Ranjan K. Singh

Structural and optical properties of sol-gel derived nanocrystalline Fe-doped ZnO

Effects of Fe doping on the structural, optical and magnetic properties of sol-gel derived Zn1−xFexO (0 ≤ x ≤ 0.06) nanoparticles have been investigated. The single-phased wurtzite structure has been characterized by XRD analysis. The nanocrystalline nature of the samples and their crystallinity has been confirmed by TEM measurements. Raman spectroscopy has been employed to study the crystalline quality and structural disorders. FTIR analysis confirms the wurtzite structure and formation of tetrahedral coordination of the oxygen ions surrounding the zinc ions. UV-Vis measurements show that the band gap is fast blue shifted in lower Fe-concentrations and after that slowly red shifted in higher Fe-concentrations. This may be attributed to the Burstein-Moss effect which is prevailed throughout the range of Fe-doping and sp-d exchange interaction that plays a role in higher Fe-concentration. Magnetic hysteresis measurement (M-H) shows that Zn0.96Fe0.04O exhibits ferromagnetic behavior at room temperature.

Raman study of vibrational dephasing in hydrogen-bonded binary and ternary complexes of C6H5Cl and methanol

Abstract The shifts and linewidth changes of three vibrational modes of C 6 H 5 Cl were studied in hydrogen-bonded binary mixtures C 6 H 5 Cl+CH 3 OH at various mole fractions of C 6 H 5 Cl, C =0.05, 0.1, 0.2, 0.3, 0.5, 0.7 and 0.9. The study reveals that the phenomena of motional narrowing and diffusion play simultaneous roles in the dephasing of the ring breathing and trigonal bending modes at ∼1000 and ∼1024 cm −1 , respectively. Minima at C =0.7 and 0.2 in the Γ L versus C plot are explained by the formation of hydrogen-bonded dimer, C 6 H 5 Cl⋯HOCH 3 , and trimer, C 6 H 5 Cl⋯(HOCH 3 ) 2 , complexes. The dephasing of a third mode at ∼700 cm −1 is explained using the indirect dephasing model of Fischer–Laubereau.

Vibrational Dynamics in Hydrogen-Bonded (Pyridine + Water) Complexes Studied by Spectrally Resolved Femtosecond CARS

The technique of femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS) was used to study the vibrational dephasing dynamics in hydrogen-bonded (pyridine + water) complexes as a function of pyridine mole fraction x(Py). By detecting the spectrally resolved CARS signal, a mapping of the vibrational coherence dynamics of ring modes at ∼990 and ∼1030 cm-1 was achieved. The quantum beatings among different modes of the hydrogen-bonded network are clearly exhibited in the transient signal. Its spectral analysis yields the involved frequencies by employing FT methods to the time domain signal. The oscillatory pattern in the CARS transients are adequately explained when contributions from several modes are accounted for. The individual vibrational dephasing times (T2) in the range ∼1.7-5.3 ps were obtained with high precision. The assumption of homogeneous broadening of the ring modes under investigation is made for those mole fractions, where the pyridine molecules have a surrounding consisting mainly of only one species. According to the concentration profile this is valid for x(Py)=1.00 (neat pyridine) and x(Py)=0.20, for which the hydrogen-bonded network is dominated by PyWn species i.e. water-water hydrogen bonding). For both mole fractions the condition (Δν˜1/2)hom=1/πcT2, which relates line width in wavenumber from Raman data and dephasing time from time-resolved data, is fulfilled. Inhomogeneous broadening is assumed for the intermediate concentration x(Py)=0.60 since several distinct species, such as: Py2W, PyW, and PyW2 are known to co-exist leading to complex chemical equilibria.

Hydrogen-bonding and self association investigated in the binary mixture (C6H5CN + CH3OH)via concentration dependent Raman study of the CN stretching mode of benzonitrile (C6H5CN) and ab-initio calculations

The Raman study of (C6H5CN + CH3OH) binary mixture has been presented. The isotropic part of the Raman spectra, Iiso are analyzed in the CN stretching region. For neat C6H5CN, the Iiso shows a double peak structure, which has been explained in terms of self association. A quantum chemical calculation on the optimized structures and wavenumbers of different modes of neat C6H5CN, self associated C6H5CN and the hydrogen-bonded C6H5CN⋯HOCH3 complex reveals that the wavenumber position of the CN stretching mode is blue shifted due to both the self association and the hydrogen-bonding with CH3OH. The Raman spectra of binary mixtures with different mole fractions of the reference system (C6H5CN), C = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, as well as neat liquid have been explained in terms of self association and hydrogen-bonding. A variation of intensity ratio of the peak assigned to the hydrogen-bonded complex to the main band with concentration exhibits a regular trend. The dephasing of the CN stretching mode in the free C6H5CN molecules seems to be governed predominantly by the concentration fluctuation model, but other effects like diffusion and motional narrowing may also have some small influence.

Synthesis, characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4′-n-alkoxybenzylideneamino)benzoate

A new series of Schiff base calamitic liquid crystal; methyl 4-(4′-n-alkoxybenzylideneamino)benzoate (MABAB), H2n+1C n OC6H4C(H)=NC6H4COOCH3 (n = 6, 8, 10, 12, 14, 16) has been synthesised and characterised by elemental analyses, Fourier transform infrared spectroscopy (FT-IR), 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy. The mesomorphic properties of these compounds were studied by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). All members of the series exhibit enantiotropic smectic A (SmA) mesophase. Temperature-dependent micro-Raman study of one of the members, MABAB-10 has been employed to identify phase transitions and the molecular rearrangement therein. Analysis of Raman marker bands; C–H in-plane bending, C–C stretching of phenyl rings and –C(H)=N– linking group of core confirms the transitions clearly as observed through DSC and POM. An in situ Raman measurement of C–H in-plane bending mode has also been performed to visualise the molecular changes more clearly. The Raman study gives an evidence of induced co-planarity of rings at Cr→SmA phase transition. The density functional theoretical (DFT) optimisation of monomer, dimer and rotational conformer of MABAB-10 also support the induced co-planarity at Cr→SmA phase transition.

A comparative laser Raman study on TB4A, TB7A and TB10A

The basic structural differences between three thermotropic liquid crystals TB4A, TB7A and TB10A have been interpreted by analysing their Raman spectra. Three spectral regions, 925-1025, 1275-1475 and 1525-1650 cm-1, have been chosen in this context to incorporate their structural dissimilarity. The induced planarity of the backbone, as an effect of increasing chain length, has been studied by observing the changes in the respective Raman spectra of the three compounds. A tentative assignment of all the modes observed in the region 400-1700cm-1 is made in this context. The molecular conformations of the three compounds have been predicted in their solution states, and are compared with the molecular environment that exists in their liquid crystalline states. Structural disorder at the solid-SmG transition is discussed and the changes are incorporated systematically.

Temperature-Dependent Raman Study of the Smectic to Nematic Phase Transition and Vibrational Analysis Using Density Functional Theory of the Liquid Crystalline System 4-Decyloxy Benzoic Acid:

Room-temperature Raman spectra of the thermotropic liquid crystalline system, 4-decyloxy benzoic acid (4DBA) have been recorded and the experimentally observed bands are assigned by density functional theory (DFT) for the first time. The C–O and C–C stretching and C–H in-plane bending modes of the phenyl ring and C=O stretching modes of the -COOH group are the marker bands for the smectic (S) → nematic (N) and nematic (N) → isotropic (I) transitions for this system. The temperature-dependent Raman spectra for these bands in the heating cycle clearly characterize the S→N and the N→I transition over a range <1 °C, which is much better than the earlier range of 23 °C for S→N and 26 °C for the N→I transition. The ∼773, ∼807, ∼881, and ∼1146 cm−1 bands disappear, whereas a band at ∼830 cm−1 appears at the S→N transition. The relative intensity of the ∼1257 and ∼1280 cm−1 bands distinguishes the three phases, namely smectic, nematic, and isotropic, in 4DBA. The variation of line width and peak wavenumber of the ∼1128 and ∼1168 cm−1 bands also clearly shows the two transitions. The molecular reorientation at the transition and the effect of local fields present in the liquid crystalline mesophases are also briefly discussed on the basis of changes in intensity, linewidth and peak wavenumber with temperature.

Ni(II) and Co(III) complexes of 5-methyl-1,3,4-thiadiazole-2-thiol: syntheses, spectral, structural, thermal analysis, and DFT calculation

Two new complexes, [Ni(en)2(mtt)2] (1) and [Co(en)2(mtt)2](mtt) (2) (Hmtt = 5-methyl-1,3,4-thiadiazole-2-thiol and en = ethylenediamine), have been synthesized and characterized by various physicochemical techniques. Complexes 1 and 2 crystallize in monoclinic and orthorhombic system with space groups P 21/n and P 21 21 21, respectively. The molecular structures of 1 and 2 show that the metal ions are six-coordinate bonded through four equatorial nitrogens of two en and two axial nitrogens of mtt ligands. The crystal structures of the complexes reveal that mtt is present in thione form and bound to the metal ion through the thiadiazole nitrogen. The crystal structures of the complexes are stabilized by various intermolecular hydrogen bonding providing supramolecular architecture. Complex 2 is also stabilized by weak π···π interactions occurring between two thiadiazole rings. The bioefficacies of the ligand and complexes have been examined against the growth of bacteria to evaluate their antimicrobial potential. The biological results suggest that 2 is more active than the ligand and 1 against the tested bacteria. The geometries of the ligand and the complexes have been optimized by the DFT method and the results are compared with the X-ray diffraction data. The Co(III) complex exhibits an irreversible Co(III)/Co(II) process while the Ni(II) complex displays quasi-reversible Ni(II)/Ni(III) redox processes with large peak separation as compared to that expected for a one electron process which is thought to be coupled with some chemical reaction. Graphical abstract

Effect of deuteration on hydrogen bonding: A comparative concentration dependent Raman and DFT study of pyridine in CH3OH and CD3OD and pyrimidine in H2O and D2O

The relative effect of hydrogen bonding of pyrimidine (Pyr) in H(2)O/D(2)O and pyridine (Py) in CH(3)OH/CD(3)OD has been analyzed using Raman Difference Spectroscopic (RDS) technique and DFT calculations. This study is focused on analyzing the concentration dependent variation of linewidth, peak position and intensity of ring breathing mode of Py and Pyr. The ring breathing mode of Pyr in H(2)O and D(2)O has three components; due to free Pyr, lighter complexes of mPyr+nH(2)O/D(2)O and heavier complexes of mPyr+nH(2)O/D(2)O. The pyridine molecules, however, show only two components in CH(3)OH and CD(3)OD. Of these two components, one corresponds to free Py and the other inhomogeneously broadened profile corresponds to all mPy+nCH(3)OH/CD(3)OD complexes. The variation of peak position and linewidth establishes the role of dipole moment of complexes and the diffusion in the mixture. In case of CD(3)OD solution splitting was observed in ∼1030 cm(-1) band of Py, where an additional band at ∼1034 cm(-1) appears at x(Py) ≤ 0.4. However, this band remains single at all concentrations in case of CH(3)OH solvent.

Solvent dependent frequency shift and Raman noncoincidence effect of SO stretching mode of Dimethyl sulfoxide in liquid binary mixtures

The isotropic and anisotropic Raman peak frequencies of S=O stretching mode of Dimethyl sulfoxide (DMSO) have been discussed in different chemical and isotopic solvent molecules using different mechanisms. The shifting of peak frequency in further dilution of DMSO with solvent molecule is observed for all solvents. Transition dipole - transition dipole interaction and hydrogen bonding may play a major role in shifting of peak frequencies. The non-coincidence effect (NCE) of DMSO was determined for all the solvents and compared with four theoretical models such as McHales model, Mirones modification of McHales model, Logans model and Onsager-Fröhlich dielectric continuum model respectively. Most of the theoretical models are largely consistent with our experimental data.