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Electron affinity of p-quinones. Improved method of electrochemical estimation
Volume
CHEMICAL PHYSICS LETTERS
127, number 2
ELECTRON IMPROVED
AFFINITY METHOD
OF p-QUINONES. OF ELECTROCHEMICAL
6 June 1986
ESTIMATION
Jan S. JAWORSKI Department
of Chemistry
University
of Warsaw, 02-093 Warsaw, Poland
Received 31 January 1986
Electron affinities of four p-quinones are estimated from enthalpy changes obtained on the basis of measured formal potentials and reaction entropies in the electroreduction process. A linear correlation between electron affinities of p-quinones and parent hydrocarbons is found.
1. Introduction A few direct methods of electron affinity (EA) measurements have been perfected in recent years, including the gas-phase thermal electron attachment, collisional ionization technique with alkali-metal beams, photodetachment and photoelectric spectroscopy (references are given, e.g., in ref. [l]). But absolute values of EA which are fundamental energetic properties associated with negative ion formation and have important applications in chemistry, physics, and biology are still rather scarce for complex organic molecules. And thus, an improvement of indirect but simple methods of evaluation can be useful. Electrochemical redox potentials of some aromatic compounds have been correlated with experimental as well as theoretical EA values [2-41. In such relationships the formal potential for the reversible oneelectron electroreduction has been considered as a sum of three terms: the gas-phase EA, the solvation free-energy change between a reactant and a product, and the term depending on the reference electrode. This last one can be kept constant for a series of compounds, and in earlier papers the solvation term was assumed to be constant, especially for aromatic hydrocarbons [2]. This assumption, however, was questioned even for hydrocarbons (cf. discussion in refs. [3,4]) and it is quite unjustifiable [4] for aromatic compounds with polar groups. The other approach was later proposed by Parker [4] on the basis of the 0 009-2614/86/$0350 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
postulate that solvation energies for positive and negative radical ions of an alternant aromatic hydrocarbon are equal. However, the gas-phase EA is rather a measure of an enthalpy change [4,5] and is usually tabulated [6] for the temperature of 0 K. And thus, enthalpy changes, not redox potentials, should preferably be correlated with gas-phase EA values as well as with parameters of quantum-mechanical calculations. The measurements of the reaction entropy for a simple electrode process in a non-isothermal cell [7] open a new way for a direct estimation of both the solvation change between reduced and oxidized forms of the redox couple and the relative enthalpy change. Svaan and Parker found [8] that in the first approximation the entropy effect can be neglected not only in the electroreduction of aromatic hydrocarbons but also for heteroaromatic compounds related to anthracene (they obtained [8] the slope of the correlation between changes of both the relative enthalpy change, AAH, and relative free-energy change or formal potential, aEf, equal to 0.907). However, from our recent measurements [9] of the reaction entropy, AS’, in the one-electron reduction for a series of pquinones, one can calculate that slopes of the similar relationships of AAH versus AEf differ considerably from unity (0.8 and 0.6 in dimethylacetamide and dimethylsulfoxide, respectively). Thus the estimation of EA values for pquinones on the basis of enthalpy changes is proposed.
133
Volume 127, number
CHEMICALPHYSICS LETTERS
2
2. Experimental Measurements were carried out in five solvents: dimethylsulfoxide (DMSO), acetonitrile (ACN), propylene carbonate (PC), dimethylformamide (DMF), and dimethylacetamide (DMA). Formal potentials of pquinones at T = 298 K were measured polarographitally using a silver reference electrode with the same solvent as in a working compartment to avoid the liquid-junction potential. The details of measurements and of purification of chemicals, as well as values of reaction entropies, have been given [9].
3. Results and discussion Table 1 contains apparent enthalpy changes in the electroreduction of 1,4-benzoquinone (BQ), 1,4naphthoquinone (NQ), 9,10-anthraquinone (AQ), and 5,12-tetracenequinone (TQ) obtained from measured formal potentials and reaction entropies [9] : mw
=Ef-TAS’.
(1)
Values of AHaPP include an unknown term which is constant in each solvent and depends on the reference electrode. Absolute values of EA were calculated from uaPpp using 2s a standard for the estimation of the above unknown term the directly measured [lo] EA value of BQ equal to 1.89~~;~ eV (this value has recently been confirmed, EA = 1.86, by a different technique [ 1 l] ). The final EA values are: 1.59 f 0.04, 1.34 + 0.04, and 1.18 f 0.06 eV for NQ, AQ, and TQ,
Table 1 Apparent nones Solvent
DMSO ACN PC DMF DMA
enthalpy
a)
change
-Lwapp
in the electroreduction
of p-qui-
b)
BQ a)
NQ
AQ
TQ
0.152 0.374 0.764 0.471 0.588
0.457 0.691 1.044 0.805 0.844
0.713 0.940 1.318 1.056 1.081
0.892 1.136 1.474 1.202 1.218
a) For abbreviations of solvents and quinones see text. b, AH app (eV) calculated from eq. (1) at T = 298 K.
134
6 June 1986
respectively. The 95% confidence levels given indicate only deviations in different solvents which are mainly due to the contribution of solvation to the enthalpy change; they are evidently within the error of the method. It should be mentioned that for NQ the absolute value of EA > 0.8 eV was estimated on the basis of direct measurements [ 1] and also that the difference between EA values of BQ and NQ equal to 0.3 eV was proposed by McIver et 21. [ 121; but the procedure used by the last authors was later questioned [l l] . On the other hand, our results are in agreement with theoretically calculated [ 131 values of 1.28, 1.60, and 1.94 for AQ, NQ, and BQ, respectively. The additive substituent effect on EA values was found for a series of n-electron compounds with in’ creasing number of CN groups [ 141: EA= EA, +cEg,
(2)
where EA, is the contribution of a molecular skeleton to the electron affinity, and Eg describes the contribution of each CN group. The substituent effect on EA values of methyl-substituted benzaldehydes has also been discussed qualitatively [ 1.51 and for substituted nitrobenzenes [ 161. In the case of the above pquinones (EAQ) one can propose on the basis of additivity the constant contribution of carbonyl groups to EAQ values and ring affinities proportional to EA values of parent hydrocarbons, EAHC. Unfortunately, it is difficult to measure in the direct experiment the precise EA value of benzene; the negative value has been considered in the literature [15,17]; however, there was a strong argument [ 181 for a positive value close to zero. Different values were also estimated using quantum-mechanical calculations: -0.057 eV by Kunii and Kuroda [ 13 1, and -0.74 eV by Dewar et al. [2]. Dewar’s EA data have recently been confirmed by the excellent correlation (for 68 hydrocarbons but without benzene) with parameters of the structure resonance theory [19]. The proposed relationships for pquinones are shown in fig. 1. There is no good correlation between EAQ and EAHC calculated by Kunii and Kuroda (the point for benzene deviates markedly). However, an excellent regression was found using Dewar’s set of EAHC :
CHEMICAL PHYSICS LETTERS
Volume 127, number 2
EA'
1.6 -
1.2 I
-1.0
-0.5
I
I
0
0.5
I
I.0
EA”’
Fig. 1. Relationships between electron affinities of pquinones, EAQ (this paper), and parent hydrocarbons, EAHC, from: ref. [ 131 (e), and ref. [2] (0) (in eV).
EAQ = 1.604 - 0.397 EAHC, with the correlation deviation of 0.013.
coefficient
(3) of 0.999 and standard
Acknowledgement The author is indebted for helpful discussion.
to Professor M.K. Kalinowski
6 June 1986
[2] M.J.S. Dewar, J.A. Hashmall and N. Trinajstic, J. Am. Chem. Sot. 92 (1970) 5555. [31 E.C.M. Chen and W.E. Wentworth, J. Chem. Phys. 63 (1975) 3183. 141 V.D. Parker, J. Am. Chem. Sot. 98 (1976) 98, and references therein. I51 P. Kebarle, in: Ions and ion pairs in organic reactions, Vol. 1, ed. M. Szwarc (Wiley-Interscience, New York, 1972) ch. 2. 161 R.C. Weast, ed., CRC handbook of chemistry and physics, 60th Ed. (CRC Press, Boca Raton, 1980) p. E-67. [71 E.L. Yee, R.J. Cave, K.L. Guyer, P.D. Tyma and M.J. Weaver, J. Am. Chem. Sot. 101 (1979) 1131. [81 M. Svaan and V.D. Parker, Acta Chem. Stand. B36 (1982) 351. [91 J.S. Jaworski, Electrochim. Acta 31 (1986), to be published. [lOI C.D. Cooper, W.T. Naff and R.N. Compton, J. Chem. Phys. 63 (1975) 2752. 1111 G. Caldwell and P. Kebarle, J. Chem. Phys. 80 (1984) 577. iI21 L.J. Raines, H.W. Moore and R.T. McIver Jr., J. Chem. Phys. 68 (1978) 3309. iI31 T.L. Kunii and H. Kuroda, Theoret. Chim. Acta 11 (1968) 97. 1141 F.M. Page and G.C. Goode, Negative ions and magnetron (Wiley-Interscience, New York, 1968). [15] W.E. Wentworth, L.W. Kao and R.S. Becker, J. Phys. Chem. 79 (1975) 1161. [ 161 E.K. Fukuda and R.T. McIver Jr., J. Phys. Chem. 87 (1983) 2993. [17] R.A. Holroyd, J. Phys. Chem. 86 (1982) 3541. [ 181 L.C. Christophorou, M.W. Grant and D.L. McCorkle, Advan. Chem. Phys. 36 (1977) 413; L.G. Christophorou, Chem. Rev. 76 (1976) 409. [ 191 A.S. Shawali, B.E. Elauadouli, C. Parkhnyi and W.C. Herndon, Bull. Sot. Chim. Belges 93 (1984) 867.
References [l] E.C.M. Chen and W.E. Wentworth, J. Phys. Chem. 87 (1983) 45.
135
CHEMICAL PHYSICS LETTERS
127, number 2
ELECTRON IMPROVED
AFFINITY METHOD
OF p-QUINONES. OF ELECTROCHEMICAL
6 June 1986
ESTIMATION
Jan S. JAWORSKI Department
of Chemistry
University
of Warsaw, 02-093 Warsaw, Poland
Received 31 January 1986
Electron affinities of four p-quinones are estimated from enthalpy changes obtained on the basis of measured formal potentials and reaction entropies in the electroreduction process. A linear correlation between electron affinities of p-quinones and parent hydrocarbons is found.
1. Introduction A few direct methods of electron affinity (EA) measurements have been perfected in recent years, including the gas-phase thermal electron attachment, collisional ionization technique with alkali-metal beams, photodetachment and photoelectric spectroscopy (references are given, e.g., in ref. [l]). But absolute values of EA which are fundamental energetic properties associated with negative ion formation and have important applications in chemistry, physics, and biology are still rather scarce for complex organic molecules. And thus, an improvement of indirect but simple methods of evaluation can be useful. Electrochemical redox potentials of some aromatic compounds have been correlated with experimental as well as theoretical EA values [2-41. In such relationships the formal potential for the reversible oneelectron electroreduction has been considered as a sum of three terms: the gas-phase EA, the solvation free-energy change between a reactant and a product, and the term depending on the reference electrode. This last one can be kept constant for a series of compounds, and in earlier papers the solvation term was assumed to be constant, especially for aromatic hydrocarbons [2]. This assumption, however, was questioned even for hydrocarbons (cf. discussion in refs. [3,4]) and it is quite unjustifiable [4] for aromatic compounds with polar groups. The other approach was later proposed by Parker [4] on the basis of the 0 009-2614/86/$0350 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
postulate that solvation energies for positive and negative radical ions of an alternant aromatic hydrocarbon are equal. However, the gas-phase EA is rather a measure of an enthalpy change [4,5] and is usually tabulated [6] for the temperature of 0 K. And thus, enthalpy changes, not redox potentials, should preferably be correlated with gas-phase EA values as well as with parameters of quantum-mechanical calculations. The measurements of the reaction entropy for a simple electrode process in a non-isothermal cell [7] open a new way for a direct estimation of both the solvation change between reduced and oxidized forms of the redox couple and the relative enthalpy change. Svaan and Parker found [8] that in the first approximation the entropy effect can be neglected not only in the electroreduction of aromatic hydrocarbons but also for heteroaromatic compounds related to anthracene (they obtained [8] the slope of the correlation between changes of both the relative enthalpy change, AAH, and relative free-energy change or formal potential, aEf, equal to 0.907). However, from our recent measurements [9] of the reaction entropy, AS’, in the one-electron reduction for a series of pquinones, one can calculate that slopes of the similar relationships of AAH versus AEf differ considerably from unity (0.8 and 0.6 in dimethylacetamide and dimethylsulfoxide, respectively). Thus the estimation of EA values for pquinones on the basis of enthalpy changes is proposed.
133
Volume 127, number
CHEMICALPHYSICS LETTERS
2
2. Experimental Measurements were carried out in five solvents: dimethylsulfoxide (DMSO), acetonitrile (ACN), propylene carbonate (PC), dimethylformamide (DMF), and dimethylacetamide (DMA). Formal potentials of pquinones at T = 298 K were measured polarographitally using a silver reference electrode with the same solvent as in a working compartment to avoid the liquid-junction potential. The details of measurements and of purification of chemicals, as well as values of reaction entropies, have been given [9].
3. Results and discussion Table 1 contains apparent enthalpy changes in the electroreduction of 1,4-benzoquinone (BQ), 1,4naphthoquinone (NQ), 9,10-anthraquinone (AQ), and 5,12-tetracenequinone (TQ) obtained from measured formal potentials and reaction entropies [9] : mw
=Ef-TAS’.
(1)
Values of AHaPP include an unknown term which is constant in each solvent and depends on the reference electrode. Absolute values of EA were calculated from uaPpp using 2s a standard for the estimation of the above unknown term the directly measured [lo] EA value of BQ equal to 1.89~~;~ eV (this value has recently been confirmed, EA = 1.86, by a different technique [ 1 l] ). The final EA values are: 1.59 f 0.04, 1.34 + 0.04, and 1.18 f 0.06 eV for NQ, AQ, and TQ,
Table 1 Apparent nones Solvent
DMSO ACN PC DMF DMA
enthalpy
a)
change
-Lwapp
in the electroreduction
of p-qui-
b)
BQ a)
NQ
AQ
TQ
0.152 0.374 0.764 0.471 0.588
0.457 0.691 1.044 0.805 0.844
0.713 0.940 1.318 1.056 1.081
0.892 1.136 1.474 1.202 1.218
a) For abbreviations of solvents and quinones see text. b, AH app (eV) calculated from eq. (1) at T = 298 K.
134
6 June 1986
respectively. The 95% confidence levels given indicate only deviations in different solvents which are mainly due to the contribution of solvation to the enthalpy change; they are evidently within the error of the method. It should be mentioned that for NQ the absolute value of EA > 0.8 eV was estimated on the basis of direct measurements [ 1] and also that the difference between EA values of BQ and NQ equal to 0.3 eV was proposed by McIver et 21. [ 121; but the procedure used by the last authors was later questioned [l l] . On the other hand, our results are in agreement with theoretically calculated [ 131 values of 1.28, 1.60, and 1.94 for AQ, NQ, and BQ, respectively. The additive substituent effect on EA values was found for a series of n-electron compounds with in’ creasing number of CN groups [ 141: EA= EA, +cEg,
(2)
where EA, is the contribution of a molecular skeleton to the electron affinity, and Eg describes the contribution of each CN group. The substituent effect on EA values of methyl-substituted benzaldehydes has also been discussed qualitatively [ 1.51 and for substituted nitrobenzenes [ 161. In the case of the above pquinones (EAQ) one can propose on the basis of additivity the constant contribution of carbonyl groups to EAQ values and ring affinities proportional to EA values of parent hydrocarbons, EAHC. Unfortunately, it is difficult to measure in the direct experiment the precise EA value of benzene; the negative value has been considered in the literature [15,17]; however, there was a strong argument [ 181 for a positive value close to zero. Different values were also estimated using quantum-mechanical calculations: -0.057 eV by Kunii and Kuroda [ 13 1, and -0.74 eV by Dewar et al. [2]. Dewar’s EA data have recently been confirmed by the excellent correlation (for 68 hydrocarbons but without benzene) with parameters of the structure resonance theory [19]. The proposed relationships for pquinones are shown in fig. 1. There is no good correlation between EAQ and EAHC calculated by Kunii and Kuroda (the point for benzene deviates markedly). However, an excellent regression was found using Dewar’s set of EAHC :
CHEMICAL PHYSICS LETTERS
Volume 127, number 2
EA'
1.6 -
1.2 I
-1.0
-0.5
I
I
0
0.5
I
I.0
EA”’
Fig. 1. Relationships between electron affinities of pquinones, EAQ (this paper), and parent hydrocarbons, EAHC, from: ref. [ 131 (e), and ref. [2] (0) (in eV).
EAQ = 1.604 - 0.397 EAHC, with the correlation deviation of 0.013.
coefficient
(3) of 0.999 and standard
Acknowledgement The author is indebted for helpful discussion.
to Professor M.K. Kalinowski
6 June 1986
[2] M.J.S. Dewar, J.A. Hashmall and N. Trinajstic, J. Am. Chem. Sot. 92 (1970) 5555. [31 E.C.M. Chen and W.E. Wentworth, J. Chem. Phys. 63 (1975) 3183. 141 V.D. Parker, J. Am. Chem. Sot. 98 (1976) 98, and references therein. I51 P. Kebarle, in: Ions and ion pairs in organic reactions, Vol. 1, ed. M. Szwarc (Wiley-Interscience, New York, 1972) ch. 2. 161 R.C. Weast, ed., CRC handbook of chemistry and physics, 60th Ed. (CRC Press, Boca Raton, 1980) p. E-67. [71 E.L. Yee, R.J. Cave, K.L. Guyer, P.D. Tyma and M.J. Weaver, J. Am. Chem. Sot. 101 (1979) 1131. [81 M. Svaan and V.D. Parker, Acta Chem. Stand. B36 (1982) 351. [91 J.S. Jaworski, Electrochim. Acta 31 (1986), to be published. [lOI C.D. Cooper, W.T. Naff and R.N. Compton, J. Chem. Phys. 63 (1975) 2752. 1111 G. Caldwell and P. Kebarle, J. Chem. Phys. 80 (1984) 577. iI21 L.J. Raines, H.W. Moore and R.T. McIver Jr., J. Chem. Phys. 68 (1978) 3309. iI31 T.L. Kunii and H. Kuroda, Theoret. Chim. Acta 11 (1968) 97. 1141 F.M. Page and G.C. Goode, Negative ions and magnetron (Wiley-Interscience, New York, 1968). [15] W.E. Wentworth, L.W. Kao and R.S. Becker, J. Phys. Chem. 79 (1975) 1161. [ 161 E.K. Fukuda and R.T. McIver Jr., J. Phys. Chem. 87 (1983) 2993. [17] R.A. Holroyd, J. Phys. Chem. 86 (1982) 3541. [ 181 L.C. Christophorou, M.W. Grant and D.L. McCorkle, Advan. Chem. Phys. 36 (1977) 413; L.G. Christophorou, Chem. Rev. 76 (1976) 409. [ 191 A.S. Shawali, B.E. Elauadouli, C. Parkhnyi and W.C. Herndon, Bull. Sot. Chim. Belges 93 (1984) 867.
References [l] E.C.M. Chen and W.E. Wentworth, J. Phys. Chem. 87 (1983) 45.
135