Prediction of Reaction Pathways and New Properties in Organosilicon Compounds Based on ab initio MO and Density Functional Theory Calculations

摘要

The paper consists of two topics. In the first part, mechanisms of 1,2-addition reactions to Si=C, Si=Si, and C=C are presented. All reactions start from electrophilic and nucleophilic initial complexes, C_e and C_N, those are related to the frontier molecular orbital theory. In the reaction of H_2Si=SiH_2 + H_2O, C_E and C_N determine the whole reaction path and finally lead to different products. In the addition reaction of acidic alcohol or hydrogen halides, only electrophilic channels are found. For the reactions of methyldisilene with water and of ethynyldisilene with water, four initial complexes and therefore four reaction channels were found. Only two complexes were found, however, in the reactions of fluorodisilene and aminodisilene with water/ hydrogen fluoride. The reaction of polar substrate, aminodisilene, shows the similar reaction profiles to that of polar substrate, silene. In addition, product switching of the stereochemistry, depending on the acidity of the reagent was found for the reaction of aminodisilene. In the second part, we describe the design of planar anionic polysilicon chains and cyclic Si_6 anions with D_(6h) Symmetry. Among 18 calculated molecules, planar equilibrium structures are obtained for seven anionic chains (Si_2H_2~(2-), Si_4H_2~(2-), Si_4H_2~(4-), Si_6H_2~(4-), S_6H_2~(6-), Si_8H_2~(4-), Si_8H_2~(6-)) and three anionic six-membered rings (c-Si_6~(2-), c-Si_6~(4-), c-Si_6~(6-)). The number of π electrons formally accommodated in the out-of-plane π orbitals is the same as the number of silicon atoms and is independent of the number of doped electrons. An anionic six-membered ring, c-Si_6~(2-), shows a large negative value of nucleus-independent chemical shifts (NICS), which indicates that c-Si_6~(2-) is an aromatic molecule.