Competing Interactions: Evolution of Inter and Intramolecular Hydrogen Bonds in Salicylic Acid at High Pressures.

Benzoic acid derivatives are important molecular systems in the pharmaceutical industry. Salicylic acid is distinct among the derivatives of benzoic acid due to the presence of an intramolecular hydrogen bond. With a view to study the evolution of inter and intramolecular hydrogen bonding at shorter length scales, in situ high pressure Raman spectroscopic measurements, angle dispersive X-ray diffraction experiments, and density functional theory (DFT) based first principle calculations have been carried out on crystalline salicylic acid. Subtle structural modifications are noted across ∼1 GPa leading to structural phase transition to a new crystalline phase above 7 GPa which is reversible. Substantial softening of the OH stretching Raman mode associated with intramolecular hydrogen bond is observed prior to the transition pressure. Possible molecular configurations associated with tautomerization are discussed with the aid of DFT calculations.

Perceptible isotopic effect in 3D-framework of α-glycine at low temperatures.

Glycine, the most fundamental amino acid, albeit studied for many decades, has kept researchers captivated with interesting structural variations relevant to important biological, astrophysical and technological applications. We report here a noticeable effect of deuteration on the three dimensional hydrogen bonding network of α-glycine using low temperature infrared absorption studies in a wide spectral range, corroborated with Raman scattering studies. These systematic studies in the range 300-4.2 K have demonstrated a relatively compact assembly of glycine molecules in the three dimensional bilayered structure of hydrogenated glycine (gly-h) at low temperatures. This is inferred from a remarkable temperature effect in the weak intra-bilayer hydrogen bond ~ along the b-axis, which strengthens upon cooling. A pronounced increase in the intensity of NH3 torsional and NH stretching modes has been observed. This is accompanied with a large rate of stiffening and softening respectively of these modes upon cooling and a change in slope across 210 K and 80 K. In contrast, the D---O hydrogen bond lengths in fully deuterated isotope (gly-d), as estimated using empirical correlation, show that the weak intra-bilayer hydrogen bond is not strengthened upon cooling down to 180 K, whereas the stronger intra-layer hydrogen bonds in the ac-plane become further strong. The ND3 torsional vibrations show no temperature effect. This implies a relatively stable two dimensional layered structure formed by strongly hydrogen bonded glycine sheets in the ac-plane. Below 180 K, similar qualitative trends have been obtained for the hydrogen bond lengths in the two isotopes. In addition, temperature induced variation of the characteristic "indicator" band of zwitterionic gly-h and gly-d has also been reported.

Proton transfer aiding phase transitions in oxalic acid dihydrate under pressure.

Oxalic acid dihydrate, an important molecular solid in crystal chemistry, ecology and physiology, has been studied for nearly 100 years now. The most debated issues regarding its proton dynamics have arisen due to an unusually short hydrogen bond between the acid and water molecules. Using combined in situ spectroscopic studies and first-principles simulations at high pressures, we show that the structural modification associated with this hydrogen bond is much more significant than ever assumed. Initially, under pressure, proton migration takes place along this strong hydrogen bond at a very low pressure of 2 GPa. This results in the protonation of water with systematic formation of dianionic oxalate and hydronium ion motifs, thus reversing the hydrogen bond hierarchy in the high pressure phase II. The resulting hydrogen bond between a hydronium ion and a carboxylic group shows remarkable strengthening under pressure, even in the pure ionic phase III. The loss of cooperativity of hydrogen bonds leads to another phase transition at ∼ 9 GPa through reorientation of other hydrogen bonds. The high pressure phase IV is stabilized by a strong hydrogen bond between the dominant CO2 and H2O groups of oxalate and hydronium ions, respectively. These findings suggest that oxalate systems may provide useful insights into proton transfer reactions and assembly of simple molecules under extreme conditions.

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure.

The study of hydrogen bonds near symmetrization limit at high pressures is of importance to understand proton dynamics in complex bio-geological processes. We report here the evidence of hydrogen bond symmetrization in the simplest amino acid-carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The dynamic proton sharing between semioxalate units results in covalent-like infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen-bonded supramolecular assemblies held together by these chains.

Polymorphic transitions of diborane at sub- and near-megabar pressures.

Recent theoretical investigations of high-pressure structures of diborane have yielded many intriguing predictions which have so far remained untested due to challenges of acquiring experimental data at extreme pressures. Here we report new pressure-induced polymorphic transformations of crystalline diborane observed between 36 and 88 GPa by in situ Raman spectroscopy and interpreted using electronic structure calculations. Two previously unknown phase transitions are identified near 42 and 57 GPa, as evidenced by significant changes in the Raman profiles. The corresponding new phases, labeled IV and V, consist of B2H6 molecules and have triclinic unit cells (P), as deduced through evolutionary structure search and comparison of experimental and simulated Raman spectra. Density-functional calculations suggest that, at pressures above 110 GPa, phase V will form new molecular structures consisting of one-dimensional (BH3)n chains and will become metallic near 138 GPa. Our findings make a significant contribution to the elucidation of the structures and properties of diborane in the near-megabar pressure region.

Hydrogen bonds and ionic forms versus polymerization of imidazole at high pressures.

Imidazole (C3H4N2) is an important biomaterial for material research and applications. Our high-pressure Raman spectroscopic investigations combined with ab initio calculations on crystalline imidazole suggest that C-H---X (X = N, π) and N-H---N intermolecular hydrogen bonding interactions largely influence the nature of its structural and polymeric transformations under pressure. At pressures around ∼10 GPa, the reduction in the N---N distances close to the symmetrization limit and the emergence of the spectral features of the cationic form indicate the onset of proton disorder. The pressure-induced strengthening of the "blue-shifting hydrogen bonds" C-H---X (X = N, π) in this compound is revealed by the Raman spectra and the ab initio calculations. No polymer phase was retrieved on release from the highest pressure of 20 GPa in this study.

Conformation and hydrogen-bond-assisted polymerization in glycine lithium sulfate at high pressures.

The conformation of glycine has been a subject of extensive research for the past several years. As glycine exists in zwitterionic form in liquids and solids, the experimental observations of its neutral conformation are very limited. The complexes of glycine are simple prototypes to study the conformational properties of glycine. We have investigated the high-pressure behavior of glycine lithium sulfate (GLS), a semiorganic complex of glycine using X-ray diffraction, Raman spectroscopy, and density functional theory (DFT)-based first principles calculations. Our Raman studies and DFT calculations suggest formation of an intramolecular hydrogen bond at higher pressures. Subsequent to a structural transformation to a new high-pressure phase at ∼9 GPa, the observed spectral changes in the Raman spectra above 14 GPa indicate possible conformational change of glycine from zwitterionic to neutral form. At pressures above 18 GPa, the characteristic features in the Raman spectra and the X-ray diffraction patterns suggest transformation to a hydrogen-bond-assisted polymeric phase with intermediate range order.

Hydrogen bonds and conformations in ethylene glycol under pressure.

Ethylene glycol (EG) is a model system for studying complex hydrogen bonding networks in biological compounds such as polysaccharides and sugars. Using in situ high-pressure Raman and infrared absorption spectroscopy, we have investigated the pressure induced variation in the conformations and hydrogen bonding interactions in this compound up to 10 GPa. The high-pressure behavior of Raman modes suggests that EG exists as a liquid with a mixture of trans and gauche conformations up to 3.1 GPa. At ∼4 GPa, a liquid-solid transition is evidenced by the appearance of external Raman modes as well as visual observation. Raman and infrared spectra of EG at high pressures indicate that new hydrogen bonding networks are formed prior to liquid-solid transition and the high pressure phase is stabilized to gauche conformation at pressures above 5 GPa.

Ring-opening polymerization in carnosine under pressure.

Our high pressure Raman scattering experiments on carnosine, a dipeptide of L-histidine and ß-alanine, show pressure induced ring-opening polymerization involving the imidazole ring. While the onset of polymeric transformation is found to be at ∼2.8 GPa, a substantial fraction of the monomeric solid becomes polymerized by 12 GPa. On release to ambient conditions, the observed Raman spectra do not contain any of the Raman modes of the ambient phase.

Pressure-induced structural transformations in bis(glycinium)oxalate.

We report in situ high-pressure Raman spectroscopic as well as X-ray diffraction measurements on bis(glycinium)oxalate, an organic complex of glycine, up to 35 GPa. Several spectral features indicate that at ∼1.7 GPa it transforms to a new structure (phase II) which is characterized by the loss of the center of symmetry and the existence of two nonidentical glycine molecules. Across the transition, all the N-H···O bonds are broken and new weaker N-H···O bonds are formed. Our high-pressure X-ray diffraction studies support the possibility of a non-centrosymmetric space group P2(1) for phase II. Across 5 GPa, another reorganization of N-H···O hydrogen bonds takes place along with a structural transformation to phase III. The C-C stretching mode of oxalate shows pressure-induced softening with large reduction from the initial value of 856 to 820 cm(-1) up to 18 GPa, and further softening is hindered at higher pressures.

Pressure-induced polymerization of acrylic acid: a Raman spectroscopic study.

We report here the first in-situ Raman spectroscopic study of pressure-induced structural and polymeric transformations of acrylic acid. Two crystalline phases (I and II) were observed upon compression to approximately 0.3 and approximately 2.7 GPa. Phase I can be characterized with a single s-cis molecular conformation with possibly a similar structure to the low-temperature phase, while phase II suggests significantly enhanced molecular interactions toward polymerization and structural disordering. Beyond approximately 8 GPa, spectroscopic features indicate the onset of polymerization. The high-pressure polymeric phase together with the pressure-quenched materials was examined and compared with two commercial acrylic acid polymers using Raman spectroscopy. The characteristics of polymeric acrylic acid and their transformation mechanisms as well as the implications of hydrogen-bonding interactions are discussed.

In situ high-pressure study of diborane by infrared spectroscopy.

As the simplest stable boron hydride in its condensed phase, diborane exhibits an interesting structural chemistry with uniquely bridged hydrogen bonds. Here we report the first room-temperature infrared (IR) absorption spectra of solid diborane compressed to pressures as high as 50 GPa using a diamond anvil cell. At room temperature and 3.5 GPa, the IR spectrum of diborane displays rich sharply resolved fundamentals and overtones of the IR active bands, consistent with the previous low-temperature IR measurements of condensed diborane at ambient pressure. When compressed stepwise to 50 GPa, several structural transformations can be identified at pressures of approximately 3.5 GPa, approximately 6.9 GPa and approximately 14.7 GPa, as indicated by the changes in the band profile as well as the pressure dependence of the characteristic IR modes and bandwidths. These transformations can be interpreted as being enhanced intermolecular interactions resulting from compression. The geometry of the four-member ring of B(2)H(6), however, does not seem to be altered significantly during the transformations and the B(2)H(6) molecule remains chemically stable up to 50 GPa.

Pressure-induced transformations in diborane: a Raman spectroscopic study.

As a classical electron-deficient molecule with unique hydrogen bridge bonding, diborane has created considerable interest in the structural chemistry. We report here the first evidence of pressure-induced structural transformations of diborane probed by in situ Raman spectroscopy. At pressures around 4 GPa, diborane undergoes a liquid-solid phase transformation to a new high-pressure phase I with a possible structure similar to the low-temperature phase. When compressed to above 6 GPa, the spectral features, such as doubling of the lattice modes, appearance of several new internal modes, and emergence of a new ring stretch mode, indicate the structural transformation to another new high-pressure phase II. This phase has a possible extended network structure of higher hydrides of borane. At pressures above 14 GPa, diborane transforms to yet another high-pressure phase III. All of the observed pressure-induced structural transformations are completely reversible upon decompression.

High pressure Raman spectroscopic study of deuterated gamma-glycine.

Raman spectroscopic investigations of deuterated gamma-glycine, carried out up to 21 GPa, indicate emergence of a new phase, which is similar to the delta-phase, reported to be formed from the undeuterated gamma-glycine at 3 GPa and the transformation to this phase is complete by 6 GPa. Observed pressure -induced variations in CD2 and N-D stretching modes indicate significant changes in the hydrogen-bonding interactions. Around approximately 15 GPa, splitting of CD2 and C-C stretching modes and discontinuous changes in the slope of CO2 and N-D stretching modes indicate another structural rearrangement across this pressure. The Raman spectra of retrieved phase at ambient conditions suggest that it may be a layered structure similar to the zeta-phase reported to be formed on decompression of the nondeuterated delta-glycine.