M. N. Deo

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.

Behaviour of NTE Material Ag3[Co(CN)6] under Pressure

Recent discovery of colossal negative thermal expansion (NTE) behaviour in Silver Hexacyanocobaltate Ag3[Co(CN)6] has triggered interest among researchers to understand the basic mechanism causing such an unusual behaviour. This report presents our results on the behaviour of Ag3[Co(CN)6] using in-situ high pressure Raman and FTIR spectroscopy. The reported trigonal-monoclinic phase transition occurring in this compound is observed at 0.2 GPa. Upon increasing the pressure further, Ag3[Co(CN)6] becomes irreversibly amorphous above 13 GPa.

Spectroscopic studies of glycinium oxalate under pressure

We report the first high pressure study on Glycinium oxalate complex up to 6 GPa. High pressure infrared and Raman spectroscopic measurements have been carried out in the spectral range 600 – 3600 cm−1. Most of the skeletal and internal vibrational modes lying in the complex 600 – 1200 cm−1 region have been reassigned and their high pressure behavior has been discussed. The spectra indicate subtle pressure induced transformations at 0.7 GPa and 2.5 GPa, with significant changes in the hydrogen bonding network.

A synchrotron infrared absorption study of pressure induced polymerization of acrylamide

The hydrogen bonded dimeric structure of the model amide based molecular crystal acrylamide has been investigated under pressure using micro-spectroscopy, employing synchrotron infrared radiation up to 24GPa at room temperature. The high pressure spectra indicate systematic evolution of new features above 4GPa, which have been identified to be due to the emergence of a polymeric phase. The polymerization gets completed up to 16.8GPa and the observed changes are found to be irreversible upon the release of pressure. The behavior of NH stretching modes indicate that the uniform inter- and intra-dimeric interactions, rather than depicting a drastic reconstruction across the phase transition, show subtle modifications and become diverse in the high pressure polymeric phase.

High pressure Raman spectroscopic studies of Pt(II) complex trans-PtCl2(PEt3)2

We report here the high pressure Raman spectroscopy of the Pt(II) complex trans-PtCl2(P(C2H5)3)2 up to 5 GPa. We have analyzed the metal-ligand stretching modes as well as ligand internal vibrational modes of the complex under pressure. Many characteristic Raman modes show pressure induced splitting at pressures as low as 1 GPa. On careful analysis of the skeletal region, the new modes appeared could be corroborated with the position of corresponding modes in the infrared spectrum, thus indicating a loss of inversion symmetry in the trans- isomer.

Pump probe based Raman spectroscopic studies of PTFE under laser driven shock compression

High pressure spontaneous Raman spectroscopic studies of poly tetra fluro ethylene (PTFE) have been carried out under laser driven shock compression in confinement geometry target. The Raman modes under shock compression as a function of pressure were measured and compared with the corresponding Raman modes in static pressure experiments. Our results indicate that PTFE undergoes transition to phase III across this pressure.

Phase transition in metal–organic complex trans-PtCl2(PEt3)2 under pressure: insights into the molecular and crystal structure

Structural studies on Pt(II) complexes which have direct correlation with their stereochemistry and microscopic interactions are of immense technological, catalytic and pharmacological importance. Here, we present high-pressure studies on trans-PtCl2(PEt3)2 (Et = C2H5) using infrared (IR) and Raman spectroscopy combined with powder X-ray diffraction (XRD) studies. The ambient structure was solved using single-crystal XRD studies and was further optimized using density functional theory (DFT) simulations. It has been shown that subtle molecular reorientations result in structural phase transition at pressures as low as ∼0.8 GPa. The emergence of Raman active modes in the IR spectra and vice versa indicates the loss of inversion symmetry across the phase transition. The crystal structure of the high-pressure phase has been found to be non-centrosymmetric using XRD studies, which suggest a change in space group from P21/n to P21 across 0.8 GPa. The spectroscopy results also indicate strengthening of inter- and intramolecular C–H⋯Cl hydrogen bonds resulting in a hydrogen bonded supramolecular network. On further compression up to 4.7 GPa, another phase transformation has been detected. The structure was completely retrieved on release of pressure. Thus, the present findings provide sound evidence of intramolecular rearrangements playing a decisive role in tuning bonding and structural characteristics and hence the physicochemical properties of Pt(II) complexes under varying environments.

High pressure infrared spectroscopy of Pt(II) complex cis-PtCl2(PEt3)2

We report here the high pressure infrared spectroscopy of the Pt(II) complex cis- PtCl2(P(C2H5)3)2 up to 12 GPa. We have analyzed the various ligand related vibrational modes of the complex under pressure. It has been observed that the cis- isomer, which preserved its structure on lowering the temperature, shows change in the rate of variation in vibrational frequencies at ~1 GPa. However, no new modes appeared upon compression, unlike in the trans isomer, up to the highest pressure. These observations indicate subtle pressure induced structural changes taking place in cis- PtCl2(P(C2H5)3)2. The highly complex C-H stretching spectral region has also been analyzed and the frequency variation of various modes has been described.

In situ high pressure study of an elastic crystal by FTIR spectroscopy

We performed an in situ high pressure FTIR spectroscopic study on a 2,3-dichlorobenzylidine-4-bromoaniline (DBA) crystal at pressures ranging from ambient pressure to 13.8 GPa at room temperature. The variations in the stretching frequency of the aromatic C–H, H–CN and C–Cl bands on compression showed significant molecular movement in the DBA crystal. Decompression was monitored on the aliphatic and aromatic C–H stretches which clearly show the reversibility of the molecular movements in the crystal lattice.

Time-Resolved Vibrational Spectroscopy of Polytetrafluoroethylene Under Laser-Shock Compression

Shock-wave-induced high pressure and nanosecond time-resolved Raman spectroscopic experiments were performed to examine the dynamic response of polytetrafluoroethylene (PTFE) in confinement geometry targets. Time-resolved Raman spectroscopy was used to observe the pressure-induced molecular and chemical changes on nanosecond time scale. Raman spectra were measured as a function of shock pressure in the 1.2–2.4 GPa range. Furthermore, the symmetric stretching mode at 729 cm–1 of CF2 was compared to corresponding static high-pressure measurements carried out in a diamond anvil cell, to see if any general trend can be established. The symmetric stretching mode of CF2 at 729 cm–1 is the most intense Raman transition in PTFE and is very sensitive to change in pressure. Therefore, it can also be utilized as a pressure gauge for large amplitude shock wave compression experiments. A maximum blueshift of 12 cm–1 for the 729 cm–1 vibrational mode has been observed for the present experimental pressure range. A comparative study on the similarities and differences from the earlier work has been done in detail. One-dimensional radiation hydrodynamic simulations were performed to validate our shock compression results and are in very good agreement.