Showing papers by "Ranjan K. Singh published in 2007"
TL;DR: In this article, an experimental and theoretical study on hydrogen-bonding between pyrimidine and water as the H-donor was performed by using polarization-resolved linear Raman spectroscopy with the pyridine ring breathing mode ν1 as a marker band.
Abstract: We present an experimental and a theoretical study on hydrogen-bonding between pyrimidine and water as the H-donor. The degree of hydrogen-bonding in this binary system varies with mixture composition. This was monitored experimentally by polarization-resolved linear Raman spectroscopy with the pyrimidine ring breathing mode ν1 as a marker band. A subsequent quantitative line shape analysis of the isotropic Raman intensity for 24 pyrimidine/water mixtures clearly revealed a splitting into three spectral components upon dilution with water. The two additional peaks have been assigned to distinct groups of hydrogen-bonded species that differ in the number of pyrimidine nitrogen atoms (N) involved in hydrogen-bonding to water hydrogen atoms (H). From the integrated Raman intensities for “free” and “hydrogen-bonded” pyrimidine, a concentration profile for these species was established. Our assignments and interpretations are supported by quantum mechanical calculations of structures and by vibrational spectra...
51 citations
TL;DR: In this article, the isotropic and anisotropic parts of the Raman spectra of NH2 bending and ν(CO) stretching modes of HCONH2 in a hydrogen-bonding solvent, methanol, at different concentrations have been analyzed carefully in order to study the noncoincidence effect.
Abstract: The isotropic and anisotropic parts of the Raman spectra of NH2 bending and ν(CO) stretching modes of HCONH2 in a hydrogen-bonding solvent, methanol, at different concentrations have been analyzed carefully in order to study the noncoincidence effect (NCE). In neat HCONH2, the experimentally measured values of noncoincidence Δνnc are ∼11 and ∼18 cm−1 for the NH2 bending and ν(CO) stretching modes, which reduce to 0.45 and 1.14 cm−1, respectively at the concentration of HCONH2 in mole fraction, χm = 0.1. The experimental results have been explained on the basis of two models, namely, the microscopic prediction of Logan and the macroscopic model of Mirone and Fini. The relative success of the two models in explaining the experimental data for both the modes have been discussed. It has been observed that in case of the ν(CO) stretching vibrational mode the Logan model can reproduce the experimental data rather precisely, whereas in the case of the NH2 bending mode, Mirone and Fini model yields more accurate results. Copyright © 2006 John Wiley & Sons, Ltd.
12 citations
TL;DR: In this paper, the vibrational spectra of pyridine, pyridazine, pyrimidine, pyrazine 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.
Abstract: 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…HO, O…HO, and O…HC 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.
10 citations
TL;DR: In this article, 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.
Abstract: 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.
10 citations
TL;DR: In this paper, a concentration-dependent Raman study of the ν(CBr) stretching and trigonal bending modes of 2- and 3-Br-pyridine (2Brp and 3Brp) in CH3OH was performed at different mole fractions of the reference molecule, 2Brp/3Brp, from 0.1 to 0.9 to understand the origin of blue/red wavenumber shifts of the vibrational modes due to hydrogen-bond formation.
Abstract: A concentration-dependent Raman study of the ν(CBr) stretching and trigonal bending modes of 2- and 3-Br-pyridine (2Br-p and 3Br-p) in CH3OH was performed at different mole fractions of the reference molecule, 2Br-p/3Br-p, from 0.1 to 0.9 in order to understand the origin of blue/red wavenumber shifts of the vibrational modes due to hydrogen-bond formation. The appearance of additional Raman bands in these binary systems at ∼617 cm−1in the case of 2Br-p and at ∼618 cm−1 in the case of 3Br-p compared to neat bromopyridine derivatives were attributed to specific hydrogen-bonded complexes formed in the mixtures. The interpretation of experimental results is supported by density functional calculations on optimized geometries and vibrational wavenumbers of 2Br-p and 3Br-p and a series of hydrogen-bonded complexes with methanol. The parameters obtained from these calculations were used for a qualitative explanation of the blue/red shifts. The wavenumber shifts and linewidth changes for the ν(CBr) stretching and trigonal bending modes as a function of concentration reveal that the caging effects leading to motional narrowing and diffusion-causing line broadening are simultaneously operative, in addition to the blue shift caused due to hydrogen bonding. Copyright © 2007 John Wiley & Sons, Ltd.
8 citations
TL;DR: In this paper, the isotropic part of the Raman bands corresponding to NH 2 bending and ν (C O) stretching modes of formamide (HCONH 2 ) at ∼1593 and 1668 cm −1, respectively, in binary mixtures with methanol (CH 3 OH) were investigated.
5 citations