4429
J. Phys. Chem. 1987, 91, 4429-4430
COMMENTS Reduction Potential of the CO,/CO,' - Couple. A Comparison with Other C, Radicals Sir: The formate radical C02'- is commonly used in aqueous radiation chemistry as a reducing agent for metalloproteins or as an intermediate in the formation of the superoxide anion. Although it is known to be a strong one-electron reductant, exact determination of the reduction potential of the couple C02/C02*is difficult because of the irreversibility' of the electron-transfer C02'-. The irreversibility might be related process C 0 2 eto the change in structure upon reduction from linear to Lilie et al.' determined polarographically that carbon dioxide is reduced at -2.22 V vs. SCE and that C02'- is oxidized at -1.34 V vs. SCE. Thus, the reduction potential is expected to lie between -1.98 and -1.10 V vs. NHE. The observed reduction4 of T1' to T1° which requires -1.9 V is consistent with a value close to the lower limit. BerdnikovSused a thermodynamic cycle involving the formation of COOH in the gas phase from carbon dioxide and hydrogen and the solvation of COOH and its ionization in order to calculate that the standard Gibbs energy change (A,,&') of reaction 1 is
+
-
C02,,
+
Hz
-
COz'-
+ Hf
(1)
41.6 kcal, from which Eo' = -1.8 V follows. Assumptions were made regarding the entropy of the COOH radical in the gas phase, and the entropy and enthalpy of hydration of COOH. Since the electron affinity of carbon dioxide is known: the only missing parameter is the Gibbs energy of solution (ASGO) of the formate radical, COz'-: E0(C02g/C02'-) = -[A@oao(CO;-)
= [EA(CO2)
- AGOg (COJ] / F
+ A,Go(C02*-)]/F
(2)
The approach in the present study is based on the observed similarity of Gibbs energies of solution of bent triatomic anions which are structurally related to CO;-. &Go of the ozonide anion is calculated from an electron affinity of 2.10 V,7,8a A@', of 163.2 kJ/mol for 03?and a A@',, of 77.4 kJ/mol for O
