Skeleton reaction profile

For the parent compound porphyrin, an Arrhenius curve pattern of the type discussed in Fig. 6.9 is observed. Noteworthy is the same low-temperature slope of the Arrhenius curves of the FIFI and DD reaction in Fig. 6.23, i.e. of the H- and D- reaction in Fig. 6.24. will be mainly caused by the asymmetry of the reaction profile because at least the energy of the cis-intermediate is required for tunneling to occur, but also the reorganization energy of the ring skeleton will contribute. Note also that the low-temperature kinetic FI/D isotope effect is smaller... 

Detailed kinetic studies of the reaction of Fe(II) in cyclophane hemes with 02 and with CO probed polarity and steric effects the effects of deformation of the porphyrin skeleton from planarity were assessed for one compound (121). Volume profiles have been established for reactions of a lacunar Fe(II) complex with CO (122 for myoglobin with 02 and with CO and for hemerythrin with 02 (123). 

These data form part of an increasing body of evidence that demonstrates the significant conformational flexibility of the porphyrin skeleton in the solid and in solution. Since a wide spectrum of photosynthetic and enzymatic reactions proceed via tt radicals of porphyrin derivatives, the consequences of the macrocyles flexibility are considered for distances and orientations between donors and acceptors, interactions with ligands, nearby charges or polar groups, electronic profiles and the gaps between the aj and a u highest occupied orbitals. 

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