where ∆Ω is the resonance frequency difference between the two spins in rad s -1 . Inversion of these relations leads to an estimate of J 12 ≃ 54.86 ±0.5 Hz for the scalar coupling constant. The resonance frequency difference ∆Ω is consistent with the 18 O-induced isotope shift estimated from the spectral splittings (see section III A).