Pauling s resonance theory

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

Pauling s resonance theory raised questions as to the ontological status of theoretical entities very similar to the problematique associated with discussions about scientific realism. Differences in the assessment of the methodological and ontological status of resonance were the object of a dispute between Pauling and Wheland, who worked towards the extension of resonance theory to organic... 

I was inspired too by Linus Pauling (1901-94), another polymath with humanistic concerns. His Nature of the Chemical Bond (1939) brought a new perspective to theories of molecular structure, and refuted the implication of a popular examination question of the time, Is inorganic chemistry a largely closed and finished subject Pauling s resonance theory, formally based on the quantum-mechanical valence-bond (VB) method for... 

The top panel in the accompanying figure depicts valence bond structures for the two major resonance contributors of benzene. Contrary to earlier notions of two rapidly fluctuating structures, Pauling s resonance theory, developed with his student George W. Wheland (1907-74), viewed benzene as represented by two idealized but fictional resonance... 

Strictly speaking, Pauling s mathematical formulation of resonance theory did not behave as its author intended (Sidebar 5.3). However, the theory was initially applied only in a qualitative empirical fashion that obscured these difficulties. Nearly a half-century elapsed before reliable polyatomic calculations allowed a rigorous test of Pauling s approximations, by which time the qualitative concepts of Pauling s resonance theory had become firmly entrenched in chemistry textbooks in more or less present form. Although some theorists continue to believe that resonance concepts... 

One of the key concepts of Pauling s quantum theory of chemical bonding, introduced in 1931, was resonance In many cases an ion or molecule could not be represented, conceptually or on paper, as one classical structure, but required what he called a hybridization of two or more of these structures. The single classical structure simply did not describe the chemical bond(s). In less than a decade he had transformed the earlier, somewhat simplistic theory of the chemical bond into a powerful, highly sophisticated theory and research tool. 

The most common example of the resonance theory is the description of the benzene structure. The experimentally precisely determined and accurately known carbon-carbon bond length is consistent with the model as average of the resonance structures. When Pauling s resonance description of the benzene structure was criticized, the physicist Edward Teller and his colleagues provided spectroscopic evidence to support it [40]. The Nobel laureate physicist Philip Anderson was oblivious of Teller s and his co-workers paper (Private communication from Philip Anderson to the author by e-mail in 2009), and 68 years after Teller s contribution, in 2008, Anderson communicated another supportive paper for Pauling s model [41]. 

In fact, this approach is sufficiently close in spirit to Pauling s resonating bond theory (where Z and play important roles) that we may regard it as an electronic realization of some of his thermochemical ideas. 

The NRT resonance weights, bond orders, and valencies are generally comparable to those of the older Pauling-Wheland theory (particularly for species of low ionicity) and can be used to rationalize chemical phenomena in a similar fashion. Pauling s classic, The Nature of the Chemical Bond, brilliantly illustrates such reasoning. 

---