Study Of Structures And Properties Of Triel Bonds Formed By Three Types Of Electron Donors

In this dissertation,the MP2/aug-cc-pVTZ theoretical method is used to study the formation,properties,and nature of triel bonds with three types of Lewis bases.The main contents are as follows:The carbene triel bond is predicted and characterized by theoretical calculations.The C lone pair of N-heterocyclic carbenes?NHCs?is allowed to interact with the central triel atom of TrR₃?Tr=B and Al;R=H,F,Cl and Br?.The ensuing bond is very strong,with an interaction energy of nearly 90 kcal/mol.Replacement of the C lone pair by that of either N or Si weakens the binding.The bond is strengthened by electron-withdrawing substituents on the triel atom,and the reverse occurs with substitution on the NHC.However,these effects do not strictly follow the typical pattern of F>Cl>Br.The TrR₃molecule suffers a good deal of geometric deformation,requiring on the order of 30kcal/mol,in forming the complex.The R?C···Tr?bond is quite short,for example,1.6?for Tr=B,and shows other indications of at least a partially covalent bond,such as a high electron density at the bond critical point and a good deal of intermolecular charge transfer.The complexes between TrR₃?Tr=B,Al,and Ga;R=H,F,Cl,and Br?and H₂X?X=O,S,and Se?is theoretically studied.The interaction energies of AlR₃···H₂X and GaR₃···H₂X are consistent with the electronegativity of the halogen atom R?R?H?,but an opposite dependence is found for BR₃···H₂X.The triel bond of TrR₃···H₂X is weaker for the heavier chalcogen donor.The dependence of triel bonding strength on the triel atom is complicated,depending on the nature of R and X.The methyl substitution of H₂X causes a substantial increase in the interaction energy from-5.74 kcal/mol to-22.88 kcal/mol,and its effect is relevant to the nature of Tr,X,and R groups.For the S and Se donors,the increased percentage of interaction energy is almost the same due to the methyl substitution,which is larger than that of the O analogue.In most triel-bonded complexes,electrostatic dominates and polarization has comparable contribution.However,polarization plays a dominant role in BR₃···R'₂S and BR₃···R'₂Se?R=Cl and Br;R'=H and Me?.The organic fluorides can engage in a triel bond with TrR₃?Tr=B,Al,and Ga;R=H,F,Cl,and Br?.The strength of triel bond is varied with the hybridization of the C atom adjoined with the F atom.The triel bond is strongest for the C_sp3-F bond,followed by C_sp2-F,and C_sp-F forms the weakest triel bond.In all cases,the triel bond is stronger in a pattern of BR₃<GaR₃<AlR₃ for the same R,which is consistent with the positive MEP on the?-hole of the triel atom.This indicates that electrostatic interaction plays an important role in the triel bond with the organic fluorides as the electron donors.In most complexes,the triel bond becomes stronger with the increase of R electronegativity.There are two exceptions.When C_sp2-F and C_sp3-F act as the electron donor,BH₃ parcipates in a stronger triel bond than does BR₃?R=halogen?.For the weakest electron donor C_sp-F,however,BR₃?R=halogen?forms a stronger triel bond for the increase of R electronegativity.