Unexpected behavior of the 3 JCH coupling constant in unsaturated compounds.

The experimental and theoretical behavior of the (OC)CCH 3 JCH coupling constant is investigated for a series of α,β-unsaturated compounds (1 to 8), and for some of them, the well-known relationship (3 JCHcis  < 3 JCHtrans ) was observed. However, for some compounds, close values for 3 JCHcis and 3 JCHtrans couplings were observed, and for aldehydes group containing compounds (7 and 8E), an inversion order is observed (3 JCHcis  > 3 JCHtrans ). In all cases where the 3 JCHcis  < 3 JCHtrans relationship it is not followed, a polar group or electronegative atom oriented in opposite direction (s-trans) to the HCC hydrogen is present, suggesting that conformational preference of such polar group or atoms are important factor on the behavior of 3 JCH couplings. Taking all of these in consideration, a new Karplus-type equation was proposed for 3 JCH couplings in α,β-unsaturated compounds, which can be used for configurational and conformational assignment on trisubstituted double bond derivatives.