I have no understanding of chemistry, but you mention that it was contamination which caused the change.

Does this imply that despite the repeated references in the article to the crystals being identical, both when tested in the 19th and 21st century (with "modern structural analysis techniques"), the crystals are in fact different on a minute undetectable level? i.e. "a single molecule in the air" which causes the change?

The paper notes that a 1-in-1000 molar concentration of acid was sufficient to trigger low-temperature melting, indicating that the amount of contamination needed is indeed very, very small. The paper does not discuss exact mechanisms of action, but one of the conjectures is that the acid takes the form of excess hydrogen atoms, which could easily tuck themselves into a crystal structure nearly undetectably.

All it takes is catalyzing a few Z->E transitions when the temperature rises to cause the crystal structure to break down, which would explain the lower melting point despite the small amount of contaminant (catalysts often reduce the energy required to start a reaction, but are not themselves consumed so they can further catalyze other reactions).

Crystals are repeating arrangements of atoms bonded in a very particular, repeating way. Modern techniques include looking at the crystal with x-rays, so any serious discrepancies in the patterns would be noticed.

The contamination, they concluded, was not structural (e.g. as a silicon semiconductor might have random atoms replaced with phosphorus or boron) but rather even more minute amounts of acid could act as a catalyst - i.e. an extra component of a reaction that participates, but doesn't actually get consumed - hence can make a difference in tiny amounts. A decent way to think of catalysts is as chemistry's matchmakers - they help reactions happen; sometimes those would happen on their own but take more time, or sometimes they wouldn't happen at all (e.g. if there's not enough energy to react without the shortcut of the catalyst).

Hope that makes some sense.

So the crystal doesn't melt at two temperatures as per the headline. Instead, it has two stable isomer ratios, which is pretty interesting - generally compounds are stable as one isomer, or the other, correct? Or is that not the case?

Also, aren't E configurations generally more stable, and therefore should have a higher melting point?

It's been years since organic chemistry so perhaps I'm way off here.

You are correct, the E configuration is generally found to be the most stable isomer. It is therefore not suprising to see that in the final liquid state a higher E isomer content is found. The final ratio of interconverting E and Z isomers that you measure in the liquid is determined mainly by the energy difference between the two and the temperature.

However, in the solid state the E isomer does not necessarily give you the more stable crystal structure since the packing of the individual molecules can be very different for each of the two isomers.

I was involved in this research project and can add that interestingly the solid form of the E isomer of this chemical has not been documented so far. It appears that under the conditions that were used, the E isomer of this molecule is found only in the liquid state.

So basically, hydrogen/protons act as a catalyst for APH, a solid phase reactant (!?) that doesn't catalyze a chemical change, but a physical phase change in the reactant (!?) (via catalyzing the isomer change). Do either of those exist anywhere else or are these properties completely novel?