Abstract: All of the possible 21 isomers for C₇₇Si, an isoelectronic molecule of C₇₈, were investigated by INDO methods based on C₇₈(C_2v). It is indicated that the most stable isomer is one in which the silicon atom locates at the hexagon C35 passing through by the x longest axis, whereas the atoms locating at the bond C1_C2 passing by the z shortest axis are difficult to substitute, and the substitution of the silicon atom makes the cage distort. The red-shift of absorption peaks of electronic spectra for C₇₇Si isomers will take place compared with that of C₇₈(C_2v) which is because of their narrower LUMO-HOMO energy gaps and there is a great difference in the characteristic absorptions between C₇₇Si35 and other isomers, which can be considered as an evidence for the formation of C₇₇Si35.

Key words: C₇₇Si, Electronic spectrum, INDO