Ringstrom bei cyclischen Polyenen mit alternierenden und ausgeglichenen Bindungsabständen

Abstract

The ring current effect in NMR has generally been used to distinguish between equal and alternant bonds in cyclic π-electron systems, assuming a strong ring current in equal-bonds-hydrocarbons and no ring current in alternant-bonds-hydrocarbons. A calculation of the ring current is presented based on the one dimensional electron gas model. The effect of bond alternation is considered by a sine curve potential. Proceeding from a model with equal bonds to a model with alternant bonds the ring current is strongly reduced in rings with more than 10 members; it is reduced by 24% and 51% only in 6 and 10 membered rings, respectively. In non-HücKEL type rings with 4 to 16 carbon atoms the ring current is directed opposite to the classical current and is strongest in cyclobutadiene. The contribution of the π-electrons to the susceptibility of these compounds is therefore paramagnetic and ring current shifts opposite to those in HÜCKEL rings are to be expected. These results are confirmed by the proton chemical shifts in the NMR spectrum of [16]-Annulene.