shows how the carboxylate group of THCA hydrogen bond to one another
resulting in a ordering of the crystal structure.
THC lacks this this carboxylate group so it can’t form this hydrogen bonds.
And then from this study.
it refers to certain cannabinoids like the THC naphthoyl ester derivative as a polymorphic crystal, having multiple crystal structures.
So I guess my question is. Is it both of these factors, the lack of the hydrogen bonding from the carboxylate group, and the possibility of THC being a crystal polymorph, that it lacks long chain ordering needed to form a nice crystal structure?
Or just the lack of the hydrogen bonding from the absent carboxylate group?
The second paper you referenced converts THC into a THC derivative. The ‘appendage’ they pop onto THC is highly crystalline itself and the molecular interactions therein allow for favourable crystal stacking. The carboxyl group in THCA is doing the save thing compared to THC - changing the intermolecular interactions such that the way the molecules stack onto each other results in a crystalline structure.
In other words, you can make crystalline THC derivatives from THC by changing its molecular structure. THC on its own is not crystalline.
Can THC form a crystal? Maybe. Will THC form a crystal? Not without some incredibly good technique and luck and even then probably not.
THC is highly prone to oxidation. Even at minor amounts these oxidative side products account for the color changes observed in the concentrated THC and prevent crystallization. Also only one report from a synthesis of THC reports a crystal with a melting point around 55 C.
I think you’re pretty much on the right track, but it’s mainly about intermolecular interactions.
THCA has that carboxylic acid group, so it can form strong hydrogen bonds. That helps the molecules pack into a stable, repeating crystal lattice. THC loses that group after decarboxylation, so it mostly relies on weaker van der Waals forces, which makes orderly packing much harder.
The polymorph idea is interesting, but it’s probably secondary since a compound needs to crystallize in the first place. THC derivatives crystallize because added groups introduce stronger interactions again.
Also, THC’s instability and impurities don’t help with forming a clean crystal structure.
So overall, the lack of hydrogen bonding is the main reason, and the rest just makes it worse.
