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epoc ABSTRACT: 1 H NMR spectroscopy was used to investigate hydrogen bonding in the structurally related (Z)-and (E)-5-substituted-2-alkylidene-4-oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these typical push-pull alkenes consist of the intramolecularly H-bonded E-isomer and intermolecularly H-bonded Z-isomer in varying proportions which depend on the solvent polarity. For a representative of the series, (E)-(5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1), the lack of a concentration and temperature dependence of the large chemical NH shift ( 12.06 ppm) in CDCl 3 indicates strong intramolecular resonance-assisted hydrogen-bond formation (RAHB). The upfield chemical shifts of the NH proton of the (Z)-1 isomer as a function of temperature increase and the large 1 H NMR Á/ÁT value (À11.82 ppb C À1 , Z/E

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