High-pressure neutron diffraction study of L-serine-I and L-serine-II, and the structure of L-serine-III at 8.1 GPa

The hydrostatic compression of L-serine-d7 has been studied to 8.1 GPa by neutron powder diffraction. Over the course of this pressure range the compound undergoes two phase transitions, the first between 4.6 and 5.2 GPa, yielding L-serine-II, and the second between 7.3 and 8.1 GPa, yielding L-serine-III. All three polymorphs are orthorhombic, P2(1)2(1)2(1), and feature chains of serine molecules connected via head-to-tail ND center dot center dot center dot O hydrogen bonds formed between ammonium and carboxylate groups. The chains are linked into a ribbon by a second set of ND center dot center dot center dot O hydrogen bonds. The hydroxyl moieties are distributed along the outer edges of the ribbon and in phase I they connect the ribbons into a layer by chains of OD center dot center dot center dot OD hydrogen bonds. The layers are connected together by a third set of ND center dot center dot center dot O hydrogen bonds, forming R-4(3)(14) rings with substantial voids at their centres. In the transition from phase I to II these voids begin to close up, but at the cost of breaking the OD center dot center dot center dot OD chains. The OD center dot center dot center dot OD hydrogen bonds are replaced by shorter OD center dot center dot center dot O hydrogen bonds to carboxylate groups. At 7.3 GPa the O center dot center dot center dot O distance in the OD center dot center dot center dot O hydrogen bonds measures only 2.516 (17) angstrom, which is short, and we propose that the phase transition to phase III that occurs between 7.3 and 8.1 GPa relieves the strain that has built up in this region of the structure. The hydroxyl D atom now bifurcates between the OD center dot center dot center dot O contact that had been present in phase II and a new OD center dot center dot center dot O contact formed to a carboxylate in another layer. Hirshfeld surface fingerprint plots show that D center dot center dot center dot D interactions become more numerous, while hydrogen bonds actually begin to lengthen in the transition from phase II to III.