Theoretical study of the N-H tautomerism in free base porphyrin

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