Analysis of 1H NMR spectra of fluorene and azafluorenes: Long-range coupling constants

Analyses of the ¹H NMR spectra of fluorene, 2-nitrofluorene, 1-azafluorene, 2-azafluorene, 4-azafluorene, 7-nitro-4-azafluorene and 1,4-diazafuorene have been carried out using computer calculations and homo double resonance techniques. The relative signs and magnitude of the ⁴J, ⁵J and ⁶J long-range coupling constants of the 9-CH₂ group protons with the pyridine and phenylene ring protons have been measured by five- and six-spin calculations. It was established that the presence of substituents and the N heteroatom has only a very weak effect on the values and signs of these coupling constants. The usual alternating signs of long-range coupling constants involving π-electron systems were confirmed, giving negative signs for even and positive signs for odd numbers of bonds separating the coupling protons.