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Chemistry

Southampton chemist solves Nobel laureate’s 120-year-old crystal mystery

Published: 7 August 2019
Threlfall Coles lab

A retired chemist at the University of Southampton has unravelled the 120-year-old mystery of a crystal that appeared to defy the laws of thermodynamics.

The curious compound, acetaldehyde phenylhydrazone (APH), had baffled scientists in the late 1800s when experiments found it would melt at different temperatures.

The puzzle was published by German chemist Emil Fischer – who would go on to win the Nobel Prize – but forgotten for over a century until it was dusted off by Terry Threlfall, a Visiting Scientist at Southampton of the past 20 years.

A repeat of the historic experiment confirmed that identical batches of the crystalline solid would melt at either 60°C or 95°C, prompting a decade-long international quest to solve the 19th century conundrum.

The laws of thermodynamics state that solids must be structurally different to possess different melting points, however modern investigations through X-ray diffraction, nuclear magnetic resonance and IR spectroscopy all declared the crystal samples to be identical.

Terry and international colleagues finally concluded that APH is the first recorded example of a solid that melts into two distinct liquids, with the process affected by a contamination so subtle that it is almost untraceable. The remarkable findings have been published in Crystal Growth and Design and come to the attention of the wider scientific community through a commentary in Physics Today.

“It is just exceedingly satisfying to be able to understand such an ancient puzzle, especially one which baffled such an eminent scientist who became a Nobel Prize winner,” Terry says. “The observation of such behaviour will be exceedingly rare because it depends on the molecules in the crystal and in the liquid having different geometries, which is unusual. Furthermore, it depends also on the conversion by acid being both possible and rapid.”

Terry first discovered the mystery in 2008 when looking through an 1896 edition of the German language scientific journal Berichte der deutschen chemischen Gesellschaft. The mystery piqued his interest and inspired a 10-year investigation that included consulting Dr Hugo Meekes of Radboud University in the Netherlands and Dr Manuel Minas da Piedade of the University of Lisbon in Portugal.

“If an element or compound can exist in two or more distinct crystalline forms, then each form will have different Gibbs energies and melt at its own distinct temperature,” article co-author Professor Simon Coles from Southampton’s School of Chemistry explains. “In this case, the molecules of the crystal are in the cis geometry - of groups pointing towards each other - and melt to an identical geometry in the absence of acid at 100°C. However, in the presence of even a trace of acid, the molecules convert on melting to the trans geometry of groups pointing away from each other. This liquid has a smaller Gibbs energy and is more stable, so the melting point becomes 65°C.

“I have been at Southampton for over 20 years and during that whole time Terry has been working in his retirement as a visitor to the group. The contributions and insights that our Emeritus Professors and retiree Visiting Scientists provide are enormous and invaluable. Furthermore, this story illustrates that there is fundamental important science in the old literature that is not explained – Terry’s encyclopaedic knowledge of the literature, curiosity and tenacity are the sole reason this 120-year-old argument is finally resolved.”

 

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