Twistane, C_(10)H_(16), is a classic D_2-symmetric chiral hydrocarbon that has been studied for decades due to its fascinating stereochemical and thermodynamic properties. Here we propose and analyze in detail the contiguous linear extension of twistane with ethano (ethane-1,2-diyl) bridges to create a new chiral, C_2-symmetric hydrocarbon nanotube called polytwistane. Polytwistane, (CH)n, has the same molecular formula as polyacetylene but is composed purely of C(sp~3)-H units, all of which are chemically equivalent. The polytwistane nanotube has the smallest inner diameter (2.6 ?) of hydrocarbons considered to date. A rigorous topological analysis of idealized polytwistane and a C_(236)H_(242) prototype optimized by B3LYP density functional theory reveals that the polymer has a nonrepeating, alternating σ-helix, with an irrational periodicity parameter and an instantaneous rise (or lead) angle near 15°. A theoretical analysis utilizing homodesmotic equations and explicit computations as high as CCSD(T)/cc-pVQZ yields the enthalpies of formation △_fH°_0(twistane)=-1.7 kcalmol~(-1) and △_fH°_0(polytwistane)= +1.28 kcal (mol CH)~(-1), demonstrating that the hypothetical formation of polytwistane from acetylene is highly exothermic. Hence, polytwistane is synthetically viable both on thermodynamic grounds and also because no obvious pathways exist for its rearrangement to lower-lying isomers. The present analysis should facilitate the preparation and characterization of this new chiral hydrocarbon nanotube.
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