Simple criteria for distinguishing between inner- and outer-sphere electrode reaction mechanisms

Abstract

4403 0 . 5 -0 0.5 1.0 conc.5'-AmP (M 1-Figure 1. Viscosity of aqueous 5'-AMP solutions as a function of concentration (J": 7.0 & 0.4). 0.1 u 0.2 0.5 1.0 2.0 Yp(CP-1)-Figure 2. Double log plot of the average longitudinal I3C relaxation rates of S'-AMP'J vs. the reciprocal viscosity of the solutions: points, average of C-I' to C-4'; crosses, average of C-2, C-8. nucleotide one calculates the rotational correlation time rc in the extreme narrowing case from3 1/Tl = Nh2yC2yH2rCH-6rc(1) with N the number of protons bound to the carbon and yc and YH the magnetogyric ratios of the two nuclear spins involved. Taking the isotropic rotating sphere as a valid approximation for the motion of 5'-AMP in water, rc is described byInserting 0.105 f 0.005 nm for r C H one obtains at all concentrations a value of a, commonly taken as the diameter of the diffusing particle, of 0.45 f 0.05 nm, which certainly is less than the actual dimensions of the single nucleotide. The relaxation times of the base carbons at the different concentrations are therefore determined by the macroscopic viscosity, and it is unnecessary to invoke any specific microscopic model to explain the experimental results. On the other hand, the relaxation rates of the sugar carbon, which possess several possibilities of internal motion, depend on two correlation times: the correlation time for overall molecular reorientation which increases with increasing viscosity and a correlation time for the internal motions which to a first approximation should not depend on the vis~osity.~J It is then clear that the effect of the internal motions can only be observed in the more concentrated solutions, that is when the overall reorientation is slowed down.Rhodes and SchimrneF measured the energy of activa-tion for the rotation around the glycosidic bond (syn e anti equilibrium) in some purine (@)ribosides to be 6.2 kcal mol-'. Therefore the segmental motion may not come from rotation around the glycosidic bond. Roder et aL7 have determined the activation energy for the conformational mobility of the furanoside ring of the ribose moiety, as for example described by the N * S model of Altona and Sundaralingams by variable temperature I3C relaxation measurements and comparison with the 2',3'-isopropylidene nucleosides to be 4.7 f 0.5 kcal mol-I. Assuming that these results are applicable to 5'-AMP one must conclude that the deviations observed in the more concentrated solutions for the relaxation rates of the sugar carbons from those of the base should not be explained by a greater diffusive mobility of the ribosephosphate moiety around the glycosidic bond but rather by transitions between the different possible conformations of the ribose ring. References and Notes(1) W. D. Hamlll, Jr., R. J. Pugmlre, and D. M. Grant, J. Am. Chem. SOC., 96, (2) A. Allerhand. D. Doddreil, and R. Komoroskl, J. Chem. Phys., 55, 189 (3) A. Abraaam, "The Principles of Nuclear Magnetism", Oxford University 2885 (1974). (1 97 1). Press, London, 1961, p 300. 5456 (1974). (4) D. Doddrell and A. Aller...

The kinetics of electroreduction of hexaamminecobalt(III) cation on Bi planes in aqueous HClO4 solutions

“…The complex ions of the transition metals constitute interesting systems for examining simple heterogeneous electron transfer reactions [1][2][3][4][5][6]. The double layer effect observed may be varied significantly by varying the chemical nature and structure of the surrounding ligands [1][2][3].…”

“…The double layer effect observed may be varied significantly by varying the chemical nature and structure of the surrounding ligands [1][2][3]. When the redox reaction occurs by the socalled outer-sphere mechanism (i.e.…”

“…When the redox reaction occurs by the socalled outer-sphere mechanism (i.e. the redox reaction is ''simple'' and the reactant does not penetrate into the inner solvent layer), the kinetic parameters (usually the experimental transfer coefficient) provide very useful information about the location of the reaction centre [1][2][3][4][5][6][7][8][9][10][11][12][13]. The substrate (electrode material) effects [11][12][13] for the outer-sphere reactions are commonly presumed to be restricted to variations in the electrostatic work terms (double layer effects), at least for an adiabatic process, in that the chemical and electronic properties of the metal surface should exert no direct influence on the electron transfer barrier [14][15][16][17][18][19][20][21][22].…”

“…Hamelin and Weaver [4] studied the one-electron electroreduction of [Co(NH 3 ) 6 ] 3þ ; [Co(NH 3 ) 5 F] 2þ ; [cisCo(en) 2 (NCS) 2 ] þ (en -ethylenediammine), and [Co(N-H 3 ) 5 OSO 3 ] þ cations in the acidified 0.1 M NaClO 4 solution. Hromadova and Fawcett [10] investigated the reduction of [Co(NH 3 ) 6 ] 3þ in perchloric acid solution (from 0.093 to 0.0093 M) as a supporting electrolyte. Some reactants ([Co(NH 3 ) 6 ] 3þ , [Co(NH 3 ) 5 F] 2þ and [Co(NH 3 ) 5 OSO 3 ] þ ) are anticipated to reduce via outersphere pathways, but with very different electrical double layer effects due to the different ionic charges [1][2][3][4]10].…”

Electroreduction of Cr(III) Ions in Methanesulphonate Solution on Pb Electrode

“…Kinetics and mechanism of Cr(III) incomplete electroreduction have been investigated both on mercury and different solid electrodes [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . Such studies have not only theoretical value but also concern an important problem of trivalent chromium electroplating.…”