Abstract
Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z' = 1 and phase III in P41212 with Z' = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.
Highlights
Pyridine (C5H5N) is one of the simplest organic compounds, but its crystal structure at ambient pressure, determined by Mootz & Wussow (1981), is unusually complicated, with four molecules in the asymmetric unit (Z0 = 4)
Heyns & Venter argued that the spectra they obtained for their ‘phase I’ were too simple to be compatible with the Z0 = 4, Pna21 crystal structure of pyridine which had been determined by Mootz & Wussow (1981) a few years previously
Phase III can be obtained by crystallization of pyridine from a solution in methanol, though this procedure can lead to the formation of a methanol solvate (Podsiadło et al, 2010)
Summary
The overall picture that has emerged from these studies is a somewhat confusing one partly because of inconsistent phase numbering In their original high-pressure Raman study, Heyns & Venter (1985) obtained a phase which they referred to as ‘phase I’ for both the h5 and d5 isotopologues at ca 1 GPa. On increasing the pressure they observed discontinuities in the trend of band frequencies versus pressure and the appearance of one new band in pyridine-h5 and two in pyridine-d5 which were taken to indicate that this phase transforms to a ‘phase II’ at 2 GPa. Heyns & Venter argued that the spectra they obtained for their ‘phase I’ were too simple to be compatible with the Z0 = 4, Pna crystal structure of pyridine which had been determined by Mootz & Wussow (1981) a few years previously. We go on to show that this phase was the one obtained along with phase II in the high-pressure Raman studies summarized above
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