Charge density experimental corrections

Table 2.4 shows a comparison of the experimental and PPP-MO calculated electronic spectral data for azobenzene and the three isomeric monoamino derivatives. It is noteworthy that the ortho isomer is observed to be most bathochromic, while the para isomer is least bathoch-romic. From a consideration of the principles of the application of the valence-bond approach to colour described in the previous section, it might have been expected that the ortho and para isomers would be most bathochromic with the meta isomer least bathochromic. In contrast, the data contained in Table 2.4 demonstrate that the PPP-MO method is capable of correctly accounting for the relative bathochromicities of the amino isomers. It is clear, at least in this case, that the valence-bond method is inferior to the molecular orbital approach. An explanation for the failure of the valence-bond method to predict the order of bathochromicities of the o-, m- and p-aminoazobenzenes emerges from a consideration of the changes in 7r-electron charge densities on excitation calculated by the PPP-MO method, as illustrated in Figure 2.14. 

Using estimates of Cd based on equation (10.6.27) or (10.6.28), the experimental capacity C may be corrected for the diffuse layer contribution to obtain the capacity of the inner layer Q. Since Cd is usually much greater than C, errors in the GC estimate of Cd are not important except in the vicinity of the PZC, where Cd falls to its lowest values. When the experimental capacity is obtained at constant electrode charge density and as a function of electrolyte concentration, the GC estimates of Cd are often used to determine whether ionic specific adsorption... 

Uncertainties of the conventional parameters of H-atoms have been addressed since the early applications of X-ray charge density method. Support from ND measurements appears to be essential, because the neutron scattering power is a nuclear property (it is independent of the electronic structure and the scattering angle). The accuracy of nuclear parameters obtained from ND data thus depends mainly on the extent to which dynamic effects (most markedly thermal diffuse scattering) and extinction are correctable. Problems associated with different experimental conditions and different systematic errors affecting the ND and XRD measurements have to be addressed whenever a joint interpretation of these data is attempted. This has become apparent in studies which aimed either to refine XRD and ND data simultaneously [59] (commonly referred to as the X+N method), or to impose ND-derived parameters directly into the fit of XRD data (X—N method) [16]. In order to avoid these problems, usually only the ND parameters of the H-atoms are used and fixed in the XRD refinement (X-(X+N) method). 

The general equation may for instance approximate to C = Cft which at moderate sol concentrations is the case for hexol nitrate, Q here being very small. Therefore hexol nitrate is a salt which is well fitted to give information on the charge density of colloids as was discussed in details in 1. In that section Q was the only quantity which really interested us, leading to the calculation of reciprocal hexol numbers. The very small and not exactly measurable Ct values had only the importance of a correction factor used in the calculation of Cf from the experimentally found C values at a few sol concentrations. 

The properties of the diffuse double layer depend directly on the surface charge density and not on the potential. In order to correct for this effect quantitatively, one needs to convert the dependence of frequency on potential A/( ), observed experimentally, to its dependence on charge density, A/( q). Having the analogous dependence A/o( q) for the supporting electrolyte, it is possible to evaluate the real response of the EQCM to specific adsorption, 5f q) = A/( ) — A ( ), and use this response for interpretation of the data obtained. This approach was taken in [74, 108,111] for several systems as seen in Figs. 9 and 10. For all cases studied, the surface excess was known from independent electrochemical experiments. 

This frequency shift should be corrected by taking into account changes in electrostatic adsorption caused by the difference in the pzc of the two metals and the differences in charge density of gold at 1.2 V (at the beginning of the experiment) and that of silver at 0.63 V, when steady state has been reached. However, this correction is much less (about 5-7 Hz) than that observed experimentally (for details, see Ref. 98). 

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