The alternation of bond lengths in long conjugated chain molecules

Abstract Ooshika (1957) has recently found, using the self-consistent molecular orbital theory, that a cyclicpolyene C2nH 2n exhibits marked bond alternation if n is very large. Here we show that, provided bond compression is taken into account, this result follows inevitably from even the simple l.c.a.o. theory, and is independent of the analytic form of either /?(r), the resonance integral, or f(r), the cr bond energy. An investigation of the linear polyenes C2nH 2n+2 and C2n+1H2n+3 leads to the same conclusions, which contradict those of Lennard-Jones (1937) and Coulson (1938) but agree with those of Ooshika (1957) and Labhart (1957). A simple calculation, based on an exponential form for leads to a value of about 0.04 A for the difference in length between adjacent bonds in the infinite chain.