π-Bond Configurations, Conformations, and Dynamic Behaviour of Benzo[c]octalene and Dibenzo[c,j]octalene

The π‐bond configurations, the conformations, and the dynamic behaviour of dibenzo [c,j]octalene (2) and of benzo [c]octalene (3) have been investigated by 13C‐NMR. spectroscopy at different temperatures. Dibenzooctalene was found to present π‐bond fixation in the octalene unit as in 2b; with this π‐bond fixation the molecule is not planar and takes two different conformations which are rapidly interconverted by inversion of one cyclooctatetraene ring. Monobenzooctalene (3) also presents π‐bond fixation in the octalene unit but exists as two valence isomers, 3b and 3c. Isomer 3c dominates the dynamic equilibrium. With this π‐bond configuration, the molecule is chiral but undergoes several isodynamic processes, namely inversion of the cyclooctatriene and/or of the cyclooctatetraene ring. The valence isomer 3b can have two different conformations which are rapidly interconverted by inversion of one cyclooctatetraene ring. The interconversion 3c ⇌ 3b implies the occurrence of a π‐bond shift process; this process affects the 13C‐NMR. lineshape above 50°.