The Ontological Status of Resonance

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 

I think that the theory of resonance is independent of the valence-bond method of approximate solution of the Schrodinger wave equation for molecules. I think that it was an accident in the development of the sciences of physics and chemistry that resonance theory was not completely formulated before quantum mechanics. It was, of course, partially formulated before quantum mechanics was discovered and the aspects of resonance theory that were introduced after quantum mechanics, and as a result of quantum mechanical argument, might well have been induced from chemical facts a number of years earlier. [25] 

In 1947, Coulson wrote an article in a semi-popular magazine on what he thought was resonance  

Is resonance a real phenomenon The answer is quite definitely no. We cannot say that the molecule has either one or the other structure or even that it oscillates between them. .. Putting it in mathematical terms, there is just one full, complete and proper solution of the Schrodinger wave equation which describes the motion of the electrons. Resonance is merely a way of dissecting this solution or, indeed, since the full solution is too complicated to work out in detail, resonance is one way - and then not the only way - of describing the approximate solution. It is a calculus , if by calculus we mean a method of calculation but it has no physical reality. It has grown up because chemists have become used to the idea of localized electron pair bonds that they are loath to abandon it, and prefer to speak of a superposition of definite structures, each of which contains familiar single or double bonds and can be easily visualizable. [30] 

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G. W. Wheland, Theory of Resonance and Its Application to Organic Chemistry (New York J. Wiley and Sons London Chapman and Hall, 1944). The Pauling-Wheland controversy on the ontological status of resonance is described in the contribution of K. Gavroglu and A. Simoes in this volume. 

In this paper we discuss a number of issues which manifest the theoretical particularity of quantum chemistry and which are usually not discussed in an explicit manner either in the historical or in the philosophical studies related to quantum chemistry. We shall focus on five issues the re-thinking of the problem of reductionism, the discourse of quantum chemistry as a confluence of the traditions of physics, chemistry, and mathematics, the role of textbooks in consolidating this discourse, the ontological status of resonance, and the more general problem of the status of the chemical bond. Finally, we shall briefly discuss the impact of large scale computing. 

Almost all of the protagonists were aware and worried about the pitfalls concerning the ontological status of the various theoretical entitles. Were orbitals real Was resonance real What was interesting was that developments did not become dependent... 

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