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-d(7) 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...O hydrogen bonds formed between ammonium and carboxylate groups. The chains are linked into a ribbon by a second set of ND...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...OD hydrogen bonds. The layers are connected together by a third set of ND...O hydrogen bonds, forming R;3_4(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...OD chains. The OD...OD hydrogen bonds are replaced by shorter OD...O hydrogen bonds to carboxylate groups. At 7.3 GPa the O...O distance in the OD...O hydrogen bonds measures only 2.516 (17) A, 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...O contact that had been present in phase II and a new OD...O contact formed to a carboxylate in another layer. Hirshfeld surface fingerprint plots show that D...D interactions become more numerous, while hydrogen bonds actually begin to lengthen in the transition from phase II to III.