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I have been studying the concepts of "resonance" and "mesomerism" recently and a common principle of these concepts is the "delocalization" (of electrons, molecular orbitals etc). Now, I have read a lot of articles on the concept of "delocalization" and how it evolved but wonder who coined the term first. Here is all the information I could find through my research (about the emergence of the "ideas" and "concepts" of delocalization along with some instances of the actual usage of the term).

Firstly, about the concept of "π-bond delocalization", it emerged as a concept in 1865 when August Kekulé proposed the structured for benzene in his paper "Sur la constitution des substances aromatiques"[1] (it is interesting to note that he revealed in "Feier der Deutschen Chemischen Gesellschaft zu Ehren August Kekulé's.", 23(1), 1305-1306 [2] that he had ideated the shape of the benzene molecule after having a day-dream of a snake eating its own tail however it is doubted as the occurrence of ring structures had been depicted before 1865 by "Archibald Couper" in 1858 [3] and "Joseph Loschmidt" in 1861[4]). Further, in 1869 Albert Ladenburg criticized Kekulé's 1865 structure in his paper "Bemerkungen zur aromatischen Theorie" [5] and in response kekulé modified his proposal in 1872[6] and suggested that the benzene molecule oscillates† between the two equivalent structures thus giving the modern-representation for benzene.

However, Kekulé’s 1872 refinement proposed a fully six-fold symmetrical, but “vibrating”, benzene structure. He did not intend the “oscillating” hypothesis others mistakenly ascribed to him later. This almost universal misinterpretation should not be repeated in the literature and in textbooks.[7]

[7] Schleyer, P. (2005). Introduction: Delocalization Pi and Sigma. Chem. Rev., 105(10), 3433–3435

Now, after the work of Kekulé came Johannes Thiele in 1899 in his paper "Zur Kenntniss der ungesattigten Verbindungen" [8] involving his application of the “partial valence” (Partialvalenz) concept upon discovering the unusual stability of the cyclopentadienyl anion to rationalize the special chemical behaviour of unsaturated compounds (which are now called “delocalized π systems”). He attributed the greater reactivity of these double bonds to “residual affinities”(Affinitätsrest) (equivalent to p orbitals), which could link across the central atoms in a “conjugated”(Conjugirte) diene explaining their extra reactivity in addition reactions (he was the first one to coin the term "conjugated systems"). This also explained the lack of reactivity in benzene with it being a "closed conjugated system" and giving the picture of having a "partial valency"(delocalized π-system) leading to the modern idea of resonance.†

This paragraph contains ideas and text from [7] which is cited above although it has been modified with additional information and rephrasing.

To discuss about the idea and evolution of delocalization in π-systems, it dates back to Heisenberg in 1926 is his paper "Mehrkorperproblem und Resonanz in der Quantenmechanik"[9 (eng. translation)] wherein he introduced the idea of "resonance"(in the physics sense) in the discussion of the quantum states of the helium atom comparing it with classical system of resonating coupled harmonic oscillators. In this case, the resulting coupling which is lower in frequency than either of the uncoupled vibrations; is analogous to a lower energy. Heitler &, London in 1927 in their paper "Wechselwirkung neutraler Atome und homöopolare Bindung nach der Quantenmechanik"[10] applied this concept to the normal state of the hydrogen molecule wherein the molecule was described as involving the resonance (exchange) of the electrons between the positions about the two nuclei(of hydrogen), and the bond energy was attributed largely to the resonance energy (exchange integral).

This concept was finally given the modern shape as we know it by Linus Pauling in 1928 through his paper "The shared-electron chemical bond"[11] which was followed by another paper in the same year, titled "The application of the quantum mechanics to the structure of the hydrogen molecule and hydrogen molecule-ion and to related problems."[12]†. This was the start of the theory of hybrid bond orbitals (i.e. the Valence Bond Theory), which was then developed further by Pauling and Slater(with works starting from 1931).

† This part is a bit confusing because it is unclear which of the two papers was published later. In a 1977 paper titled, "The theory of resonance in chemistry" [13] Pauling mentions:

"A similar treatment, involving resonance of one electron between the 1s orbitals about the two nuclei, was developed for the hydrogen molecule ion (Pauling 1928a). The theory of resonance was then used (Pauling 1928b) to explain....."

where "Pauling 1928b" refers to "The shared-electron chemical bond" which was published earlier although mentioned to be used after "Pauling 1928a" referring to "The application of the quantum mechanics to the structure of the hydrogen molecule and hydrogen molecule-ion and to related problems." which was published later.

After reviewing multiple papers from Pauling, Slater, Hückel, etc., I couldn't find any mention of the word "delocalization" in the π-system sense. Finally, the earliest mention of the word 'delocalization' (in the sense of 'π-bond delocalization') that I could find was in a paper by Klaus Ruedenberg in 1962, titled "The Physical Nature of the Chemical Bond"[14] (interestingly there is a mention by Ruedenberg in his autobiography wherein he says about himself; "defined the real meaning of delocalization in conjugated, especially aromatic,π-systems...." ). There is another mention by Ruedenberg in 1963 in an article titled "Localized Atomic and Molecular orbitals"[15].

Secondly, the concept of 'σ-bond delocalization' emerged in Robert Mulliken's paper in 1941 named "Hyperconjugation*"[16](with A. Rieke and W. G. Brown). In this paper, is the earliest occurrence of the word delocalization that I could find (pg. 42) (it is also interesting to note that the first use of the term localized† I could find is from "Friedrich Hund" in 1931[17] and of the term non-localized† {the predecessor of the term "delocalized"} from "Robert Mulliken" in 1935[18]).

* I think most of the information in this question is correct, still if there are any corrections/edits and revisions/suggestions, they will be appreciated! Also, you may think this question as kind of pointless but I have wondered for a long time about the origin of the term "delocalization" so any help will be very appreciated. Thanks!


Question:

So, finally my question is that who is person who coined the term "delocalization"(in the sense of chemistry) and who used the term for the first time in that sense? Also, is it somehow related to the people I mentioned above (specifically Ruedenberg and Mulliken as they were the only people whose publications contained the actual word "delocalization" and not just the idea concerning it)?

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  • $\begingroup$ I think most of the information in this question is correct, still if there are any corrections/edits and revisions/suggestions, they will be appreciated! Also, you may think this question as kind of pointless but I have wondered for a long time about the origin of the term "delocalization" so any help will be very appreciated. Thanks! $\endgroup$ Jan 23 at 23:47
  • $\begingroup$ This self-comment should be added to the post, try to edit it in. $\endgroup$
    – Mauricio
    Jan 24 at 8:38
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    $\begingroup$ @AChem Oh thank you for the suggestion sir! You said rightly about the interest in history, it is just that whenever I study concepts in science in general I just instinctively lean towards finding the history of the concept first as it "doesn't feel right" studying that concept without knowing about it first {I don't think that makes any sense, also it puts me behind in following my school curriculum :(( } $\endgroup$ Jan 24 at 22:30
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    $\begingroup$ Bhavya Jain, Reading the history first of any topic is not the right approach. First master the concept at your educational level and then go about exploring its history. I had realized that as a school student you were bogged down historical nitty gritty. The problem with history is that knowing the oldest literature does not mean it was correct. Many ideas were corrected in science. Focus on learning the concepts and later, as time permits, read about history from serious works like Partington's History of Chemistry book. Same applies to math. We cannot start how Newton started calculus. $\endgroup$
    – AChem
    Jan 25 at 0:13
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    $\begingroup$ @AChem Yes sir I'll try to change my approach and focus on studying the concept first and reading about it's history if time permits. Thank you for the suggestion (I'll try to read it asap) and the encouragement sir. $\endgroup$ Jan 26 at 0:37

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