Abstract: The structure and stability of all possible isomers of C50X(X=SiH2, PH, S) were investigated by ab initio HF/3-21G calculations. The calculation results indicate that the preferred position for cycloaddition of SiH2, PH, and S on C50 is consistent, i.e. on the C3—C4 and C4—C4 sites of C50, and the resultant most stable cycloaddition isomers are [5,6]-closed and [5,5]-closed structures. The main driving force governing the stability of the C50X(X=SiH2, PH, S) isomers is the strain, or the conjugative effect, or the combination of strain and conjugative effect, depending on the addition positions and the types of the attacked C—C bonds. Further, the structures and stabilities of C50X(X=SiH2, PH, S) are compared with those of the C50X(X=CH2, NH, O), and a general conclusion are drawn: the size of the attacking atom and the type of the attacked C—C bond are two decisive factors for the formation of open or closed structure of the C50X isomer.

Key words: C50X(X=SiH2, PH, S), Cycloaddition derivative, Structure, Stability