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Metallization of a Hypervalent Radical Dimer: Molecular and Band Perspectives

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posted on 07.04.2010, 00:00 by John S. Tse, Alicea A. Leitch, Xueyang Yu, Xuezhao Bao, Sijia Zhang, Qingqing Liu, Changqing Jin, Richard A. Secco, Serge Desgreniers, Yasuo Ohishi, Richard T. Oakley
Variable pressure and temperature conductivity measurements on the bisthiaselenazolyl radical dimer [1a]2 have established the presence of a weakly metallic state over the pressure range 5−9 GPa. To explore the origin of this metallization we have examined the crystal and molecular structure of [1a]2 as a function of pressure. At ambient pressure the dimer consists of two radicals linked by a hypervalent 4-center 6-electron S···Se−Se···S σ-bond into an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the x-direction of the monoclinic (space group P21/c) unit cell. Pressurization to 4 GPa induces little change in the molecular structure of [1a]2 or in the slipped π-stack crystal architecture. Near 5 GPa, however, stress on the dimer leads to buckling of the two halves of the molecule and a contraction in the metrics of the S···Se−Se···S unit. These structural changes can be understood in terms of an electronic configurational switch from a 4-center 6-electron σ-bonded dimer to a more conventional π-bonded arrangement. At the same time the slipped π-stack arrays undergo a concertina-like compression, and the crystal structure experiences highly anisotropic changes in cell dimensions. DFT calculations on the molecular electronic structure of the dimer indicate a marked decrease in the HOMO−LUMO gap as the dimer buckles. Related solid-state calculations indicate a rapid closure of the valence/conduction band gap in the same pressure region and the formation of a quasi-metallic state. Metallization of [1a]2 thus arises as much from intramolecular changes, which give rise to a collapse of the HOMO−LUMO gap and near coalescence of the valence and conduction bands, as from increased intermolecular interactions, which cause widening and overlap of the band edges.