Charge displacement, second-order effects

Let us consider another, so-called, second-order effect of an external electric field E on a given molecule M2 This field influences the molecular charges, electTOTis, and nuclei, causing their displacements, and as a result, there appears the induced dipole moment d . ... 

A more widely used approach for organic molecules is based on second-order perturbation theory. Here the dipolar contribution to the field induced charge displacement is calculated by inclusion of the optical field as a perturbation to the Hamiltonian. Since the time dependence of the field is included here, dispersion effects can be accounted for. In this approach the effect of the external field is to mix excited state character into the ground state leading to charge displacement and polarization. The accuracy of this method depends on the parameterization of the Hamiltonian in the semi-empirical case, the extent to which contributions from various excited states are incorporated into the calculation, and the accuracy with which those excited states are described. This in turn depends on the nature of the basis set and the extent to which configuration interaction is employed. This method is generally referred to as the sum over states (SOS) method. 

The second consideration in CE sample preparation is the importance of sample pH. In capillary zone electrophoresis (CZE), analyte mobility is based on mass and charge, so migration velocity is highly dependent on the pH of the medium. In electrokinetic injection, variations in sample pH can change analyte mobility and therefore affect injection efficiency. In displacement injection, a differential in sample and run buffer pH is sometimes used to effect zone sharpening or stacking in order to increase detection sensitivity [40] variations in sample pH alter stacking efficiency. 

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