Extended hückel theory—v

Abstract Approximate calculations of the extended Huckel type are reported for ground and excited states of cumulenes, polyenes, polyacetylenes and Cn. Bond length variation in these molecules is studied by an iterative method which relates bond distance to Mulliken overlap population. For the cumulenes, simple aninons and cations as well as the first excited state prefer some twisting of the terminal CH2 groups from the ground state geometry. In the case of allene skeletal bending is also indicated in the excited state. An alternation of torsional barriers in the cumulene series is predicted. For the ground state of butadiene, one calculates a potential curve to twisting around the central single bond which has the s-cis form as a potential maximum, and which shows no activation energy to cis-trans interconversion. The first excited state of butadiene prefers a geometry with one CH2 group twisted out of plane. Distinct cis and trans bending tendencies in various excited acetylene states are obtained. Bond alternation is found for polyacetylenes, polyenes, essentially none for cumulenes. For linear Cn an intermediate situation occurs; bond alternation appears in short chains, is not expected to persist for large n, but a large “end effect” is predicted. Cn rings with n = 4q + 2 become more stable than the corresponding linear chains for n≥10.