Unexpected behavior of the ³ J_CH coupling constant in unsaturated compounds

The experimental and theoretical behavior of the (OC)CCH ³ J_CH 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 J_CHcis  < ³ J_CHtrans ) was observed. However, for some compounds, close values for ³ J_CHcis and ³ J_CHtrans couplings were observed, and for aldehydes group containing compounds (7 and 8E), an inversion order is observed (³ J_CHcis  > ³ J_CHtrans ). In all cases where the ³ J_CHcis  < ³ J_CHtrans 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 J_CH couplings. Taking all of these in consideration, a new Karplus-type equation was proposed for ³ J_CH couplings in α,β-unsaturated compounds, which can be used for configurational and conformational assignment on trisubstituted double bond derivatives.