A theoretical study of H(S2N2)YH (Y = 1, 2, 3, 4, 5):: Modeling the superconducting polymer (SN)X

The bonding and energetics in H(S2N2)(Y)H are analyzed within two models of conductivity. The effective exchange and mixed-valence models for conductivity predict similar geometric changes. The effects of hydrogen capping on the geometries were removed by averaging the central bond lengths and angles in the larger oligomers. The extrapolated singlet-triplet energy gap, predicted to be between 2.44 and -2.54 kcal mol(-1) for the polymer, accounts for conductivity along the chain axis, the suppression of a Peierls instability and the failure to synthesize single strand (SN)(X). Extrapolated values for the ionization potential, between 5.61 and 5.17 eV, electron affinity, between 3.40 and 4.08 eV, and disproportionation energy, between 2.14 and 0.98 eV, are consistent with previous estimates. The perpendicular conductivity can be related to the small, but non-negligible, disproportionation energy. An analysis of the charge distributions reveal that both models of conductivity have a common electron-phonon mechanism, suggesting that the remarkable superconductivity in (SN)(X) is best explained as a combination of the effective exchange and mixed-valence modes of conductivity. (C) 2008 Elsevier B.V. All rights reserved.