X-ray crystallography is a well-utilized tool in chemistry to determine the atomic and molecular structures of crystals. Dame Kathleen Lonsdale (1903-1971) was one of the most prominent x-ray crystallographers of the 20th century, contributing to the field in numerous fields.
First, here’s some background on how x-ray crystallography works. Essentially, scientists expose a crystallized sample of a specific molecule to x-rays, usually through an x-ray camera which takes a photograph of it. This photograph shows the pattern of diffracted x-rays as they pass through the crystallized molecule. This pattern is specific for each substance, allowing scientists to map the substance’s molecular structure using math.
Lonsdale’s first contribution to crystallography came from her study of benzene. Benzene is an important carbon compound that’s found in crude oil and gasoline and is the starting material for a number of other substances, including plastic and pesticides.
In 1865, August Kekulé (1829-1896), a prominent German organic chemist who taught chemistry at Ghent University, proposed benzene’s structure to be a six-membered ring of carbon atoms. Kekulé also postulated that the ring of carbon atoms alternated single and double bonds. The structure was drawn as a hexagon. However, by 1925, there was still no confirmation of this ring structure of benzene.
At the time, Lonsdale was working in the physics department at the University of Leeds in England. There, she was given crystals of hexamethybenzene, C6(CH3)6 for x-ray analysis work. This was important because benzene itself was a liquid, meaning it couldn’t be studied through x-ray crystallography, which requires crystallized forms. However, in 1928, Lonsdale found that benzene existed in the crystals of hexamethybenzene as a separate identity. So, benzene could be studied through this substance.
Lonsdale found that the benzene carbon atoms were arranged in a ring formation, and the ring was hexagonal and planar. Additionally, Lonsdale found several similarities between benzene and graphite. For example, they had similarly sized rings of carbon atoms.
After this discovery, Lonsdale continued her research by performing an x-ray analysis of hexachlorobenzene C6H6Cl6. During this procedure, Lonsdale used the same technique as her hexamethybenzene analysis but also used a Fourier analysis. The Fourier analysis is a mathematical method in which general functions are represented in terms of sums of trigonometric functions. It essentially performs calculations that aid in the research.
While Lonsdale’s results did not match her results from her hexamethybenzene study, she was the first person to use the Fourier method in the structural analysis of an organic compound, and her work on benzene greatly increased the credibility of crystallography.