“…It is noteworthy that in the conformationally labile selenophenes 4-7, all vicinal couplings, 3 J(Se,H-3), 3 J(Se,H-4) and 3 J(Se,H X ), differ noticeably in the s-cis and s-trans conformers, and this difference is mostly pronounced for the latter, 3 J(Se,H X ), amounting to ca 12 Hz in 4 and to ca 14 Hz in 7. Apparently, the reason of this striking difference originates in the orientation of the substituent bearing the C = X (X = O, N) bond.…”