Studies On Electronic Structure And Noncovalent Interaction Of Boron-based Clusters

The boron-based clusters exhibit unique electronic structure,bonding characteristics and physical and chemical properities.Such a topic has always been internationally recognized and focused.Among them,B₁₂X₁₂^2-(X=H)borane and its halogenated derivatives(X=F,Cl,Br,I)represented by closed cage-like boron clusters have a highly symmetrical I_h geometry and exhibit thermodynamic and thermochemical stability.Furthermore,the delocalized B-X?electron system can stabilize the extra two electrons.They possess the potential applications in nonlinear optics,catalysis,energy storage,biomedicine and other fields,which has attracted great attention in many scientific fields.However,many questions related to boron-based clusters have not yet been answered at the microscopic quantum level,such as the nature of borane-related dihydrogen bonds(DHB),the internal driving force of supramolecular recombination,and the host-guest field-induced Coulomb repulsion barrier.To address the above challenges,based on the B₁₂X₁₂^2-·n H₂O(X=H,F,I;n=1–6)and?CD·B₁₂X₁₂^2–(?=?,?,?;X=H,F,Cl,Br,I)cluster,we combine photoelectron spectroscopy and theoretical methods to explore their electronic structures and the noncovalent interactions of boron clusters at molecular level and to provide reliable microscopic information.The main research results are summarized as follows:1.Using photoelectron spectroscopy and theoretical methods to explore the electronic structure of B₁₂X₁₂^2-·n H₂O(X=H,F,I;n=1–6),where the pure traditional hydrogen bonding and double hydrogen bonding interaction modes in monohydrate B₁₂X₁₂^2-·H₂O(X=H,F,I)clusters serve as ideal models to measure and compare the relative bonding strengths.The experimental and theoretical methods innovatively reveal that the double hydrogen bond BH···HO DHB is 1.15(expt.)/1.14(theo.)and4.61(expt.)/5.57(theo.)kcal/mol stronger than the prototype strong HBs of B-F···H-O,and weak B-I···H-O respectively.The energy decomposition analysis using symmetry-adapted perturbation theory shows that,compared with the typical strong hydrogen bonds B-F···H-O,the interaction strength advantage of double hydrogen bonds B-H···H-O DHB is contributed by the induction and the dispersion term;while compared with the weak hydrogen bonds B-I···H-O HB,the strength advantage of DHB is reflected in the electrostatic interaction.Different borane substrates have different effects on the growth path of water clusters from monomer to hexamer,showing obvious solute specificity.Driven by the DHB B-H···H-O,the added water molecules are arranged in a unified binding mode manipulated in B₁₂H₁₂^2-·n H₂O,especially B₁₂H₁₂^2-·5H₂O,and the water pentamer is arranged in the form of a five-membered ring.2.In order to further explore the electronic structure of borane-cyclodextrin functional clusters and the intrinsic driving force of supramolecular self-assembly,the photoelectron spectroscopy and quantum chemical theoretical exploration were performed for?CD·B₁₂X₁₂^2–(?=?,?,?;X=H,F,Cl,Br,I)clusters.The results show that the electronic structure stability of borane significantly increases with the complexation of cyclodextrin,compared with isolated B₁₂X₁₂^2–,the increasment of ionization energy is largest at?CD·B₁₂H₁₂^2–and gradually decreases with the increase of the size of halide.This is consistent with the theoretical prediction that the strongest non-covalent interactions in the CDs·B₁₂H₁₂^2–boron cluster with DHBs.The B-H···H-C/O multiple interlocking mode in the borane-cyclodextrin complex cluster results in the weak interaction of?CD·B₁₂H₁₂^2–with 1.1 e V stronger than that of the F series,and the superiority of the intrinsic driving forces of DHB further makes for the molecular self-assembly of borane-cyclodextrin clusters.The energy decomposition analysis shows that the electrostatic term is not the main factor responsible for the larger BE of the X=H series compare to X=F series,but comes from the strong polarization and dispersion terms.3.Based on the borane-cyclodextrin clusters,the intrinsic repulsive Coulomb barrier was extended to supramolecular host-guest complexes.And the regulation effect of host and guest on the RCB were quantitatively revealed by photoelectron spectroscopy and theoretical methods.The results show tat the regulatory effect of the host molecule on RCB weakenes as the polarizability of the borane guest molecule increases.The existence of RCB₂ in the photoionization process of host molecule was confirmed by both experimental and theoretical methods.Analysis using constrained density functional theory shows that the polarization effect of guest dianions in the host-guest cluster system leads to the special RCB₂ in the ionization process of neutral host molecule.In addition,the experimental and theoretical method reveals the main reason why induced RCB₂ is lower than RCB₁ during electron loss process.Essentially,it can be attributed to the short-range binding force of the positive charged host fragment and attractive potential derived from induced?⁺hole,and the superposition of the two factors reduces the RCB₂.Surface analysis of electrostatic potential reveals a counter-intuitive physical picture of the‘like-charge attraction'between the outgoing electron and the remaining system.It is essentially induced by the classic electrostatic attraction between the positive surface electrostatic potential in monovalent anion and ejected electron.