Examination of Electron Transfer Self-Exchange Rates Using NMR Line-Broadening Techniques: An Advanced Physical Inorganic Laboratory Experiment

2000 ◽  
Vol 77 (1) ◽  
pp. 88 ◽  
Author(s):  
Donald L. Jameson ◽  
Rajan Anand
Keyword(s):  
Electron Transfer ◽  
Exchange Rates ◽  
Laboratory Experiment ◽  
Line Broadening

Study of the exchange rates of ethanol and water on ferriprotoporphyrin IX by n.m.r. line broadening

1969 ◽  
Vol 47 (17) ◽  
pp. 3217-3224 ◽  
Author(s):  
N. S. Angerman ◽  
B. B. Hasinoff ◽  
H. B. Dunford ◽  
R. B. Jordan
Keyword(s):  
Exchange Rates ◽  
Temperature Jump ◽  
Aqueous Ethanol ◽  
Ion Pair ◽  
Solvent Composition ◽  
Water Proton ◽  
Solvent Exchange ◽  
Line Broadening ◽  
Kcal Mole ◽  
Ethanol Molecule

The nuclear magnetic resonance (n.m.r.) line broadening technique has been used to determine the rate of solvent exchange from the first coordination sphere of ferriprotoporphyrin IX (hemin) in aqueous ethanol. The line broadenings of the methyl and methylene protons of ethanol and the water protons have been studied as a function of solvent composition at 35° and of temperature in 48.8 mole % ethanol.The average values of the kinetic parameters for ethanol molecule exchange, taken from the CH3 and CH2 proton broadening are ΔH≠ = 6.2 ± 1.0 kcal mole−1 and ΔS≠ = −9.1 ± 4.4 cal mole−1 deg−1. Both proton transfer and water molecule exchange contribute to the water proton line broadening with a net value of ΔH≠ = 6.3 ± 0.5 kcal mole−1 and ΔS≠ = −3.8 ± 2.0 cal mole−1 deg−1.From the n.m.r. study at varying solvent composition, it is concluded that monomeric and dimeric hemin undergo solvent exchange at the same rate. The equilibrium constant for formation of the monoethanol solvated hemin from hydroxyhemin is determined as 8.7 at 35° from this study.A comparison of the rate of ethanol exchange (1.8 × 106 s−1 at 25°) and the rate of imidazole binding to hemin (from a separate temperature jump study) indicates that the imidazole substitution occurs through an SN1 ion pair mechanism.

Pressure effects on the rate of electron transfer between tris(1,10-phenanthroline)iron(II) and -(III) in aqueous solution and in acetonitrile

1988 ◽  
Vol 66 (11) ◽  
pp. 2763-2767 ◽  
Author(s):  
Hideo Doine ◽  
Thomas Wilson Swaddle
Keyword(s):  
Aqueous Solution ◽  
Electron Transfer ◽  
Ionic Strength ◽  
Classical Theory ◽  
Activation Parameters ◽  
Pressure Effects ◽  
Line Broadening ◽  
Water As Solvent ◽  
Aqueous Solvent ◽  
Mean Pressure

Proton nmr line-broadening experiments at ambient and elevated (to 215 MPa) pressures show that the rate of electron transfer between Fe(phen)32+ and Fe(phen)33+ as bisulfates in D2O/D2SO4 is represented by the activation parameters (at ionic strength I ~ 0.4 mol kg−1) ΔH≠ = 1.6 ± 0.5 kJ mol−1, ΔS≠ = −102.2 ± 1.6 JK−1mol−1, k(276 K) = 1.31 × 107 kg mol−1s−1, and (at I ~ 0.3 mol kg−1 and a mean pressure of 100 MPa) ΔV≠ = −2.2 ± 0.1 cm3mol−1. For the same reaction of the perchlorate salts (total [Fe] 0.046–0.065 mol kg−1) in CD3CN, ΔH≠ = 11.0 ± 1.0 kJ mol−1, ΔS≠ = −72.5 ± 3.6 J K−1 mol−1, k(277 K) = 8.0 × 106 kgmol−1s−1, and ΔV≠ = −5.9 ± 0.5 cm3 mol−1. For water as solvent, ΔV≠ is satisfactorily accounted for by a classical theory of the Stranks–Hush–Marcus type. Volumes of activation for electron self-exchange are shown to provide criteria for non-adiabaticity and for dominance of (non-aqueous) solvent reorganization dynamics; on this basis, it is seen that neither of these factors is important in the title reactions.

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