At high pressure the urea crystal abruptly collapses in two stages. When transforming from phase I to III at 0.48 GPa, the crystal volume is abruptly compressed by 7.3%. At this transition a huge abrupt linear compression to 65% along a and to 95% along c is partly compensated by an unprecedented abrupt negative linear compression (i.e., expansion) to 148% along b. At another discontinuous transformation, at 2.8 GPa to phase IV, the crystal still displays an abrupt negative linear compression to 106% along c. The intermediate phase III between 0.48 and 2.80 GPa increases the contribution of the zwitterionic mesomeric structure of urea molecules and the formation of weak NH•••N bonds has been evidenced in phase III. The formation of phase II, above 373 K and above 0.6 GPa reported by Bridgman, has not been confirmed.
High pressure destabilizes the high Z′
polymorph of 3-hydroxy-4,5-dimethyl-1-phenylpyridazin-6-one (1α); however, recrystallization is needed for obtaining
a low Z′, more dense polymorph. Three polymorphs
α, β, and γ can be monotonically compressed to 2.0
GPa at least. At ambient pressure 1 crystallizes in space
group C2/c, Z′
= 4 as polymorph 1α, or as lower-density polymorph 1β, of space group P21/c and Z′ = 1. Polymorph β
is metastable, and after about one year, it transforms to phase α.
The isochoric recrystallization above 0.40 GPa yields a new polymorph
γ of space group P21/a and Z′ reduced to 1. The γ polymorph
retrieved to ambient conditions for months has showed no signs of
transformations. The main motif of OH···O bonded chains
is retained in all three phases, but high pressure enforced identical
conformation of closely packed molecules and their identical crystal
environment in phase γ.
High pressure increases the temperature of the spontaneous resolution of 1,1'-binaphthyl conformational enantiomers in the crystalline state, which confirms that the enantiomers and racemates are stabilized in the molecular environments in compressed structures. The established pressure-temperature (p-T) preference diagram for the racemate-enantiomer spontaneous crystallization corresponds to a boundary between solid phases, as it is consistent with the Clausius-Clapeyron equation, however, the hysteresis of such a solid-state transformation extends to very high pressure, to 3 GPa, at least according to this study. High-pressure X-ray diffraction study on single crystals of 1,1'-binaphthyl racemate and enantiomer reveals their monotonic compression and structural changes up to 3 GPa. It also reveals the increasing role of intermolecular interactions for stabilizing the structures, despite the exceptionally large density difference between the racemate (1.277 g cm) and enantiomers (1.183 g cm).
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