Role of Cu+ association on the formamide → formamidic acid → (aminohydroxy)carbene isomerizations in the gas phase

The structures, relative stabilities, and bonding characteristics of complexes of formamide, formamidic acid, and (aminohydroxy)carbene with Cu+ have been investigated through the use of high-level density functional theory (DFT) calculations. Geometries were optimized at the B3LYP/6-311G(d,p) level, while the final energies were obtained using a 6-311+G(2df,2p) basis set expansion. Contrarily to what happens upon association with alkali metal cations, the energy gaps between formamide and its two isomers decrease significantly upon Cu+ association, because formamidic acid and (aminohydroxy)carbene exhibit higher Cu+ binding energies than formamide. This behavior is similar to the one exhibited in the protonation processes, indicating that the neutral-Cu+ interactions have a sizable covalent character. Nevertheless, the isomerization processes among the systems studied (formamide, formamidic acid, (aminohydroxy)carbene) are not catalized by Cu+ ions, since in general the hydrogen transfers involved in such isomerization processes result in a weakening of the interaction between the neutral and the metal cation. The only exception to this general behavior corresponds to the isomerization between the nitrogen attached formamide complex and the corresponding enolic species, because in this particular case the hydrogen shift enhances the ion-neutral interaction.