Quasireversibility

To extend the applicability of the SECM feedback mode for studying ET processes at ITIES, we have formulated a numerical model that fully treats diffusional mass transfer in the two phases [49]. The model relates to the specific case of an irreversible ET process at the ITIES, i.e., the situation where the potentials of the redox couples in the two phases are widely separated. A further model for the case of quasireversible ET kinetics at the ITIES is currently under development. For the case where the oxidized form of a redox species, Oxi, is electrolytically generated at the tip in phase 1 from the reduced species, Red], the reactions at the tip and the ITIES are ... 

The quasireversible case. When Eq. (5.4.22) cannot be simplified as described under alternatives 2 and 3, then it should be considered that the first term on its right-hand side increases compared to the second term as the potential becomes more positive and decreases at more negative potentials. As the velocity of convection increases, the second term increases compared to the first term. 

It is not uncommon that in electron transfer processes one observes that at low scan rates the process behaves reversibly, whereas at high scan rates the process behaves irreversibly (such behaviour is more easily seen for processes that are not complicated by coupled reactions). Processes occurring in the transition zone between reversible and irreversible behaviour are called quasireversible. 

A quasireversible process occurs when the rate of the electron transfer Ox + ne — Red is of the same order of magnitude as the mass transport (concomitantly, the inverse reaction Ox + we — Red has a non-negligible rate). 

Since in cyclic voltammetry the potential scan rate commonly ranges from 0.02 V s-1 to 50 V s-1, it follows that for a quasireversible process ... 

Coming to the mathematical treatment of the quasireversible case, the solution of the differential equations for Ox and Red needs, as boundary condition at the electrode surface, the relation ... 

The first effect that one notices in the voltammetric response is that quasireversibility induces a separation between the forward and the reverse peaks i.e. the peak-to-peak separation A p) much greater than that of a reversible process (to determine this parameter correctly one must use instrumentation able to compensate for the solution resistance). 

In order to understand better the distinction between reversibility, quasireversibility and irreversibility in cyclic voltammetry, the typical cyclic voltammetric profiles for the three cases are represented in Figure 8. 

Diagnostic Criteria to Identify a Quasireversible Process. A quasireversible process is characterized by determining either the thermodynamic parameter E° or the kinetic parameters a and k°. 

On the basis of the previous discussion, the criteria for defining a process as quasireversible are the following. 

In acetonitrile solution, the Cu Cu1 reduction of [CunL]2+ appears as a quasireversible process, giving rise to a forward-backward peak-system with AEp = 114 mV, at a scan rate of 0.1 V s l. 

In contrast, let us examine a case of very important structural consequences. As we will discuss in Chapter 7, Section 2.4, the carbonyl cluster [Os6(CO)18] undergoes a two-electron reduction with marked features of electrochemical quasireversibility AEp = 255 mV at 0.01 Vs 1. 

We must conclude that the marked electrochemical quasireversibility is due to the remarkable geometrical reorganization from the bicapped tetrahedron of [Os6(CO)18] to the octahedron of [Os6(CO)i8]2 occurring upon the two electron addition. 

It is important to underline finally that quasireversibility is an electrochemical criterion and it does not means partial chemical reversibility . 

Finally, if the reduction process of the adsorbed species is quasireversible the curve will exhibit the reverse peak, but the peak-system will not be completely symmetric. 

As expected, it shows in dichloromethane solution a quasireversible one-electron oxidation ( ° = + 0.54 V, vs. Ag/AgCl).47... 

These 3d1 complexes display a quasireversible one-electron oxidation corresponding to the chemically reversible step [VOivL]/[VOvL] + (L = Schiff base ligand). As an example, Figure 3 shows the cyclic voltammetric response of [VO(acen)].7 The relative potential values are reported in Table 3. 

It should be emphasized that [VO(salen)] in its oxidized form [VO(salen)]+ ([C104]- counteranion) maintains the original square-pyramidal geometry, but, in accord with the electrochemical quasireversibility of the couple Viv/Vv, there are some variations in bond... 

From a speculative viewpoint, the quasireversibility of the electrochemical reduction Viv/Vm could account for such structural rearrangements. 

In the absence of crystallographic data one cannot discuss in detail the structural variations triggered by these reduction processes, but their electrochemical reversibility, or quasireversibility, suggests that there are not significant structural rearrangements. 

As a further confirmation that cuboidal tetramanganese complexes can be redox active, the phosphinate derivative [Mn404(02PPh2)6] illustrated in Figure 46 displays in dichloromethane solution a quasireversible Mn2inMn2IV-MnUIMn3IV oxidation ( ° = +1.08 V ra. SCE).65... 

The electrochemical behaviour of this acetato complex has not been reported, but the analogous benzoato derivative [Mn402(C>2CPh)7 (bipy)2]+ in dichloromethane solution displays a quasireversible one-electron reduction to the corresponding neutral derivative MnnMn3in E°< = + 0.28 V vs. SCE) and a quasireversible one-electron oxidation to the corresponding dication Mn3mMnIV ( 4/ = +1.34 V).66... 

The complex in which two molecules of trifluoromethanesulfonate of the preceding compound have been substituted by two molecules of water, namely [Mn402(tpdp)2(H20)4]5 +, has an analogous crystal structure,76 and analogously exhibits a quasireversible one-electron oxidation (E01 = +0.93 V vs. SCE) and a quasireversible one-electron reduction (E° = +0.39 V).76... 

In CH2C12 solution the last complex displays a partially chemically reversible one-electron oxidation (E01 = +0.81 V vs. SCE) and a quasireversible one-electron reduction.82... 

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