• R R Choudhury

      Articles written in Pramana – Journal of Physics

    • Phase transition in triglycine family of hydrogen bonded ferroelectrics: An interpretation based on structural studies

      R R Choudhury R Chitra P U Sastry Amit Das M Ramanadham

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      Using the crystal structure, a comprehensive interpretation of the origin of ferroelectricity in the hydrogen bonded triglycine family of crystals is given. Our detailed analysis showed that the instability of nitrogen double well potential plays a driving role in the mechanism of the ferroelectric transitions in these crystals.

    • Zinc (tris) thiourea sulphate (ZTS): A single crystal neutron diffraction study

      P U Sastry R Chitra R R Choudhury M Ramanadham

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      The crystal structure of ZTS has been determined by neutron diffraction with a finalR-value of 0.026. Using the structural parameters, the contributions from the structural groups to the linear optical susceptibility and linear electro-optic coefficients have been evaluated. Results showed a significant contribution from the hydrogen bonds in the structure.

    • Hydrogen bonding in oxalic acid and its complexes: A database study of neutron structures

      R Chitra Amit Das R R Choudhury M Ramanadham R Chidambaram

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      The basic result of carboxylic group that the oxygen atom of the -OH never seems to be a hydrogen bond acceptor is violated in the cases, namely urea oxalic acid and bis urea oxalic acid complexes, where the hydroxyl oxygen atom is an acceptor of a weak N—H… O hydrogen bond. The parameters of this hydrogen bond, respectively in these structures are: hydrogen acceptor distance 2.110 Å and 2.127 Å and the bending angle at hydrogen, 165.6° and 165.8°. The bond strength around the hydroxyl oxygen is close to 1.91 valence units, indicating that it has hardly any strength left to form hydrogen bonds. These two structures being highly planar, force the formation of this hydrogen bond. As oxalic acid is the common moiety, the structures of the two polymorphs, α-oxalic acid and β-oxalic acid, also were looked into in terms of hydrogen bonding and packing.

    • Structural changes during the unfolding of Bovine serum albumin in the presence of urea: A small-angle neutron scattering study

      Amit Das R Chitra R R Choudhury M Ramanadham

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      The native form of serum albumin is the most important soluble protein in the body plasma. In order to investigate the structural changes of Bovine serum albumin (BSA) during its unfolding in the presence of urea, a small-angle neutron scattering (SANS) study was performed. The scattering curves of dilute solutions of BSA with different concentrations of urea in D2O at pH 7.2 ± 0.2 were measured at room temperature. The scattering profile was fitted to a prolate ellipsoidal shape (a, b, b) of the protein witha = 52.2 Å andb = 24.2 Å. The change in the dimensions of the protein as it unfolds was found to be anisotropic. The radius of gyration of the compact form of the protein in solution decreased as the urea concentration was increased.

    • Thiourea-doped ammonium dihydrogen phosphate: A single crystal neutron diffraction investigation

      A Jayarama M R Suresh Kumar S M Dharmaprakash R Chitra R R Choudhury

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      Thiourea-doped ammonium dihydrogen phosphate (TADP) exhibits nonlinear optical property and the second harmonic generation efficiency of these crystals is three times that of pure ammonium dihydrogen phosphate (ADP) crystal. In this context, the study of structural distortion in the thiourea-doped ADP crystal is significant, hence single neutron diffraction investigations were undertaken. The final 𝑅-factors are: $R[F^{2} > 2\sigma(F^{2})] = 0.11$, Goodness of fit $(S) = 1.15$. Though the dopant could not be located from the difference Fourier map, the cell parameters ($a = b = 7.531(3)$ Å, $c = 7.544(5)$ Å) were found to be significantly greater than that of pure ADP at RT ($a = b = 7.502(1)$ ̊Å, $c = 7.546(1)$ ̊Å). This indicates that the dopant concentration in the crystals is small but enough to bring changes in the overall average structure.

    • Phase transitions in liquid crystal 6O.4 (p-𝑛-hexyloxybenzylidine-pˊ-𝑛-butylaniline)

      Siddharth Gautam R R Choudhury L Panicker S Mitra R Mukhopadhyay

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      DSC measurements on p-𝑛-hexyloxybenzylidine-pˊ-𝑛-butylaniline (6O.4) showed that the crystalline to liquid crystalline ($K–S_{\text{H}}$) transition at 33.7°C observed in the heating cycle does not revert even when the sample is cooled down to −100°C. Hence it is inferred that a physically stable supercooled liquid crystalline phase is formed on cooling 6O.4. To investigate the $K–S_{\text{H}}$ transition further the techniques of polarized microscopy and X-ray diffraction were used which concurred with the DSC results. Quasi-elastic neutron scattering measurements carried out to study the re-orientational motions in the ordered phases of 6O.4 (𝐾 and $S_{\text{H}}$) show that while in the crystalline phase (at RT) the re-orientational motion is found to involve only the core of the molecule, in the $S_{\text{H}}$ phase (at 45°C) the dynamics involves the whole molecule and this motion is found to persist even when the sample cools back to room temperature corroborating the results of the DSC, microscopy and X-ray diffraction.

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