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Electrode reactions of organic compounds as a function of pressure
Electroanalytical Chemistry and Interracial Electrochemistry, 60 (1975) 313-321
313
Z2) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ELECTRODE REACTIONS OF ORGANIC COMPOUNDS AS A FUNCTION OF PRESSURE
M. FLEISCHMANN, W. B. GARA* and G. J. HILLS
Chemistry Department, U'niversity of Southampton, Southampton S09 5NH (England) (Received 13th December 1974)
INTRODUCTION
A major aim of this investigationhas been the development of instrumentation for the study of electrode reactions in non-aqueous media as a function of hydrostatic pressure up to 2000 atm. In this paper we report results obtained of cyclic voltammetry for the reversible formation of a number of radical ions. Previous work on the pressure coefficient of electrode processes of organic species in aqueous media by means of polarography has been reviewed by Hills et al. 1-4. Claesson et al. s have examined the effect of pressure on the solvation of ion-pairs of the fluorenyl radical anion and calculated the value of AV° for the change from a tight ion-pair at low pressures to a loose ion-pair at high pressures. Peover has investigated and reviewed 6'7 the effect of solvation on the oxidation and reduction potentials of polycyclic aromatic hydrocarbons and other organic compounds. THE PRESSURE COEFFICIENT OF CYCLIC VOLTAMMETRIC PEAK POTENTIALS AS A MEANS OF ESTIMATING AVe
The formation of charged species in solution is accompanied by electrostriction of the solvent so that the ion has a lower potential energy in solution as compared to in vacuo. It is evident that changes in the solvation energy will affect the standard potential E e. For the process O+ne~R the peak potential, Ep, is related to Ee by E e = E r + (R T/nF) [ln (Do~DR) + In (ao/a,) ] + 0.029/n
(1)
Since AGe= - nFE e
(2)
and
(3)
(~3AGe/SP)r = AVe
we obtain (Ee)i,2 - (Ee)p, -- - (A Ve/nF)(P2 - P1) * Department of Chemistry, University College, London WC1H OAJ, England.
(4)
314
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
As the "internal pressure" of the system will be several orders of magnitude higher than the applied hydrostatic pressure we may assume that the ratio of the compressibilities of the solvated radical ions to that of the parent hydrocarbons will not change markedly up to 1000 atmospheres. Therefore the ratio Do/D R will stay sensibly constant. It has also been suggested 4 that the activities do not change significantly in this range. Therefore we obtain from eqns. (1) and (4) (Ep)e2 - (Ep)e, = - (A Ve/nF)(P2 - P1)
(5)
It is particularly convenient to use E v in these measurements since there is no accumulation of products, the reaction being reversible on the time scale of these experimentsS-, o. EXPERIMENTAL
The cell developed for this investigation is shown in Fig. 1. The electrodes sealed through glass were fitted into the cell metal body using sets of O-rings; one in each set was inert to the electrolyte solution and the other to the hydraulic fluid (Shell Risella 23) used to transmit the hydrostatic pressure via the metal bellows. The design of the hydraulic system was conventional I the pressure being monitored by Bourdon gauge (Budenberg) graduated in steps of 500 psi (34 atm). The pressure vessel was in turn thermostatted in an oil bath at 25°C.
A H
R
B C
D
E
N
M
P
F
D
K Fig. 1. High pressure electrochemical cell. (A) Upper block (stainless steel); (B) "O'-ring (EP 127/75); (C) "O"-ring (PB 80); (D) washer (stainless steel); (E) screw cap (brass); (F) counter electrode body (glass); (G) heavy walled capillary (glass); (H) working electrode body (glass); (I) bellows (stainless steel); (K) reference electrode body ( glass); (L) reference solution; (M) electrolyte solution; (N) working electrode (platinum button); (O) counter electrode (platinum coil); (Q) capillary opening; (R) platinum wire; (S) lower block (stainless steel).
The working electrode consisted of a platinum button sealed into glass and polished so as to expose a flat circular area of diameter 1 mm. The platinum counter electrode was connected to the working electrode via capillary of inner diameter 2.5
VOLUMES OF ACTIVATION
315
ram. The thin layer of electrolyte trapped in the gap between the two tubes served as the capillary probe connection to the reference electrode. This consisted of the I3, Iz/I- couple in contact with a platinum electrode. This design proved to be an adequate compromise between the need to transmit the pressure throughout the cell and the need to minimise diffusion of the reference system to the working electrode, Adequate uniformity of the electric field was ensured by making the capillary connection between the working and counter electrodes 10 mm in length. Residual ohmic losses were compensated for by using positive feedback in the potentiostat. In order to optimise signal to noise the potentiostat was built from integrated circuits in a screened box mounted on the lid of the pressure vessel. The well known De Ford ~~ configuration was used. Voltage wave forms for cyclic voltammetry were obtained from Chemical Electronics RB [ generator modified so as to give single sweeps. The output from the current follower was offset and stored in a Hi-Tek .," ........ ,,,%.
(•)
•
I
%.. '",,°.,.
.' ,,.
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." A -'
,..o..,,. .'""
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.,-
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....'"
.o.
i I
I
-1,875
•
-'L995
'
I
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-2.149
-2.259
." ,,,..,...,
,,.,,.,,,,
E/V
• '.,,, .,,.. "..,
~ ,..,..,.,.,.
..'
(b) IO0}JA
D s.~ ¢" ....-
I
-1,995
E
E/V
1
-2.149
Fig. 2. Cyclic voltammetry of perylene at 2000 atm. Sweep rate, 6 V s-~; no. of sweeps averaged, 8. (A) Forward sweep; (B) reverse sweep; (C) background current (horizontal sensitivity, 3 mV per data point), (D) expansion of A in the region indicated; (E) background current (horizontal sensitivity, 0.6 mV per data point). The solid horizontal lines represent zero current.
signal averager type AA1. The 256 data points were divided so as to read 128 points on both the forward and reverse .sweeps; an example is shown in Fig. 2A. By introducing a time delay and more closely spaced data points (0.6 mV/data point)
316
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
the peak potential could be determined more accurately (Fig. 2B). A 4 chart paper was used throughout for the X-Y display. The signal to noise could be improved further by averaging repeated scans. Using this technique and the precautions outlined below the reproducibility was + 1 mV over 12 h. Voltammograms were recorded at a sweep rate 6 V s- ~ and the reversibility of each system was checked using the range 0.6-60 V s-x at pressures of 1, 1000 and 2000 atmospheres. The absence of systematic errors was checked by measuring AEp versus P both for increasing and decreasing pressures. All reactions except the oxidation of 9,10-dimethylanthracene were fully reversible at 6 V s -1 while this latter oxidation in turn became reversible at 60 V s-1. The polynuclear hydrocarbons [9,10-diphenylanthracene (9,10-DPA), perylene (Koch Light)and 9,10-dimethylanthracene (9,10-DMA) (Aldrich)], fluorenone and ferrocene (B.D.H.) were all of high purity and were used as received. Tetraethylammonium borofluoride (from tetraethylammonium bromide and sodium borofluoride) was purified by repeated recrystallisation from water until the product gave a negative halide test, and dried in a vacuum oven at 80 ° for 2 days at 0.1 Torr, and kept over P205. Acetonitrile (B.D.H.) was purified as described by Walter and Rameley (method B) a2 and stored in a glove box containing P205 and an atmosphere of high purity nitrogen at positive pressure. Acetonitrile purified in this manner gave very satisfactory background voltammograms when tested under experimental conditions over the whole potential and pressure ranges used in the experiments described below. Aluminium oxide was prepared from alumina, activated by heating at 300°C for 5 h, allowed to cool in vacuo and stored in the drybox. Tetra-n-butylammonium triiodide was kindly supplied by Dr. P. J. Ovenden. The cells and volumetric flasks containing the weighed out base electrolyte were dried in a vacuum oven at 100°C and 0.1 Tort for 4 h. All further operations were carried out in the glove box. All solutions (excepting those used in the reference electrode) were stirred magnetically over activated alumina. Ten per cent by volume of activated alumina was also placed in the cell. RESULTS AND DISCUSSION
The pressure coefficients were measured for the following systems: Pt
9,10-DPA (satd), 0.3 M Et4NBF 4 in acetonitrile solution
0.3 M Et4NBF 4+0.001 M Bu4NI3 in acetonitrile solution
Pt
Pt
perylene (satd), 0.3 M Et4NBF4 in acetonitrile solution
0.3 M Et4NBF 4 +0.001 M Bu4NI 3 in acetonitrile solution
Pt
Pt
9,10-DMA (satd), 0.3 M Et4NBF4 in acetonitrile solution
0.3 M Et4NBF 4 +0.001 M Bu4NI3 in acetonitrile solution
I Pt
J
317
VOLUMES OF ACTIVATION
fluorenone (0.001 M), 0.3 M Et4NBF~ in acetonitrile solution
Pt
I I 0.3 M Et4NBF4
I Pt
II +0.001 M Bu4NI3 II in acetonitrile solution
I
ferroccne (0.001 M), 0.3 M Et4NBF 4 I I 0.3 M Et4NBF 4 in acetonitrile solution II +0.001 M Bu4NI3 II in acetonitrile solution
Pt
I Pt
[
For the first three systems the changes in peak potential [bEp=(Ep)xatm(Ep)l,,m] t;s. pressure are shown both for their one-electron reduction and oneelectron oxidation reactions in Fig. 3 (A to F). For the one-electron reduction of fluorenone and one-electron oxidation of ferrocene the plots are shown in Fig. 3 (G and H). 20G .
lO
A
5Ep/ /my o10-
C
E
"',,.
"'""""".
"''..
,
:::::i .............. "'".°, ,.
"'''.
• , . , ... H
"i"
20-
1000
2000
Au
P/ATM
1000
2200 000
~) . . . .
P/ATM
1OOO i . . . . 2 0 0 0i
YATM
O ....
1OOO i ....
2 0 0r0
P/ATM
Fig. 3. Plots of peak potential change vs. pressure (uncorrected). Reductions: (A) 9,10-DPA, (C) perylene, (E) 9,10-DMA. (G) fluorenone; oxidations: (B) 9,10-DPA, (C) perylene, (E) 9,10-DMA, (H) ferrocene. ~Ep = (Ep) ..... - - ( E p ) l atm"
The actual values of Ep at 1 atm v s . the 13, I2/I-/Pt reference electrode are summarised in Table 1. There is, however, some uncertainty relating to the values of the peak potential at pressures greater than 1 atm. This is because the potentials were obtained in relation to a reference electrode potential, which is conventionally taken as zero volts. It cannot, however, be assumed that the potential of the reference electrode is independent of pressure. It was with this in mind that the determination of 6Ep v s . TABLE ]
Compound
Ep ( oxidation )/V"
Ep (reduction)~ V"
9,10-DPA Perylene 9,10-DMA Fluorenone Ferrocene
+0.892 + 0.697 +0.746
-2.261 - 2.105 -2.394 - 1.670
+ 0.072
a To obtain potentials vs. aq. SCE add + 0.30 V.
318
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
pressure for ferrocene (Fig. 4H) was undertaken• Ferrocene has been used to allow for changes in liquid junction potentials when the same reference electrode was used in a range of solvents a3. It is clear that the changes in potential shown in Fig. 3 (A to G) are due to the combined effects at the working and reference electrodes. It has been suggested that the ferrocene/ferrocinium couple is essentially unaffected by changes of solvent in work at ambient pressures, i.e. the solvation energy is small and constant. In order to make a first estimate of the changes in the 13, 12/I- potential with changes in pressure we have made the analogous assumption of constancy of the ferrocene/ ferrocinium couple in these experiments• Within the limits of this approximation Fig. 3H may be attributed to the changes at the reference electrode• It can be seen that there is a marked decrease in 6Ep with P at ~ 1000 atm which is probably due to complete dissociation of the triiodide ion (I;)+,,,~ ~ (I2),,,,~+ (I-)+o,,, If we take the potentials shown in Fig. 3H as the baseline, we obtain the corrected data in Fig. 4 (A to G) which can now be attributed to the formation of the 2010.
C
~Ep/ /mY
O-
E
,L." "°
L~
.• ....••..•..... •,..•,.,..
''"'"•.
',..,
10.
•
•.
¥'
6 20I ' '
0
'
+
[
;
1000
I
I
I
J
2000 0
I
t
''
I
'
1000
'
i
,
I
'
2000 0
'
' , I
',
1000
'
'
I
I
2000 0
I
'
'
'
I
'
1000
r
'
J
I
2000
Fig. 4. Plots of peak potential change vs. pressure, after correction for the pressure dependence of the reference potential. Reductions: (A) 9,10-DPA, (C) perylene, (E) 9,10-DMA, (G) fluorenone; oxidations; (B) 9,10-DPA, (D) perylene, (F) 9,10-DMA. 6Ep= (Ep)+,,m.-(Ep)I +,m"
radical ions themselves. The validity of this argument can be assessed by comparing Figs. 3E and 4E; clearly it would be virtually impossible to devise a model which would give a positive AV for the formation of an ion as implied by Fig. 3E. The initial slopes of the plots in Fig. 4 ( P < 1000 atm) have been used to estimate AVe according to A V e = +_ 6Ep × 96,490 x 10 9 cm 3mol - 1 999 x 101,325
(6)
where the positive sign refers to oxidations. Values are shown in Table 2. The magnitude of the volume changes observed is reasonable, i.e. in keeping
319
VOLUMES OF ACTIVATION
with the volume changes found in simple ionisation reactions 1.'15 in solution provided that there is substantial charge localisation on these large radical ions. All such volume changes are the result of changes in the free volume of the solvent since the molecular species are themselves virtually incompressible*. TABLE2 Compound
Oxidation
6Ep°/V
Reduction
A V~/
6Eft~V
cm 3 mol- a
Perylene 9,10-DMA 9,10-DPA Fluorenone
-0.0135 -0.0055 -0.0105 --
" 6E,=(E,),oooa,m-(Ep)l
....
- 13 -5 - 10 --
A V~/ crn 3 m o l - t
+0.0105 +0.0070 +0.0175 + 0.0200
- 10 -7 - 17 - 19
In agreement with the hypothesis that charge localisation is an important factor it is observed that the formation of the fluorenone radical anion has the largest iAVel of the systems investigated. However, the sequence of ]AFOrs: [fluorenone] "- > [9,10-DPA]'- > [perylene]'- > [9,10-DMA]'- is surprising since it would be predicted that charge localisation would be in the order: [fluorenone]'-> [9,10-DMA]',-> [9,10-DPA]'- > [perylene]'-. It appears essential therefore to assume that ]AV~I can be reduced from the values appropriate for the "free ions"; ion pairing is the most likely factor since this reduces the effective charge seen by the solvent. Ion pairing with the even larger tetrabutylammonium ions has previously been noted by Peover as. The effect of ion pairing would be expected to follow that of charge localisation. However, it is known from e.s.r, spectra that the electrolytically generated fluorenone ketyl is essentially a free anion 192° in the presence of tetraalkylammonium salts, the charge being localised on the heteroatom. The effect of ion pairing would be expected to be relatively small in the case of [9,10-DPA]'- because of the blocking effect of out of plane phenyl groups located at the centres of the high charge density 21. In the case of the oxidations the order of [AVers is: [perylene] "+ > [9,10-DPA] "+ > [9,10-DMA] "+. Ion pairing of BF£ with [9,10-DMA] "+ and to a lesser extent with [9,10DPA] "+ would satisfactorily account once more for the relative sizes of their JAI,~[. It has been shown by e.s.r, spectroscopy that the degree of localisation of charge in [9,10-DPA]'- is substantially larger than for [9,10-DPA] "+, due to a greater angle of twist of 68 ° of the phenyl rings compared with 61 ° for the cation2L On the other hand, examination of the e.s.r, spectra of perylene radical cations and anions 22 suggests a somewhat greater degree of localisation of charge in the cation. These latter two facts then account for the relative change in the magnitudes of IAV~I of 9,10-DPA and of perylene from that observed for the reductions. Equally they would explain the larger IAVel for [perylene] "+ than for [perylene]'- as well as the * T h e effect of the c h a n g e in b o n d o r d e r on the skeletal size 16"17 will be a m i n o r term.
320
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
relatively smaller IAVel for [9,10-DPA] "+ than for [9,10-DPA]'-. The fact that IAVelfor the formation of 9,10-DMA and 9,10-DPA radical cations is smaller than for the respective anions supports the idea that ion-pairing of aromatic radical cations is more important than for the radical anions 6'?. It can be seen from Fig. 4 that the plots of 6 E v vs. P for the formation of the radical anions are curved. This is to be expected since the compressibility of the solvent decreases with increasing pressure. It is perhaps the more remarkable that the plots for the formation of the radical cations are linear. A possible explanation is that the relation is again a function of two independent effects namely the pressure dependence of the charge transfer reaction and that of the degree of ion pair dissociation. These two effects would act in opposite senses. SUMMARY
A study has been made of the electrochemical oxidation and reduction of four polynuclear aromatic hydrocarbons as a function of hydrostatic pressure up to 2000 atm. A high pressure three-electrode cell was designed and a range of cyclic voltammetric experiments has been carried out using platinum electrodes. The variation of the I3, I2/I- reference electrode potential was studied by examining the pressure dependence of the ferrocene/ferrocinium potential and all the values of ~Ep/OPa r e referred to this. The values of AVe for both the oxidation and the reduction reactions have been found to be negative. Whereas A Ve for the formation of the radical cation is essentially constant in the pressure range investigated, that for the formation of the radical anions decreases with pressure. The relative magnitudes and changes of AVe are discussed in terms of competitive solvation and ion pairing of the radical ions; it is shown that the extent of charge delocalisation is an important parameter.
REFERENCES 1 G.J. Hills in R. S. Bradley (Ed.), Advances in High Pressure Research, Vol. 2, Academic Press, LondonNew York, 1969, pp. 225 55. 2 G. J. Hills, Talanta, 12 (1965) 1317. 3 G.J. Hills and P. J. Ovenden in P. Delahay and C. W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 4, Interscience, New York, 1966. 4 G. J. Hills and D. R. Kinnibrugh, J. Electrochem. Soc., 113 (19667 1111. 5 S. Claesson, B. Lundgren and M. Szwarc, Trans. Faraday Soc., 66 (1970) 3053. 6 M. E. Peover in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Dekker, New York, 1967, pp. 11-15. 7 M. E. Peover, private communications. 8 R. S. Nicholson and I. Shain, Anal. Chem., 36 (1964) 706. 9 R. S. Nicholson, Anal. Chem., 37 (1965) 1351. 10 P. E. Whitson, H. W. Vanden Born and D. H. Evans, Anal. Chem., 45 (1973) 1298. l l E. R. Brown, T. G. McCord, D. E. Smith and D. D. de Ford, Anal. Chem., 38 (1966) 1119. 12 M. Walter and L. Rameley, Anal. Chem., 45 (1973) 165. 13 M. E. Peover, J. Chem. Soc., (1962) 4540. 14 B. B. Owen and S. R. Brinkley, Chem. Rev., 24 (1941) 461. 15 B. B. Owen, U.S. Natl, Bur. Std. Circ., (1953) 524. 16 R. Dandel, R. Lefebore and C. Moser, Quantum Chemistry, Interscience, New York, 1969, Ch. 5. 17 C. A. Coulson and A. Golebiewsky, Proc. Phys. Soc. (London), 78 (1961) 1310.
VOLUMES OF ACTIVATION 18 19 20 21 22
321
M. E. Peover, Discuss. Faraday Soc., 45 (1968) 186. N. Hirota in E. T. Kaiser and L. Kevan (Eds.), Radical Ions, Interscience, New York, 1968, pp. 4248. R. Dehl and G. K. Fraenkel, J. Chem. Phys., 39 (1963) 1793. L. O. Wheeler, K. S. V. Santhanam and A. J. Bard, J. Phys. Chem., 71 (1967) 2223. L. C. Snyder and T. Amos, J. Chem. Phys.. 42 (1965) 3670.
313
Z2) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ELECTRODE REACTIONS OF ORGANIC COMPOUNDS AS A FUNCTION OF PRESSURE
M. FLEISCHMANN, W. B. GARA* and G. J. HILLS
Chemistry Department, U'niversity of Southampton, Southampton S09 5NH (England) (Received 13th December 1974)
INTRODUCTION
A major aim of this investigationhas been the development of instrumentation for the study of electrode reactions in non-aqueous media as a function of hydrostatic pressure up to 2000 atm. In this paper we report results obtained of cyclic voltammetry for the reversible formation of a number of radical ions. Previous work on the pressure coefficient of electrode processes of organic species in aqueous media by means of polarography has been reviewed by Hills et al. 1-4. Claesson et al. s have examined the effect of pressure on the solvation of ion-pairs of the fluorenyl radical anion and calculated the value of AV° for the change from a tight ion-pair at low pressures to a loose ion-pair at high pressures. Peover has investigated and reviewed 6'7 the effect of solvation on the oxidation and reduction potentials of polycyclic aromatic hydrocarbons and other organic compounds. THE PRESSURE COEFFICIENT OF CYCLIC VOLTAMMETRIC PEAK POTENTIALS AS A MEANS OF ESTIMATING AVe
The formation of charged species in solution is accompanied by electrostriction of the solvent so that the ion has a lower potential energy in solution as compared to in vacuo. It is evident that changes in the solvation energy will affect the standard potential E e. For the process O+ne~R the peak potential, Ep, is related to Ee by E e = E r + (R T/nF) [ln (Do~DR) + In (ao/a,) ] + 0.029/n
(1)
Since AGe= - nFE e
(2)
and
(3)
(~3AGe/SP)r = AVe
we obtain (Ee)i,2 - (Ee)p, -- - (A Ve/nF)(P2 - P1) * Department of Chemistry, University College, London WC1H OAJ, England.
(4)
314
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
As the "internal pressure" of the system will be several orders of magnitude higher than the applied hydrostatic pressure we may assume that the ratio of the compressibilities of the solvated radical ions to that of the parent hydrocarbons will not change markedly up to 1000 atmospheres. Therefore the ratio Do/D R will stay sensibly constant. It has also been suggested 4 that the activities do not change significantly in this range. Therefore we obtain from eqns. (1) and (4) (Ep)e2 - (Ep)e, = - (A Ve/nF)(P2 - P1)
(5)
It is particularly convenient to use E v in these measurements since there is no accumulation of products, the reaction being reversible on the time scale of these experimentsS-, o. EXPERIMENTAL
The cell developed for this investigation is shown in Fig. 1. The electrodes sealed through glass were fitted into the cell metal body using sets of O-rings; one in each set was inert to the electrolyte solution and the other to the hydraulic fluid (Shell Risella 23) used to transmit the hydrostatic pressure via the metal bellows. The design of the hydraulic system was conventional I the pressure being monitored by Bourdon gauge (Budenberg) graduated in steps of 500 psi (34 atm). The pressure vessel was in turn thermostatted in an oil bath at 25°C.
A H
R
B C
D
E
N
M
P
F
D
K Fig. 1. High pressure electrochemical cell. (A) Upper block (stainless steel); (B) "O'-ring (EP 127/75); (C) "O"-ring (PB 80); (D) washer (stainless steel); (E) screw cap (brass); (F) counter electrode body (glass); (G) heavy walled capillary (glass); (H) working electrode body (glass); (I) bellows (stainless steel); (K) reference electrode body ( glass); (L) reference solution; (M) electrolyte solution; (N) working electrode (platinum button); (O) counter electrode (platinum coil); (Q) capillary opening; (R) platinum wire; (S) lower block (stainless steel).
The working electrode consisted of a platinum button sealed into glass and polished so as to expose a flat circular area of diameter 1 mm. The platinum counter electrode was connected to the working electrode via capillary of inner diameter 2.5
VOLUMES OF ACTIVATION
315
ram. The thin layer of electrolyte trapped in the gap between the two tubes served as the capillary probe connection to the reference electrode. This consisted of the I3, Iz/I- couple in contact with a platinum electrode. This design proved to be an adequate compromise between the need to transmit the pressure throughout the cell and the need to minimise diffusion of the reference system to the working electrode, Adequate uniformity of the electric field was ensured by making the capillary connection between the working and counter electrodes 10 mm in length. Residual ohmic losses were compensated for by using positive feedback in the potentiostat. In order to optimise signal to noise the potentiostat was built from integrated circuits in a screened box mounted on the lid of the pressure vessel. The well known De Ford ~~ configuration was used. Voltage wave forms for cyclic voltammetry were obtained from Chemical Electronics RB [ generator modified so as to give single sweeps. The output from the current follower was offset and stored in a Hi-Tek .," ........ ,,,%.
(•)
•
I
%.. '",,°.,.
.' ,,.
loO)JA
"'..,o,.,..°,.
." A -'
,..o..,,. .'""
•
., . . . . * " ' ' ' ' ' " '
.,-
,..,
....'"
.o.
i I
I
-1,875
•
-'L995
'
I
p
-2.149
-2.259
." ,,,..,...,
,,.,,.,,,,
E/V
• '.,,, .,,.. "..,
~ ,..,..,.,.,.
..'
(b) IO0}JA
D s.~ ¢" ....-
I
-1,995
E
E/V
1
-2.149
Fig. 2. Cyclic voltammetry of perylene at 2000 atm. Sweep rate, 6 V s-~; no. of sweeps averaged, 8. (A) Forward sweep; (B) reverse sweep; (C) background current (horizontal sensitivity, 3 mV per data point), (D) expansion of A in the region indicated; (E) background current (horizontal sensitivity, 0.6 mV per data point). The solid horizontal lines represent zero current.
signal averager type AA1. The 256 data points were divided so as to read 128 points on both the forward and reverse .sweeps; an example is shown in Fig. 2A. By introducing a time delay and more closely spaced data points (0.6 mV/data point)
316
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
the peak potential could be determined more accurately (Fig. 2B). A 4 chart paper was used throughout for the X-Y display. The signal to noise could be improved further by averaging repeated scans. Using this technique and the precautions outlined below the reproducibility was + 1 mV over 12 h. Voltammograms were recorded at a sweep rate 6 V s- ~ and the reversibility of each system was checked using the range 0.6-60 V s-x at pressures of 1, 1000 and 2000 atmospheres. The absence of systematic errors was checked by measuring AEp versus P both for increasing and decreasing pressures. All reactions except the oxidation of 9,10-dimethylanthracene were fully reversible at 6 V s -1 while this latter oxidation in turn became reversible at 60 V s-1. The polynuclear hydrocarbons [9,10-diphenylanthracene (9,10-DPA), perylene (Koch Light)and 9,10-dimethylanthracene (9,10-DMA) (Aldrich)], fluorenone and ferrocene (B.D.H.) were all of high purity and were used as received. Tetraethylammonium borofluoride (from tetraethylammonium bromide and sodium borofluoride) was purified by repeated recrystallisation from water until the product gave a negative halide test, and dried in a vacuum oven at 80 ° for 2 days at 0.1 Torr, and kept over P205. Acetonitrile (B.D.H.) was purified as described by Walter and Rameley (method B) a2 and stored in a glove box containing P205 and an atmosphere of high purity nitrogen at positive pressure. Acetonitrile purified in this manner gave very satisfactory background voltammograms when tested under experimental conditions over the whole potential and pressure ranges used in the experiments described below. Aluminium oxide was prepared from alumina, activated by heating at 300°C for 5 h, allowed to cool in vacuo and stored in the drybox. Tetra-n-butylammonium triiodide was kindly supplied by Dr. P. J. Ovenden. The cells and volumetric flasks containing the weighed out base electrolyte were dried in a vacuum oven at 100°C and 0.1 Tort for 4 h. All further operations were carried out in the glove box. All solutions (excepting those used in the reference electrode) were stirred magnetically over activated alumina. Ten per cent by volume of activated alumina was also placed in the cell. RESULTS AND DISCUSSION
The pressure coefficients were measured for the following systems: Pt
9,10-DPA (satd), 0.3 M Et4NBF 4 in acetonitrile solution
0.3 M Et4NBF 4+0.001 M Bu4NI3 in acetonitrile solution
Pt
Pt
perylene (satd), 0.3 M Et4NBF4 in acetonitrile solution
0.3 M Et4NBF 4 +0.001 M Bu4NI 3 in acetonitrile solution
Pt
Pt
9,10-DMA (satd), 0.3 M Et4NBF4 in acetonitrile solution
0.3 M Et4NBF 4 +0.001 M Bu4NI3 in acetonitrile solution
I Pt
J
317
VOLUMES OF ACTIVATION
fluorenone (0.001 M), 0.3 M Et4NBF~ in acetonitrile solution
Pt
I I 0.3 M Et4NBF4
I Pt
II +0.001 M Bu4NI3 II in acetonitrile solution
I
ferroccne (0.001 M), 0.3 M Et4NBF 4 I I 0.3 M Et4NBF 4 in acetonitrile solution II +0.001 M Bu4NI3 II in acetonitrile solution
Pt
I Pt
[
For the first three systems the changes in peak potential [bEp=(Ep)xatm(Ep)l,,m] t;s. pressure are shown both for their one-electron reduction and oneelectron oxidation reactions in Fig. 3 (A to F). For the one-electron reduction of fluorenone and one-electron oxidation of ferrocene the plots are shown in Fig. 3 (G and H). 20G .
lO
A
5Ep/ /my o10-
C
E
"',,.
"'""""".
"''..
,
:::::i .............. "'".°, ,.
"'''.
• , . , ... H
"i"
20-
1000
2000
Au
P/ATM
1000
2200 000
~) . . . .
P/ATM
1OOO i . . . . 2 0 0 0i
YATM
O ....
1OOO i ....
2 0 0r0
P/ATM
Fig. 3. Plots of peak potential change vs. pressure (uncorrected). Reductions: (A) 9,10-DPA, (C) perylene, (E) 9,10-DMA. (G) fluorenone; oxidations: (B) 9,10-DPA, (C) perylene, (E) 9,10-DMA, (H) ferrocene. ~Ep = (Ep) ..... - - ( E p ) l atm"
The actual values of Ep at 1 atm v s . the 13, I2/I-/Pt reference electrode are summarised in Table 1. There is, however, some uncertainty relating to the values of the peak potential at pressures greater than 1 atm. This is because the potentials were obtained in relation to a reference electrode potential, which is conventionally taken as zero volts. It cannot, however, be assumed that the potential of the reference electrode is independent of pressure. It was with this in mind that the determination of 6Ep v s . TABLE ]
Compound
Ep ( oxidation )/V"
Ep (reduction)~ V"
9,10-DPA Perylene 9,10-DMA Fluorenone Ferrocene
+0.892 + 0.697 +0.746
-2.261 - 2.105 -2.394 - 1.670
+ 0.072
a To obtain potentials vs. aq. SCE add + 0.30 V.
318
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
pressure for ferrocene (Fig. 4H) was undertaken• Ferrocene has been used to allow for changes in liquid junction potentials when the same reference electrode was used in a range of solvents a3. It is clear that the changes in potential shown in Fig. 3 (A to G) are due to the combined effects at the working and reference electrodes. It has been suggested that the ferrocene/ferrocinium couple is essentially unaffected by changes of solvent in work at ambient pressures, i.e. the solvation energy is small and constant. In order to make a first estimate of the changes in the 13, 12/I- potential with changes in pressure we have made the analogous assumption of constancy of the ferrocene/ ferrocinium couple in these experiments• Within the limits of this approximation Fig. 3H may be attributed to the changes at the reference electrode• It can be seen that there is a marked decrease in 6Ep with P at ~ 1000 atm which is probably due to complete dissociation of the triiodide ion (I;)+,,,~ ~ (I2),,,,~+ (I-)+o,,, If we take the potentials shown in Fig. 3H as the baseline, we obtain the corrected data in Fig. 4 (A to G) which can now be attributed to the formation of the 2010.
C
~Ep/ /mY
O-
E
,L." "°
L~
.• ....••..•..... •,..•,.,..
''"'"•.
',..,
10.
•
•.
¥'
6 20I ' '
0
'
+
[
;
1000
I
I
I
J
2000 0
I
t
''
I
'
1000
'
i
,
I
'
2000 0
'
' , I
',
1000
'
'
I
I
2000 0
I
'
'
'
I
'
1000
r
'
J
I
2000
Fig. 4. Plots of peak potential change vs. pressure, after correction for the pressure dependence of the reference potential. Reductions: (A) 9,10-DPA, (C) perylene, (E) 9,10-DMA, (G) fluorenone; oxidations; (B) 9,10-DPA, (D) perylene, (F) 9,10-DMA. 6Ep= (Ep)+,,m.-(Ep)I +,m"
radical ions themselves. The validity of this argument can be assessed by comparing Figs. 3E and 4E; clearly it would be virtually impossible to devise a model which would give a positive AV for the formation of an ion as implied by Fig. 3E. The initial slopes of the plots in Fig. 4 ( P < 1000 atm) have been used to estimate AVe according to A V e = +_ 6Ep × 96,490 x 10 9 cm 3mol - 1 999 x 101,325
(6)
where the positive sign refers to oxidations. Values are shown in Table 2. The magnitude of the volume changes observed is reasonable, i.e. in keeping
319
VOLUMES OF ACTIVATION
with the volume changes found in simple ionisation reactions 1.'15 in solution provided that there is substantial charge localisation on these large radical ions. All such volume changes are the result of changes in the free volume of the solvent since the molecular species are themselves virtually incompressible*. TABLE2 Compound
Oxidation
6Ep°/V
Reduction
A V~/
6Eft~V
cm 3 mol- a
Perylene 9,10-DMA 9,10-DPA Fluorenone
-0.0135 -0.0055 -0.0105 --
" 6E,=(E,),oooa,m-(Ep)l
....
- 13 -5 - 10 --
A V~/ crn 3 m o l - t
+0.0105 +0.0070 +0.0175 + 0.0200
- 10 -7 - 17 - 19
In agreement with the hypothesis that charge localisation is an important factor it is observed that the formation of the fluorenone radical anion has the largest iAVel of the systems investigated. However, the sequence of ]AFOrs: [fluorenone] "- > [9,10-DPA]'- > [perylene]'- > [9,10-DMA]'- is surprising since it would be predicted that charge localisation would be in the order: [fluorenone]'-> [9,10-DMA]',-> [9,10-DPA]'- > [perylene]'-. It appears essential therefore to assume that ]AV~I can be reduced from the values appropriate for the "free ions"; ion pairing is the most likely factor since this reduces the effective charge seen by the solvent. Ion pairing with the even larger tetrabutylammonium ions has previously been noted by Peover as. The effect of ion pairing would be expected to follow that of charge localisation. However, it is known from e.s.r, spectra that the electrolytically generated fluorenone ketyl is essentially a free anion 192° in the presence of tetraalkylammonium salts, the charge being localised on the heteroatom. The effect of ion pairing would be expected to be relatively small in the case of [9,10-DPA]'- because of the blocking effect of out of plane phenyl groups located at the centres of the high charge density 21. In the case of the oxidations the order of [AVers is: [perylene] "+ > [9,10-DPA] "+ > [9,10-DMA] "+. Ion pairing of BF£ with [9,10-DMA] "+ and to a lesser extent with [9,10DPA] "+ would satisfactorily account once more for the relative sizes of their JAI,~[. It has been shown by e.s.r, spectroscopy that the degree of localisation of charge in [9,10-DPA]'- is substantially larger than for [9,10-DPA] "+, due to a greater angle of twist of 68 ° of the phenyl rings compared with 61 ° for the cation2L On the other hand, examination of the e.s.r, spectra of perylene radical cations and anions 22 suggests a somewhat greater degree of localisation of charge in the cation. These latter two facts then account for the relative change in the magnitudes of IAV~I of 9,10-DPA and of perylene from that observed for the reductions. Equally they would explain the larger IAVel for [perylene] "+ than for [perylene]'- as well as the * T h e effect of the c h a n g e in b o n d o r d e r on the skeletal size 16"17 will be a m i n o r term.
320
M. FLEISCHMANN, W. B. GARA, G. J. HILLS
relatively smaller IAVel for [9,10-DPA] "+ than for [9,10-DPA]'-. The fact that IAVelfor the formation of 9,10-DMA and 9,10-DPA radical cations is smaller than for the respective anions supports the idea that ion-pairing of aromatic radical cations is more important than for the radical anions 6'?. It can be seen from Fig. 4 that the plots of 6 E v vs. P for the formation of the radical anions are curved. This is to be expected since the compressibility of the solvent decreases with increasing pressure. It is perhaps the more remarkable that the plots for the formation of the radical cations are linear. A possible explanation is that the relation is again a function of two independent effects namely the pressure dependence of the charge transfer reaction and that of the degree of ion pair dissociation. These two effects would act in opposite senses. SUMMARY
A study has been made of the electrochemical oxidation and reduction of four polynuclear aromatic hydrocarbons as a function of hydrostatic pressure up to 2000 atm. A high pressure three-electrode cell was designed and a range of cyclic voltammetric experiments has been carried out using platinum electrodes. The variation of the I3, I2/I- reference electrode potential was studied by examining the pressure dependence of the ferrocene/ferrocinium potential and all the values of ~Ep/OPa r e referred to this. The values of AVe for both the oxidation and the reduction reactions have been found to be negative. Whereas A Ve for the formation of the radical cation is essentially constant in the pressure range investigated, that for the formation of the radical anions decreases with pressure. The relative magnitudes and changes of AVe are discussed in terms of competitive solvation and ion pairing of the radical ions; it is shown that the extent of charge delocalisation is an important parameter.
REFERENCES 1 G.J. Hills in R. S. Bradley (Ed.), Advances in High Pressure Research, Vol. 2, Academic Press, LondonNew York, 1969, pp. 225 55. 2 G. J. Hills, Talanta, 12 (1965) 1317. 3 G.J. Hills and P. J. Ovenden in P. Delahay and C. W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 4, Interscience, New York, 1966. 4 G. J. Hills and D. R. Kinnibrugh, J. Electrochem. Soc., 113 (19667 1111. 5 S. Claesson, B. Lundgren and M. Szwarc, Trans. Faraday Soc., 66 (1970) 3053. 6 M. E. Peover in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Dekker, New York, 1967, pp. 11-15. 7 M. E. Peover, private communications. 8 R. S. Nicholson and I. Shain, Anal. Chem., 36 (1964) 706. 9 R. S. Nicholson, Anal. Chem., 37 (1965) 1351. 10 P. E. Whitson, H. W. Vanden Born and D. H. Evans, Anal. Chem., 45 (1973) 1298. l l E. R. Brown, T. G. McCord, D. E. Smith and D. D. de Ford, Anal. Chem., 38 (1966) 1119. 12 M. Walter and L. Rameley, Anal. Chem., 45 (1973) 165. 13 M. E. Peover, J. Chem. Soc., (1962) 4540. 14 B. B. Owen and S. R. Brinkley, Chem. Rev., 24 (1941) 461. 15 B. B. Owen, U.S. Natl, Bur. Std. Circ., (1953) 524. 16 R. Dandel, R. Lefebore and C. Moser, Quantum Chemistry, Interscience, New York, 1969, Ch. 5. 17 C. A. Coulson and A. Golebiewsky, Proc. Phys. Soc. (London), 78 (1961) 1310.
VOLUMES OF ACTIVATION 18 19 20 21 22
321
M. E. Peover, Discuss. Faraday Soc., 45 (1968) 186. N. Hirota in E. T. Kaiser and L. Kevan (Eds.), Radical Ions, Interscience, New York, 1968, pp. 4248. R. Dehl and G. K. Fraenkel, J. Chem. Phys., 39 (1963) 1793. L. O. Wheeler, K. S. V. Santhanam and A. J. Bard, J. Phys. Chem., 71 (1967) 2223. L. C. Snyder and T. Amos, J. Chem. Phys.. 42 (1965) 3670.