Pauling resonance theory

SOME BASICS OF NATURAL RESONANCE THEORY VERSUS PAULING RESONANCE THEORY... 

As suggested by the original resonance theory of Pauling and Wheland,51 such delocalization effects appear to represent some type of average of multiple resonance structures. A general goal of resonance theory is to represent each property (P)true of the true delocalized system in resonance-averaged form... 

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. 

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. 

L. Pauling I agree that one must be cautious. For twenty-five years I have been troubled by the statements made about resonance theory by people who did not understand it very well, who did not have enough background of experience to apply it in a satisfactory way. 

The resonance theory of Pauling and Wheland is brought into readier contact with classical ideas with the introduction of the resonance-theoretic wavefunction Ansatz appropriate for alternant systems. This is simply an equal-weighted sum over Kekule structures... 

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... 

Nevertheless, acknowledging or denying the existence of differences between resonance theory and classical structural theory was dependent on their different assessments of the role of alternative methods to study molecular structure. Wheland equated resonance theory to the valence bond method and viewed them as alternatives to the molecular orbital method. Pauling conceded that the valence bond method could be compared with the molecular orbital method, but not with... 

It is interesting to note the initiative of the New York Chapter of the National Council of Arts, Sciences and Professions to organize a meeting on the subject. It was proposed that the meeting have the form of a debate where N. D. Sokolov from Moscow, Coulson, and Pauling would each contribute a paper and there would follow a discussion of the points raised in the communications. Coulson felt that the best way would be for Sokolov and Pauling to present their viewpoints and that he would make a series of comments. Each party would be asked to provide answers to the following questions What is the resonance theory What is the evidence in... 

With the advent of the computer era, it is now possible to reexamine and rethink the resonance theory at the ab initio level. For example, throughout Pauling and Wheland s books, benzene is supposed to be a hybrid of two Kekule structures, by noting that Dewar and other ionic structures make little contribution to the resonance in benzene. However, classical ab initio VB calculations with all possible 175 resonance structures by Norbeck et al. [51] and Tantardini et al. [3], where strictly atomic orbitals are used to construct VB functions, manifested that the five covalent Kekule and Dewar structures make even less contribution to the ground state of benzene than the other 170 ionic structures. This prompts us to reconsider the mathematical formulations for resonance structures [52]. 

We have applied Pauling s theory to the molecular hydrogen cluster as follows the nonmetallic cluster is well described by the usual Kekule structure (1-2 3-4) (Fig.2), where orbitals 1 and 2 are at one hydrogen molecule and 3 and 4 are at the other one. The synchronized resonance is the mechanism in which the system alternates between structures (1-2 3-4) and (1-4 2-3) (anti-Kekule) (Fig.2), breaking simultaneously the two original covalent bonds and forming two new ones. 

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... 

Pauling, L. In Perpectives in Organic Chemistry, pp. 1-8 (A. Todd, Ed.), New York Interscience, 1956. In this paper Pauling proffered the opinion that the resonance theory is an extension of the classical structural theory, rather than quantum-mechanical in character. 

Another rather interesting relation between the resonance theory and elementary MO theory was established by Ham and Ruedenberg 107>. Let Pp be the Pauling bond-order matrix known from resonance theory 179>. If the adjacency matrix of the -th Kekute graph is Aj, then... 

---