Ab initio SCF MO calculations on triaza and tetraaza cyclic amines

Abstract

Molecular orbital ab initio calculations have been performed on some triaza and tetraaza cyclic amines aiming at the determination of their minimum energy geometries and of the relative conformational energies in different protonation states.

The results clearly show two factors governing the conformational preferences in these systems: the formation of intramolecular hydrogen bonds and the repulsion between two nitrogen protons or two nitrogen lone pairs.

For each protonation state there are distinct conformers depending on the number of NH bonds pointing ‘inwards’ relative to the macrocyclic cavity and on their ability to form intramolecular hydrogen bonds. All the systems display a marked preference for a helical geometry, with the ethylenediamine bridges in a gauche conformation. The triaza systems having propylenediamine bridges display an additional conformational freedom due to the large number of configurations available for these bridges. The calculated structures and energies are in good agreement with the experimental X-ray and NMR spectroscopy data available.