Theoretical studies on the self-exchange electron-transfer reaction between 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radical anion DDQ(-.)

The structure of 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radi cal anion was optimized by semi-empirical AM1, PM3, ab initio HF/3-21G, 6-3 1G, 6-31G(d), 6-31+G(d), and density functional B3LYP/6-31G(d), 6-31+G(d) m ethods. Nelsen's model was used to calculate the internal reorganization en ergy hi. The results of hi from the AM1, PM3 and B3LYP methods are reasonab le, while those from the HF method are too large because of not considering the effect of electron correlation. The linear reaction coordinate R was u sed to construct the double-well potential surface for the donor and accept or, and then the transition state was determined at the point of R = 0.50. Then Marcus's two-sphere model was applied to estimate the solvent reorgani zation energy lambda (o) in different solvents CH3CN, benzonitrile, acetone , CHCl3, and CH2Cl2. The electron transfer (ET) matrix element V-rp was cal culated using two-state model. The self-exchange ET reaction rate constants k(ET) in different solvents were calculated and the results were consisten t with the experimental values. The reason why the behavior of self-exchang e ET reaction between DDQ and DDQ(-.) does not correlate with Marcus's theo ry was discussed as well. (C) 2001 Elsevier Science B.V. All rights reserve d.