Abstract
Electron densities on the olefinic carbons of some dimethylene compounds are calculated by semiempirical model PM3, and the values are compared with those of the corresponding monomethylene compounds. A relationship is observed between the change of electron density Δ(C) and the change of 13C NMR chemical shift Δδ. A correlation is also found between Δ(C) and Δ(O), i.e. the carbon and oxygen atoms, of methylene-ketones. The electron density differences are induced by three types of transannular interactions: electrostatic force, orbital interactions through bond (TB) and orbital interactions through space (TS). Electrostatic force is the most prominent factor for ketones in which the high polarity of carbonyl group can apply a strong field effect onto nearby atoms. For nonpolar methylene groups, orbital interactions can be transmitted either TS or TB more effectively than electrostatic force. A conformational analysis, performed on a dimethylene compound 4e, confirms the existence of TS interaction between the double bonds.
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