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Abstract

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.