Pauling’s structure

In the present chapter, we explore the Pauling ideas in the context of an ab initio valence bond formalism. We do so by admitting extra orbitals in the constituent atoms of molecular systems, and devising the new types of VB structures that can be set allowing the atoms to form more bonds than their "atomic valences" admit. We will see that these Pauling s structures, as we named them, are important to describe some kinds of delocalized bonds and that they give some information about possible processes of charge transfer in the molecular systems. 

There are several types of structures the conventional covalent (also called Kekule structures), the ionic and the newest Pauling s structures (metallic structures). Below, the main structures of each small anionic cluster, shown in Fig.5, are discussed. 

It is interesting to notice that as the systems become bigger and more compact, a larger number of VB structures becomes necessary to describe them. It means that the metallic character of these systems is increasing (that is why we usually say that small lithium clusters could be prototypes of metals). At the same time, Pauling s structures are showing their importance. From the C2v Li4 ... 

Following Pauling, we have admitted extra orbitals on monovalent atoms involved in molecular systems with some metallic character or very delocalized bonds. In conjunction with a spin-free valence bond formalism, these extra orbitals have allowed us to devise new kinds of VB structures, the Pauling s structures, as we call them. These structures permit the monovalent atoms to form two covalent bonds simultaneously, as a consequence of electron transfer from neighbors and, thus, give information about delocalization of charge in the system, that is not directly inferred from the usual Kekule or ionic structures. Therefore, the Pauling s structures complement the VB description of molecular systems. 

In the case of the hydrogen cluster, we have obtained, through the Pauling s structures, a description of the metallization of the system under pressure, that is completely consistent with other sophisticated ab initio calculations and with experiments. 

For the lithium clusters, we have seen that the Pauling s structures can have the highest weights in the VB wave function, e.g. the C2v Li4" and C2v Li5 clusters. The structures are certainly very important when three-body bonds are present. According to our results, a VB description of the anionic or neutral lithium clusters without the Pauling s structures would be incomplete or would result in a less compact wave function. 

Pauling s structures, the Blomstrand-Mendelejeff unbranched ones, are at present substantiated through X-ray work. Also, the chemistry of the polythionates is fairly well understood (37, 40, 52, 82, 87, 96) on the basis of unbranched sulfur chain structures. 

For biographical details see Paradowski (1991), Hager (1995, 1998), and for a discussion of Pauling s structural chemistry see Paradowski (1972). A complete bibUography of Pauling s papers from 1920 to 1990 compiled by Z. S. Hermann and D. B. Munro is contained in Mead (1991). See also Rich and Davidson (1968). 

Griffith (1956) duely pointed out that in addition to Pauling s structures, the oxygen molecule can be bound to the Fe(II)-ion in hemoglobin by three-center bonds as illustrated by structure (III). 

The discovery of the ci-helix structure was only one of many achievements that led to Pauling s Nobel Prize in chemistry in 1954. The official citadon for the prize wa.s for his research into the nature of die chemical bond and its application to die elucidadon of the structure of complex substances. ... 

The crystal structures in Chapters 5 and 6 were determined by x-ray diffraction, and the papers illustrate Pauling s approach to this experimental technique, including his most notable methodological contributions—the coordination method (SP 42) and the stochastic method (SP 47). In its day, SP 47 was a tour de force in the determination of a complex crystal structure. SP 46 contains Pauling s famous discovery of two quite different crystal structures giving the same x-ray diffraction pattern, which violated the then-current conventional wisdom in x-ray crystallography. 

Chapter 9, on entropy and molecular rotation in crystals and liquids, is concerned mostly with statistical mechanics rather than quantum mechanics, but the two appear together in SP 74. Chapter 9 contains one of Pauling s most celebrated papers, SP 73, in which he explains the experimentally measured zero-point entropy of ice as due to water-molecule orientation disorder in the tetrahedrally H-bonded ice structure with asymmetric hydrogen bonds (in which the bonding proton is not at the center of the bond). This concept has proven fully valid, and the disorder phenomenon is now known to affect greatly the physical properties of ice via the... 

---