Deepa Singh | Central Michigan University (original) (raw)

Address: Mount Pleasant, Michigan, United States

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Papers by Deepa Singh

Research paper thumbnail of Raman and DFT study of static, dynamic interactions and isotope effect in pyridazine + H 2O/D 2O systems

Vibrational Spectroscopy, 2009

Research paper thumbnail of Concentration-dependent surface-enhanced Raman scattering and molecular dynamic study of dimethyl formamide

Journal of Raman Spectroscopy, 2007

A concentration-dependent Raman study of dimethyl formamide (DMF) in Ag nanocolloidal solution wa... more 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.

Research paper thumbnail of Raman and DFT study of hydrogen-bonded 2- and 3-chloropyridine with methanol

Journal of Raman Spectroscopy, 2008

Precise polarized Raman measurements of 2-chloropyridine (2Clpy) in the region 560–1060 cm−1 and ... more 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 and methanol-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 other modes, 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.

Research paper thumbnail of DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment

Journal of Molecular Structure-theochem, 2007

... DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment. Deepa... more ... DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment. Deepa Singh a , Sunil K. Srivastava a , Animesh K. Ojha b , BP Asthana a and Ranjan K. Singh a , Corresponding Author Contact Information , E-mail The Corresponding Author. ...

Research paper thumbnail of Raman and DFT study of static, dynamic interactions and isotope effect in pyridazine + H 2O/D 2O systems

Vibrational Spectroscopy, 2009

Research paper thumbnail of Concentration-dependent surface-enhanced Raman scattering and molecular dynamic study of dimethyl formamide

Journal of Raman Spectroscopy, 2007

A concentration-dependent Raman study of dimethyl formamide (DMF) in Ag nanocolloidal solution wa... more 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.

Research paper thumbnail of Raman and DFT study of hydrogen-bonded 2- and 3-chloropyridine with methanol

Journal of Raman Spectroscopy, 2008

Precise polarized Raman measurements of 2-chloropyridine (2Clpy) in the region 560–1060 cm−1 and ... more 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 and methanol-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 other modes, 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.

Research paper thumbnail of DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment

Journal of Molecular Structure-theochem, 2007

... DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment. Deepa... more ... DFT study of hydrogen bond bridging mode of pyridine and diazenes in water environment. Deepa Singh a , Sunil K. Srivastava a , Animesh K. Ojha b , BP Asthana a and Ranjan K. Singh a , Corresponding Author Contact Information , E-mail The Corresponding Author. ...

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