1 H, 13 C, 17 O and 19 F NMR spectroscopic study of isomeric ring‐substituted monofluoro‐( E )‐3‐phenylpropenals

Isomeric ring substituted monofluoro‐( E )‐3‐phenylpropenals (cinnamaldehydes) were synthesized in order to provide, with the NMR‐active 19 F nucleus, an insight into the influence of substituents and the transmission of electronic effects between the aromatic ring and the side‐chain. Their 1 H, 13 C and 17 O NMR chemical shifts and their n J (H,H), n J (H,F), n J (C,H) and n J (C,F) values were determined. Generally, the n J (C,F) coupling constants correlate with the length of the coupling route and thus simplify the assignments of the 13 C NMR spectra. Only the side‐chain carbons C‐2 and C‐3 in the ortho ‐substituted compounds showed an exception to this rule. Many long‐range n J (H,F) ( n = 4–7) values were easily observed. The 17 O NMR chemical shift of the carbonyl oxygen in the o ‐ and m ‐fluoro derivatives show clear substituent effects, whereas p ‐fluorocinnamaldehyde shows a value comparable to that of that parent compound.