Ground and ionized state intramolecular hydrogen bonds in azomethine compounds

Abstract

The electronic structure of 2-hydroxybenzylideneamine and of 3-iminoprop-1-enol, a model system for intramolecularly hydrogen-bonded azomethine compounds, was investigated by means of ab initio calculations at STO-3G level. The results show that the removal of a N_2p non-bonding electron destabilizes the hydrogen-bonded structure. Ionization from the O_2p-based molecular orbital gives an ion state which is stabilized by the hydrogen bonding, although the hydrogen-bonding energy is predicted to be smaller than in the neutral molecule.

ΔSCF N_2p and O_2p ionization energies both increase on hydrogen-bond formation, thus being in agreement and apparently at variance, respectively, with known experimental trends. Relative molecular orbital energies are significantly dependent on the individual relaxation energy, which is calculated to be smaller in the bonded than in the non-bonded structure. The Mulliken population analysis shows that the decrease in the hydrogen-bonding energy in the O⁺_2p ion, with respect to the neutral molecule, can be traced back to a decrease in the positive charge on the proton and in the reduced N⋯O electrostatic repulsion.

The hydrogen-bonding destabilization in the N⁺_2p ion can be associated with a reverse shift in the proton charge with respect to the neutral molecule and with O—H bond polarization and N⋯O coulombic repulsion in the non-bonded conformation being smaller than in the neutral molecule.