Polarographic wave interpretation

Methoxyazocines exhibit a two-electron polarographic wave corresponding to conversion to the lOir-electron aromatic dianion (7UA161). The HMO delocalization energy for the planar azocinyl dianion has been calculated to be -5.1 /S, compared to -3.7 /S for cyclotetraenide dianion. Voltammetric data for 2-methoxy-3,8-dimethylazocine in acetonitrile-tetramethylammonium fluoroborate have been interpreted in terms of the direct formation of the dianion at the electrode followed by rapid protonation of the strongly basic dianion (76ACS(B)773). In contrast, the reduction of cyclooctatetraene under these conditions takes place in two steps. 

On the basis of this assumption, the authors studied the equilibrium concentrations of all eight of these aldoses at three total concentrations (0.10, 0.25, and 0.5 M) and at three pH values (6.5, 7.0, and 7.5), and calculated the percent concentrations of the reducible forms. On the basis of the data obtained on the presumed concentrations of the acyclic forms, a relationship between their stability and structure was sought. Although their interpretation of the data was incorrect (because the polarographic waves do not have diffusion character in these cases), the data nevertheless provided qualitative information as to the stability of the cyclic forms of the aldoses studied, as may be seen from a comparison of the order of aldose stability obtained in this work with the values obtained later (see Table II p. 143). 

Having proved that fortuitous phenomena and methodical imperfections have been eliminated, we shall try to propose an interpretation of the observed facts. The question of the increment in albumin due to denaturation seems to us to have been satisfactorily answered. However, the question of globulins is different their polarographic wave heights are increased by addition of alkali hydroxide only when the filter paper has remained in the solution serving as eluent. Let us discuss all possibilities concerning the behavior of globulins of diseased subjects, as follows ... 

The polarographic behavior of all the bipyridines with the exception of the 3,4 -isomer has been investigated in aqueous solution. The difficulty of reduction increases in the order 4,4 -, 2,4 -, 2,3 -, 2,2 -, and 3,3 -bipyr-idine. Most of the work has been concerned with 2,2 - and 4,4 -bipyridines. Both isomers are reduced in aqueous solution by a two-electron process, sometimes observed as two separate one-electron steps, which is pH dependent because of the formation of a dihydro derivative. Complications in interpretation of the waves due to prewaves, catalytic hydrogen waves, wave maxima, and adsorption phenomena have frequently been observed. ° Polarography and cyclic voltammetry investigations have been extended to the study of 2,2 - and 4,4 -bipyridines... 

The polarographic behavior of 1,10-phenanthroline,150-152 1,7-phenanthroline,153 and 4,7-phenanthroline154 has been studied in aqueous solution, but the interpretation of the reduction waves is not always certain because of complications due to adsorption and catalytic hydrogen waves. Some substituted 1,10-phenanthrolines have also been investigated in this way.151,155 Two clear reduction waves were obtained with 1,10-phenanthroline in dimethylformamide,156 however, and an attempt was made to correlate the reduction potentials with the energy levels of the molecule. Other studies in nonaqueous solvents with 1,10-, 1,7-, and 4,7-phenanthrolines also gave distinct waves.151,157... 

In solutions of histidine and histamine containing pyruvic acid, another reaction takes place (9) in addition to the formation of Schiff base. With increasing concentration of these amino acids apart from the formation of the wave of the Schiff base at a more positive potential, a decrease of the total limiting current can be observed (Fig. 5). The decrease corresponds to the formation of a polarographically inactive compound this may be interpreted by reaction (6) ... 

Another system in which ring-formation has been considered to be manifested on polarographic curves is the reduction of pyridoxal (77, 80). The reduction wave of this compound changes with pH and the observed plot is similar to that shown in Fig. 22. This dependence can be explained either by hydration (as for other pyridine carboxaldehydes), or by hemiacetal formation. The same two interpretations can be applied to electronic spectra. A comparison with the behaviour of pyridoxal-5-phosphate can contribute to the solution of this problem. With this ester the formation of the hemiacetal form is impossible and practically no current decrease in acidic solutions can be observed. Hence it can be concluded that the decrease in the limiting current of pyridoxal is due to ring formation. Nevertheless, the possibility of some participation by a dehydration reaction cannot be completely excluded, for it is possible to assume that the introduction of a phosphoric acid residue into position 5 either shifts the equilibrium towards the dehydrated form or increases the rate of dehydration. 

One well-established observation is that, under conditions where single-stranded polynucleotides give rise to a d.c. polarographic reduction wave, both native DNA and other double-helical natural and synthetic polynucleotides are inactive 22 23,46-47, 58,59,61) Tjjjs js rea(ji]y interpretable in that, in such helical structures, the adenine and cytosine residues are located in the interior of the helix, and hydrogen bonded in complementary base pairs (see below). Z-DNA, in which cytosine residues are at the surface of the helix, is of obvious interest in this regard, and the B - Z transition in the synthetic poly(dG dC) has been investigated with the aid of differential pulse polarography and UV spectroscopy 60). 

As the polarographic curve, furthermore, can be influenced by certain compounds or inhibitors, adsorption phenomena can complicate the interpretation of the curves, and catalytic waves may suggest further reductions than those found by macroelectrolysis, a certain caution must be exercised in evaluating the voltammetric data. In most cases, however, no complications arise, and with a little experience the differences mentioned above are not serious drawbacks, but are of value as the combination of polarography and macroelectrolysis then throws light on one or more of the steps in the reaction. 

Conventional polarographic measurements are usually less definitive—one-, two-, three- or four-electron transfers have been associated with the first cathodic wave of various acetylenes (Table 7). In the case of diphenylacetylene,. several polarographic studies still do not permit a clear choice between a one- or a two-electron first wave . As for the dianions of equation (75), it is probable that they are present in some cathodic processes. Since the evidence for them generally depends on the interpretation of polarographic and product data, their intermediacy is less certain. 

In acid solution, pyrimidine [312-316] is reduced polarographically in two one-electron waves two two-electron waves are observed in neutral medium, whereas a four-electron reduction occurs in alkaline solution. These findings have been interpreted in the following way ... 

The applicability of donicities to cation-solvent interactions is most convincingly demonstrated by the polarographic reduction of various metal ions in solvents of different donicity. The observed variation of half-wave potentials with solvent donicity can be explained neither in terms of the Born equation nor by simple microscopic electrostatic models in view of the magnitude of the dipole moments of solvent molecules. The concept also provides the basis for an interpretation of complex formation reactions and the behaviour of electrolytes (ion pair equilibria) in a large number of EPD solvents. 

The shapes and positions of irreversible waves can furnish only kinetic information. One may be able to determine such parameters as kf, 1, or a, but thermodynamic results, such as and free energies, are not available (28, 33, 34). As a rule of thumb, a system with > 2 X 10 cm/s appears reversible on the classical polarographic time scale of a few seconds when D is on the order of 10 cm /s. A heterogeneous charge transfer with < 3 X 10 cm/s will behave in a totally irreversible manner under the same conditions, and one can evaluate the rate parameters as described above. Systems with between these limits are quasireversible, and some kinetic information can be obtained from them through the treatment prescribed by Randles (33, 34). Naturally, the precision of the kinetic information deteriorates as the reversible limit is approached. See Section 5.5.4 for much more information about the interpretation of irreversible waves. 

Square wave voltammetry is normally carried out at a stationary electrode such as an HMDE, and involves the waveform and measurement scheme shown in Figure 7.3.13. As in other forms of pulse voltammetry, the electrode is taken through a series of measurement cycles however there is no renewal of the diffusion layer between cycles. In contrast to NPV, RPV, and DPV, square wave voltammetry has no true polarographic mode. The waveform can be viewed as a special case of that used for DPV (Figure 7.3.9), in which the preelectrolysis period and the pulse are of equal duration, and the pulse is opposite from the scan direction. However, the interpretation of results is facilitated by considering the waveform as consisting of a staircase scan, each tread of which is superimposed by a symmetrical double pulse, one in the forward direction and one in the... 

When the polarographic reduction of phenyl(4-methylphenyltelluro)acetylene was examined in MeCN in the presence of benzoic acid, the first reduction wave corresponding to the 2-electron electrochemical cleavage of the Csp—Te bond was progressively superseded by the 4-electron process of the hydrogenation of the triple bond. This reduction has been interpreted as involving an electron transfer to the adsorbed substrate yielding an anion radical, which after protonation and further reduction would form the saturated telluride (Scheme 1). 

The results obtained by Cantor and Peniston became the starting point for the correct interpretation of the polarographic currents of aldopentoses and aldohexoses, and contributed to the discovery (and theoretical treatment) of kinetic currents, which will be considered in the next Section (see p. 135). In contrast to aldopentoses and aldohexoses, the corresponding ketoses exhibit mixed diffusion-kinetic currents, and, for this reason, it is diflBcult to obtain the required data and treat them mathematically. On the other hand, their waves have greater analytical importance than those of the corresponding aldoses. 

Attempts to reduce the known cycloocta-l J S-triene-5 8-dione (11) electrochemically failed to produce any interpret able esr signals. The reduction polarographically involved a clean 2-electron wave (-1.0 V. relative to s.c.e.) apparently the result of 8 being more easily reduced than the parent dione. 

From a polarographic point of view semi- and thiosemicarba-zones resemble the structurally related arylhydrazones. Earlier papers reported only isolated data on the reduction of aldehyde and ketone semicarbazones [145-147] and p-acetaminobenzaldehyde thio-semicarbazone [148, 149] at the dropping mercury electrode. The latter compound has been studied in more detail by Dusinsky [150], who observed a four-electron wave on the polarograms in acidic solutions at pH 7.0 this wave decreased almost to zero. A two-electron wave, the limiting current of which did not depend on pH, appeared in alkaline solution. These results were interpreted on the assumption that tautomeric equilibrium existed between the ionic form in acidic solutions and the thiol form in alkaline solutions. The opinions of the various authors differed on the question of the number of electrons involved in the electrode reactions with semi- and thiosemi-carbazones. 

Pfister-Guillouzo et al. have performed CNDO/S calculations (spd model) on the 1,2-dithiolium cations (87) and suggested that the spectral absorptions in the u.v. region may be completely interpreted by 7r - 7r transitions. This is in agreement with a former study involving PPP calculations which provided an adequate description of the absorption spectrum of (87) and rationalized the effects of methyl substituents on (86)—(88) (i.e. spectral shifts and changes in the polarographic half-wave potentials). A linear correlation has been found between the reduction potentials and the lowest free MO s (see also ref. 41). 

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