Browsing by Author "K. Vikram"

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A new low temperature solid modification in 1-isothiocyanato-4-(trans‑4-propylcyclohexyl)benzene (3CHBT) probed by Raman spectroscopy and quantum chemical calculations
(Elsevier B.V., 2018) K. Vikram; Ranjan K. Singh; Satyendra Nath Gupta
Raman spectra of 1-isothiocyanato-4-(trans 4-propylcyclohexyl)benzene (3CHBT) were studied in the region, 1450–2300 cm− 1 at twenty two different temperatures in the range, 83–293 K in cooling and heating cycles. All bands in this region were critically evaluated in term of linewidth, peak position and relative intensity. Raman bands at ~ 2085 cm− 1 and ~ 2120 cm− 1 shows clear evidence of a solid modification through anomaly in temperature dependence peak positions and linewidths variation in the temperature range 173–203 K. A detailed analysis of the variation of the linewidth and peak position of the two component bands leads to the conclusion that the molecular/dimer arrangement in crystalline packing changed between 173 K and 203 K. This solid modification was also analyzed at the molecular level. The ~ 2085 cm− 1 and ~ 2120 cm− 1 bands were corresponded as parallel and anti-parallel dimers of 3CHBT, which are identified as dimer I (D1) and dimer II (D2), respectively. The structures of both the dimers have been optimized by quantum chemical calculations employing density functional theoretic (DFT) methods. © 2017
Crystal→nematic phase transition in the liquid crystalline system 1-isothiocyanato-4-(trans-4-propylcyclohexyl) benzene (3CHBT) probed by temperature-dependent micro-Raman study and DFT calculations
(John Wiley and Sons Ltd, 2010) K. Vikram; Nicolae Tarcea; S.K. Srivastava; B.P. Asthana; J. Popp; Ranjan K. Singh
Raman spectra of 3CHBT in unoriented form were recorded at 14 different temperature measurements in the range 25-55°C, which covers the crystal→nematic (N) phase transition, and the Raman signatures of the phase transition were identified. The wavenumber shifts and linewidth changes of Raman marker bands with varying temperature were determined. The assignments of important vibrational modes of 3CHBT were also made using the experimentally observed Raman and infrared spectra, calculated wave numbers, and potential energy distribution. The DFT calculations using the B3LYP method employing 6-31G functional were performed for geometry optimization and vibrational spectra of monomer and dimer of 3CHBT. The analysis of the vibrational bands, especially the variation of their peak position as a function of temperature in two different spectral regions, 1150-1275 cm-1 and 1950-2300 cm-1, is discussed in detail. Both the linewidth and peak position of the (-C-H-) in-plane bending and ν(N=C=S) modes, which give Raman signatures of the crystal → N phase transition, are discussed in detail. Themolecular dynamics of this transition has also been discussed. We propose the co-existence of two types of dimers, one in parallel and the other in antiparallel arrangement, while going to the nematic phase. The structure of the nematic phase in bulk has also been proposed in terms of these dimers. The red shift of the ν(N=C=S) band and blue shift of almost all other ring modes show increased intermolecular interaction between the aromatic rings and decreased intermolecular interaction between two -NCS groups in the nematic phase. © 2009 John Wiley & Sons, Ltd.
Dynamics and mechanism of the Crystal II → smecticG phase transition in TB7A by a temperature-dependent micro-Raman study and DFT calculations
(John Wiley and Sons Ltd, 2009) K. Vikram; Sunil K. Srivastava; Animesh K. Ojha; S. Schlücker; W. Kiefer; Ranjan K. Singh
The solid to smecticG (SmG) phase transition in a Schiff base liquid crystalline compound, terepthal-bis-heptylaniline (TB7A), is monitored in situ by temperature-dependent Raman microspectroscopy, using the band of a C-H in-plane bending mode as a marker. Contrary to the earlier report of a sudden wavenumber shift, the in situ measurement shows very clearly that a new Raman band at ~1160 cm-1 appears at the Crystal II → SmG transition. The dynamics of this phase transition is discussed in terms of a triple well potential below 210 K and a double well potential above 210 K. The phase transition essentially takes place as a result of intra-molecular rotation about the long molecular axis. The optimization energy at various fixed dihedral angles, (-C-C-C = N - ) are calculated using density functional theory (DFT) at the B3LYP/6-31G* level of theory. The relative energy at each dihedral angle is calculated relative to optimization energy obtained without any constraints and plotted as a function of dihedral angle (φ) between the adjacent phenyl ring planes, which also shows a double well potential at room temperature. © 2009 John Wiley & Sons, Ltd.
Fluorescence quenching of molybdenum disulfide quantum dots for metal ion sensing
(Springer, 2020) Himanshu Mishra; Vijay K. Singh; Rashid Ali; K. Vikram; Jai Singh; Arvind Misra; Hirdyesh Mishra; Anchal Srivastava
Abstract: In the present work, we have used hydrothermally synthesized in situ functionalized MoS2-QDs for a sensitive (limit of detection ~ 2.06 µM) and selective detection of Fe3+ ions. A detailed study of fluorescence quenching behavior for MoS2-QDs in the presence of Fe3+ ions has been performed using the Stern–Volmer plot, modified Stern–Volmer plot, and time-resolved photoluminescence measurements. Absorption based titration spectra and time-resolved photoluminescence measurements confirmed the fluorescence quenching is static with three decay times originated from the three different fluorescing sites. Interestingly, it is found that emission spectra consist of three bands at positions ~ 450 nm (P1, ~ 2.76 eV), ~ 475 nm (P2, ~ 2.61 eV), and ~ 503 nm (P3, ~ 2.46 eV). These peaks show a systematic quenching with the increasing concentration of Fe3+ ions. Quenching constants corresponding to these emission bands are found of the order of ~ 103 M−1. Large values of bimolecular quenching constants (~ 1011 M−1 s−1) suggest a strong binding interaction between MoS2-QDs and Fe3+ ions. Furthermore, to understand the fluorescence quenching of MoS2-QDs in the presence of Fe3+ ions, a ground-state complex formation-based mechanism has been proposed and elucidated in detail. Graphic abstract: [Figure not available: see fulltext.]. © 2020, Springer-Verlag GmbH Austria, part of Springer Nature.
Investigation of phase transition in the binary mixture of thermotropic liquid crystalline systems 4-(Heptylox) benzoic acid and 4-(Undecyloxy) benzoic acid using Raman and DFT studies
(Elsevier B.V., 2023) Chandan Bhai Patel; K. Vikram; Pankaj Kumar Tripathi; Rajib Nandi; K. Ummer; Shashank Shekhar; Ranjana Singh; Ranjan K. Singh
A binary mixture Uba@Hba of liquid crystalline systems; 4-(Undecyloxy) benzoic acid and 4-(Heptylox) benzoic acid was prepared, and their liquid crystalline properties were investigated through differential scanning calorimetry and polarized optical microscopy at the bulk level and temperature dependent Raman spectroscopic technique as well as density functional calculations at molecular level. The polarized optical microscopic studies revealed crystal, smectic A, nematic and isotropic phases in the Uba@Hba. The phase transition temperatures of the binary mixture Uba@Hba was identified by the temperature dependent Raman study which was found almost close to phase transitions observed in differential scanning calorimetry and polarized optical microscopy. The detailed Raman & DFT studies provide information occurring in phase transitions at molecular level. The changes in inter/intra-molecular interactions at crystal → smectic A and smectic A → nematic transitions have been discussed in detail. © 2023 Elsevier B.V.
Low temperature Raman and DFT study of creatinine
(2012) K. Vikram; Shivangi Mishra; S.K. Srivastava; Ranjan K. Singh
The Raman spectra of creatinine powder in the wavenumber region of 500-3500 cm -1 have been recorded as a function of temperature ranging from 293 K to 83 K and different clusters of creatinine were optimized with DFT method to determine hydrogen bonding network in crystalline creatinine. The accurate vibrational assignment of all vibrational bands of creatinine has been done on the basis of the calculated vibrational spectra and the potential energy distribution. The calculated Raman spectra of hydrogen bonded tetramer cluster matches nicely with the experimentally observed spectra confirming four molecules in unit cell as reported by crystallographic data. In addition signature of dimer, which is just half of the tetramer, has also been observed. The bands associated with CH 3, CH 2, ν(CO), ν(CN) and ν(CH) have been given more focus to see the behavior of hydrogen bond network with temperature. The analysis of these bands reveals that a gradual change (increase/decrease) in linewidth and peak position occurs with temperature as a consequence of the anharmonic coupling of internal modes with phonon mode and intermolecular interaction. The behavior of hydrogen bond network on cooling has been explained in terms of contraction of unit cell. © 2012 Elsevier B.V. All rights reserved.
Low temperature Raman study of 1-isothiocyanato-4-(trans-4- propylcyclohexyl) benzene (3CHBT)
(2010) K. Vikram; Ranjan K. Singh
[No abstract available]
Low temperature Raman study of a liquid crystalline system 4-Decyloxy benzoic acid (4DBA)
(Elsevier B.V., 2013) K. Vikram; Rajib Nandi; Ranjan K. Singh
The Raman spectra of a liquid crystalline system, 4-Decyloxy benzoic acid (4DBA) have been recorded at different temperatures within the interval 300-78 K in order to identify the structural changes in crystalline state of a nematogen and to understand the molecular alignment therein. The earlier predicted dimer structure of 4DBA was optimized with DFT method and the theoretical Raman spectra of dimer as well as monomer have been calculated for comparison with the experimental spectra. The mode specific quartic coupling coefficient; Aω,x and phonon frequency; ωi have been calculated using temperature dependent anharmonic perturbation theory. The precise band shape analysis of Raman bands at ∼807, ∼881, ∼1255, ∼1282, ∼1436, ∼1576, ∼1604, ∼2881 and ∼3081 cm-1 gives signature of temperature induced slow crystal modification. The structural changes leading to crystal modification have been discussed. © 2013 Elsevier B.V. All rights reserved.
pH dependent luminescence switching of tin disulfide quantum dots
(Elsevier B.V., 2019) Rohit Ranjan Srivastava; Himanshu Mishra; Vijay K. Singh; K. Vikram; Rajesh Kumar Srivastava; S.K. Srivastava; Anchal Srivastava
Designing a molecule with pH, temperature and ion concentration sensitive luminescence properties is always fascinating. Tin disulfide (SnS2), a member of layered metal dichalcogenides (LMDs)family, is explored much for their commendable applications, however, its fluorescence property is still less explored. Present study reports a facile and eco-friendly bottom up synthesis route for tin disulfide quantum dots (SnS2-QDs). Transmission electron microscopic (TEM), and Atomic force microscopic (AFM)analysis revealed an average particle size of ∼3.6 nm. Our reported synthesis method provide the in-situ functionalization of quantum dots (QDs)making it highly sensitive to its environment. SnS2-QDs are found to be pH sensitive and hence a detailed pH dependent luminescence study has been performed. Interestingly it was found that SnS2-QDs possess ∼16 fold enhanced luminescence intensity in acidic condition (pH ∼ 1, QY = 5.32%)rather its basic condition (pH ∼ 12, QY = 1.17%). To explain this pH dependent behavior of SnS2-QDs, a mechanism has been proposed where this luminescence switching is mainly supposed due to protonation and deprotonation between –NH2 and –COOH groups. We believe present study may provide an insight for the development of pH sensor using SnS2-QDs for practical applications. © 2019 Elsevier B.V.
Probing the mesophase formation in thermotropic liquid crystal HBDBA using temperature-dependent Raman spectroscopy and DFT method
(Elsevier B.V., 2024) Chandan Bhai Patel; Satyabratt Pandey; Sachin K. Singh; K. Vikram; Ranjan K. Singh
Liquid crystalline properties of the synthesized liquid crystal (LC) N-(o-hydroxybenzylidene)-N′-(4-n-alkoxybenzylidene) azines (HBDBA) are probed thoroughly using the comprehensive array of techniques e.g. differential scanning calorimetry (DSC), differential thermal analysis (DTA), polarizing optical microscopy (POM), temperature-dependent Raman spectroscopy and density functional theory (DFT) method. In this study, intricate molecular interactions crucial for mesophase formation of liquid crystalline system HBDBA and molecular rearrangement that occurs during LC transitions are unravelled comprehensively. Remarkably, at the Cr → SmA phase transition, the peak position, linewidth, and intensity of signature Raman bands are prominently changed. A thorough analysis of Raman marker bands and DFT calculation confirm the disruption of intramolecular hydrogen bonds in HBDBA at the Cr → SmA transition. The conclusion of the present study enriches the understanding of the underlying mechanisms of mesophase formation and intricate molecular interactions and arrangement at the molecular level of the thermotropic LC. © 2024 Elsevier B.V.
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 signature of the interaction between functionalized MWCNT and the liquid crystalline system (4DBA)
(American Institute of Physics, 2024) Ummer Bashir Khoja; Chandan Bhai Patel; Rajesh Srivastava; Vijay K. Mishra; Pankaj Tripathi; Amit Raj Singh; Ranjan K. Singh; K. Vikram
Liquid crystalline systems provide the best molecular matrix as a host for carbon nanotubes (CNTs) due to their fluidity character that controls the molecular ordering. The anisotropic shape of the CNTs makes them highly suitable nanoparticles that can be embedded in liquid crystalline systems to achieve uniform tunable dispersion. The uniform and stable dispersion of CNTs in the liquid crystalline host is challenging due to strong Vander Waal forces between CNTs. The functionalization of CNTs is one of the most popular techniques to achieve the dispersion system in an LC host due to the covalent bonding between CNT and LC systems. The COOH functionalized multi-wall carbon nanotubes (F-MWCNT) provide effective sites to interact with LC systems with the carboxylic group at the terminal. Raman spectroscopy is used as a probe to investigate the interaction between 4DBA and F-MWCNT through COOH - -COOH dimer formation. Comparative Raman analysis of 4DBA, 4DBA + bare MWCNT, and 4DBA + F-MWCNT spectra show bands associated with the COOH group giving strong evidence of interaction between 4DBA and F-MWCNT. Raman study reveals the mechanism of dimer formation between F-MWCNT and 4DBA. © 2024 Author(s).
Synthesis and mesomorphic investigation of calamitic liquid crystalline system ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB): A temperature dependent micro-Raman study and DFT calculations
(2013) Rajib Nandi; K. Vikram; Sachin Kumar Singh; Bachcha Singh; Ranjan K. Singh
A new liquid crystalline material containing diester linking group ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB) was synthesized. The phase transition temperatures were noted by differential scanning calorimetry (DSC), the texture pattern were observed by polarizing optical microscopy (POM) and temperature dependent Raman study was employed to observe the transitions as well as to understand the molecular rearrangement during phase transition. The transitions were observed with all the three techniques but the Raman signature of crystal → smectic A transition is many fold and more precise and accurate. The correlation between intermolecular interaction and phase behaviour has been discussed using temperature dependence Raman data of CH in-plane bending and CO stretching vibrations. With the help of DFT method the possible dimers of 4-EDBB were optimized and the rotational isomers were also investigated. There exists weak hydrogen bonds at room temperature, which breaks as the temperature is increased causing the CH in-plane bending to shift lower and CO stretching vibrations to shift higher. The discussion of the temperature dependent Raman data reveals that at crystal → smectic A transition as a result of intra-molecular rotation the molecules transform from trans- to cis- conformer. © 2013 Elsevier B.V.
Synthesis, characterisation and mesomorphic properties of ester containing aroylhydrazones and their nickel(II) complexes
(2011) Sachin Kumar Singh; K. Vikram; Bachcha Singh
A new series of mesogenic aroylhydrazone-based ligands, N-[4-(4'-alkoxy)benzoyloxybenzylidene]-N'-[4'-alkoxybenzoyl] hydrazine with either the same or different peripheral alkyl chains, and nickel(II) complexes of some of them have been synthesised. They were characterised by elemental analyses, Fourier transform infrared, proton and carbon nuclear magnetic resonance and ultraviolet-visible spectroscopy. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry and polarising optical microscopy. All the aroylhydrazones, except those with no lateral chains on either end of the molecule and where m = n = 14, 16, exhibit a monotropic or enantiotropic smectic C mesophase, which are almost insensitive to the peripheral alkoxy chain length. The square planar nickel(II) complexes of the ligands show only an isotropic phase at higher temperature (>175°C) and no mesogenic nature is observed. Density functional theory calculations have been performed using the GAUSSIAN-03 program at the Becke, three-parameter, Lee-Yang-Parr level to obtain the stable electronic structure of the ligand. © 2011 Taylor & Francis.
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.
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
(Society for Applied Spectroscopy, 2010) K. Vikram; Nicolae Tarcea; J. Popp; Ranjan K. Singh
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. © 2010 Society for Applied Spectroscopy.