Der ‘Push‐Pull’‐Effekt von ‘Push‐Pull’‐Oligoacetylenen. Eine 13 C‐NMR‐Untersuchung

The ‘Push‐Poll’ Effect of ‘Push‐Pull’ Oligoacetylenes. A 13 C‐NMR Investigation According to 13 C‐chemicaI shifts of ‘push‐pull’ oligoacetylenes 1 – 4 , the ‘push‐pull’ effect ( i.e. π delocalization induced by ‘push‐pull’ substituents) rapidly decays in this series. To correct for other than π ‐charge‐density effects, Δδ values of symmetrically placed C‐atoms of the oligoacetylene chain are discussed. Stereoelectronic resteffects (SER) of the substituents on terminal C‐atoms of PP‐ketones 1a – 3a and PP ‐esters 1b – 4b are estimated from the residual Δδ of the asymptotes of Fig. 3. Fig. 4 convincingly shows that Δδ values are dramatically decreasing with increasing number n of acetylene units between the push and pull substituents. Assignment problems of ‘push‐pull’ triacetylenes 3 have been solved by 13 C labelling of the CO group of 3a .