Calculated molecular properties of tautomeric species

The geometries, tautomerization energies, and dipole moments of the tautomeric species of the 2-amino-2-oxazoline heterocycle were calculated with full geometry optimization using semiempirical (MNDO, AM1 and PM3) and nb initio (RHF/6-31G) methods. The planar conformation of the amino form is well reproduced by the semiempirical and by the db initio methods as compared with X-ray structures in which this form is found. However the exocyclic nitrogen atom is pyramidalized in the semiempirical geometries whereas the ab initio method reproduces a sp hybridization state. The twisted conformation of the imino form is better reproduced by the ab initio calculation. All methods (semiempirical and nb initio) show a stability energy value in favour of the amino tautomer of the neutral heterocycle. On the contrary, the dominant species of the charged heterocycle is the imino tautomer, protonated on the exocyclic nitrogen atom. Molecular electrostatic potential calculations confirmed this tendency. Finally the 3D-lipophilicity profile of the heterocycle completed the investigation of the tautomerism.