The sign of the long-range spin–spin coupling constant between 19F and the aldehydic 1H in p-fluorobenzaldehyde: evidence for a negative QCCHO

Abstract

The spin–spin coupling constant of −0.50 ± 0.03 Hz between the 19F nucleus and the aldehydic proton in p-fluorobenzaldehyde contrasts with a positive coupling between 19F and methyl protons in p-fluorotoluene derivatives. If the former coupling is dominated by a π electron mechanism, it implies a negative hyperfine interaction constant between the aldehydic proton and the aromatic π system.