POLYTHIENE, A NOVEL HYPOTHETICAL CARBON-SULFUR POLYMER

We propose a hypothetical carbon-sulfur polymer (CS), which conceptually derives from both the experimentally known trithiapentalene molecule (TTP) and the theoretically suggested ''polyketone''. Using the extended Huckel method, we examine the electronic structure of this system first in a planar conformation. The polytrithiapentalene formulation, with electron-rich multicenter S-S bonding and six electrons in the pi system of each C2S2 repeat unit, fits best the electronic structure calculated. The pi band filling suggests both that the carbon sublattice will undergo a Peierls distortion and that the intriguing multicenter sulfur bonding of TTP carries over into the polymer; however, half-filled sulfur lone pair bands imply that a pairing distortion is likely in the sulfur sublattice too. We contrast this with planar polyketone, where there is no electronic reason for pairing but rather a driving force to distort into a helix. Studying the likely pairing distortions of carbon and sulfur sublattices in polythiene, we conclude that pairing of the carbons is marginally energetically favorable, whereas pairing of the sulfurs is strongly favorable. Two possible combinations of carbon and sulfur pairing compete for being the lowest-energy conformation. We examine helical (i.e., nonplanar) conformations and find an energy minimum for a very slightly twisted conformation. Finally we discuss the role, we think small, of 3d basis functions on sulfur.