Browsing by Author "P.R. Alapati"

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A new phase transition in supercooled TBDA probed by low-temperature Raman study
(John Wiley and Sons Ltd, 1999) S.K. Dash; Ranjan K. Singh; B.P. Asthana; P.R. Alapati; A.L. Verma
Raman spectra of terephthalidenebis-p-n-decylaniline (TBDA) were recorded in the regions 925-1000 and 1120-1220 cm-1 from room temperature down to 20 K during both cooling and heating cycles. The subtle changes in the spectral features of the bands at ca 975 and ca 1195 cm-1 at 47 K were attributed to a hitherto unreported stable-metastable phase transition in TBDA. The dynamics of the new phase transition are explained in terms of splitting of the non-planar mode at ca 975 cm-1 due to strong steric hindrances caused by increased intermolecular interactions due to close packing in the low-temperature phase. Copyright © 1999 John Wiley & Sons, Ltd.
Solvent dependent frequency shift and Raman noncoincidence effect of S=O stretching mode of Dimethyl sulfoxide in liquid binary mixtures
(2013) Ganesh Upadhyay; Th. Gomti Devi; Ranjan K. Singh; A. Singh; P.R. Alapati
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 McHale's model, Mirone's modification of McHale's model, Logan's model and Onsager-Fröhlich dielectric continuum model respectively. Most of the theoretical models are largely consistent with our experimental data. © 2013 Elsevier B.V. All rights reserved.
Temperature dependent Raman study of SB → SC transition in liquid crystalline compound N-(4-n-pentyloxybenzylidene)-4′-heptylaniline (5O.7)
(2010) K. Vikram; P.R. Alapati; Ranjan K. Singh
Temperature dependent Raman study of C-H in-plane bending mode (∼1163 cm-1 and ∼1190 cm-1) and C-C stretching mode of phenyl ring (∼1571 cm-1 and ∼1594 cm-1) of N-(4-n-pentyloxybenzylidene)-4′-heptylaniline (5O.7) has been done. Vibrational assignment and potential energy distribution (PED) of individual modes have been calculated employing density functional theory (DFT) for the first time. The SB → SC transition is nicely depicted in the variation of the linewidth of the ∼1163 cm-1 band and the peak position of ∼1594 cm-1 band with temperature. Because of a small amount of charge density transfer from the core part to the alkyl chain region, the ∼1163 cm-1 band shifts towards lower wavenumber side whereas the ∼1190 cm-1 band towards higher wavenumber side at SB → SC transition. The ∼1571 cm-1 and ∼1594 cm-1 bands are assigned as 8a and 8b modes, whose relative intensity variation with temperature gives the evidence of increased possibility of C-H bending motion of the linking group and the C-C stretching of the alkyl chain in SC phase. © 2010.