Electronic structures and infrared spectra of C120XY molecules (X, Y=O, S) and some of the corresponding ions are investigated using PM3 semi-empirical molecular orbital calculations with full optimization of geometrical structures. It is found that the energy penalty is about 30-42 kJ/mol due to introducing a double bond in the five-membered ring except for C120O2- and triplet C120O22-. It is also found that the structures of neutral molecules and the corresponding ions are almost the same; for instance, the change of bond length is less than 0.001 nm. The change of frontier orbits from oxide to sulfide is little as well. The triplet states of C120O22- and C120OS2- are more stable than their singlet states, which means that C120O22- and C120OS2- follow the Hund’s rule. The vibration analysis showed that the infrared spectra of neutral C120O2 and C120OS molecules are in good agreement with the experimental results. Compared with the neutral molecule, vibration frequencies of triplet C120O22- change littl Electronic structures and infrared spectra of C120XY molecules (X, Y = O, S) and some of the corresponding ions are investigated using PM3 semi-empirical molecular orbital calculations with full optimization of geometrical structures. It is found that the energy penalty is about 30 -42 kJ / mol due to introducing a double bond in the five-membered ring except for C120O2- and triplet C120O22-. It is also found that the structures of neutral molecules and the corresponding ions are almost the same; for instance, the change of bond length is less than 0.001 nm. The change of frontier orbits from oxide to sulfide is little as well. The triplet states of C120O22- and C120OS2- are more stable than their singlet states, which means that C120O22- and C120OS2- follow the Hund’s rule. The vibration analysis showed that the infrared spectra of neutral C120O2 and C120OS molecules are in good agreement with the experimental results. Compared with the neutral molecule, vibration frequencies of triplet C1 20O22- change littl