On Isosteric Isomers: An Important Early Paper

Ava Helen, Linus Jr. and Linus Pauling on a family hike, 1926.

In 1926 life was going well for twenty-five year old Linus Pauling – he had been married for a couple of years, had a healthy one year old son, and was quickly establishing himself as one of the top chemists in the United States. His primary research topic at the time was structural chemistry, and his hard work in the laboratory had already resulted in a good number of publications.

Nonetheless, Pauling had yet to publish a paper dedicated entirely to the subject that would soon become synonymous with his name: the chemical bond. This finally changed in 1926, when he and Sterling B. Hendricks  (who considered himself to be “Linus’ first student”) published a paper titled “The Prediction of the Relative Stabilities of Isosteric Isomeric Ions and Molecules.”

In this publication, Pauling and Hendricks calculated the potential energies of similar ions and similar molecules in order to predict the most favorable structure of a given ion or molecule. For example, the atoms in carbon dioxide (CO2) can only be arranged in two configurations, OCO and COO. By comparing the potential energy values measured for each structure, it is possible to determine which is more likely to naturally form – the structure with the lower potential energy.

In their 1926 paper, Pauling and Hendricks first found it necessary to define the terms “isosteric” and “isomeric.” Isosteric refers to “molecules or ions that contain the same numbers of atomic nuclei and the same numbers of electrons, but differ in the positive charges on the nuclei.” Some examples of isosteric molecules are:

:N:::N: :C:::O: (:C:::N:) (:C:::C:)

Isomers, on the other hand, are molecules or ions that contain the same atoms in a different arrangement. In the carbon dioxide example given above, COO is one isomer and OCO is another. Other examples include the cyanate ion (NCO) and the fulminate ion (CNO) as well as the NON and NNO configurations of nitrous oxide (NO2).

Although two molecules or ions that are isosteric aren’t always isomeric and vice versa, Pauling and Hendricks were only interested in substances that fulfill both conditions. Because isomeric and isosteric substances are so similar in many ways, Pauling and Hendricks argued that their differences in stability could be attributed almost exclusively to differences in potential energy. Using a rather complex equation that is explained in the paper, Pauling and Hendricks determined the most stable structures for a variety of isomeric and isoteric substances. Their results are displayed in the table reproduced below.

As the authors stressed in their paper, two points are particularly important to note: 1) the significance of relative values (as opposed to absolute values) of potential energy, and 2) that the configuration with the lowest potential energy is favored. Therefore, for carbon dioxide, the OCO configuration is favored over COO; for nitrous oxide, the NNO configuration is favored over NON, and so on. As it turned out, the two men’s results agreed very well with prior experimental and chemical evidence, with a few specific exceptions.

Although this paper is not generally counted among Pauling’s most important contributions, it does stand as an undoubtedly strong start to the chemical bond work that would win him the 1954 Nobel Prize in Chemistry. For more information on Pauling’s breakthroughs in structural chemistry, please visit the website Linus Pauling and the Nature of Chemical Bond: A Documentary History.

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