Unexpected behavior of the 3JCH coupling constant in unsaturated compounds

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