ION-PAIR FORMATION BETWEEN THE ELECTROGENERATED 2,3-DICHLORO-5,6-DICYANO-1,4-BENZOQUINONE DIANION AND THE SODIUM-ION AT PLATINUM SURFACES

The electrochemical reduction at platinum electrodes of 2,3-dichloro-5 ,6-dicyano-1,4-benzoquinone (DDQ) in the presence of Na+ in acetonitri le solution was studied using various electrochemical measurements und er conditions with significant mass transport, namely channel flow- an d sono-voltammetries. In channel electrode voltammetry, the presence o f Na+ was observed to change the voltammetric response from that of pu re DDQ alone. In particular, at potentials corresponding, in the absen ce of Na+, to the two electron reduction step of DDQ, a spike-like inc rease and decrease of reductive current was observed on negative going potential scans after which the current returned to the level expecte d for a one-electron reduction. This behavior can be explained by the following reaction scheme with the formation of a passivated surface d ue to the adsorption of Na-2(+) DDQ(2-). DDQ(2-) + Na+ --> Na+ DDQ(2-) Na+ DDQ(2-) --> Na+ DDQ(ads)(2-) Na+ DDQ(ads)(2-) + Na+ DDQ(ads)(2-) --> Na-2(+) DDQ(ads)(2-) + DDQ(2-) This scheme was found to be consist ent with independent sono-voltammetric data and with in situ electroch emical ESR measurements. (C) 1998 Elsevier Science S.A. All rights res erved.