Browsing by Author "Deepa Singh"

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Concentration-dependent surface-enhanced Raman scattering and molecular dynamic study of dimethyl formamide
(John Wiley and Sons Ltd, 2007) Shivangi Mishra; Animesh K. Ojha; Deepa Singh; Rajeev R. Prasad; Sunil K. Srivastava; Ranjan K. Singh
A concentration-dependent Raman study of dimethyl formamide (DMF) in Ag nanocolloidal solution was carried out in order to observe the effect of concentration on the surface enhancement mechanism. The Raman spectra in the region 900-2200 cm-1 comprising four prominent Raman modes were measured experimentally and analyzed at five different concentrations: 1, 3, 5, 7, 10 mM, and in neat DMF. In order to find the possible configurations of DMF + Ag complexes, density functional theory (DFT) calculations were carried out taking one, three and five Ag atom clusters. The Raman spectra of unconjugated DMF, DMF + Ag and DMF + 3Ag complexes were calculated theoretically to assign the vibrational modes under consideration more accurately and to understand the wavenumber shift and change in intensity observed in experimental measurements. Water present in the colloidal solution may also conjugate with DMF and its complexes with Ag. In order to see the influence of water on the wavenumber shift and intensity changes, we have also obtained the optimized structures and Raman modes of DMF + water and DMF + water + Ag complexes. Good agreement between the experimental and theoretical wavenumber shifts has been obtained by using B3LYP functional theory and CEP-31G basis set for the DMF + Ag complex. The experimental results suggest that the SERS enhancement is concentration-dependent. The concentration-dependent linewidth shows the existence of the phenomena of motional narrowing and diffusion dynamics in the colloidal solution. Copyright © 2007 John Wiley & Sons, Ltd.
DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment
(2007) Deepa Singh; Sunil K. Srivastava; Animesh K. Ojha; B.P. Asthana; Ranjan K. Singh
Optimized geometries and vibrational spectra of pyridine (Py), pyridazine (Py1,2), pyrimidine (Py1,3), pyrazine (Py1,4) and their complexes with water molecules have been calculated with a view to look into the strength of hydrogen bonds, binding energies and vibrational wavenumbers of hydrogen bond bridging modes. The counterpoise corrected binding energies for different complexes are also calculated. The results of DFT calculations have been used to draw the potential energy curve and to calculate the amplitude of stretching vibration of hydrogen bond bridging modes. The three types of hydrogen bonds: N...H{single bond}O, O...H{single bond}O, and O...H{single bond}C have been studied. The relative hydrogen bond strengths and vibrational wavenumbers of these bridging modes have been explained in terms of redistribution of electronic charges on individual atoms. © 2007 Elsevier B.V. All rights reserved.
Effect of neodymium doping on structural, electrical and magnetic properties of multiferroic GdMn2O5
(Springer New York LLC, 2017) Deepa Singh; Vandana Gupta; Ranjan K. Singh; K.K. Bamzai
Neodymium doped gadolinium manganate with general composition (Nd0.1Gd0.9Mn2O5) was prepared by co-precipitation method. Microstructural and compositional analysis has been carried out by X-ray diffraction and scanning electron microscopy. The optical studies have been carried out by Raman and FTIR. The electrical properties studied include dielectric constant, dielectric loss, ac conductivity and activation energy in the temperature range 20–400 °C. The shift in the dielectric peak towards higher temperature side with increasing frequency indicates frequency dispersion and suggesting the relaxational behaviour of the material. Frequency dependence of ac conductivity obeys the universal power law. The value of activation energy depends on increase in frequency. The room temperature magnetic behaviour has been analyzed from the magnetic field dependent magnetization curve. The grown material exhibits the paramagnetic behavior at room temperature. © 2017, Springer Science+Business Media New York.
Effect of Ni addition on PNZT tungsten bronze structure
(Springer New York LLC, 2018) Rashmi Gupta; Deepa Singh; Ranjan K. Singh; K.K. Bamzai
Lead niobate zirconate titanate (PNZT) with general formula 0.5PbNb2O6–0.5Pb(ZrxTi1-x)O3 (where x = 0.15, 0.25, 0.35) and lead nickel niobate zirconate titanate (PNNZT) with general formula Pb(NixNb0.5-xZr0.25Ti0.25)O3 (where x = 0.167, 0.250, 0.333) have been prepared by conventional solid state reaction. The phase formation has been studied by X – ray diffraction analysis. Raman investigations have been carried out in the 200–1000 cm−1 wave number range. The structural changes observed in the X – ray diffraction pattern has been further confirmed by Raman analysis. The major changes in the E(1TO), A1(1TO) and E + B1 phonon modes suggest the structural change from cubic to rhombohedral with increase in Ni concentration in PNNZT. The optical band gap has been calculated from UV – Vis absorption spectra. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
Fourier transform raman and DFT study of blue shift C-H stretching vibration of diazines on hydrogen bond formation
(Walter de Gruyter GmbH, 2011) Deepa Singh; Juergen Popp; Ranjan K. Singh
Diazines form various types of hydrogen bonds with water, wherein C-H groups are sometimes involved directly and sometimes indirectly. On hydrogen bond formation, the wavenumber of C-H stretching vibration are usually red shifted (normal hydrogen bond) but blue shift of C-H modes (anomalous hydrogen bond) is also possible in some cases. The Raman spectra of the C-H stretching bands of three diazines; pyrimidine, pyridazine, and pyrazine in pure form and at many concentrations in mole fractions of diazines in the mixture of diazines + water have been measured to analyze the wavenumber shifts experimentally. Theoretical wavenumber shifts have been calculated and NBO analysis has been performed using DFT methods to understand the cause of the shifts. All the four C-H stretching bands of diazines are blue shifted on dilution with water both experimentally and theoretically. The NBO calculations reveal that the cause of the shift is decrease in the charge density in the antibonding orbital of C-H bond on complex formation. © by Oldenbourg Wissenschaftsverlag, München.
Raman and DFT study of hydrogen-bonded 2- and 3-chloropyridine with methanol
(John Wiley and Sons Ltd, 2008) Deepa Singh; K. Vikram; Dheeraj K. Singh; W. Kiefer; Ranjan K. Singh
Precise polarized Raman measurements of 2-chloropyridine (2Clpy) in the region 560-1060 cm-1 and 3-chloropyridine (3Clpy) in the region 680-1080 cm-1 at different concentrations in mole fraction of methanol were made to calculate the isotropic part of the Raman spectra, which has contributions only from vibrational dephasing. A detailed analysis of the Raman spectra was carried out to see the variation of peak position and linewidth. The dephasing is mode specific. The trigonal bending mode of 3Clpy has two components when it is mixed with methanol. The relative intensities of these two bands are used to calculate the equilibrium constants. The ring-breathing mode of 3Clpy, on the other hand, remains single in the mixture. The appearance of a new band corresponding to the trigonal bending mode, as well as the nonappearance of that of the ring-breathing mode, is also shown by the density functional theory (DFT) study of gas phase andmethanol-solvated complexes. The vibrational dephasing time for the hydrogen-bonded ring-breathing mode is calculated from the linear Raman linewidth and peak position data. For othermodes, it was not possible to calculate the dephasing time because of the nonavailability of a suitable theoretical model. Contrary to 3Clpy, in 2Clpy the ring-breathing mode becomes a doublet but the trigonal bending mode remains single. It is seen that the hydrogen-bonding capacity of chloropyridines is highly influenced by the position of the Cl atom. Single and double components of these modes are also explained by DFT calculations. We obtained excellent match of the experimental and theoretical spectra with the B3LYP/6-31+G (d,p) method. Copyright © 2008 John Wiley & Sons, Ltd.
Raman and DFT study of static, dynamic interactions and isotope effect in pyridazine + H2O/D2O systems
(2009) Deepa Singh; Animesh K. Ojha; W. Kiefer; Ranjan K. Singh
Polarized Raman and density functional theory (DFT) approach have been applied to study the static and dynamic properties of pyridazine (PRD) in H2O(W) and D2O(D) environment. The possible hydrogen bonded (HB) complexes of PRD with H2O in gas phase and in the water solvation (using IEF-PCM and Onsager models) have been calculated using a B3LYP functional and 6-31+G(d,p)/6-311++G(d,p) basis sets. The static interaction in the PRD + H2O complex leads to a blue shift in all the Raman modes of PRD and red shift in the O-H modes of water. The IEF-PCM solvation model gives the Raman wavenumbers closest to the experimental values. Raman spectra of ∼962 and 1061 cm-1 mode of PRD in the mixture of PRD + H2O and PRD + D2O at different mole fractions of PRD (x) have been measured. A difference in the wavenumber shift of the two modes of PRD is observed experimentally when PRD is diluted with H2O and D2O. The wavenumber shift at maximum dilution (x = 0.1), however, takes the same value in both H2O and D2O. In view of the similar chemical properties of H2O and D2O, the difference in the trend of the wavenumber shift is not trivial. It has been explained on the basis of relative values of dipole moments of H2O, D2O, and conjugated molecules of PRD with H2O/D2O calculated theoretically and the role of larger diffusive property of H2O compared to D2O. The dynamical process in the mixture of PRD+ H2O/D2O is discussed by studying the variation of the linewidth with concentration. A theoretical model, which is based on the fact that the concentration in microscopic volume fluctuates, fits the experimental results nicely. © 2008 Elsevier B.V. All rights reserved.