Vicinal proton coupling constants and the electronegativity of substituents: evidence for an effect alternating with the number of intervening bonds

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Abstract
It is well known that in the fragment XHC1-HC2-HC3-HC4 the vicinal coupling constant between a proton on C1 and a proton on C2,J 1 v, decreases as the electronegativity, Ex , of the substituent atom, X, increases, and this has been explained in terms of the effects of rehybridization and induction which should vary monotonically down the fragment. However, from J v taken from the literature for a series of substituted hydrocarbons, it is apparent that J 2 v increases with Ex while J 3 v again decreases as Ex increases.