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Expe ri ments var ying the 4KI con cen tra ti on did not show any de pen den ce of Ep on CA in theran ge analy zed, CA
= 0.05-5 mol dm-3, ν = 0.1 V s-1, (see Ta ble 1) whi le the
peak cur rent was found to be pro por ti o nal to ν1/2.
This be ha vi or of 4KI at low swe ep ra tes is in di ca ti ve of a cou pling of the ani on-radical, A•, to CO2. The ex pe ri men
-tal re sults show that the cou pling can be des cri bed by an ECE (steps 1, 2, 3, and 4) or DISP1 (steps 1, 2, 3’, and 4) me cha nism (see be low), the fun da men tal dif fe ren ce be ing the way the se cond elec tron up ta ke oc curs: he te ro ge ne ous (ECE) or ho mo ge ne ous (DISP). The ECE-DISP1 pro blem has been the sub ject of de ta i led the o re ti cal analy sis for se -ve ral elec tro che mi cal tech ni ques6,15,16. Cyclic vol tam -metry is a very po wer ful tech ni que in me cha nis tic analy sis but, in this par ti cu lar case it is not able to dis cri mi na te bet -we en ECE/DISP1, this im pos si bi lity re sults from the fact that the rds (step 2) is the same for both me cha nisms as sta -ted be low.
A + e A• (1)
A• + CO2 (ACOO-)• (2)
(ACOO-)• + e (ACOO-)- (3)
(ACOO-)• + A• (ACOO-)- + A (3’)
(ACOO-)- + CO
2 A(COO-)2 (4)
Furt her ad van ce in the me cha nis tic for mu la ti on was at temp ted by per for ming dou ble po ten ti al step chro no am pe -ro me tric ex pe ri ments. The po ten ti al was step ped f-rom an ini ti al res ting po ten ti al of -1.20 V to a po ten ti al of -1.70 V, whe re it was kept for a time θ, fi nally be ing step ped back to -1.20 V. Fi gu re 4 shows the the o re ti cal i(2)/i(θ) vs. log θ cur ves for the ECE/DISP1 ca ses22 as well as the ex pe ri
-men tal data ob ta i ned for 4KI + CO2. The ex pe ri men tal re
-sults were fit ted to the wor king cur ve by dis pla ce ment of the x axis. Whi le no ac cep ta ble fit can be ob ta i ned bet we en the ex pe ri men tal data and the the o re ti cal cur ve for an ECE me cha nism, a more than re a so na ble fit, at le ast in the log θ
≥ 2,0 re gi on, is ob ta i ned for the DISP1 me cha nism. Com pa ri son of the the o re ti cal wor king cur ve and the ex pe ri -men tal po ints le ads to k1 = 0.35 s-1. DISP1 ki ne tics were
ob ser ved for the re duc ti ve cou pling of ot her clas ses of c o m p o u n d s w i t h C O2 1 4 , 2 4. T h e r e d u c t i o n o f
1,4-benzoquinone24 sho wed an even slo wer cou pling cons -tant. When we com pa re the ra di cal-anion for med in the first elec tron trans fer of both com pounds one can con clu de that 4KI is much more uns ta ble than the BQ, so the cou pling cons tant is re la ted to the life time of the ani on-radical. This slow cou pling re ac ti on with the CO2 mo le cu le is pre dic ted
by Mar cus the ory. The cou pling rate of the ani on-radical (step 2) is sub ject to lar ge char ges due to the re or ga ni za ti o -nal energy of the sol vent as the li ne ar CO2 mo le cu le will
have to bend in or der to re act with the ani on-radical.
For the fast po ten ti os ta tic step re gi on (0.1 < θ < 10 ms) a mi xed be ha vi or is ob ser ved. In this re gi on, be si des the slow re duc ti ve ad di ti on of CO2 (step 2) the re ver se re ac ti
-on (step 1 of the me cha nism) starts to com pe te with the CO2 ad di ti on ex pla i ning the not to tally sa tis fac tory fit ting
ob ser ved for θ < -2 bet we en the ex pe ri men tal data and the DISP1 si mu la ted cur ve (Fig. 4). The cyclic vol tam metry for high swe ep ra tes, dis cus sed be low, sho wed the same evi den ces.
For hig her swe ep ra tes (ν > 0.8 V s-1) less dras tic chan -ges are ob ser ved in the C.V. in the pre sen ce of CO2. The
du pli ca ti on of the cur rent of the first cat ho dic peak, ve ri fi ed at lo wer swe ep ra tes (see Fig. 2B), is no lon ger ob ser -ved ap pro a ching the peak cur rent va lu es much clo ser to the ones ob ta i ned in the ab sen ce of CO2. A per fect fit bet
-we en cur ves A and B (Fig. 5) is not ob ser ved be ca u se it was not pos si ble to achi e ve high enough swe ep ra tes to
Fi gu re 5. Cyclic vol tam mo grams of 1 mmol dm-3 of 4KI in AN/0.1 mol dm-3 TBAP: (a) in the ab sen ce; (b) in the pre sen ce of 0.14 mol dm-3 CO2, ν = 5.0 V s-1.
Fi gu re 4. De pen den ce of i(2θ)/iθ on log θ. (•) ex pe ri men tal data for 1 mmol dm-3 4KI in AN/0.1 mol dm-3 TBAP in the pre sen ce of CO2 (0.14 mol dm-3). Con ti nu ous cur ves: the o re ti cal be ha vi or for: (a) DISP 1; (b) ECE me cha nisms.
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