REACTIVE INORGANIC OXY-SPECIES OF C, N, S, AND P - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 209

1.0

2.0

0.8

1.5

0.6

1.0

0.4

0.5

0.2

0.0
400 500 600 700 800

wavelength (nm)

0.0

0 2 4 6 8 10

pH

Figure 5.4. Fraction of the total carbonate radical that is present as the basic form,
CO •− , as a function of the pH. Inset: the spectrum observed at pH 10 (filled circles), at
pH 2.7 (open squares), and pH 0.8 (open diamonds) (adapted from [33] with the per-
mission of the American Chemical Society).

resonance Raman spectroscopy [36]. The radical has C 2v symmetry, suggesting
some distortion from the predicted d 3h structure. This study found no change
in the Raman spectrum between pH 7.5 and 13.5, consistent with the absorp-
tion spectrum shown in Figure 5.4. The EPR spectrum with g -factor and line
width values of 2.0113 and 5.0 g, respectively, has also been reported [26, 37].
The spectral studies concluded the carbonate radical anion is an electrophilic
oxygen-centered radical.
The self-recombination of CO •− is second order [38, 39]. The kinetics of the
reaction was studied with temperatures of up to 250°C to understand
the mechanism, which suggests oxygen transfer reactions [38]. Moreover, the
chemistry of CO •− at high temperature is relevant in designing an efficient
nuclear reactor and in destroying pollutants under supercritical water condi-
tions [39]. The mechanism involves a pre-equilibrium formation of the dimer,
C O
2

2− , which dissociates to the peroxymonocarbonate anion and CO 2 (reac-
tion 5.5). The CO •− radical reacts with the peroxymonocarbonate anion, yield-
ing a peroxymonocarbonate radical (reaction 5.6), which can further react with
CO •− to form C O

6

2− (reaction 5.7):

2 7

•− •−

2

− 2 − 6

CO + CO C O → O COO + CO k = 4 25 10 . × /M/s (5.5)

3 3 2 2 2 5