Theoretical Study On The Structures,Properties,and Nature Of π-hole And σ-hole Triel Bonds

Triel bond is an attractive interaction between a molecule composed of a Ⅲ main group atom and an electron donor,and it plays an important role in crystal materials and chemical reactions.Therefore in this thesis,theoretical calculations have been performed to study triel bonds with different types of electron donors in views of their geometries,strength,properties,and nature.The maincontents arefollowed:First,the replacement of a CH group of benzene by a triel（Tr）atom places a positive region of electrostatic potential near the Tr atom in the plane of the aromatic ring.Thisσ-hole can interact with an X lone pair of XCCH（X=F,Cl,Br,and I）to form a triel bond（Tr B）.The interaction energy between C₅H₅Tr and FCCH lies in the range between 2.2 and 4.4 kcal/mol,in the order Tr=B+cation above the ring pulls density toward itself and thus magnifies the Trσ-hole.The Tr B to the XCCH nucleophile is thereby magnified as is the strength of the Tr B.This positive cooperativity is particularly large for Tr=B.Then,the ability of B atoms on two different molecules to engage with one another in a noncovalent diboron bond is studied by ab initio calculations.Due to electron donation from its substituents,the trivalent B atom of BYZ₂（Z=CO,N₂,and CNH;Y=H and F）has the ability to in turn donate charge to the B of a BX₃molecule（X=H,F,and CH₃）,thus forming a B…B diboron bond.These bonds are of two different strengths and character.BH（CO）₂and BH（CNH）₂,and their fluorosubstituted analogues BF（CO）₂and BF（CNH）₂,engage in a typical noncovalent bond with B（CH₃）₃and BF₃,with interaction energies in the 3-8 kcal/mol range.Certain other combinations result in a much stronger diboron bond,in the 26-44 kcal/mol range,and with a high degree of covalent character.Bonds of this type occur when BH₃is added to BH（CO）₂,BH（CNH）₂,BH（N₂）₂,and BF（CO）₂,or in the complexes of BH（N₂）₂with B（CH₃）₃and BF₃.The weaker noncovalent bonds are held together by roughly equal electrostatic and dispersion components,complemented by smaller polarization energy,while the stronger ones are with the largest contribution from polarization.Finally,（BH）₂（NHC）₂（NHC=nitroheterocyclic carbin）acts as the electron donor to form a triel bond with TrPhX₂（Tr=B,Al,and Ga;X=F,Cl,Br,CH₃,and OH）.The larger total interaction can make thetwo-body structure quitestable,and when the substituent X in the electron acceptor is same,the change of total interaction intensity is related to the Tr atom.When Tr are Al and Ga,the action energy is stronger than that of B.As the electron withdrawing ability of the substituents increases,the total interaction energies show different changes.When Tr=B and Al,as electronegativity of the halogen atoms increases,the change order of the total interaction energy is TrPhBr₂>TrPhCl₂>TrPhF₂,which is different from the change order of their most positive electrostatic potential values.When Tr=Ga,the total action energy does not change much as the electronegativity of the halogen atom increases.For CH₃and OH,the intensity changes for both are same,and both have the largest total action energy for OH substitution.The main roles in the system are triel bonds and X…H hydrogen bonds.When the substituents are the same,the action energy of the triel bond increases in the order of AlPhX₂22,which is different from the order of the most positive electrostatic potential on the Tr atom in TrPhX₂.When X is a halogen atom,the action energy of the triel bond decreases in the order of Br>Cl>F,which is just opposite to the most positive electrostatic potential on the Tr atom in TrPhX₂.OH substitution has greater effect energy than CH₃substitution.In most complexes,the interaction energy of hydrogen bonds is smaller than that of triel bonds.And there is no hydrogen bond in the methyl-substituted complexs.In general,the energy of hydrogenbonding increases with theelectronegativity of thehalogen atoms.