Binding energies of metal monocations to Β-lactones and Β-lactams. A theoretical study of cyclization effects

Abstract

Ab initio calculations have been performed to study the association ofΒ-propiolactam andΒ-propiolactone and their aliphatic analoguesN-methyl acetamide and methyl acetate with different metal monocations: Li⁺, Na⁺, Mg⁺, and Al⁺, in an effort to investigate cyclization effects on the gas-phase basicities of amides and esters, when the reference acid is not a proton. Similarly to what was found for protonation,N-methyl acetamide andΒ-propiolactam are more basic than methyl acetate andΒ-propiolactone, when the reference acids are the aforementioned metal monocations. However, cyclization effects on the corresponding binding energies for both kind of compounds do not parallel those observed for protonation energies, andΒ-lactone is as basic as methyl acetate when the reference acid is Li⁺ and slightly more basic than methyl acetate when the attaching ion is Na⁺. This implies that when the interactions of the bases with the reference acids are essentially electrostatic the reactivity patterns change with respect to those observed when the interactions are essentially covalent.