Discovery of High-Pressure Polymorphs for a Typical Polymorphic System: Oxalyl Dihydrazide

Abstract

The variation of pressure is an effective experimental technique to explore new polymorphs of organic crystals. At ambient condition, oxalyl dihydrazide (C 2 N 4 O 2 H 6 , ODH) exhibits five polymorphs: α, β, δ, γ, and ε. Here we report the high-pressure response of the existed five forms of ODH by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 20 GPa. Highpressure experimental results show that all five polymorphs undergo phase transitions to new phases at different pressures, respectively. We propose that the special molecular conformation yields several geometric constructions for hydrogenbonding arrangements. The detailed mechanisms of the phase transition and the high-pressure behaviors of the polymorphs are analyzed by considering molecular stacking.

Overcoming the difficulties of predicting conformational polymorph energetics in molecular crystals via correlated wavefunction methods

“…8. 87 Five additional high-pressure polymorphs have been reported, 136 but their structures are unknown and they are not considered here. Experimentally, the a, 3, and d forms are the most stable polymorphs, though the ranking among those three is uncertain.…”

“…137 Finally, the b form has proved difficult to produce and characterize, and it converts readily to the a form. 87,136 Therefore, it is assumed to be the least stable form. Overall, the inferred lattice energy ranking is (from most to least stable): a, d, 3 < g < b.…”

Behavior of intermolecular interactions in α-glycine under high pressure

“…The intermolecular interactions of organic molecules change drastically with compression, leading to pressureinduced structural changes and chemical reactions (e.g., Refs. [1][2][3][4][5][6][7][8][9]. Amino aids, also known as the building blocks of proteins, contain both the amino and carboxyl groups.…”

Pressure‐induced Fermi resonance between fundamental modes in 7,7,8,8‐tetracyanoquinodimethane

“…It is generally accepted that the frequency of Raman modes shifts to higher wavenumber region under external compression. As a result, the frequency changes in response of the Raman mode with pressure are a good indicator of a possible phase transition . Figure shows the Raman frequency–pressure relationship of TCNQ.…”