The Structure of Acetaldehyde Enolate Anion

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Abstract
Non-empirical SCF–MO computations have been performed on [CH2CHO]−, the simplest enolate anion. Following a preliminary search of the conformational hypersurface using an STO-3G basis set to determine the C—C and C—O bond lengths as a function of rotation about the C—C bond, the rotation–inversion surface has been generated with an STO-4G basis set. The energy minimum of this surface is the all-planar anion whose C—C and C—O bond lengths are entirely compatible with the usual description [Formula: see text] The planar structure results from extensive conjugative stabilization amounting to over 30 kcal/mol (based on the C—C rotational barrier). When the CH2 plane is perpendicular to the CHO plane, the conjugative interaction is symmetry forbidden and the electron-withdrawing inductive effect of the formyl substituent leads to a pyramidal carbanion. Experimental implications of these results are discussed.

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