Coarctate and Möbius: The Helical Orbitals of Allene and Other Cumulenes

As brought to the attention of the community by Hendon et al. and noted by previous workers, the π orbitals of the equilibrium geometry odd-carbon (even number of double bonds = <i>n</i>) [<i>n</i>]­cumulenes may be written in either rectilinear or helical form. We trace the origins and detailed composition of the helical orbitals of cumulenes, which emerge in the simplest Hückel model and are not much modified in advanced computations. For the α,ω-disubstituted even [<i>n</i>]­cumulenes, the helical representation is obligatory as the symmetry is reduced from <i>D</i>_2<i>d</i> to <i>C</i>₂. A relationship is apparent between these helical orbitals of the even [<i>n</i>]­cumulenes, seen as a Herges coarctate system, and the corresponding Möbius cyclic polyene orbitals. The twist of the orbitals varies in interesting ways along the helix, and so does the contribution of the component atomic orbitals. Though the electronic structures of even [<i>n</i>]­cumulenes and Möbius cyclopolyenes are closely related, they differ for higher <i>n</i> in intriguing ways; these are linked to the constrained rotation of the basis orbitals along the helical twist itinerary. Relations are constructed between the level patterns of the π-systems of even [<i>n</i>]­cumulenes and ideas of Hückel and Möbius aromaticity

Extreme Conductance Suppression in Molecular Siloxanes

Single-molecule conductance studies have traditionally focused on creating highly conducting molecular wires. However, progress in nanoscale electronics demands insulators just as it needs conductors. Here we describe the single-molecule length-dependent conductance properties of the classic silicon dioxide insulator. We synthesize molecular wires consisting of Si–O repeat units and measure their conductance through the scanning tunneling microscope-based break-junction method. These molecules yield conductance lower than alkanes of the same length and the largest length-dependent conductance decay of any molecular systems measured to date. We calculate single-molecule junction transmission and the complex band structure of the infinite 1D material for siloxane, in comparison with silane and alkane, and show that the large conductance decay is intrinsic to the nature of the Si–O bond. This work highlights the potential for siloxanes to function as molecular insulators in electronics