THE MECHANISM OF INNER-HYDROGEN MIGRATION IN FREE-BASE PORPHYRIN - AB-INITIO MP2 CALCULATIONS

Ab initio RHF SCF geometry optimizations and MP2 limited geometry opti mizations are used to investigate the mechanism of inner-hydrogen migr ation in free base porphyrin. Previous approximate SCF results using A M1, MNDO, and PM3 methods predict the existence of a highly stable cis isomer close in energy to the most stable trans form and that trans r eversible arrow trans interconversion proceeds in an asynchronous two- step fashion via this intermediate. However, the calculated activation energies are very much greater than those observed. Here, at the ab i nitio MP2 level of correlation, it is found that all (classical) barri ers are substantially reduced in height, becoming compatible with expe riment: the trans to cis activation energy is 16.7 kcal/mol, the trans to trans saddle energy is 19.3 kcal/ mol, and the relative energy of the cis isomer is 10 kcal/mol. Hence, an asynchronous path remains pre ferred. The reaction coordinate at small displacements is seen to corr elate with the pyrrolic hindered rotation nu(35), observed at 109 cm(- 1); for use in one-dimensional models, we find that the involvement of NH bending and stretching motions in the reaction at large displaceme nts results in a globally optimized effective reactant reaction coordi nate frequency of 600 cm(-1). Using PM3, zero-point energy considerati ons are shown to account for just over half of the observed inner-hydr ogen isotope effect and to lower the classical activation energy by ca . 5 kcal/mol. The net activation energy is thus estimated to be 12 kca l/mol, no doubt fortuituously close to the value of 13 kcal/mol which has been deduced from experiment using simple tunneling theories.