COMPETITIVE RESONANCE AT THE CARBONYL GROUP AS VISUALIZED BY THE NATURAL BOND ORBITAL ANALYSIS

Geometries and electron densities of 28 acyl compounds including aldeh ydes, ketones, carboxylic acids, esters, amides and ureas were studied by means of ab initio calculations at the Becke3LYP/6-31G level with the aid of localized bond orbitals. Methyl, phenyl, -OR and -NR2 subs tituents at a carbonyl carbon have been proved to donate electrons int o the pi(C=O) antibonding orbital of the carbonyl group to an increas ing amount, yielding a rising resonance at this group. The competitive behaviour of the two substituents at a carbonyl carbon is responsible for a high resonance stabilization of benzamides and ureas. These com pounds show only a low delocalization of lone-pair electrons at their nitrogen-atoms. All acyl derivatives which have been studied can be qu antitatively classified by this rule, independent of the nature of the electron donating substituent. The concept of competitive resonance i s verified by studies on 2-chloro-6-NR2-pyran-4-ones (NR2 = pyrrolidin o, dimethylamino, morpholidino, piperidino) at the HF/6-31G level. Th ese compounds contain several potent electron donating atoms, which ar e not directly connected to the keto group. The amount of conjugation of the keto group with the amine fragment and the chlorine atom, respe ctively, was found to depend on the nature of the NR2 moiety and its r otation relating to the common plane of resonance. (C) 1997 Elsevier S cience B.V.