An n→π* interaction reduces the electrophilicity of the acceptor carbonyl group.

Carbonyl-carbonyl (C=O···C'=O') interactions are ubiquitous in both small and large molecular systems. This interaction involves delocalization of a lone pair (n) of a donor oxygen into the antibonding orbital (π*) of an acceptor carbonyl group. Analyses of high-resolution protein structures suggest that these carbonyl-carbonyl interactions prefer to occur in pairs, that is, one donor per acceptor. Here, the reluctance of the acceptor carbonyl group (C'=O') to engage in more than one n→π* electron delocalization is probed using imidazolidine-based model systems with one acceptor carbonyl group and two equivalent donor carbonyl groups. The data indicate that the electrophilicity of the acceptor carbonyl group is reduced when it engages in n→π* electron delocalization. This diminished electrophilicity discourages a second n→π* interaction with the acceptor carbonyl group.