Electron-phonon coupling in the monoanions of cubic molecules such as cubane (CH)(8) and octasilacubane (SiH)(8) (cubane-series) are studied. The vibrational frequencies and the orbital vibronic coupling constants are computed and analyzed and the electron-phonon coupling constants are estimated. The results for cubane-series are compared with those for acene-(acene-series) and phenanthrene-edge-type hydrocarbons (phenanthrene-series). The T-2g modes of 687 and 848 cm(-1), the E-g modes of 922 and 1125 cm(-1), and the A(1g) mode of 1021 cm(-1) strongly couple to the t(1u) lowest unoccupied molecular orbital (LUMO) in cubane, on the other hand, only T-2g mode of 197 cm(-1) and the E-g mode of 403 cm(-1) strongly couple to the t(1u) LUMO in octasilacubane. The calculated total electron-phonon coupling constant for the monoanion of cubane (0.495 eV) is much larger than that for the monoanion of octasilacubane (0.262 eV). The relationships between the HOMO-LUMO gaps and the total electron-phonon coupling constants in the monoanions of acene-, phenanthrene-, and cubane-series are investigated. The plot of the total electron-phonon coupling constants against the HOMO-LUMO gap is found to be linear and the slope of the linear plots would be approximately 0.05 for the monoanions of nanosized molecular systems. We suggest the precondition under which the monoanions of nanosized molecular crystals can exhibit high-temperature superconductivity on the basis of the hypothesis that vibronic interactions between the LUMO and intramolecular vibrations would play an essential role in the occurrence of possible superconductivity in negatively charged nanosized molecules. (C) 2003 American Institute of Physics.