“Cogitate, calculate:
Dame Kathleen Lonsdale,
Through X-ray spectroscopy,
Compound discerns.
Insight incipient:
Hex-methyl-ation will
Benzene’s geometry
Flatly confirm.“
As a new year and new semester are now officially underway, I will return to the weekly routine of these posts. The 11 April 2022 poem began the 2022 week of “Twitter biographies.” The first was a pseudo-double-dactyl poem summarizing a key experimental insight in chemistry from Kathleen Lonsdale, who lived from 1903-1971.
“Cogitate, calculate: /
Dame Kathleen Lonsdale, /
Through X-ray spectroscopy, /
Compound discerns…”
Dame Kathleen Lonsdale was the first woman elected as president of the International Union of Crystallography, in addition to many, many other honors.
X-ray crystallography is a technique in which, by sending high-energy X-rays at a sample of a compound, a chemist can examine how those X-rays are scattered: a useful analogy might be inferring the shape of an object from the shadow it casts, although X-ray crystallography techniques are far more involved and exacting. Many compounds’ structures have been discerned through this technique, generalized in the poem as “X-ray spectroscopy” (again, a less precise characterization than is ideal, this time for the sake of the meter).
“Insight incipient: /
Hex-methyl-ation will /
Benzene’s geometry /
Flatly confirm.”
The specific experiment commemorated in this poem was Lonsdale’s use of X-ray crystallography to determine the geometry of benzene, a compound which had interested chemists for many years. Before this insight, it was known that a benzene molecule contained six carbon atoms and six hydrogen atoms and arranged these atoms cyclically, in a ring. However, scientists had still disagreed for decades as to its planarity: was the ring flat? (Did it have all of its carbon atoms in the same plane?)
Lonsdale determined an answer to this question by analyzing a derivative of benzene called hexamethylbenzene, which has a methyl group (-CH3) attached to each carbon in the benzene ring. She noted that the central benzene ring had to be flat to account for the results seen via her X-ray crystallography experiment. Thus, the geometry was “flatly confirm[ed]”: benzene was shown to be planar, via significant and convincing evidence.
Chemphasis 