Unexpected behavior of the 3JCH coupling constant in unsaturated compounds

The experimental and theoretical behavior of the (O(sic)C)-C(sic)C-H (3)J(CH) coupling constant is investigated for a series of alpha,beta-unsaturated compounds (1 to 8), and for some of them, the well-known relationship ((3)J(CHcis) 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) J(CHtrans) relationship it is not followed, a polar group or electronegative atom oriented in opposite direction (s-trans) to the H-C(sic)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 alpha,beta-unsaturated compounds, which can be used for configurational and conformational assignment on trisubstituted double bond derivatives.