文摘
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.