Direct Observation of Contact Ion-Pair Formation in La3+ Methanol Solution.

An approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to carry out a comparative study about the solvation properties of dilute La(NO3)3 solutions in water and methanol, with the aim of elucidating the still elusive coordination of the La3+ ion in the latter medium. The comparison between these two systems enlightened a different behavior of the nitrate counterions in the two environments: while in water the La(NO3)3 salt is fully dissociated and the La3+ ion is coordinated by water molecules only, the nitrate anions are able to enter the metal first solvation shell to form inner-sphere complexes in methanol solution. The speciation of the formed complexes showed that the 10-fold coordination is preferential in methanol solution, where the nitrate anions coordinate the La3+ cations in a monodentate fashion and the methanol molecules complete the solvation shell to form an overall bicapped square antiprism geometry. This is at variance with the aqueous solution where a more balanced situation is observed between the 9- and 10-fold coordination. An experimental confirmation of the MD results was obtained by La K-edge XAS measurements carried out on 0.1 M La(NO3)3 solutions in the two solvents, showing the distinct presence of the nitrate counterions in the La3+ ion first solvation sphere of the methanol solution. The analysis of the extended X-ray absorption fine structure (EXAFS) part of the absorption spectrum collected on the methanol solution was carried out starting from the MD results and confirmed the structural arrangement observed by the simulations.

On the Coordination Chemistry of the lanthanum(III) Nitrate Salt in EAN/MeOH Mixtures.

A thorough structural characterization of the La(NO3)3 salt dissolved into several mixtures of ethyl ammonium nitrate (EAN) and methanol (MeOH) with EAN molar fraction χEAN ranging from 0 to 1 has been carried out by combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS). The XAS and MD results show that changes take place in the La3+ first solvation shell when moving from pure MeOH to pure EAN. With increasing the ionic liquid content of the mixture, the La3+ first-shell complex progressively loses MeOH molecules to accommodate more and more nitrate anions. Except in pure EAN, the La3+ ion is always able to coordinate both MeOH and nitrate anions, with a ratio between the two ligands that changes continuously in the entire concentration range. When moving from pure MeOH to pure EAN, the La3+ first solvation shell passes from a 10-fold bicapped square antiprism geometry where all the nitrate anions act only as monodentate ligands to a 12-coordinated icosahedral structure in pure EAN where the nitrate anions bind the La3+ cation both in mono- and bidentate modes. The La3+ solvation structure formed in the MeOH/EAN mixtures shows a great adaptability to changes in the composition, allowing the system to reach the ideal compromise among all of the different interactions that take place into it.

Anatomy of a deep eutectic solvent: structural properties of choline chloride : sesamol 1 : 3 compared to reline.

The structural properties of the deep eutectic solvent (DES) formed by choline chloride (ChCl) and sesamol in 1 : 3 ratio have been investigated and compared to those of reline (ChCl : urea 1 : 2). An integrated approach combining small and wide angle X-ray scattering with molecular dynamics simulations has been employed and the simulation protocol has been validated against the experimental data. In the ChCl : sesamol DES, strong hydrogen bonds (HBs) are formed between the chloride anion and the hydroxyl groups of the choline and of sesamol molecules. Conversely, choline-choline, choline-sesamol and sesamol-sesamol interactions are negligible. A more extended interplay between the constituents is observed in reline where, besides the HBs involving the chloride anion, the eutectic formation is favored also by strong choline-urea and urea-urea interactions. The three-dimensional arrangement around the individual components shows that, in the ChCl : sesamol DES, the cholinium cations and the sesamol molecules are packed in such a way to maximize the interactions with the chlorine anion. This structural arrangement may favor the π-π interactions between the sesamol molecules and the aromatic species mediated by the chloride ions, providing an interpretation for the high separation rates previously observed for phenolic DESs towards aromatic compounds.

Elusive Coordination of the Ag+ Ion in Aqueous Solution: Evidence for a Linear Structure.

X-ray absorption spectroscopy (XAS) has been employed to study the coordination of the Ag+ ion in aqueous solution. The conjunction of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) data analysis provided results suggesting the preference for a first shell linear coordination with a mean Ag-O bond distance of 2.34(2) Å, different from the first generally accepted tetrahedral model with a longer mean Ag-O bond distance. Ab initio molecular dynamics simulations with the Car-Parrinello approach (CPMD) were also performed and were able to describe the coordination of the hydrated Ag+ ion in aqueous solution in very good agreement with the experimental data. The high sensitivity for the closest environment of the photoabsorber of the EXAFS and XANES techniques, together with the long-range information provided by CPMD and large-angle X-ray scattering (LAXS), allowed us to reconstruct the three-dimensional model of the coordination geometry around the Ag+ ion in aqueous solution. The obtained results from experiments and theoretical simulations provided a complex picture with a certain amount of water molecules with high configurational disorder at distances comprised between the first and second hydration spheres. This evidence may have caused the proliferation of the coordination numbers that have been proposed so far for Ag+ in water. Altogether these data show how the description of the hydration of the Ag+ ion in aqueous solution can be complex, differently from other metal species where hydration structures can be described by clusters with well-defined geometries. This diffuse hydration shell causes the Ag-O bond distance in the linear [Ag(H2O)2]+ ion to be ca. 0.2 Å longer than in isolated ions in solid state.

Unraveling the solvation geometries of the lanthanum(III) bistriflimide salt in ionic liquid/acetonitrile mixtures.

A synergic approach combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS) has been used to investigate the structural properties of the La(Tf2N)3 salt (where Tf2N = bistriflimide or bis(trifluoromethansulfonyl)imide) dissolved into several mixtures of acetonitrile and the 1,8-bis(3-methylimidazolium-1-yl)octane bistriflimide (C8(mim)2(Tf2N)2) ionic liquid (IL), with the IL molar fraction (χIL) ranging from 0 to 1. The XAS and MD results show that major changes take place in the La3+ first solvation shell when moving from pure acetonitrile to pure C8(mim)2(Tf2N)2. With increasing the IL concentration of the mixture, the La3+ first shell complex progressively loses acetonitrile molecules to accommodate more and more oxygen atoms of the Tf2N- anions. Except in pure C8(mim)2(Tf2N)2, La3+ is always able to coordinate both acetonitrile and Tf2N- anions, with a ratio between the two different ligands strongly dependent on the IL content. Moreover, the La3+ ion prefers to form a 10-coordinated first shell complex in all the investigated systems, with a slightly different geometry of the cluster depending on the composition of the La3+ first solvation shell. In particular, when moving from pure acetonitrile to pure C8(mim)2(Tf2N)2, the La3+ first solvation shell passes from a bicapped square antiprism geometry where all the Tf2N- anions act only as monodentate ligands, to a "1 + 5 + 4" structure in which the Tf2N- anion binds La3+ both in a monodentate and bidentate fashion. The great adaptability shown by the La3+ solvation structure allows it to reach the optimal balance among many different forces at play involving all of the different species present in the mixtures.

Unraveling the Hydration Properties of the Ba2+ Aqua Ion: the Interplay of Quantum Mechanics, Molecular Dynamics, and EXAFS Spectroscopy.

The structural and dynamic properties of the Ba2+ cation in water have been studied by combining quantum mechanical (QM) calculations, molecular dynamics (MD) simulations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. An effective Ba2+-water interaction potential, to be used in the MD simulation of a Ba2+ aqueous solution, has been developed by means of QM methods, and the validity of the whole procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. By combining distance and angular distribution functions it was possible to unambiguously identify the geometry adopted by the water molecules surrounding the ion in the solution. The Ba2+ ion was found to preferentially form an 8-fold first shell complex with a bicapped trigonal prism (BTP) geometry. The 8-fold complex is in equilibrium with a 9-fold structure having a tricapped trigonal prism (TTP) geometry, and the hydration shell is very diffuse and flexible, being characterized by a very fast solvent exchange process on the picosecond time scale.

Solvation structure of lanthanide(iii) bistriflimide salts in acetonitrile solution: a molecular dynamics simulation and EXAFS investigation.

A synergic approach combining molecular dynamics (MD) and extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate weak-concentrated (0.1 M) acetonitrile solutions of La(Tf2N)3 and Dy(Tf2N)3 salts (where Tf2N is the bis(trifluoromethanesulfonyl)imide). The MD simulations show that contact ion pairs between the Ln3+ cations and the Tf2N- anions are formed in the solutions. This finding has been experimentally confirmed by the analysis of the Ln K-edge EXAFS experimental signals of the two solutions. Both La3+ and Dy3+ ions preferentially form a 10-fold first shell complex composed of acetonitrile molecules and Tf2N- counterions with a bicapped square antisprism (BSAP) geometry. As a consequence of lanthanide contraction, the Dy3+ cation binds the inner shell solvent molecules at shorter distances as compared to La3+ and the high charge density of Dy3+ allows the coordination with additional ligands at longer distances. On the other hand, the bigger La3+ ion forms a very crowded coordination shell with a larger average distance and with the capped molecules at distances from the ion more similar to the inner shell ones. This peculiar coordination structure could explain the high catalytic activity of the Ln-Tf2N complexes and the high Lewis acidity of the lanthanide center.

On the coordination of Zn2+ ion in Tf2N- based ionic liquids: structural and dynamic properties depending on the nature of the organic cation.

A synergic approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy has been used to investigate diluted solutions of zinc bis(trifluoromethanesulfonyl)imide (Zn(Tf2N)2) in Tf2N- based ionic liquids (ILs) having different organic cations, namely the 1-butyl-3-methylimidazolium ([C4(mim)]+), 1,8-bis(3-methylimidazolium-1-yl)octane ([C8(mim)2]2+), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([Choline]+) and butyltrimethylammonium ([BTMA]+) ions. All of the ILs tend to dissolve the Zn(Tf2N)2 species giving rise to a different structural arrangement around the Zn2+ as compared to that of the salt crystallographic structure. A quantitative analysis of the Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the microscopic description of the systems derived from the MD simulations. A very good agreement between theoretical and experimental EXAFS signals has been obtained, allowing us to assess the reliability of the MD structural results for all the investigated solutions. The Zn2+ ion has been shown to be coordinated by six oxygen atoms of the Tf2N- anions arranged in an octahedral geometry in all the Tf2N- based ILs, regardless of the organic cation of the IL solvent. However, the nature of the organic cation has a small influence on the overall spatial arrangement of the Tf2N- anions in the Zn2+ first solvation shell: two different Zn-Tf2N complexes are found in solution, a 5-fold one, with one bidentate and four monodentate Tf2N- anions, and a 6-fold one with only monodentate ligands, with the ratio between the two species being slightly dependent on the IL cation. The IL ion three-dimensional arrangements in the different IL solutions were also investigated by carrying out a thorough analysis of the MD simulations, highlighting similarities and differences between imidazolium and ammonium based IL systems.

How Does CeIII Nitrate Dissolve in a Protic Ionic Liquid? A Combined Molecular Dynamics and EXAFS Study.

A diluted solution of Ce(NO3 )3 in the protic ionic liquid (IL) ethylammonium nitrate (EAN) was investigated using molecular dynamics (MD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. For the first time polarizable effects were included in the MD force field to describe a heavy metal ion in a protic IL, but, unlike water, they were found to be unessential. The CeIII ion first solvation shell is formed by nitrate ions arranged in an icosahedral structure, and an equilibrium between monodentate and bidentate ligands is present in the solution. By combining distance and angular distribution functions it was possible to unambiguously identify this peculiar coordination geometry around the ions dissolved in solution. The metal ions are solvated within the polar domains of the EAN nanostructure and the dissolved salt induces almost no reorganization of the pre-existing structure of EAN upon solubilization.

Development of Lennard-Jones and Buckingham Potentials for Lanthanoid Ions in Water.

New sets of Lennard-Jones and Buckingham potentials have been developed to be used in classical molecular dynamics simulations of Ln3+-containing systems for the whole lanthanoid series. The force-field parameters have been refined by directly comparing the hydration structure obtained from the simulations with the extended X-ray absorption fine structure (EXAFS) experimental data, in order to reproduce Ln3+-water EXAFS experimentally inferred mean distances. Analysis of the simulation results has shown that both Lennard-Jones and Buckingham potentials are able to properly describe the radial distribution of water molecules around the Ln3+ ions, the smooth decrease of the hydration number along the lanthanoid series, as well as the geometry of the first-shell hydration complex formed by Ln3+ ions in water. The newly optimized interaction potential parameters can be used in conjunction with force fields available in the literature to investigate the solvation properties of Ln3+ ions in different disordered systems.

Structure of Water in Zn2+ Aqueous Solutions from Ambient Conditions up to the Gigapascal Pressure Range: A XANES and Molecular Dynamics Study.

The structural modifications induced on a 0.5 M Zn2+ aqueous solution by increasing the pressure to 6.4 GPa were investigated using a combination of X-ray absorption near edge structure (XANES) spectroscopy and molecular dynamics (MD) simulations. The Zn K-edge XANES experimental spectra show two different trends depending on the pressure and temperature conditions of the system. On the one hand, when the pressure is increased to 1.0 GPa while keeping the temperature at 300 K, the highly structured nature of Zn2+ second hydration shell is preserved. On the other hand, when the Zn2+ aqueous solution is simultaneously pressurized and heated to follow the melting curve above 1.0 GPa, the Zn2+ second shell loses its high degree of structuring and becomes much more disordered and unstructured. These results are confirmed by the analysis of MD simulations of Zn2+ aqueous solutions under high pressure. By combining distance and angular distribution functions it is possible to highlight the loss of water structuring in the Zn2+ second coordination shell that takes place upon pressurization and heating. A progressive crowding of the Zn2+ second shell is observed with increasing pressure; the water structure becomes remarkably different from that found at ambient conditions, and for pressure values higher than 1.0 GPa the tetrahedral arrangements of water molecules is highly distorted. Moreover, MD simulations of Zn2+ aqueous solutions performed at 1.0 GPa and at increasing temperature values have shown that the loss of water structuring in the Zn2+ second coordination shell observed by simultaneously pressurizing and heating is due to a combined effect of pressure and temperature, both producing an increase of the Zn2+ second-shell disorder.

On the development of polarizable and Lennard-Jones force fields to study hydration structure and dynamics of actinide(III) ions based on effective ionic radii.

In this contribution, we show how it is possible to develop polarizable and non-polarizable force fields to study hydration properties of a whole chemical series based on atomic properties such as ionic radii. In particular, we have addressed the actinide(III) ion series, from U3+ to Cf3+, for which X-ray absorption data and effective ionic radii are available. A polarizable force field has been re-parameterized improving the original one [M. Duvail et al., J. Chem. Phys. 135, 044503 (2011)] which was based on solid state ionic radii. The new force field does not depend on solid state properties but directly on the liquid phase ones, and it can be used to study these ions in liquid water without any ambiguity. Furthermore, we have shown that it is possible to parameterize also a non-polarizable potential using standard Lennard-Jones and Coulombic forces, which can be transferred to other systems in condensed phase. The structural and dynamical properties of these two force fields are compared to each other and with data available in the literature, providing a good agreement. Moreover, we show the comparison with experimental X-ray absorption data that are very well reproduced by both force fields.

Unraveling the Sc(3+) Hydration Geometry: The Strange Case of the Far-Coordinated Water Molecule.

The hydration structure and dynamics of Sc(3+) in aqueous solution have been investigated using a combined approach based on quantum mechanical (QM) calculations, molecular dynamics (MD) simulations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. An effective Sc-water two-body potential has been generated from QM calculations and then used in the MD simulation of Sc(3+) in water, and the reliability of the entire procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. The outstanding outcome of this work is that the Sc(3+) ion forms a well-defined capped square antiprism (SAP) complex in aqueous solution, where the eight water molecules closest to the ion are located at the vertexes of a SAP polyhedron, while the ninth water molecule occupying the capping position is unusually found at a very long distance from the ion. This far-coordinated water molecule possesses a degree of structure comparable with the other first shell molecules surrounding the ion at much shorter distances, and its presence gave us the unique opportunity to easily identify the geometry of the Sc(3+) coordination polyhedron. Despite very strong ion-water interactions, the Sc(3+) hydration shell is very labile, as the far-coordinated ligand allows first shell water molecules to easily exchange their positions both inside the solvation shell and with the rest of the solvent molecules.

Structural properties of geminal dicationic ionic liquid/water mixtures: a theoretical and experimental insight.

The structural behavior of geminal dicationic ionic liquid 1,n-bis[3-methylimidazolium-1-yl] alkane bromide ([Cn(mim)2]Br2)/water mixtures has been studied using extended X-ray absorption fine structure (EXAFS) spectroscopy in combination with molecular dynamics (MD) simulations. The properties of the mixtures are investigated as a function of both water concentration and alkyl-bridge chain length. The very good agreement between the EXAFS experimental data and the theoretical curves calculated from the MD structural results has proven the validity of the theoretical framework used for all of the investigated systems. In all the solutions the water molecules are preferentially coordinated with the Br(-) ion, even if a complex network of interactions among dications, anions and water molecules takes place. The local molecular arrangement around the bromide ion is found to change with increasing water content, as more and more water molecules are accomodated in the Br(-) first coordination shell. Moreover, with the decrease of the alkyl-bridge chain length, the interactions between dications and anions increase, with Br(-) forming a bridge between the two imidazolium rings of the same dication. On the other hand, in [Cn(mim)2]Br2/water mixtures with long alkyl-bridge chains peculiar internal arrangements of the dications are found, leading to different structural features of geminal dicationic ionic liquids as compared to their monocationic counterparts.

Local order and long range correlations in imidazolium halide ionic liquids: a combined molecular dynamics and XAS study.

A thorough characterization of the structural properties of alkylimidazolium halide ionic liquids (ILs), namely 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br with n = 5, 6, 8, 10) and iodide ([C6mim]I), has been carried out by combining molecular dynamics simulations and EXAFS spectroscopy. The existence of a local order in [Cnmim]Br ILs has been evidenced, with anions and imidazolium head groups forming a local three-dimensional bonding pattern that is common to all the [Cnmim]Br IL family, regardless of the length of the alkyl chain attached to the cation. On the other hand, upon alkyl chain elongation significant differences have been highlighted in the long-range structure of these ILs. Theoretical X-ray structure factors have been calculated from MD simulations and a low q peak has been found for all [Cnmim]Br ILs, indicating the existence of long-range structural correlations. The low q peak moves to smaller q values corresponding to longer distances, increases in intensity and sharpens with increasing alkyl chain length on the cation. Similarities and differences between the ion three-dimensional arrangements in [C6mim]Br and [C6mim]I were highlighted and the structural arrangement of Br(-) and I(-) was found to be different in the proximity of the most acidic hydrogen atom of the imidazolium ring: the I(-) ion is preferentially located above and below the ring plane, while the Br(-) ion has a high probability also to be coplanar with the imidazolium ring. A quantitative analysis of the Br and I K-edge EXAFS spectra of alkylimidazolium halide ILs has been carried out based on the microscopic description of the systems derived from MD simulations. A very good agreement between theoretical and experimental EXAFS signals has been obtained, allowing us to assess the reliability of the MD structural results for all the alkylimidazolium halide ILs investigated in this work.

Combining EXAFS spectroscopy and molecular dynamics simulations to understand the structural and dynamic properties of an imidazolium iodide ionic liquid.

The structural properties of liquid 1-butyl-3-methylimidazolium iodide [C4mim]I have been investigated using an integrated approach that combines EXAFS spectroscopy and molecular dynamics (MD) simulations. A well defined first coordination shell composed on average of 4.5 I(-) ions around the imidazolium cation has been evidenced, and the structural arrangement of the I(-) ions has been found to be different in the proximity of the most acidic hydrogen atom of the imidazolium ring, as compared to the other two ring protons: in the former case the I(-) ion is not coplanar with the imidazolium ring plane, but it prefers to be above and below the plane itself, while in the latter the anion has the same probability of being or not being coplanar with the plane. A quantitative analysis of the I K-edge EXAFS spectrum of liquid [C4mim]I has been carried out starting from the structural information on the system derived from the MD simulation. This combined approach allows one to reduce the number of correlated model parameters required in the fitting of the experimental data and to increase the reliability of the EXAFS data analysis that represents a non-trivial task when dealing with disordered systems. Moreover, the good agreement between the EXAFS experimental and theoretical spectra of liquid [C4mim]I has proven the reliability of the MD results and force field employed.

Quantitative analysis of deconvolved X-ray absorption near-edge structure spectra: a tool to push the limits of the X-ray absorption spectroscopy technique.

A deconvolution procedure has been applied to K-edge X-ray absorption near-edge structure (XANES) spectra of lanthanoid-containing solid systems, namely, hexakis(dmpu)praseodymium(III) and -gadolinium(III) iodide. The K-edges of lanthanoids cover the energy range 38 (La)-65 (Lu) keV, and the large widths of the core-hole states lead to broadening of spectral features, reducing the content of structural information that can be extracted from the raw X-ray absorption spectra. Here, we demonstrate that deconvolution procedures allow one to remove most of the instrumental and core-hole lifetime broadening in the K-edge XANES spectra of lanthanoid compounds, highlighting structural features that are lost in the raw data. We show that quantitative analysis of the deconvolved K-edge XANES spectra can be profitably used to gain a complete local structural characterization of lanthanoid-containing systems not only for the nearest neighbor atoms but also for higher-distance coordination shells.

Unraveling halide hydration: A high dilution approach.

The hydration properties of halide aqua ions have been investigated combining classical Molecular Dynamics (MD) with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Three halide-water interaction potentials recently developed [M. M. Reif and P. H. Hünenberger, J. Chem. Phys. 134, 144104 (2011)], along with three plausible choices for the value of the absolute hydration free energy of the proton (ΔG [minus sign in circle symbol]hyd[H+]), have been checked for their capability to properly describe the structural properties of halide aqueous solutions, by comparing the MD structural results with EXAFS experimental data. A very good agreement between theory and experiment has been obtained with one parameter set, namely LE, thus strengthening preliminary evidences for a ΔG [minus sign in circle symbol]hyd[H] value of -1100 kJ mol(-1) [M. M. Reif and P. H. Hünenberger, J. Chem. Phys. 134, 144104 (2011)]. The Cl(-), Br(-), and I(-) ions have been found to form an unstructured and disordered first hydration shell in aqueous solution, with a broad distribution of instantaneous coordination numbers. Conversely, the F(-) ion shows more ordered and defined first solvation shell, with only two statistically relevant coordination geometries (six and sevenfold complexes). Our thorough investigation on the effect of halide ions on the microscopic structure of water highlights that the perturbation induced by the Cl(-), Br(-), and I(-) ions does not extend beyond the ion first hydration shell, and the structure of water in the F(-) second shell is also substantially unaffected by the ion.

Hydration properties of the Zn2+ ion in water at high pressure.

The structure and dynamics of water in ionic solutions at high pressure have been investigated using a combined approach based on extended X-ray absorption fine structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. Modification of the hydration properties of the Zn(2+) ion induced by a pressure increase from ambient condition up to ∼6.4 GPa has been revealed and accurately analyzed. With increasing pressure the first hydration shell of the Zn(2+) ion has been found to retain an octahedral symmetry with a shortening of the Zn-O distance up to 0.09 Šand an increased width associated with thermal motion, as compared to the ambient condition hydration complex. A very interesting picture of the dynamic behavior of the first hydration shell has emerged from the analysis of the simulations: up to 2.5 GPa no exchange events between first and second shell water molecules occurred, while above this pressure value several exchange events take place in the solution following an associative interchange mechanism. This result can be explained by the very high compression and packing of the solvent which force second shell water molecules to enter the Zn(2+) first hydration shell. MD simulations indicate a strong pressure effect also on the structure of the second coordination shell which is compressed and becomes more disordered and less structured with increasing pressure. The water mobility and the ion diffusion coefficient have been found to increase in the high density conditions, as a consequence of the rupture of the hydrogen bond network caused by pressure.