[Part 6 of 7]
“The Nature of the Chemical Bond. VI. The Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures.” The Journal of Chemical Physics, July 1933.
In paper number five in his Nature of the Chemical Bond series, Linus Pauling argued that the theory of resonance could be used to accurately discern the structure of many compounds, and he used Valence Bond theory to substantiate that claim. However, much of the argumentation put forth in the paper relied upon fairly generalized calculations, some of which were subsequently shown to be in error.
In his sixth paper, published one month later, Pauling put forth more definitive calculations that used thermochemical data that were more empirically based, and therefore less prone to errors. As with the previous publication, this paper was co-authored. However, instead of G.W. Wheland, Pauling’s collaborator this time around was Jack Sherman, a theoretical chemist who had received his PhD from Caltech the year before.
The data used in the paper weren’t anything new; in fact, they had been used by chemists for years to calculate energy values and to determine bond energies. However, in many cases these calculations failed because chemists, who were rooted in classic organic or physical models, always assumed that the molecules under study were consistently similar.
Relying instead on a quantum mechanical approach, Pauling and Sherman argued that compounds could (and should) be organized into two broad categories. In one group, there resided those molecules that were well-approximated by their Lewis structures (classical representations of molecules using lines and dots to represent bonds and electrons). The other group consisted of compounds whose structures could only be accurately explained through resonance.
By organizing compounds into these two discrete bins, Pauling and Sherman were then able to make more accurate calculations of bond energies. More specifically, the duo was able to calculate energies of formation for various molecules by using extant experimental data on heats of combustion. Pretty quickly they realized that energies of formation could be accurately calculated for Lewis structure (non-resonating) compounds.
For resonating compounds however, the tandem found that calculated energies of formation were much higher than what would have been predicted by theory. Higher energies of formation yield more stable molecules, and the co-authors concluded that the “difference in energy is interpreted as the resonance energy of the molecule among several electronic structures” and that “in this way, the existence of resonance is shown for many molecules.”