Theoretical Study On The Structure And Bonding Character Of The CH₂=XR₂ And R₂C=XH₂

Some heavy analogues of olefins do not exhibit the classical planar geometry but rather have trans-bent structures.Ethylene is planar geometry,but a significant number of stable compounds with M=C and M=M(M=Si,Ge,Sn,Pb)have trans-bent structures.These amusing chemical bond has attracted some interest.In order to explain this phenomenon,theoretical chemist researched this bonding and stability of the doubly bonded molecule using all kinds of manners:Apeloig and Karni advanced such reversed polarity of theÏ€-bond.They proposed that it is the most important electronic factor that enhances the kinetic stability.Carter and Goddard,Malrieu and Trinquier have interpreted the bond strength and structure of R₂X=YR₂ species in terms of the valence states of the R₂X:and YR₂:fragment.Chen Wei-Chih has studied the substituent effects on A₂Si=SiH₂,A₂Ge=GeH₂,A₂Si=GeH₂,A₂Ge=SiH₂.From their analysis,they predicted that for the disubstituted olefin systems a more electronegative substituent will lead to a largerâ–³E_stand,in turn,will decrease the strength of theÏ€-bond and enlarge the degree of distortion from planarity.On the other hand,from their conclusions they predict that a more electropositive substituent will result in a smallerâ–³E_stand will then increase the strength of theÏ€-bond.Also,a smaller singlet-triplet splitting can result in a relatively higher thermodynamic stability of the doubly bonded molecule,making such a species synthetically accessible.Ottosson demonstrated theoretically thatÏ€-electron-donating substituents on the carbon site cause substantial pyramidalization at the the Si atom.Lately,Cheng and Chu discussedÏ€-electron-donating substituents in C=Si and Si=Ge compounds. They obtained that substitution at the carbon atom is more effective than that at the Si atom at inducing a bent structure.In a word,effects of substituents on the structure and thermodynamic stabilities of unsaturated compounds of group 14 elements has interested many generations of chemists.In the paper,on the basis of the molecular orbital theory,the tradition transition state theory as well as quantum chemistry theory,the systems choosed have been investigated using Density Functional(DFT)Methods and the Natural Bond Orbital analysis.Substituent effects on the disubstituted heavier analogues of olefin R₂X=YH₂ (X,Y = C,Ge;R = H,OH,NH₂,SH,PH₂,F,Cl,Br,(NHCH)₂,CH₃,(CH)₂) were investigated using density functional theory(B3LYP/6-31+G(d,p)).The conclusion is that substitution at the carbon atom is more effective than that at the Ge atom at inducing a bent structure,the stronger theÏ€-donation the substituents,the larger theâ–³E_STof CR₂ and the stronger theÏ€-electron reversed-polarization effect. As theÏ€-electron reversed-polarization effect increased,the distortion of GeH₂ fragment increased,the stability of the bent structure increased.In addition,theÏ€-electron-donating substitution at the carbon atom was more effective at inducing a bent structure.The whole paper consists of five chapters.Chapter 1 describes the progeess and application of quantum chemistry as well as the development.At the same time,the development and the current situation of olefin are explained.Chapter 2,simply introduced elementary theory,mainly contained tradition transition state theory,ab initio theory,density functional theory(DFT),and rules for intersystem crossing.The contents of the two chapters were the basis and background of our studies and offer us with useful and reliable quantum methods.In Chapter3,theoretical study on the structure and bonding character of the R₂Ge=CH₂ and R₂C=GeH₂ has been carried in theory.In Chapter 4,substituent effects on the geometries of the R₂Sn=CH₂ and R₂C=SnH₂ has been studied.