Abstract
Extremely low rotational barriers for ethylenic C=C double bonds in the range 5–17 kcal/mol have been measured by low temperature 1H and 13C NMR spectroscopy and confirmed by calculations at the M06‐2X level for 5‐methylene‐1,3‐dioxane‐4,6‐diones (Meldrum's acid derivatives) and 4‐methyleneisoxazol‐5(4H)‐ones. The barriers are ascribed to the push‐pull (donor‐acceptor) nature of the alkenes. A correlation between the calculated C=C bond lengths and the ratios of electron occupancies in the ethylenic π and π* orbitals (the π/π* quotient) constitutes a useful measure of the push‐pull character.
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