MNDO, AM1, and PM3 study of the protonation tautomerization and valence tautomerization equilibria of some pyrrole macrocycles

The geometries, relative stabilities, proton affinities for tautomers of 1,3,6,10,12,15-hexamethyl-6,7,15,16-tetrahydropyrrolo[3,4-a]pyrrolo[3,4-i][16]annulene (1) and its possible fixed models (in which proton migration is eliminated) of 1,2,3,6,10,11,12,15-octamethyl-6,7,15,16-tetrahydropyrrolo[3,4-a]pyrrolo[3,4-i][16]annulene, of 1,2,3,6,8,10,12,15-octamethyl-6,7,8,15,16-pentahydro-2H-pyrrolo[3,4-a]pyrrolo[3,4-i][16]annulene, 1,3,5,7,10,12,14,16-octamethyl-5,6,7,14,15,16-hexabydro-2H-pyrrolo[3,4-a]2H-pyrrolo[3,4-i][16]annulene, molecules (2) were calculated with fully geometry optimization using MNDO, AM1 and PM3 methods in the gas and aqueous phases. A parallelism between the experimental acidity constant values pK(a) and of the computed acidity constants, pK(a) was search to clearing the tautomeric forms. (C) 2002 Elsevier Science B.V. All rights reserved.