suggests that B₁₀ is non-fluxional with a rotation barrier of13.32 kcal mol^-1.However,the B₁₀ fragment in binary B₁₀Ca species exhibits dynamic fluxionality,with its peripheral B₈ ring rotating freely around an inner B₂ core.During the structural evolution process,the GM rotates via two transition states,with the barriers being 0.95 and 1.25 kcal mol^-1,respectively.Thus,a single Ca atom activates and drives the in-plane rotation of B₁₀cluster.NBO charge distribution shows that Ca atom donates its two electrons to B₁₀,and the GM of B₁₀Ca can be formulated as[Ca]²⁺[B₁₀]^2-.Bonding analysis suggests that electron delocalization decreases the rotation barrier of B₁₀ cluster.5.Nanoscale Compass:Mg₂B₈Boron-based binary cluster Mg₂B₈ is shown to adopt a compass structure via global searches,featuring an Mg₂ dimer as the needle and a disk-shaped B₈ molecular wheel as baseplate.BOMD simulations indicate that Mg₂B₈ is structurally fluxional with the needle rotating freely on baseplate,analogous to a compass.The rotational barrier is only 0.1 kcal mol^-1 at single-point CCSD?T?.Bonding analyses suggest that the cluster compass can be formulated as[Mg₂]²⁺[B₈]^2-;that is,the baseplate and needle are held together primarily through ionic interactions.The baseplate is doubly aromatic with 6?and 6?electrons,which provides dilute,continuous,and delocalized electron cloud,lubricating the dynamics of nanocompass.6.Nanoscale Umbrella:Li₄B₇^-A boron-based binary cluster,Li₄B₇^-,is shown to adopt an umbrella-like structure via quantum chemistry calculations,featuring a B₇ molecular wheel as core,a Li₃ trimer and a single Li atom as two metal ligands,respectively.BOMD simulation indicates that Li₄B₇^-is structurally fluxional with the Li₃rotating freely above the B₇ wheel,being akin to a nanoscale umbrella.The rotational barrier is only 0.37 kcal mol^-1 at CCSD?T?.Bonding analysis suggests that the umbrella can be described as[Li₃]⁺[B₇]^3-[Li]⁺,in which the B₇ wheel,Li₃ trimer and single Li atom are held together through ionic interactions.The Li₄B₇^-cluster is triply aromatic with 6?and 6?electrons in B₇^3-wheel and 2?electrons in Li₃⁺trimer.This unique bonding pattern is the key to dynamical fluxionality of Li₄B₇^-cluster.7.The Smallest Borospherenes:Sea-Shell B^-₂₈ and B₂₈ CagesFree-standing boron nanocages or borospherenes have been observed recently for B^-₄₀ and B₄₀.There is evidence that a family of borospherenes may exist.However,the smallest borospherene is still not known.Here,we report experimental and computational evidence of a seashell-like borospherene cage for B^-₂₈ and B₂₈.Photoelectron spectroscopy?PES?of B^-₂₈ indicates contributions from different isomers.Theoretical calculations show that seashell-like B^-₂₈ borospherene is competing for the GM with a planar isomer.The former is shown to be present in cluster beam,contributing to the observed photoelectron spectrum.The seashell structure is found to be the GM for neutral B₂₈,and the B^-₂₈ cage represents the smallest borospherene being observed to date.It is composed of two triangular close-packed B₁₅ sheets,interconnected via three corners by sharing two boron atoms.The B₂₈ borospherene have nine delocalized?bonds,which follow the 2?n+1?² electron-counting rule for spherical aromaticity.