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Solvent effect on the redox potential of quinone-semiquinone systems
J. Electroanal. Chem., 105 (1979) 329--334 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
329
SOLVENT EFFECT ON THE REDOX POTENTIAL OF QUINONE--SEMIQUINONE SYSTEMS
JAN S. JAWORSKI, EWA LESNIEWSKA and MAREK K. KALINOWSKI Department of Chemistry, University of Warsaw, 1 Pasteur Street, 02-093 Warsaw (Poland) (Received 15th May 1979)
ABSTRACT Polarographic reduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone and 9,10-anthraquinone was studied in pyridine, acetone, hexamethylphosphoramide, dimethylformamide, dimethylacetamide, acetonitrile, dimethylsulfoxide and propylene carbonate. The variation of the E1/2-values proper for the first electron transfer step (measured against oxidation potential of ferrocene) with the properties of solvent is analyzed in terms of the donor--acceptor concept. Linear correlations of Et/2 vs. acceptor number are observed; the slopes of corresponding lines correlate with the spin densities at oxygen atoms of semiquinones.
INTRODUCTION O n e o f t h e m a j o r p r o b l e m s o f organic e l e c t r o c h e m i s t r y is t h e d e s c r i p t i o n o f t h e m e d i u m ( s u p p o r t i n g e l e c t r o l y t e a n d solvent) e f f e c t o n t h e c h a r a c t e r i s t i c s o f e l e c t r o c h e m i c a l processes. F r o m t h e t h e o r e t i c a l v i e w p o i n t , t h e energetics o f reversible o n e - e l e c t r o n r e d u c t i o n ( n o t a c c o m p a n i e d b y a n y p r e c e d i n g a n d f o l l o w i n g c h e m i c a l step) c a n b e c o m p a r e d w i t h t h e gas p h a s e e l e c t r o n a f f i n i t y ( E A ) o f t h e r e a c t a n t a n d w i t h t h e d i f f e r e n c e in free s o l v a t i o n energies o f t h e r e a c t a n t a n d t h e c o r r e s p o n d i n g radical a n i o n (AGsolv). 0 The half-wave potential is given b y 0 --nFE,n = EA + (G01)Hg+ AGso,v
(I)
where (G°l)Hgis the free energy of the electron in an electrode. Equation (1) was first analyzed by Hoijtink and van Schooten for the electroreduction of aromatic hydrocarbons [ 1], and essentially the same derivation has been also used by more recent authors (see e.g. refs. 2 and 3 and corresponding references therein). In practice, it is not easy to estimate the real AG~olvvalues [4], therefore the influence of solvent properties on redox potentials of organic compounds is usually considered in terms of empirical equations based on a linear free energy relationship. Such equations have been proposed [ 5,6], e.g. for the description of the solvent effect on the half-wave potential of the process in which ion pairs are formed between radical anions and background electrolyte cations. The purpose of this paper is to describe the influence of non-protic solvents
"330
on the redox potentials of quinone--semiquinone systems in the absence of ion pairs formation. Quinones were chosen as the model systems because their electrochemical reactions have been well characterized in non-protic media [7]. Semiquinones are normally stable under these conditions and the rates of heterogeneous electron transfer are large enough to ensure reversible behavior of quinone--semiquinone systems in polarographic experiments [8]. EXPERIMENTAL
The polarographic and voltammetric measurements were carried o u t as in our previous paper [9]. According to the suggestion made b y Strehlow [10] all potentials measured by us are referred to the oxidation potential of ferrocene
[111. All solvents applied were dried and purified immediately before use according to conventional methods [12]. Addition of neutral alumina [13] caused practically no change of the essential characteristics of t h e first waves of investigated compounds. Tetraethylammonium perchlorate was prepared as described elsewhere [14], recrystallized three times from triply distilled water and vacuum dried (50 ° C). 1,4-Benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and 9,10-phenanthrenequinone (PQ) were commercial products and were purified by vacuum sublimation. Numerical calculations were made with an ODRA-1305 computer. RESULTS AND DISCUSSION
In all the solvents applied in this work quinones reduce polarographically in two one-electron steps. The firststep is reversible;corresponding a E / a l o g [ i / ( i d - - i ) ] , ipa/ipc and E p a - E,c values are theoretical for fast one-
TABLE 1 Half-wave potentials of the first polarographic waves of quinones Solvent
Dimethylsulfoxide (DMSO) Acetonitrile (ACN) Propylene carbonate (PC) Dimethylformamide (DMF) Pyridine (Py) Dimethylacetamide (DMA) Acetone (Ac) Hexamethylphosphoramide (HMPA)
D a
46.7 37.5 36.7 36.7 13.2 38.9 21.3 30
AN b
19.3 18.9 18.3 16.0 14.2 13.8 12.5 10.8
_ E l / 2/V c
BQ
NQ
PQ
AQ
0.76 0.79 0.81 0.91 0.98 1.00 1.03 --
1.06 1.08 1.08 1.15 1.19 1.20 1.24
1.04 1.09 1.05 1.13 1.15 1.17 1.20
1.28 1.33 1.31 1.36 1.40 1.40 1.43
--
1.24
1.48
a Dielectric permittivity [ 12 ]. b Acceptor number [15]. c E1/2_values are given against the potential of ferrocene (see text).
331 -E,/~/V I
I
I I
HMPA
Ac
DIVIAPy
DIVlF
I
I
i4 - -
' 16
I
L
PC ACN DNS
1.0
A-
NO
O-PQ L~- BQ
0510
' 12
i8
AN
Fig. 1. Variation in half-wave potentials of the first polarographic waves of quinones (measured against an o x i d a t i o n potential of ferrocene) with the a c c e p t o r n u m b e r of the solvents. F o r abbreviations see Table 1. The correlation coefficients of linear regression are given in Table 2.
electron transfer. The half-wave potentials are independent of the depolarizer concentrations (in the range 5 X 10-5--1 × 10 -3 M). In the case of BQ, NQ and AQ they are also independent of the concentration of tetraethylammonium perchlorate {from 0.02 to 0.2 M), therefore the ion-pairing phenomena between corresponding semiquinones and (C2Hs)4N÷ cations may be neglected. The very weakly associated ion pairs have been detected only for phenanthrenesemiquinone in weakly electron-donating solvents. The obtained half-wave potentials are presented in Table 1, together with some physicochemical characteristics of solvents [15]. As seen from Table 1, E~n-values proper for the reduction of quinones to semiquinones vary considerably with the nature of solvent. Plots of the E1/2 vs. the reciprocal of dielectric permittivities showed no correlations, whereas linear relationships have been obtained with the acceptor numbers of solvents (Fig. 1). Then, it is obvious that the Born equation does not apply to the solvation effects in the systems under investigation, but the results presented in this report can be simply explained on the basis of the donor--acceptor concept proposed by Gutmann for solvent--solute interaction [15]. Taking into account the results reproduced in Fig. 1, the solvent effect on the first electroreduction step of quinones can be quantitatively described by the empirical equation El~2 = E ° n + a A N
(2)
332 TABLE 2 a-values of eqn. (2) for e l e c t r o r e d u c t i o n of q u i n o n e s t o s e m i q u i n o n e s Compound
a
BQ NQ PQ AQ
0.040 0.026 0.022 0.021
+- 0 . 0 0 2 -+ 0.001 _+ 0 . 0 0 2 + 0.002
r a
q0 b
0.997 0.997 0.974 0.981
0.196 0.167 0.148 0.146
a r is a c o r r e l a t i o n c o e f f i c i e n t o f linear regression. b q0 d e n o t e s spin d e n s i t y at t h e o x y g e n a t o m p o s i t i o n o f t h e s e m i q u i n o n e .
where E°:2 denotes an intercept (the value of E~/2 corresponding to a solvent with A N = 0) and a is the slope of the regression line, i.e. the sensitivity of halfwave potential to the solvent effect described in terms of Lewis acidity *. It can be pointed out here that the linear dependence between Ell2 and A N was also observed in the case of the electroreduction of hexacyanomanganate(III) [19]. The values of a corresponding to the c o m p o u n d s investigated b y us are listed in Table 2. This also contains the spin densities at the position of the oxygen atoms of corresponding free radicals. The data for para-semiquinones were extracted from the paper of GendeU et al. [20] who investigated ESR spectra of these radicals. The corresponding value for phenanthrenesemiquinone was unknown, therefore we have calculated it using the m e t h o d of Hiickel and McLachlan with parametrization suggested b y Dehl and Fraenkel [21]. Let us compare the results presented in Table 2. Analyzing the electron charge density distribution in semiquinones, it m a y be anticipated that according to the donor--acceptor model, the interaction between the molecules of the radicals and solvents mentioned should be related to electron density at the oxygen atom of the carbonyl group. Such a supposition is in good agreement with many ESR measurements (its application in the study of the solvation processes is discussed in detail in ref. 22); recent investigations indicated clearly that in semiquinones alcohols mainly solvate the oxygen atoms [ 23--25]. In that case, as electron density can be empirically related to spin density and, hence, to the corresponding hyperfine splitting constant in ESR spectra, the avalues from eqn. (2) should be dependent on q0. In Fig. 2 the relationship between both parameters is presented and a satisfactory correlation can readily be seen. Moreover, taking into account the results reported by Gulick and Geske [26] who determined the hyperfine splitting constant for ~70 atoms (A0) in the spectra of 1,4-benzosemiquinone, we have also obtained a good correlation between A0 and the A N of the solvents used {Fig. 3).
* It s h o u l d be also n o t e d t h a t we h a v e a n a l y z e d t h e r e l a t i o n s h i p i n c l u d i n g R e i c h a r d t a n d D i m r o t h E w values [ 1 6 , 1 7 ] i n s t e a d A N (see e.g. [ 18 ]). In this case, h o w e v e r , t h e c o r r e l a t i o n s are m u c h worse ( c o r r e l a t i o n c o e f f i c i e n t s are 0 . 7 9 3 , 0 . 8 3 7 , 0 . 8 0 2 a n d 0 . 8 2 1 for BQ, NQ, PQ a n d AQ, respectively}.
333
-A o 'gauss T1
I
ACNDMSO a OO4
I
C2H50H
H20
9.5 /
BQ
0,03
9,0 ~pQ NQ
0.02
__ 0.10
AQ " 0.15 '
i
0.20
q-o-
8.5 10
,
30
,
5~0
AN
Fig. 2. Relationship between the a-values of eqn. (2) and spin density (q0) at the oxygen atom position of semiquinones. The correlation coefficient of linear regression is r = 0.985. Fig. 3. Relationship between hyperfine splitting constants proper for the 170 atom (A0) in the ESR spectra of 1,4-benzosemiquinone (from ref. 26) and the acceptor number of the solvents. The correlation coefficient of linear regression is r = 0.999.
F r o m t h e s e c o n s i d e r a t i o n s it f o l l o w s that t h e d o n o r - - a c c e p t o r c o n c e p t is very u s e f u l for t h e d e s c r i p t i o n o f s o l v e n t - - s o l u t e i n t e r a c t i o n in t h e electroc h e m i s t r y o f q u i n o n e s . O b v i o u s l y , t h e d o n o r properties o f s e m i q u i n o n e s are greater in c o m p a r i s o n w i t h t h o s e for parent m o l e c u l e s . It m e a n s that t h e r e d u c e d f o r m s are m o r e s t r o n g l y a f f e c t e d b y t h e i n t e r a c t i o n w i t h t h e s o l v e n t t h a n t h e o x i d i z e d f o r m s , and h e n c e a shift in h a l f - w a v e p o t e n t i a l t o w a r d t h e p o s i t i v e d i r e c t i o n is o b s e r v e d w i t h an increase o f a c c e p t o r n u m b e r , i.e. w i t h an increase o f Lewis a c i d i t y o f t h e solvent. Further investigations are in progress. ACKNOWLEDGEMENT This w o r k w a s s u p p o r t e d b y t h e Polish A c a d e m y o f S c i e n c e s as a part o f t h e MR I-9 project. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
G . J . H o i j t i n k and J. van S c h o o t e n , R e c . T r a y . C h i m . P a y s - B a s , 7 1 ( 1 9 5 2 ) 1 0 8 9 . M.J,S. D e w a r , J . A . H a s h m a l l a n d N. T r i n o j s t i c , J. A m . C h e m . S o c . , 9 2 ( 1 9 7 0 ) 5 5 5 5 . V . D . P a r k e r , J. A m . C h e m . S o c . , 9 8 ( 1 9 7 6 ) 9 8 . B. Case, N.S. H u s h , R . Parsons and M.E. P e o v e r , J. E l e c t r o a n a l . C h e m . , 1 0 ( 1 9 6 5 ) 3 6 0 . T.M. K r y g o w s k L J. E l e c t r o a n a l . C h e m . , 3 5 ( 1 9 7 2 ) 4 3 6 . T.M. K r y g o w s k i and W.R. F a w e e t t , J. A m . C h e m . S o c . , 9 7 ( 1 9 7 5 ) 2 1 4 3 . J . Q . C h a m b e r s in S. P a t a i (Ed.)~ T h e C h e m i s t r y o f Q u i n o n o i d s , P a r t 2, Wiley, N e w Y o r k , 1 9 7 4 , C h . 14. R.W. R o s a n s k e and D . H . E v a n s , J. E l e c t r o a n a l . C h e m . , 7 2 ( 1 9 7 6 ) 2 7 7 . J.S. J a w o r s k i a n d M . K . K a l i n o w s k i , J. E l e e t r o a n a l . C h e m . , 7 6 ( 1 9 7 7 ) 3 0 1 . H. S t r e h l o w in J . J . L a g o w s k i ( E d . ) , T h e C h e m i s t r y o f N o n - A q u e o u s S o l v e n t s , P a r t 1, A c a d e m i c Press, N e w Y o r k , 1 9 6 6 , C h . 4. M . K . K a l i n o w s k i a n d I. S z e l a z e k , Pol. J . C h e m . , 5 2 ( 1 9 7 8 ) 1 8 5 3 . C.K. M a n n in A . J . B a r d ( E d . ) , E l e c t r o a n a l y t i c a l C h e m i s t r y , P a r t 3, M a r c e l D e k k e r , N e w Y o r k , 1 9 6 9 , Ch. 2. O. H a m m e r i c h and V . D . P a r k e r , E l e c t r o c h i m . A c t a , 1 8 ( 1 9 7 3 ) 5 3 7 . I.M. K o l t h o f f and J . F . C o e t z e e , J. A m . C h e m . S o c . , 7 9 ( 1 9 5 7 ) 8 5 0 .
334
1 5 V. G u t m a n n , T h e D o n o r - - - A c c e p t o r A p p r o a c h t o M o l e c u l a r I n t e r a c t i o n s , P l e n u m Press, N e w Y o r k , 1978. 1 6 C h . R e i c h a r d t a n d K. D i m r o t h , T o p . C u r t . C h e m . , 11 ( 1 9 6 8 ) 1. 17 Ch. R e i c h a r d t , L S s u n g s m i t t e l E f f e k t e in d e r o r g a n i s c h e n C h e m i e , V e r l a g C h e m i e , W e n h e i m , 1 9 7 3 . 1 8 T.M. K r y g o w s k i a n d M. L i p s z t a j n , W i a d . C h e m . , 31 ( 1 9 7 7 ) 7 3 9 . 1 9 G. G r i t z n e r , K. Danksagn~iHler a n d V . G u t m a n n , J . E l e c t r o a n a l . C h e m . , 9 0 ( 1 9 7 8 ) 2 0 3 . 2 0 J. G e n d e l l , J . M . F r e e d a n d G . K . F r a e n k e l , J. C h e m . P h y s . , 37 ( 1 9 6 2 ) 2 8 3 2 . 21 R. D e h l a n d G . K . F r a e n k e l , J. C h e m . P h y s . , 3 9 ( 1 9 6 3 ) 1 7 9 3 . 2 2 K.S. C h e n a n d N. H i r o t a in G.S. H a m m e s ( E d . ) , I n v e s t i g a t i o n s o f R a t e s a n d M e c h a n i s m s o f R e a c t i o n s , P a r t 2, Wiley, N e w Y o r k , 1 9 7 3 , Ch. 14. 2 3 K . S . C h e n a n d N. H i r o t a , J. A m . C h e m . S o c . , 9 4 ( 1 9 7 2 ) 5 5 5 0 . 2 4 K . S . C h e n a n d N. H i r o t a , J. P h y s . C h e m . , 8 2 ( 1 9 7 8 ) 1 1 3 3 . 2 5 M. B r u s t o l o n , L. P a s i m e n i a n d C. C o r v a j a , J. C h e m . Soe. F a r a d a y T r a n s . II, 71 ( 1 9 7 5 ) 1 9 3 . 2 6 W.M. G u l i c k , Jr., a n d D . H . G e s k e , J. A m . C h e m . Soc., 8 8 ( 1 9 6 6 ) 4 1 1 9 . -
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329
SOLVENT EFFECT ON THE REDOX POTENTIAL OF QUINONE--SEMIQUINONE SYSTEMS
JAN S. JAWORSKI, EWA LESNIEWSKA and MAREK K. KALINOWSKI Department of Chemistry, University of Warsaw, 1 Pasteur Street, 02-093 Warsaw (Poland) (Received 15th May 1979)
ABSTRACT Polarographic reduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone and 9,10-anthraquinone was studied in pyridine, acetone, hexamethylphosphoramide, dimethylformamide, dimethylacetamide, acetonitrile, dimethylsulfoxide and propylene carbonate. The variation of the E1/2-values proper for the first electron transfer step (measured against oxidation potential of ferrocene) with the properties of solvent is analyzed in terms of the donor--acceptor concept. Linear correlations of Et/2 vs. acceptor number are observed; the slopes of corresponding lines correlate with the spin densities at oxygen atoms of semiquinones.
INTRODUCTION O n e o f t h e m a j o r p r o b l e m s o f organic e l e c t r o c h e m i s t r y is t h e d e s c r i p t i o n o f t h e m e d i u m ( s u p p o r t i n g e l e c t r o l y t e a n d solvent) e f f e c t o n t h e c h a r a c t e r i s t i c s o f e l e c t r o c h e m i c a l processes. F r o m t h e t h e o r e t i c a l v i e w p o i n t , t h e energetics o f reversible o n e - e l e c t r o n r e d u c t i o n ( n o t a c c o m p a n i e d b y a n y p r e c e d i n g a n d f o l l o w i n g c h e m i c a l step) c a n b e c o m p a r e d w i t h t h e gas p h a s e e l e c t r o n a f f i n i t y ( E A ) o f t h e r e a c t a n t a n d w i t h t h e d i f f e r e n c e in free s o l v a t i o n energies o f t h e r e a c t a n t a n d t h e c o r r e s p o n d i n g radical a n i o n (AGsolv). 0 The half-wave potential is given b y 0 --nFE,n = EA + (G01)Hg+ AGso,v
(I)
where (G°l)Hgis the free energy of the electron in an electrode. Equation (1) was first analyzed by Hoijtink and van Schooten for the electroreduction of aromatic hydrocarbons [ 1], and essentially the same derivation has been also used by more recent authors (see e.g. refs. 2 and 3 and corresponding references therein). In practice, it is not easy to estimate the real AG~olvvalues [4], therefore the influence of solvent properties on redox potentials of organic compounds is usually considered in terms of empirical equations based on a linear free energy relationship. Such equations have been proposed [ 5,6], e.g. for the description of the solvent effect on the half-wave potential of the process in which ion pairs are formed between radical anions and background electrolyte cations. The purpose of this paper is to describe the influence of non-protic solvents
"330
on the redox potentials of quinone--semiquinone systems in the absence of ion pairs formation. Quinones were chosen as the model systems because their electrochemical reactions have been well characterized in non-protic media [7]. Semiquinones are normally stable under these conditions and the rates of heterogeneous electron transfer are large enough to ensure reversible behavior of quinone--semiquinone systems in polarographic experiments [8]. EXPERIMENTAL
The polarographic and voltammetric measurements were carried o u t as in our previous paper [9]. According to the suggestion made b y Strehlow [10] all potentials measured by us are referred to the oxidation potential of ferrocene
[111. All solvents applied were dried and purified immediately before use according to conventional methods [12]. Addition of neutral alumina [13] caused practically no change of the essential characteristics of t h e first waves of investigated compounds. Tetraethylammonium perchlorate was prepared as described elsewhere [14], recrystallized three times from triply distilled water and vacuum dried (50 ° C). 1,4-Benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and 9,10-phenanthrenequinone (PQ) were commercial products and were purified by vacuum sublimation. Numerical calculations were made with an ODRA-1305 computer. RESULTS AND DISCUSSION
In all the solvents applied in this work quinones reduce polarographically in two one-electron steps. The firststep is reversible;corresponding a E / a l o g [ i / ( i d - - i ) ] , ipa/ipc and E p a - E,c values are theoretical for fast one-
TABLE 1 Half-wave potentials of the first polarographic waves of quinones Solvent
Dimethylsulfoxide (DMSO) Acetonitrile (ACN) Propylene carbonate (PC) Dimethylformamide (DMF) Pyridine (Py) Dimethylacetamide (DMA) Acetone (Ac) Hexamethylphosphoramide (HMPA)
D a
46.7 37.5 36.7 36.7 13.2 38.9 21.3 30
AN b
19.3 18.9 18.3 16.0 14.2 13.8 12.5 10.8
_ E l / 2/V c
BQ
NQ
PQ
AQ
0.76 0.79 0.81 0.91 0.98 1.00 1.03 --
1.06 1.08 1.08 1.15 1.19 1.20 1.24
1.04 1.09 1.05 1.13 1.15 1.17 1.20
1.28 1.33 1.31 1.36 1.40 1.40 1.43
--
1.24
1.48
a Dielectric permittivity [ 12 ]. b Acceptor number [15]. c E1/2_values are given against the potential of ferrocene (see text).
331 -E,/~/V I
I
I I
HMPA
Ac
DIVIAPy
DIVlF
I
I
i4 - -
' 16
I
L
PC ACN DNS
1.0
A-
NO
O-PQ L~- BQ
0510
' 12
i8
AN
Fig. 1. Variation in half-wave potentials of the first polarographic waves of quinones (measured against an o x i d a t i o n potential of ferrocene) with the a c c e p t o r n u m b e r of the solvents. F o r abbreviations see Table 1. The correlation coefficients of linear regression are given in Table 2.
electron transfer. The half-wave potentials are independent of the depolarizer concentrations (in the range 5 X 10-5--1 × 10 -3 M). In the case of BQ, NQ and AQ they are also independent of the concentration of tetraethylammonium perchlorate {from 0.02 to 0.2 M), therefore the ion-pairing phenomena between corresponding semiquinones and (C2Hs)4N÷ cations may be neglected. The very weakly associated ion pairs have been detected only for phenanthrenesemiquinone in weakly electron-donating solvents. The obtained half-wave potentials are presented in Table 1, together with some physicochemical characteristics of solvents [15]. As seen from Table 1, E~n-values proper for the reduction of quinones to semiquinones vary considerably with the nature of solvent. Plots of the E1/2 vs. the reciprocal of dielectric permittivities showed no correlations, whereas linear relationships have been obtained with the acceptor numbers of solvents (Fig. 1). Then, it is obvious that the Born equation does not apply to the solvation effects in the systems under investigation, but the results presented in this report can be simply explained on the basis of the donor--acceptor concept proposed by Gutmann for solvent--solute interaction [15]. Taking into account the results reproduced in Fig. 1, the solvent effect on the first electroreduction step of quinones can be quantitatively described by the empirical equation El~2 = E ° n + a A N
(2)
332 TABLE 2 a-values of eqn. (2) for e l e c t r o r e d u c t i o n of q u i n o n e s t o s e m i q u i n o n e s Compound
a
BQ NQ PQ AQ
0.040 0.026 0.022 0.021
+- 0 . 0 0 2 -+ 0.001 _+ 0 . 0 0 2 + 0.002
r a
q0 b
0.997 0.997 0.974 0.981
0.196 0.167 0.148 0.146
a r is a c o r r e l a t i o n c o e f f i c i e n t o f linear regression. b q0 d e n o t e s spin d e n s i t y at t h e o x y g e n a t o m p o s i t i o n o f t h e s e m i q u i n o n e .
where E°:2 denotes an intercept (the value of E~/2 corresponding to a solvent with A N = 0) and a is the slope of the regression line, i.e. the sensitivity of halfwave potential to the solvent effect described in terms of Lewis acidity *. It can be pointed out here that the linear dependence between Ell2 and A N was also observed in the case of the electroreduction of hexacyanomanganate(III) [19]. The values of a corresponding to the c o m p o u n d s investigated b y us are listed in Table 2. This also contains the spin densities at the position of the oxygen atoms of corresponding free radicals. The data for para-semiquinones were extracted from the paper of GendeU et al. [20] who investigated ESR spectra of these radicals. The corresponding value for phenanthrenesemiquinone was unknown, therefore we have calculated it using the m e t h o d of Hiickel and McLachlan with parametrization suggested b y Dehl and Fraenkel [21]. Let us compare the results presented in Table 2. Analyzing the electron charge density distribution in semiquinones, it m a y be anticipated that according to the donor--acceptor model, the interaction between the molecules of the radicals and solvents mentioned should be related to electron density at the oxygen atom of the carbonyl group. Such a supposition is in good agreement with many ESR measurements (its application in the study of the solvation processes is discussed in detail in ref. 22); recent investigations indicated clearly that in semiquinones alcohols mainly solvate the oxygen atoms [ 23--25]. In that case, as electron density can be empirically related to spin density and, hence, to the corresponding hyperfine splitting constant in ESR spectra, the avalues from eqn. (2) should be dependent on q0. In Fig. 2 the relationship between both parameters is presented and a satisfactory correlation can readily be seen. Moreover, taking into account the results reported by Gulick and Geske [26] who determined the hyperfine splitting constant for ~70 atoms (A0) in the spectra of 1,4-benzosemiquinone, we have also obtained a good correlation between A0 and the A N of the solvents used {Fig. 3).
* It s h o u l d be also n o t e d t h a t we h a v e a n a l y z e d t h e r e l a t i o n s h i p i n c l u d i n g R e i c h a r d t a n d D i m r o t h E w values [ 1 6 , 1 7 ] i n s t e a d A N (see e.g. [ 18 ]). In this case, h o w e v e r , t h e c o r r e l a t i o n s are m u c h worse ( c o r r e l a t i o n c o e f f i c i e n t s are 0 . 7 9 3 , 0 . 8 3 7 , 0 . 8 0 2 a n d 0 . 8 2 1 for BQ, NQ, PQ a n d AQ, respectively}.
333
-A o 'gauss T1
I
ACNDMSO a OO4
I
C2H50H
H20
9.5 /
BQ
0,03
9,0 ~pQ NQ
0.02
__ 0.10
AQ " 0.15 '
i
0.20
q-o-
8.5 10
,
30
,
5~0
AN
Fig. 2. Relationship between the a-values of eqn. (2) and spin density (q0) at the oxygen atom position of semiquinones. The correlation coefficient of linear regression is r = 0.985. Fig. 3. Relationship between hyperfine splitting constants proper for the 170 atom (A0) in the ESR spectra of 1,4-benzosemiquinone (from ref. 26) and the acceptor number of the solvents. The correlation coefficient of linear regression is r = 0.999.
F r o m t h e s e c o n s i d e r a t i o n s it f o l l o w s that t h e d o n o r - - a c c e p t o r c o n c e p t is very u s e f u l for t h e d e s c r i p t i o n o f s o l v e n t - - s o l u t e i n t e r a c t i o n in t h e electroc h e m i s t r y o f q u i n o n e s . O b v i o u s l y , t h e d o n o r properties o f s e m i q u i n o n e s are greater in c o m p a r i s o n w i t h t h o s e for parent m o l e c u l e s . It m e a n s that t h e r e d u c e d f o r m s are m o r e s t r o n g l y a f f e c t e d b y t h e i n t e r a c t i o n w i t h t h e s o l v e n t t h a n t h e o x i d i z e d f o r m s , and h e n c e a shift in h a l f - w a v e p o t e n t i a l t o w a r d t h e p o s i t i v e d i r e c t i o n is o b s e r v e d w i t h an increase o f a c c e p t o r n u m b e r , i.e. w i t h an increase o f Lewis a c i d i t y o f t h e solvent. Further investigations are in progress. ACKNOWLEDGEMENT This w o r k w a s s u p p o r t e d b y t h e Polish A c a d e m y o f S c i e n c e s as a part o f t h e MR I-9 project. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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