AB-INITIO SCF-MO CALCULATIONS ON TRIAZA AND TETRAAZA CYCLIC AMINES

Molecular orbital ab initio calculations have been performed on some t riaza 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 formati on of intramolecular hydrogen bonds and the repulsions between two nit rogen protons or two nitrogen lone pairs. For each protonation state t here are distinct conformers depending on the number of NH bonds point ing 'inwards' relative to the macrocyclic cavity and on their ability to form intramolecular hydrogen bonds. All the systems display a marke d preference for a helical geometry, with the ethylenediamine bridges in a gauche conformation. The triaza systems having propylenediamine b ridges display an additional conformational freedom due to the large n umber of configurations available for these bridges. The calculated st ructures and energies are in good agreement with the experimental X-ra y and NMR spectroscopy data available.