THEORETICAL-STUDY OF THE N-H TAUTOMERISM IN FREE-BASE PORPHYRIN

The N-H tautomerism of free base porphyrin is investigated at the semi empirical spin-unrestricted AM1 (UAM1) and ab initio RHF/3-21G levels. The UAM1 method provides delocalized geometries for all stationary st ructures without imposing any symmetry constraint. RHF/3-21G geometry optimizations have to be performed under symmetry restrictions to ensu re that realistic delocalized structures are obtained. Both the semiem pirical and the ab initio calculations predict that the interconversio n between trans tautomers proceeds in an asynchronous two-step process via intermediate cis tautomers. The cis tautomers are characterized a s minima in the potential energy surface and are 8-10 kcal mol(-1) hig her in energy. The activation energy for the trans --> cia interconver sion is calculated to be approximately 23 kcal mol(-1) at the 3-21G le vel. The activation energy for the synchronous trans --> trans interco nversion is higher and has a value of 30.5 kcal mol(-1). The activatio n energies obtained at the semiempirical UAM1 level are twice as large as the ab initio values.