CALCULATED MOLECULAR-PROPERTIES OF TAUTOMERIC SPECIES

The geometries, tautomerization energies, and dipole moments of the ta utomeric species of the 2-amino-2-oxazoline heterocycle were calculate d with full geometry optimization using semiempirical (MNDO, AM1 and P M3) and nb initio (RHF/6-31G) methods. The planar conformation of the amino form is well reproduced by the semiempirical and by the db initi o methods as compared with X-ray structures in which this form is foun d. However the exocyclic nitrogen atom is pyramidalized in the semiemp irical geometries whereas the ab initio method reproduces a sp hybridi zation state. The twisted conformation of the imino form is better rep roduced by the ab initio calculation. All methods (semiempirical and n b initio) show a stability energy value in favour of the amino tautome r of the neutral heterocycle. On the contrary, the dominant species of the charged heterocycle is the imino tautomer, protonated on the exoc yclic nitrogen atom. Molecular electrostatic potential calculations co nfirmed this tendency. Finally the 3D-lipophilicity profile of the het erocycle completed the investigation of the tautomerism.