Electron–phonon interactions in charged cubic fluorocarbon cluster, (CF)8

Abstract

Electron–phonon interactions in the charged cubic fluorocarbon, ( CF ) 8 are studied, and compared with those in charged ( CH ) 8 and ( CD ) 8 . The A 1g mode of 1470 cm−1 much more strongly couples to the a 1g lowest unoccupied molecular orbitals (LUMO) than the A 1g mode of 554 cm−1 in ( CF ) 8 . The T 2g mode of 1030 cm−1, the E g mode of 980 cm−1, and the A 1g mode of 1470 cm−1 strongly couple to the t 2u highest occupied molecular orbitals (HOMO) in ( CF ) 8 . The total electron–phonon coupling constants for the monoanion (l −1 ) and monocation (l +1 ) of ( CF ) 8 are estimated to be 0.932 and 0.585 eV, respectively. The logarithmically averaged phonon frequencies for the monoanion (ω ln ,−1 ) and monocation (ω ln ,+1 ) of ( CF ) 8 are estimated to be 1365 and 998 cm−1, respectively. The l −1 and ω ln ,−1 values increase much more significantly by H–F substitution than by H–D substitution in cubane. The larger displacements of carbon atoms in the high frequency vibronic active mode in ( CF ) 8 than those in ( CD ) 8 due to larger atomic mass of fluorine than that of deuterium, and the unchanged electron distributions in the LUMO somewhat localized on carbon atoms as a consequence of H–F and H–D substitution in cubane, are the main reason why the l −1 and ω ln ,−1 values increase much more significantly by H–F substitution than by H–D substitution. The l +1 and ω ln ,+1 values less significantly change than the l −1 and ω ln ,−1 values by H–F substitution as well as by H–D substitution in cubane. This is because the t 2u HOMO in ( CF ) 8 and the t 2g HOMO in ( CH ) 8 are somewhat localized on fluorine atoms, and thus, the high frequency vibronic active modes in which the displacements of carbon atoms are large cannot necessarily very strongly couple to the HOMO somewhat localized on fluorine atoms in ( CF ) 8 .