The relative signs of geminal and vicinal proton spin coupling constants

The 60 Mc/s proton resonance spectrum of 2,3-dibromopropionic acid is reported and analysed as an ABC system. The relative signs of the geminal and vicinal coupling constants observed have been determined by the double irradiation method and confirmed by experiments at 15·086 Mc/s. The two types of coupling constant are found to have opposite signs and to be large, the geminal one being ∓9·9 c.p.s. and the vicinal ones ±10·8 c.p.s. and ±4·6 c.p.s.