Classification of atomic interactions

Properties of the charge density at the (3, — 1) critical point for a number of diatomic molecules are given in Table 7.4 and, for some polyatomic molecules, in Table 7.5. Contour and relief maps of the Laplacian distributions for some of these molecules are shown in Figs 7.15-7.17. 

The second limiting type of atomic interaction is that occurring between closed-shell systems, such as found in noble gas repulsive states, in ionic 

Molecule and state R V p(re) Eigenvalues of Hessian of Pit.) Kinetic energy contribution Net charge on A in AB 

IThe almost complete interatomic transfer of one electronic charge indicated in Table 7.4 for the ionic systems is verified by the nodal structure of the corresponding Laplacian maps. The cations, Li , Na , and K , all lack the outer nodes associated with the valence density distribution of the isolated atom. Thus, Li in LiCl has but one negative region rather than two, Na in NaCl has two rather than three, and K in KF has three rather than four he reader is referred to Fig. E7.2 for displays of the charge distributions and interatomic surfaces for some of these systems. Another characteristic of a closed-shell interaction exemplified by the alkali halides and discussed in Section E7.1 is the separate localization of the electrons within the basin of each atom, as determined by the spatial localization of the Fermi hole. 

The Laplacian map for the dimer of HF is topologically equivalent (i.e. continuously deformable) into that for Ncj. Why certain closed-shell interactions are stable with respect to separated species while others are not is related to the extent of interatomic charge transfer and to the polarization of the atomic distribution, a topic discussed in Section 7.5. 

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Table I. Classification of atomic interactions on the basis of local topological and energetic properties at the bond critical point...

When photons penetrate a substance, they can interact with the nuclei or with the shell electrons. O Table 8.7 shows a classification of the interactions of photons (X-ray and 7 radiation) in matter. The dependence of the cross section cr of the interaction on the atomic number Z of the medium is also shown. Note that the Compton effect is inelastic for the atom but it is treated theoretically as elastic scattering on free electrons as the binding energy of the atomic electron is much smaller than that of the scattered electron. 

Many observations concerning these trends had been made over the years, and in the 1950s S. Ahrland, J. J. Chatt, and M. Davies presented a classification of metals based on their preferred interaction with donor atoms. Class A metals are those that interact preferentially when the donor atom is in the first row of the periodic table. For example, they prefer to bond to N rather than P donor atoms. Class B metals are those which interact better when the donor atom is in the second row of the periodic table. For example, a class B metal would bond better to P than to N. The following table summarizes the behavior of metal atoms according to this classification. 

Following our general classification of hydrogen bonds, a negatively polarized hydrogen atom in H-Y, where Y is a less electronegative element than H (e.g., B, Al, Re, K, Mg, etc.), could be considered an unusual or nonconventional proton acceptor. Then the interaction... 

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