Theoretical Investigations of the Acetylene Analogues of Group 14 Elements E2X2 (E=Si-Pb, X=F-I)
In this study, the isomers of E2X2 (E=Si-Pb, X=H, F, Cl, Br and I) were investigated. In most structures, the E-E bond length and the E-X-E angle correlate with the halogen atoms. The relative energies showed that the non-planar doubly bridged structures A are global minima for all investigated E2X2...
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|In this study, the isomers of E2X2 (E=Si-Pb, X=H, F, Cl, Br and I) were investigated. In most structures, the E-E bond length and the E-X-E angle correlate with the halogen atoms. The relative energies showed that the non-planar doubly bridged structures A are global minima for all investigated E2X2 molecules except for Si2F2, although the global minimum for Si2F2 is the vinylidene structure in the triplet state. The linear isomers G are always found as energetically higher lying transition states. The investigation of the EX fragments presented that the molecules need very high excitation energies from the X2Π ground state to the a4Σ- first excited state. Due to the large excitation energies, the bond dissociation energies of the linear XEEX structures are very small, or negative, and it is found that the E-E triple bond formation is quite unfavorable.
The Energy Decomposion Analyses presented that the stability of the E2X2 molecules correlate with the bond dissociation energies. The non-planar doubly bridged structures A show the largest dissociation energies. The large dissociation energies of A stem from the large dipole-dipole electrostatic interactions and the effective three-center-two-electron orbital interaction. The preparation energies of the bridged structures are larger than those of the bent structures due to the elongation of E-X bond to form the E-X-E bridged structure. The ring structures of Si2X2 show large preparation energies and those of Si2F2 are especially quite large. Due to that, the doubly bridged structures cannot compensate the energetic loss of the E-X bond elongation to form bridged structures. As a result, the dissociation energies of SiA for Si2F2 become smaller and the isomer cannot be the global minimum.