AM1 andab initio studies on the internal reorganization energy of self-exchange electron transfer reactions of several quinone derivatives

Abstract

The semiempirical AM1 method, ab initio (HF/3-21G, 6-31G, 6-31G(d), 6-31+G(d)) and DFT (B3LYP/6-31G(d), 6-31+G(d)) methods were used to optimize the geometry of DDQ and its anion radical ⋯ ⋯ Nelsen’s model was used to calculate the internal reorganization energy λi of self-exchange electron transfer (ET) reactions. The calculated λi results of DDQ/DDQ⊸ by AM1 and B3LYP/ 6-31G(d), 6-31+G(d) methods are close to each other and consistent with the reported values; while those from Hartree-Fock methods are too large because of not considering the effect of electron correlation. The structure and ET behavior of MQ0/MQ 0 ⊸ couple were studied by AM1 and DFT (B3LYP/6-31G(d), 6-31+ G(d, p)) methods, and those of MQ0/MQ n ⊸ (n = 1–7) were studied by AM1 method for the first time. The results indicate that the values of the heat of formation of MQn increases with the increasing of the length of the isoamylene substituent chains. It also shows that the length of substituent has little effect on the bond lengths, bond angles of ring A, ring B and the dihedral angles of ring B, but it has considerable effect on the dihedral angles between the plane of ring A and oxygen atoms. The calculated λi results of MQ0/MQ 0 ⊸ and MQn/MQ n ⊸ (n = 1–7) indicate that not only the changes of high frequency vibrations, corresponding to the changes of bond lengths and bond angles, contribute to λi of MQn/MQ n ⊸ (n = 1–7), but also do those of the low frequency vibrations, corresponding to the changes of dihedral angles of MQn and MQ n ⊸ . The contribution from the change of low frequency vibrations between the plane of ring A and oxygen atoms to the λi values of MQn/MQ n ⊸ (n = 1–7) is greater than 1.60 kJ/mol; din addition, the contribution from the changes of the low frequency vibrations of the isoamylene substituent to the λi values of MQ4/MQ 4 ⊸ and MQ6/MQ 6 ⊸ is about 2 kJ/mol.