Low Temperature and High-Pressure Study of Bending L-Leucinium Hydrogen Maleate Crystals

: The polymorphism of molecular crystals is a well-known phenomenon, resulting in modiﬁcations of physicochemical properties of solid phases. Low temperatures and high pressures are widely used to ﬁnd phase transitions and quench new solid forms. In this study, L-Leucinium hydrogen maleate (LLHM), the ﬁrst molecular crystal that preserves its anomalous plasticity at cryogenic temperatures, is studied at extreme conditions using Raman spectroscopy and optical microscopy. LLHM was cooled down to 11 K without any phase transition, while high pressure impact leads to perceptible changes in crystal structure in the interval of 0.0–1.35 GPa using pentane-isopentane media. Surprisingly, pressure transmitting media (PTM) play a signiﬁcant role in the behavior of the LLHM system at extreme conditions—we did not ﬁnd any phase change up to 3.05 GPa using parafﬁn as PTM. A phase transition of LLHM to amorphous form or solid–solid phase transition(s) that results in crystal fracture is reported at high pressures. LLHM stability at low temperatures suggests an alluring idea to prove LLHM preserves plasticity below 77 K.

One of the methods to understand the nature of an important property of molecular crystal is to apply significant outside impact-low temperature, hydrostatic pressure, mechanochemical stress, etc. [36][37][38][39]. These methods help to follow the behavior of the systems at changing environments at macroscopic (thermodynamics) and microscopic (molecular contacts) levels, depending on the availability of experimental and theoretical techniques [40][41][42][43][44][45][46][47]. Moreover, low temperatures and high pressures often trigger phase transitions, resulting in new solid forms, including polymorphs. The application of extreme conditions is a powerful tool to find and sometimes stabilize new forms of organic molecules.
Thus, studies of new forms of molecular crystals using extreme conditions [11,37,48] as well as a search of new bending crystals [30,49,50] are of great interest for modern science. Nevertheless, the interrelation of these two areas is just an emerging field. There are very few works that search for new forms of plastic crystals at high pressures or low temperatures [34,35,51]. To fulfill this gap, in this work we applied low temperatures and high pressures to crystals of LLHM, chasing new forms of these bending crystals. In this work we continue examination of the LLHM system at different conditions, reporting experimental behavior of this crystal at extreme conditions. L-Leucinium hydrogen maleate crystals were grown for the first time by Arkhipov et al. [52] as an individual system in a series of amino acid maleates. Providing single crystal X-ray diffraction (SCXRD) experiments, the plasticity of LLHM was noted and formulated as "interesting mechanical behavior: mechanical action on crystals of (LLHM) results in elastic, and then plastic bending". Further, a detailed study showed that crystals preserve their plasticity at temperatures down to 77 K. It was proven using video and photo recording of the bending process [34]. Using SCXRD, authors showed that no significant changes in crystal structure of LLHM occurred during the cooling down to 100 K, preserving the layered structure [34]. The system behavior on bending was also investigated using optical and scanning electron microscopy, as well as SCXRD [35]. Scrupulous analysis of crystal structure allows authors to provide a simple model for bending LLHM crystals-layers of L-Leucinium cations and maleic acid anions connected via H-bonds (forming bc planes) were stacked over a direction (interacting with weak VdW interactions). This results in the possible slipping of layers along b direction ( Figure 1).  [34,35]). Layers' shifts are marked with an arrow sign depending on the direction of displacement.
Nevertheless, no experiments below 100 K and high pressures were provided before, leaving an opportunity for a combined study of possible phase transitions at extreme conditions using powerful methods of XRD and Raman spectroscopy.

Crystal Growth
Crystals of LLHM were obtained by slow evaporation of an equimolar aqueous solution of L-leucine and maleic acid using the 'sitting-drop' approach [53] as described in previous work [34]. L-leucine (>98%, HPLC) and maleic acid (>99%, HPLC) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification. The crystallization procedure leads to 'hedgehog' polycrystalline material as presented in [35]. This bulk material was used to find and cut single crystals for low-temperature and high-pressure experiments. Crystals were operated with care at every stage of isolation and setting to the diamond anvil cell (DAC) to avoid accidental bending.

High-Pressure Generation and Measurement
Hydrostatic pressure was generated in diamond-anvil cells (DAC) of 'Almax-Boehler' type without beryllium backing plates [54] with natural diamonds suitable both for X-ray diffraction and for Raman experiments. Paraffin (ROTH GmbH, Karlsruhe, Germany, hydrostatic limit~3 GPa) and a 1:1 stoichiometric pentane-isopentane mixture (PIP) (hydrostatic limit 7 GPa) were used as a pressure-transmitting medium (PTM) in two separate experiments [55]. A special chamber was used to facilitate the DAC loading [56]. Pressure measurement was done with a precision of 0.05 GPa using the ruby fluorescence method [57,58].

Optical Microscopy
Optical microscopy was provided using the OLYMPUS BX41 microscope (Olympus, Tokyo, Japan) with a MPlan N 10×/0.25 FN22 objective. Euromex fiber optic light source EK-1 was used for illumination.

Single-Crystal X-ray Diffraction (SCXRD)
Data were collected using an Oxford Diffraction Gemini R Ultra X-ray diffractometer (Crawley, Australia) with a CCD area detector and Mo Kα radiation. The quality of data was not high enough to refine the atomic coordinates or determine unit cell parameters, mainly because of the DAC usage. It was mentioned before that the diffraction of LLHM crystals is not very high for accessible laboratory instruments either. [34]

Raman Spectroscopy
Raman experiments were performed for low-temperature and high-pressure samples. Raman spectra were recorded using a LabRam HR 300 spectrometer from HORIBA Jobin Yvon (Edison, NJ, USA) with a CCD detector. For spectral excitation, a 488 nm line of an Ar + laser was used with a beam size of~1 µm at the surface of the sample and a power of 8 mW. All data were collected using a Raman microscope in backscattering geometry. The spectral resolution was~2 cm −1 providing seven scans 30 s each for every spectrum.
Raman spectra of LLHM were recorded in the temperature range of 300-11 K during cooling without repeating on heating.
Two distinct experiments were performed for samples at high pressures. Raman spectra for LLHM in PIP were recorded at pressures of 1. 35

Computational Methods
Gas-phase calculations of vibrational spectra (both IR and Raman) were done for L-leucine cation, Maleic acid anion, and LLHM dimer to provide a more reasonable assignment of experimental modes. Ions of L-Leucine, maleic acid, and their dimer were extracted from the LLHM crystal structure from [34] and were further freely optimized at B3LYP/6-311+G(d,p) level of theory, providing vibrational calculations subsequently. None of the atoms or groups were fixed for gas-phase optimization, making possible the ion formation of L-Leucine and Maleic acid in the gas phase calculations. Gaussian09 package was used for all calculations [59] Solid state calculations, as suggested in literature, were attempted to perform for the simulation of high-pressure behavior [3,7,46] and the vibrational band assignment. Nevertheless, even the usage of supercomputers (80 cpu, 384 Gb RAM, max time for task without interruption-240 h) did not allow for the performance of such calculations in reasonable time, providing the 'simple' optimization of one full unit cell in several months. Thus, only gas-phase calculations were used for this work.

Results
Crystals of LLHM were grown as described in the' Section 2. One crystal was used for the low temperature experiment, and another two were used in high pressure experiments in PIP and paraffin PTM. All crystals were selected using optical polarized microscopy.

Room Temperature Raman Spectra and Vibrational Mode Assignment
The crystal structure of LLHM contains six molecules in the asymmetric unit cell, resulting in 24 molecules in the full unit cell (Figure 2). Multiple H-bonds are located in directions not coinciding with cell axes or crystal faces, which limits the utility of polarized Raman spectroscopy. The complicated structure of LLHM significantly constrains precise band assignment in Raman spectra. Nevertheless, literature data and calculated gas-phase vibrational spectra of L-Leucine and maleic acid ions and their dimer helps to assign main band regions of obtained experimental spectra (Figure 3) [60][61][62][63][64][65][66]. All literature data except ref. [67], which contains multiple inaccuracies, confirm the suggested band assignment. Main changes are expected in the region before 200 cm −1 (lattice vibrations) and 3000-3400 cm −1 (valence -OH and -NH 3 vibrations). A significant change in mode position or appearance of new modes is key evidence of phase transition, while slight changes in peak positions or their intensities are traditional for low temperature or high-pressure behavior of the original phase.

Low Temperature Study
Low temperature Raman spectra of LLHM crystal were recorded in the temperature range of 300-11 K, cooling down the sample. No additional peaks of a new phase were found in spectra (Figure 4). No crystal changes were also found using optical microscopy on cooling. Wide bands split on cooling due to the thermal motion decrease. At low temperatures, the motions of functional groups in the molecule (e.g., different CH 3 groups motion in L-Leucine ion) become distinguishable in Raman spectra. Moreover, vibrations of the same groups in symmetry unequal molecules are close in spectra, but they have slightly different frequencies (2-10 cm −1 ) because of the crystalline environment and become distinguishable at low temperatures as well. This is a typical behavior of molecular crystals, especially with a high Z' number [68][69][70]. Low and high-frequency regions are shown in detail in Figure S2. Stretching vibration of CO 2 − group (1699 cm −1 and 1734 cm −1 at 298 K, see Figure 3) has a different intensity ratio, which changes at 160 K and 80 K, and may be evidence of a phase transition, e.g., a doubling of the unit cell. Nevertheless, we assume change of this vibration intensity is the result of structure shrinking and corresponding intermolecular interaction changes, but not phase transition. SCXRD in ref. [34] showed no phase transition at 160 K or at the 0-200 cm −1 region in Raman spectra. Based on scrupulous analysis of Raman spectra at low temperatures, one can suggest no phase transition on cooling. This coincides well with our previous X-Ray study in the temperature range of 100-300 K [34]. Summing up these two experiments, LLHM plasticity preservation can be proposed below 77 K down to liquid helium temperatures.

High Pressure Study
High pressure experiments were provided in different PTMs using DAC as reported in the Section 2. Usage of SCXRD was limited due to the poor diffraction data, which is a result of the nature of the LLHM crystal structure (defining plasticity) and DAC construction. Thus, only Raman spectra were recorded for all high-pressure experiments.
Surprisingly, the behavior of LLHM crystals differs significantly in paraffin and PIP. An effect of PTM, as well as experiment protocol on molecular crystals phase transition at high pressure, is a documented phenomenon [43,71,72]. The low-frequency region in Raman spectra shows no phase transition in paraffin up to 2.9 GPa (close to pressure limit for paraffin) and some changes in LLHM crystal in PIP at 1.35 GPa ( Figure 5). The high-frequency Raman region is not very informative, lacking -OH and -NH 3 vibrational modes information at high pressures [73,74]. Nevertheless, some changes in -CH x vibration modes confirm crystal changes in PIP in contrast to paraffin (full Raman spectra at all pressures are shown in Figure S3). We also report spectra of LLHM crystal in PIP at 0 GPa before pressure impact and after relaxation from 6.15 Gpa ( Figure 6). Significant background level (halo) may be explained with possible amorphization of plastic LLHM crystal at high pressure or luminescence, which was not observed before pressure impact.
Possible phase transition in PIP was additionally confirmed by optical microscopy, which recorded crystal destruction at 1.35 GPa (Figure 7). No obvious changes of LLHM crystal in paraffin occurred in the whole pressure range according to optical microscopy. This confirms different behavior of LLHM crystals under pressure in different media.
An absence of SCXRD did not allow for the report of phase transition in LLHM crystal at high pressure in PIP unequivocally. Nevertheless, relevant changes in optical microscopy and Raman spectra allowed us to speculate about crystal destruction because of one or a cascade of phase transitions in the crystal structure or amorphization of the LLHM sample.

Discussion
A search of new phases of molecular crystals at extreme conditions is an important area of modern chemistry. In this work, a unique bending crystal of LLHM, which preserves its plasticity at 77 K, was studied at low temperatures and high pressure. Based on literature data and computational vibrational spectra, major band regions were assigned to follow their changes at extreme conditions. Raman spectra of LLHM were recorded down to 11 K, showing no phase transition. Based on this experimental fact, we suggest the possibility to preserve LLHM plasticity below liquid nitrogen temperature. High pressure experiments showed different behavior of LLHM crystal in paraffin and PIP used as PTM. We found no phase transition using paraffin up to 2.95 GPa. Raman spectra and optical microscopy gave reasons to postulate the LLHM phase transition, phase transition cascade, or amorphization at high pressure, which results in crystal destruction at PIP PTM at 1.35 GPa. Additional XRPD or SCXRD using synchrotron radiation may be applied to highlight additional details of LLHM high pressure behavior in future studies. Acknowledgments: Denis Rychkov acknowledge Elena Boldyreva for basic idea to study the crystals of amino acids and their salts, including L-Leucinium Hydrogen Maleate system, at low temperatures and high pressures. All the experiments were performed at REC-008 "Molecular design and ecologically safe technologies" of the Novosibirsk State University using its instruments: The crystals of L-Leucinium Hydrogen Maleate were grown by T. Nguyen (under joint guidance of S. Arkhipov and D. Rychkov), S. Arkhipov has collected and preliminary processed Raman spectroscopy data at low temperature. E. Losev performed high-pressure X-ray diffraction and Raman spectroscopy experiments and provided the optical microphotographs. S. Arkhipov and E. Losev have performed preliminary data analysis. The authors thank S. Arkhipov and E. Losev for fruitful discussions. The authors are also grateful to A. A. Iurchenkova for assistance in preparing figures. The Siberian Branch of the Russian Academy of Sciences (SB RAS) Siberian Supercomputer Center is gratefully acknowledged for providing supercomputer facilities. The authors also acknowledge the Supercomputing Center of the Novosibirsk State University (http://nusc.nsu.ru) for providing computational resources.