Mechanism and quantum mechanical tunneling effects on inner hydrogen atom transfer in free base porphyrin: A direct ab initio dynamics study

A theoretical study of the mechanism and kinetics of the inner double hydro gen atom-transfer process in free base porphyrin is presented. Our analysis reveals that the stepwise mechanism first requires the porphyrin ring to c ompress at an approximate cost of 8.7 kcal/mol, followed by transfer of a H atom with an additional energy requirement of 8.2 kcal/mol. Solvent effect s were investigated using a dielectric continuum model and found to be smal l. The forward and reverse rate constants for the hydrogen atom-transfer pr ocess of transisomer <-> cis-isomer were calculated using a canonical varia tional transition-state theory augmented by multidimensional semiclassical tunneling approximations in the temperature range of 200-1000 K. The calcul ated activation energy of 10.8 kcal/mol in the temperature range of 200-300 K agrees well with the available experimental data. We found that tunnelin g is significant for both the forward and reverse trans cis tautomerization processes, especially in the low-temperature range.