Hybridization possible concepts

The representation of non-bonding orbitals on an atom again uses the concept of. T-systems, though they may have any kind of hybridization (p, sp etc.), In Figure 2-56 the three possibilities arc shown lone pairs, radicals, and orbitals without electrons can be accommodated by this eoneept. 

To deal with circumstances such as the bonding in ozone, the notion of resonance between Lewis structures was developed. According to the resonance concept, when more than one Lewis structure may be written for a molecule, a single structure is insufficient to describe it. Rather, the true structure has an electron distribution that is a hybrid of all the possible Lewis structures that can be written for the molecule. In the case of ozone, two equivalent Lewis structures may be written. We use a double-headed aiTow to represent resonance between these two Lewis structures. 

However, we should emphasize that the NBO/NRT concepts of hybridization, Lewis structure, and resonance differ in important respects from previous empirical usage of these terms. In earlier phases of valence theory it was seldom possible to determine, e.g., the atomic hybridization by independent theoretical or experimental procedures, and instead this term became a loosely coded synonym for the molecular topology. For example, a trigonally coordinated atom might be categorized as sp2-hybridized or an octahedrally coordinated atom as d2sp3-hybridized, with no supporting evidence for the accuracy of these labels as descriptors of actual... 

It is also possible that orbitals of different kinds on the two atomic centers such as s-pz, p -d , d,z-p, etc. can combine to generate the MO for the diatomic molecules. As the one-center atomic orbitals are not orthogonal in molecules, for the depiction of electronic structure, the concept of hybridization is quite useful. 

Long before it was. possible to perform MO calculations on even the simplest molecules, the equivalence of the bonds led to the development of a different conception of the bonding in AB molecules, in which nonequivalent AOs on the central atom are combined into hybrid orbitals. These hybrid orbitals provide a set of equivalent lobes directed at the set (or subset) of symmetry equivalent B atoms. It is therefore obvious that all A—B bonds to all equivalent B atoms will be equivalent. 

Animal models of disease, whether naturally occurring or artificially induced, provide valuable insights into human disease and allow rigidly controlled studies that are not possible in humans. Probably the most intensively studied model of an immune complex disease is the spontaneous lupuslike disease that occurs in certain inbred strains of mice, especially the (NZB x NZW) F, hybrid. Its study has provided insights with respect to genetic predisposition (H25), the role of endogenous retroviruses (D8), the influence of sex hormones (R9), the identification of T suppressor cell abnormalities (T3), and the propensity to form certain autoantibodies, especially to double-stranded DNA (S36) and to the RNA-protein complex, Sm (E2). Though animal studies are not the subject of this review, we emphasize that the entire concept and framework by which we view immune complex disease in humans are based on initial observations in animals. 

---