异质富勒烯C_(59)Si与C_(69)Si的理论研究

Theoretical Studies on the Heterofullerenes C_(59)Si and C_(69)Si

利用MNDO,AM1和PM3半经验量子化学计算方法对硅杂富勒烯C_(59)Si和C_(69)Si进行了系统的理论研究.计算了稳定构型、生成热、前沿轨道能级差、电离势、电子亲和势、绝对电负性和整体硬度.结果表明,硅杂富勒烯的稳定性虽然低于全碳富勒烯,但也具有相当的稳定性.C_(59)Si的稳定性比已经合成的C_(58)X_2(X=B,N)高,C_(69)Si与C_(70)的稳定性差异也很小,因此在适宜条件下文中所讨论的硅杂富勒烯是应该能够合成的.在C_(69)Si各异构体中,取代位置在赤道的异构体具有最低的能量和最大的前沿轨道能级差,也是最稳定的异构体.与全碳富勒烯C_(60)和C_(70)相比,C_(59)Si和C_(69)Si具有较小的电离势和电子亲和势,表明硅杂富勒烯容易被氧化,而被还原的难度要大些,但是仍容易发生还原反应而生成负离子.因此硅原子的掺杂能够使富勒烯的氧化还原性能得以改善.C_(59)Si和C_(69)Si更容易与亲电试剂反应,而发生亲核反应的活性要相对小一些.硅杂富勒烯C_(59)Si和C_(69)Si的绝对电负性和硬度都小于相对应的全碳富勒烯,对电子的束缚力要相对小一些.

A systematic investigation on the molecular structures of silicon - substituted fullerenes C59Si and C69Si has been performed using semiempirical MNDO, AMI and PM3 methods. The equilibrium geometrical structures, heats of formation, HOMO - LUMO gap energies, heats of atomization, ionization potentials, electron affinities, absolute electronegativities and globle hardnesses of C59Si and C69Si have been studied in this paper. The calculation results obtained indicate that both C59Si and C69Si are less stable compared with C60 and C70, but the stability of C59Si is higher than those of the already successfully synthesized C58N2 and C58B2, which means that it can be synthesized experimentally. For C69Si, both the heat of formation and the HOMO - LUMO splitting suggest that the isomer E, in which the silicon atom is located in the equator, is in the ground state. The heterofullerenes C59 Si and C69 Si have smaller ionization potentials (IP) and electron affinities ( EA) as compared with their all - carbon analogues. Thus the redox characteristics of C60 and C70 can be enhanced by doping. The reactivities of the silicon substituted fullerenes towards an electrophile or a nucleophile are considered based on the frontier molecular orbital (FMO) theory and the HSAB principle. The absolute electronegativities (x) and globalhardnesses (η) of the silicon substituted fullerenes are both smaller than those of C60 and C70. Thus the abilities of C59Si and C69Si to attract electrons to themselves are relatively weaker, and the resistance of the chemical potential to change in the number of electrons are also weaker as compared with their all carbon analogues.