How Pauling and Wheland Translated Quantum Mechanics for Chemists
The most important idea in chemistry, once described as a symphony of different structures blending together into one stable molecule.
Have you ever tried to explain a complex scientific idea using simple drawings or analogies? This exact challenge—translating the bizarre world of quantum mechanics into language chemists could understand—faced Linus Pauling and George Wheland in the 1930s. Their solution, the theory of resonance, would revolutionize chemistry, but only after they solved the puzzle of how to teach it.
Between 1930 and 1950, chemistry underwent a dramatic transformation. The new science of quantum mechanics, developed by physicists, promised to explain the very nature of the chemical bond. Yet for most chemists, the complex mathematics was an impenetrable barrier. Enter Linus Pauling and his collaborator George Wheland, who became "chemical translators," bridging the disciplinary divide between physics and chemistry. Their concept of resonance offered a powerful new way to understand molecular structure and stability, but its adoption came with serious philosophical questions and widespread misunderstandings that they worked tirelessly to clarify 1 4 .
In the mid-1920s, the development of quantum mechanics created both excitement and anxiety in chemical circles. X-ray crystallography and other spectroscopic methods were revealing the three-dimensional arrangements of atoms in molecules, but chemists lacked a robust theoretical framework to explain these structures. The mathematics that came so naturally to physicists proved daunting to many practicing chemists 4 .
Pauling, then a young professor at Caltech, recognized this growing divide. During his time in Europe, he had immersed himself in the new quantum mechanics and recognized its profound implications for chemistry.
Pauling made a crucial decision: he would translate the mathematical formalism of quantum mechanics into the visual, intuitive language that chemists preferred. As he later stated, "the principal contribution of quantum mechanics to chemistry" was the concept of resonance 4 . Between 1931 and 1933, he published seven groundbreaking papers titled "The Nature of the Chemical Bond," with the final three devoted almost entirely to resonance theory 3 .
Heitler and London implement Lewis' electron-pair bond ideas into quantum mechanics .
Pauling publishes his seven groundbreaking papers "The Nature of the Chemical Bond" 3 .
Pauling and Wheland develop and refine resonance theory as a bridge between physics and chemistry.
Nowhere was the power of resonance theory more dramatically demonstrated than in solving the long-standing puzzle of benzene's structure. For decades, chemists had struggled to explain how this simple six-carbon ring (C₆H₆) could be so unexpectedly stable 3 .
The prevailing model, proposed by August Kekulé, featured alternating double bonds that oscillated between two possible arrangements. But this explanation had serious flaws: no evidence of the oscillating structures had ever been found, and the model predicted instability contrary to benzene's observed robust nature 3 .
Pauling's revolutionary insight was that benzene's true structure wasn't rapidly switching between forms but was rather a quantum-mechanical blend of all possible structures. He identified five "canonical forms" for benzene and calculated both the energy of each structure and the combined resonance energy. The result perfectly matched experimental data—benzene's extra stability came from resonance energy 3 .
Kekulé Structure
Resonance Hybrid
Pauling boldly asserted that "it is the resonance among these structures which imparts to the molecule its peculiar aromatic properties" 3 . This elegant solution stood in stark contrast to the approach of German chemist Erich Hückel, whose molecular orbital calculations were so cumbersome they couldn't be applied to larger aromatic compounds. Pauling's valence bond approach, by contrast, could handle all aromatics, even naphthalene with its forty-two canonical structures 3 .
Pauling and Wheland's approach to calculating resonance energy involved several key steps:
The resonance theory successfully explained not only benzene's exceptional stability but also its uniform bond lengths—all carbon-carbon bonds in benzene are identical, unlike the alternating single and double bonds in the Kekulé model. This reflected the true nature of benzene as a hybrid with electron density perfectly delocalized around the ring 3 .
When Pauling and Wheland applied the same methodology to naphthalene, with its forty-two canonical structures, they found similar resonance stabilization. Pauling confidently noted that the approach could be extended to even larger systems like anthracene and phenanthrene, though "not without considerable labor" 3 .
Pauling and Wheland shared a commitment to making resonance theory accessible, but they employed notably different teaching strategies that reflected their philosophical differences about the very nature of resonance.
Pauling took what might be called a "robust" approach—flamboyant and physical in his exposition. He presented resonance as a real phenomenon with tangible consequences, using vivid analogies and visual representations. His teaching style was charismatic and persuasive, making the complex concept feel intuitive and concrete 1 4 .
Wheland, by contrast, adopted a more cautious approach, emphasizing that resonance was a mathematical device rather than a physical phenomenon. In his 1944 book "The Theory of Resonance and Its Application to Organic Chemistry," he took pains to clarify that resonance was not like tautomerism (where molecules genuinely oscillate between forms) but was instead a conceptual tool for approximating the quantum mechanical reality 5 .
| Aspect | Pauling's Approach | Wheland's Approach |
|---|---|---|
| Presentation Style | Flamboyant, physical illustrations | More cautious, emphasizing limitations |
| View of Resonance | Real phenomenon with tangible consequences | Mathematical device, not physical reality |
| Primary Concern | Making concept accessible and intuitive | Preventing confusion with tautomerism |
| Underlying Philosophy | Resonance describes an actual phenomenon | Resonance is a calculative method |
These different strategies reflected the challenge of "translating" across disciplinary boundaries. As one observer noted, "For the sake of persons of different types of mind scientific truth should be presented in different forms" 4 .
To work with resonance theory, chemists needed to master several new conceptual tools:
These are the hypothetical, contributing structures that differ only in the arrangement of electrons, not atoms. They represent extreme bonding patterns that don't actually exist independently but contribute to the true structure 3 .
The actual molecule is a hybrid of all canonical structures, with properties that are weighted averages of the contributing forms. This hybrid represents the most stable, lowest-energy state 3 .
The extra stability gained by molecules with resonance, quantified as the energy difference between the actual molecule and the most stable canonical structure. For benzene, this is approximately 57.5 kcal/mol 2 .
The quantum mechanical phenomenon where electrons are spread over multiple atoms rather than localized between specific pairs, leading to enhanced stability.
A modern computational approach that evaluates the original Pauling-Wheland adiabatic resonance energy (ARE), allowing precise calculation of resonance stabilization 2 .
| Concept | Application | Significance |
|---|---|---|
| Adiabatic Resonance Energy (ARE) | Energy difference between real system and most stable resonance structure | Provides quantitative measure of resonance stabilization 2 |
| Extra Cyclic Resonance Energy (ECRE) | Characterizes extra stabilization in conjugated rings | Measures aromaticity; for benzene, ranges from 25.7-36.7 kcal/mol depending on reference 2 |
| Bond Orders | Semiquantitative theory using Pauling bond orders | Allows prediction of bond lengths and properties in conjugated systems 6 |
The reception of resonance theory in the chemical community was not without controversy. The similarity between resonance and the familiar concept of tautomerism led to serious misunderstandings, despite Pauling and Wheland's teaching efforts 1 . Additionally, resonance theory faced a strong challenge from molecular orbital theory, developed by Robert Mulliken and others .
For approximately two decades, VB theory and resonance dominated chemical thinking, but by the 1950s, MO theory began to eclipse it, particularly as computational methods advanced . However, resonance theory never disappeared—it remained an essential conceptual tool for practicing chemists, even as computational chemistry increasingly relied on MO approaches.
In recent decades, valence bond theory has enjoyed a remarkable renaissance, with new computational methods and conceptual frameworks reestablishing its relevance and power . Modern adaptations of Pauling-Wheland resonance energies continue to provide quantitative measures of aromaticity and antiaromaticity, demonstrating the enduring legacy of their work 2 .
The story of resonance theory represents more than just a historical episode—it illustrates the ongoing challenge of translating across scientific paradigms and the importance of effective communication in advancing science.
Pauling and Wheland's strategies for teaching resonance remind us that scientific progress depends not only on discovery but on making those discoveries accessible to others.
As Pauling himself reflected, their work established "a way of thinking that might not have been introduced by anyone else, at least not for quite a while" 3 —a way of thinking that continues to shape how chemists understand the molecular world.