High Pressure Studies On Hydrogen-Bonded Supramolecular Structure: Guanidinium Nitrate

Supramolecular chemistry has provided a wide canvas for a variety of studies of molecular materials in the solid state. Supramolecular interactions, which include hydrogen bonding, play a vital role in the design of supramolecular architectures. The contribution of hydrogen bonds to the area of supramolecular chemistry is definitely outstanding owing to the reversibility, specificity, directionality, and better tuning of stabilization. The hydrogen bonding interactions between the individual building blocks and the resulting lattice architecture substantially determine the properties of supramolecules. Moreover, hydrogen bonds which define the aggregation state can be easily altered by compression as the Coulombic interaction between the protons and acceptor atoms is relatively weak compared to covalent bonds. This means that pressure has the ability to control the strength of hydrogen bonding interaction. Thus, the study of hydrogen bonds under high pressure can give valuable information to probe the stability of supramolecular system.Pressure, temperature, composition are three parameter for substance. The change of temperature and composition is the usual method to study the characters and improve the performance. Pressure is suitable for altering hydrogen bonding, as small variations in the applied forces typically results in large changes in intermolecular separations, which often trigger dramatic reorganization of crystal packing, without encountering the major perturbations produced by changes in temperature and chemical composition. Guanidinium nitrate (C(NH2)3+·NO3-, GN) is a representative supramolecular architecture of hydrogen-bonded rosette network which is built of sheets of guanidinium and nitrate ions linked by N–H···O hydrogen bonds.19-21 According to the reports, GN crystals reveal two temperature-induced phase transition. The low-temperature phase, GN1, on heating at T12=296 K, undergoes a transition to phase GN2; Then a continuous phase transition happens at T23=384 K, with the crystals transforming to GN3 phase.22,23 The structures of these three phases are two dimensional hydrogen-bonded layer motifs with different space groups at ambient pressure.21 The GN2 phase exists at ambient condition and has a monoclinic structure belonging to C2 spacegroup with the unit cell parameters a=12.545(5) ?, b=7.303(4) ?, c=7.476(4) ?,β=124.93(5)°at 292 K. Because of its relative simple structure, GN2 can be considered as a model system for us to investigate the influence of high pressure on the hydrogen-bonded supramolecular structures. Furthermore, the structure and stability of GN2 at high pressure is determined mainly by the balance between hydrogen bonding and electrostatics interaction.21 Therefore, high-pressure studies on GN2 are also valuable for comprehending the relationship between long–range electrostatics forces and geometric factors. In the present paper, we have performed in situ XRD as well as Raman spectroscopy measurements to detect the phase transition and the motions of molecular fragments in crystalline GN2 at high pressure, and to see which consequences the hydrogen bonds will have for the stability of the structure with respect to phase transitions. Moreover, we performed ab initio calculations to provide the mechanism of this phase transition. The results followed after:1. Considerable changes at ~1 GPa in the spectrum were observed by use of in situ Raman spectroscopy measurements on GN crystal. There was a lattice mode arising at 117 cm-1. At the same time, the bands (540 and 724 cm-1 at ambient pressure) related to CN3 deformation motion and NH2 bending showed markedly twofold splitting. When the pressure went beyond 1.1 GPa, we could observe that NH2 stretching modes exhibited obvious changes in intensity and position. The phase transition was apparent owing to the emergence of the changes. At sufficiently high pressure (~1 GPa), the increased energy of interionic interactions may rotate ions or molecular fragments and distorted the hydrogen bonded networks. The observed abrupt changes of the N–H stretching modes at ~ 1 GPa implied that there was a considerable rearrangement of hydrogen bonds. With further increase of pressure, all of NH2 stretching modes remained undergoing a considerable red shift up to ~8 GPa, which indicated that hydrogen bonds between neighboring molecules still existed in GN-HP phase and all the N–H bonds participated in hydrogen bonds as before. However, at ~8 GPa these frequencies suddenly started to increase at different rates of pressure shifts. The blue shift was primarily due to the significant enhancement of hydrogen bond strength. Thus, N–H bonds participated in strong hydrogen bonds at higher pressure. Fortunately, the hydrogen-bonded framework of GN crystal was preserved through the phase transition. With further increase of pressure up to 21.5 GPa, spectrum remains essentially similar to that at 1.1 GPa. Beyond the phase transition,all of Raman bands in this internal region exhibited shifts towards to the high frequencies owing to the decrease in the bond distance and increase in the effective force constants. The high pressure phase reverted to the ambient pressure phase upon total release of pressure.2. For a further understanding of the GN2 transition caused by pressure, we carried out X-ray diffraction experiments. An abrupt change of the diffraction pattern, which is the most convincing evidence for the pressure-induced phase transition, was observed at 0.9 GPa. We believed that it was a new phase compared to the XRD patterns of the other two temperature-induced phases of GN. With increasing pressure, the GN-HP phase remained stable up to the highest pressure. The intensity refinements of the X-ray diffraction data suggested that the most likely structure of high-pressure phase was in space group P21. In comparison with the ambient spacegroup, the high pressure spacegroup P21 had lower symmetry than C2. The symmetry lowering of high pressure crystal structure provided a circumstantial evidence for the splitting of Raman modes in the Raman spectra of GN-HP phase. Moreover, in contrast to low-temperature phase GN1 which crystallizes in Cm spacegroup, ions adopt different relative orientation in GN-HP phase, which led to a more complicated crystal structure.3. To understand what changes are happening of hydrogen bonding networks under high pressure, we performed ab initio calculations based on density functional theory. The calculated results revealed that the hydrogen-bonded sheet was distorted to wave-shaped structure at higher pressure. It is worthy to note that guanidinium cations were modulated into tilt and displacement from the average plane of sheet. Based on the calculated hydrogen bonding model, we propose that the occurrence of phase transition was because the balance between hydrogen bonding interactions and electrostatic forces within sheets are disturbed.In summary, pressure caused the occurrence of phase transition of GN crystal, which engendered an unreported new phase. These studies will be important to further develop an understanding of hydrogen bonds and stability of hydrogen-bonded supramolecular systems under high pressure conditions.