Effective charge cloud

Both of the above approaches rely in most cases on classical ideas that picture the atoms and molecules in the system interacting via ordinary electrical and steric forces. These interactions between the species are expressed in terms of force fields, i.e., sets of mathematical equations that describe the attractions and repulsions between the atomic charges, the forces needed to stretch or compress the chemical bonds, repulsions between the atoms due to then-excluded volumes, etc. A variety of different force fields have been developed by different workers to represent the forces present in chemical systems, and although these differ in their details, they generally tend to include the same aspects of the molecular interactions. Some are directed more specifically at the forces important for, say, protein structure, while others focus more on features important in liquids. With time more and more sophisticated force fields are continually being introduced to include additional aspects of the interatomic interactions, e.g., polarizations of the atomic charge clouds and more subtle effects associated with quantum chemical effects. Naturally, inclusion of these additional features requires greater computational effort, so that a compromise between sophistication and practicality is required. 

This moment measures the extent and direction of the shift of an atom s electronic charge cloud with respect to the nucleus. The quantity M(fi) can effectively be regarded as an intra-atomic dipole moment. The intra-atomic dipole moment of each atom contributes to the... 

Interpretating Alkali Iodide Data. The alkali iodide data given above show that the idealized model of ionic crystals is inadequate since 8 9 constant and hp 9 0. To interpret the data one must consider the effects on the iodine 5p population by covalency, deformation of the charge cloud by electrostatic interaction, and deformation by overlap. 

C) Charge-cloud overlap repulsions The existence of some type of repulsive force follows from the fact that all the other contributory effects lead to bonding and shortening of the H-bond. But these forces must be fairly strong, as can be seen from the distances involved. For example, in the 0X —H 02 system, we find 0X 02 distances as low as 2-5 A, yet the closest 0 0 distance found in crystals, where no H-bond exists, is about 3 0 A. Alternatively, the sum of the van... 

This process is a relatively fast one and the diffusion coefficient determined from the correlation time of the fluctuations in the scattering can be too large202) giving too small radii, tm- To avoid this effect, Mazer et al.33) made measurements on sodium dodecyl sulphate in high salt concentration to contract the counterion charge cloud. 

In the same way that electronegativities determine the polarity of diatomic interactions, ionization radii should define the effective electronic charge clouds that interpenetrate to form diatomic molecules, as shown schematically in Figure 5.3. The overlap of two such spheres defines a lens of focal lengths fixed by the ionization radii, r and r2, at an interatomic distance d = x i + x2-... 

In an environment of atoms in collision, interatomic contacts consist of interacting negative charge clouds. This environment for an atom is approximated by a uniform electrostatic held, which has a well-defined effect on the phases of wave functions for the electrons of the atom. It amounts to a complex phase (or gauge) transformation of the wave function ... 

A further point of interest regarding this problem has been raised by Bell et al. (1971). Calculations based on an electrostatic charge cloud model indicate that the variation in kH/kD is primarily determined by the tunnel correction. Different reactions will have different barrier widths, hence different tunneling probabilities, and, in the context of this hypothesis, different variations of isotope effects. The hypothesis still predicts, however, that for a given system kH/kD will have maximum value for the symmetrical transition state where the probability of tunneling is highest. 

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