The electrochemical (EC) reduction mechanism of methylcobalamin (Me-Cbl) in a mixed DMF/MeOH solvent in 0.2 M tetrabutylammonium fluoroborate electrolyte was studied as a function of temperatureand solvent
ratio vs a nonaqueous Ag/AgCl/Cl
- reference electrode. Double-potential-step chronoamperometry allowed the rate constant of the subsequent homogeneous reaction to be measured over thetemperature range from 0 to -80
![](/images/entities/deg.gif)
C in 40:60 and 50:50 DMF:MeOH
ratios. Activation enthalpies are 5.8± 0.5 and 7.6 ± 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively.
Digital simulationand curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 × 10
3s
-1 give
E ![](/images/entities/deg.gif)
' = -1.466 V,
k ![](/images/entities/deg.gif)
= 0.016 cm/s, and
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= 0.77 at 20
![](/images/entities/deg.gif)
C for a quasi-reversible electrode process.Digital simulation of the results of Lexa and Savéant (
J. Am. Chem. Soc. 1978,
100, 3220-3222) showsthat the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible stepfollowing the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants ofthese reactions. An electron coupling matrix element of
kA = (4.7 ± 1.1) × 10
-4 eV (~46 J/mol) is calculatedfor the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy forhomolytic cleavage of Me-Cbl is calculated as 31 ± 2 kcal/mol. The voltammetry of the ethyl-,
n-propyl-,
n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentialswere correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL
2DZ) calculations.