High-pressure Investigations Of Typical Hydrogen-bonded Organic Crystals

As a pervasive noncovalent interaction, hydrogen bonding has significantimportance in structure, function, and dynamics of chemical systems. Pressure is acrucial thermodynamic parameter. Compression of materials can facilitate closepacking, which alters the cooperativity between hydrogen bonding, van der Waalsforce, electrostatic interaction and π stacking interactions. Compared to covalentbonds, hydrogen bonds can easily be tuned by compression in organic systems. Thus,hydrogen bonds will have rich changes under pressure, such as breakage andformation, symmetrization and proton disorder, as well as rearrangement of thehydrogen-bonded networks. The changes in hydrogen bonding will ultimately resultin structure variations. The physicochemical properties of solid materials are stronglyrelated to their structures. Hence, high-pressure investigations of hydrogen-bondedmolecular crystals are essential to acquire further knowledge on structure-propertyrelationships. This research also provides a better understanding of the nature ofhydrogen bonds, as well as the cooperativity of various noncovalent interactions. Suchinvestigations are also of fundamental and practical significance for physics,chemistry, life science and materials science.We present high-pressure investigations on the simple hydrogen-bonded organiccrystals. Oxamide and Biurea crystals have been studied by in situ Ramanspectroscopy and synchrotron X-ray diffraction (ADXRD) in a diamond anvil cell.Oxamide, one of the simplest amides, exhibits a typical layered structure of organic molecules with two-dimensional hydrogen-bonded sheets. Within the plane, eachmolecule is linked to four neighbor molecules by hydrogen bonds, while there isrelatively weak van der Waals interaction between these planes only. The Ramanexperimental results reveal that the discontinuity in Raman shifts around9.6GPaindicates a pressure-induced structural phase transition. N–H stretching vibrationsexperience dramatic changes in Raman shifts. Meanwhile, NH2rocking modedisplays an abrupt red shift at9.6GPa, and N–H bending (amide II) mode shifts tolower frequency constantly after9.6GPa. Such variations indicate this phasetransition is attributed to the distortions of the hydrogen-bonded networks. This phasetransition is confirmed by the changes of ADXRD spectra with the symmetrytransformation from P-1to P1. On total release of pressure, the diffraction patternreturns to its initial state, implying this transition is reversible. Oxamide has a highand anisotropic compressibility under high pressure. The compression ratio of thea-axis is much higher than the b-axis and c-axis. The model of the high-pressurephase was provided by Rietveld refinement. The two molecules in high-pressurephase separate from the bc-plane, twist, and are not parallel anymore. In terms ofstructure, biurea can be regarded as two urea molecules connected via the removal oftwo H atoms. In view of its simple structure, biurea is a model system for studying thestructure and phase stability of hydrogen-bonded molecular crystals under highpressures. Biurea also exhibits ordinary three-dimensional hydrogen-bonded networkswhere molecules are held together by extensive N–H…O hydrogen bonds. Significantchanges in Raman spectra provide evidence for a pressure-induced structural phasetransition in the range of0.6to1.5GPa. Modes related to N H…O hydrogen bonds,including C=O in-plane bending, O=C N in-plane bending, NH2out-of-planebending, NH2scissoring, and NH2stretching, exhibit abrupt changes. This providessufficient evidence that the phase transition observed in this study is attributed torearrangement of the hydrogen-bonded networks. The softening of the NH2stretchingmodes correspond to the hardening of the external modes, indicating the hydrogenbonding strengthens continuously by the application of pressure. ADXRDexperiments confirm this reversible phase transition by symmetry transformation from C2/c to a possible space group P2/n.We conduct high-pressure studies on the hydrogen-bonded organic polymorphicsystems. Two polymorphs of p-aminobenzoic acids (PABA) and two forms ofCinchomeronic Acid (CA) have been studied by in situ Raman spectroscopy in adiamond anvil cell. Experimental results reveal that α-PABA and β-PABA remainstable up to13GPa. Ab initio calculations are performed to account for the changes inunit cell parameters, molecular arrangements, and hydrogen bonds. Polymerization isobserved in α-PABA through the calculated geometric parameters of hydrogen bonds.Hydrogen bonds are approximately equal to the length of the covalent bonds. Theelectron cloud of H atom is almost equally distributed between the two O atoms; thehydrogen bonds possess a substantial covalent characteristic. Therefore, a newphenomenon that hydrogen bonds transform into covalent bonds is observed. Basedon a systematic comparison of the subtle structural changes, anisotropic characteristic,and various interactions of the two polymorphs, we propose that the stability ofα-form crystals is associated with the special dimer structure. The stability of theβ-form is attributed to the hydrogen-bonded networks with four-membered ringconstruction. The Raman experimental results of the CA polymorphism reveal that theForm I phase undergoes a phase transition at6.5GPa. The phase transition isattributed to the competition between hydrogen bonds and van der Waals interactionunder high pressures. Hydrogen bonds have been demonstrated to evolve into adistorted state through the phase transition, evidenced by the variations in the C Hstretching vibrations. The phase transition of Form II in the range11.3–12.9GPa hasbeen corroborated by Raman experimental results. Modes related to C H…Ohydrogen bonds, including C H wagging, C O stretching, COO-asymmetricstretching, C H stretching, show obvious changes. This indicates that the phasetransition is proposed to stem from the rearrangement of the hydrogen bonds.We investigate the high-pressure behavior of hydrogen-bonded energeticmaterial carbohydrazide (CHZ) via in situ Raman spectroscopy and angle-dispersiveX ray diffraction (ADXRD) in a diamond anvil cell with~15GPa at roomtemperature. CHZ can be used as an important component of rocket propellants. The N H…O and N H…N hydrogen bonds in its structure are linked with neighbormolecules to form hydrogen-bonded sheets. The reversible phase transition in therange~8–10.5GPa has been indicated by Raman experimental results and is proposedto stem from the rearrangements of the hydrogen bonds. Further analysis of ADXRDpatterns illustrates the new phase has P-1symmetry. The density in phase P-1hasbeen increased by~23.1%compared to ambient phase P21/n. Phase P-1could beexpected to release more chemical energy due to the higher density. Therefore, phaseP-1may play a bigger role in the subsequent process of rocked propellantcombustion.