Direct evidence for mode-specific vibrational energy relaxation fromquantum time-dependent perturbation theory. II. The v_4and v_7modes of iron-protoporphyrin IX and iron porphine

摘要

The mode-specific vibrational energy relaxation (VER) of the iron-protoporphyrin IX (the heme)was studied using a non-Markovian time-dependent perturbation theory at the UB3LYP/6-31G(d)level. The derived VER time constants of the excited v4 and v7 modes, 1.2 ± 0.1 and 2.1 ± 0.1 ps,respectively, agree well with previous experimental results for MbCO (1.1 ± 0.6 ps for the v4 modeand 1.9 ± 0.6 ps for the 1,7 mode). The energy transfer pathways from the excited v4 and v7 modeswere identified. The different symmetries of the v4 and 1,7 modes are reflected in distinct relaxationpathways. No direct energy transfer between the 1,4 and 117 modes was observed. The overtone of the—350 cm-1 iron out-of-plane y7 mode was observed to be strongly coupled to the v7 mode andessential to its relaxation. The two isopropionate side chains of the heme were found to play anessential role in the VER mechanism for both v4 and v7 modes, providing the mode-specific levelexplanation to previous observations. Comparison of the results for VER in iron porphine (FeP) toresults for the imidazole-ligated porphine model (FeP-Im) demonstrates that the axial Im ligand haslittle effect on the v4 or v7 mode relaxation processes. By considering the VER process as amultistep reaction and the third order Fermi resonance parameters the reaction rate constants, theVER kinetics of FeP was examined by solving the master equation.