B. P. Asthana

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.

pH-dependent Raman study of pyrrole and its vibrational analysis using DFT calculations.

Raman spectra of pyrrole in aqueous medium at different pH values, 2.5, 5.5, 7.5 and 10.5 were recorded in the two spectral regions, 1,040-1,160 cm(-1) and 3,300-3,360 cm(-1) and pH dependence of the linewidth, peak position and intensity of the Raman bands corresponding to the ring breathing and symmetric nu(N-H) stretching modes were examined. A linear pH dependence of the peak positions for the ring breathing mode and a maximum at nearly neutral pH (7.5) for the symmetric nu(N-H) normal mode is observed, whereas the linewidth (FWHM) shows almost no variation with the change of pH. A slight decrease in the wavenumber position of the nu(N-H) mode at pH value >7.5 indicates that the influence of deprotonation is small, which results from a weak interaction between the reference molecule and the surrounding environment. The density functional theory (DFT) calculations were made primarily to obtain the optimized geometry and vibrational spectra of pyrrole in the ground electronic state using B3LYP functional and the highest level basis set 6-311++G(d,p). The assignments of the normal modes of pyrrole were made on the basis of potential energy distribution (PED). The calculations were also performed on protonated and deprotonated structures of pyrrole.

Antagonistic properties of a natural product-Bicuculline with the gamma-aminobutyric acid receptor: studied through electrostatic potential mapping, electronic and vibrational spectra using ab initio and density functional theory.

(+)-Bicuculline (hereinafter referred to as bicuculline), a phthalide isoquinoline alkaloid is of current interest as an antagonist of gamma-aminobutyric acid (GABA). Its inhibitor properties have been studied through molecular electrostatic potential (MEP) mapping of this molecule and GABA receptor. The hot site on the potential surface of bicuculline, which is also isosteric with GABA receptor, has been used to interpret the inhibitor property. A systematic quantum chemical study of the possible conformations, their relative stabilities, FT-Raman, FT-IR and UV-vis spectroscopic analysis of bicuculline has been reported. The optimized geometries, wavenumber and intensity of the vibrational bands of all the conformers of bicuculline have been calculated using ab initio Hartree-Fock (HF) and density functional theory (DFT) employing B3LYP functional and 6-311G(d,p) basis set. Mulliken atomic charges, HOMO-LUMO gap ΔE, ionization potential, dipole moments and total energy have also been obtained for the optimized geometries of both the molecules. TD-DFT method is used to calculate the electronic absorption parameters in gas phase as well as in solvent environment using integral equation formalism-polarizable continuum model (IEF-PCM) employing 6-31G basis set and the results thus obtained are compared with the UV absorption spectra. The combination of experimental and calculated results provides an insight into the structural and vibrational spectroscopic properties of bicuculline.

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.

Raman study of the concentration dependence of the frequency shifts and linewidth changes in a(3Cl-pyridine + methanol) system

Abstract The frequency shifts and linewidth changes of the two vibrational bands of 3Cl-pyridine (3Cl-p) at ≈ 729 and ≈1031 cm −1 have been studied in a binary mixture of (3Cl-p + CH 3 OH) with varying concentrations of 3Cl-p in mole fractions C = 0.1, 0.2, 0.3, 0.5, 0.7, 0.8 and 0.9. A plot of linewidth vs. C depicts a minimum around C = 0.7 which has been explained in terms of two competing phenomena, namely diffusion and motional narrowing. The frequency shifts, which exhibit almost negligibly small changes with varying C , are most probably caused due to opposing effects of the CI substituent at position 3 and an N atom sitting in the ring.

Vibrational spectra of 2 (3H) benzofuranone studied by Raman, IR spectroscopy and AM1 semiempirical molecular orbital calculations

Raman spectra of 2 (3H) benzofuranone have been recorded in the region 400-3200 cm(-1) and the IR spectra have been recorded in the region 200-4000 cm(-1). Vibrational frequencies for the fundamental modes of this bicyclic heteroatomic molecule have also been calculated using Austin method 1 (AM1) semiempirical molecular orbital method. Vibrational assignments have been made for the fundamental modes and the observed combination and overtone bands are also assigned. A splitting in the carbonyl group (C=O stretching) frequency observed at 1640-1660 cm(-1) in both Raman and IR spectra, is explained as Fermi-resonance. Net atomic charges for each atom of this molecule along with its heat of formation were also calculated. It is evident from the calculations that the 2 (3H) benzofuranone is more stable than the 3 (2H) benzofuranone in contrast to earlier estimates.

Infrared and Raman spectra of 8-azaguanine and 8-azaadenine

Abstract The IR and Raman spectra of 8-azaguanine and 8-azaadenine have been studied. The spectra of 8AG have been critically examined and compared with those of guanine and 9-methylguanine. Similarly, the spectra of 8-azaadenine have been examined and compared with those of adenine and 9-methyladenine. It has been possible to refute or accept the assignments for certain frequencies proposed earlier in the parent molecules by other workers, and thus almost unambiguous assignments for several frequencies in guanine and adenine as well as in 8AG and 8AA have been obtained. Several frequencies have been explained as combinations involving a 130 cm−1 vibration which has been assigned to the ring-folding mode. It is found that not as many combination bands involving this mode occur in 8AA as those in 8AG. These observations indicate that guanine and 8AG are geometrically more flexible than adenine and 8AA.

Study of hydration of sarcosine, formation of its zwitterion and their different oligomers in aqueous media: a Raman spectroscopic and theoretical study.

Raman spectra of the biologically important molecule sarcosine (SAR) (C3H7NO2) were studied experimentally in aqueous solution at different concentrations. These spectra were also calculated theoretically using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Further, all the observed normal modes were assigned through potential energy distribution (PED). Geometry optimization of SAR produced its three conformers with slightly different energies. The lowest energy conformer of SAR was selected for a systematic solvation study wherein different numbers of water molecules (nW, n=1-9) were placed near it. In the SAR-9W complex, the SAR molecule is located almost at the center of the cage of 9 water molecules. Geometries of different oligomers of SAR (dimer, trimer, tetramer and pentamer) were also optimized in aqueous media taking the input structures from crystallographic data and using the polarizable continuum (PCM). Proton transfer required for the formation of the zwitterionic form of SAR was found to occur when the number of water molecules in the first hydration shell was six or more.

Study of Hydrogen Bonding Patterns of a Pharmaceutically Active Drug Molecule Paraldehyde: a Raman and DFT Study

Abstract Raman spectra of neat paraldehyde (prldh) and its binary mixtures with methanol (M) with varying mole fractions of prldh from 0.1 to 0.8 were recorded in order to explore the hydrogen bonding patterns of prldh and their influence on spectral features of some selected vibrational bands of prldh in the region 400–1200 cm−1. Only few vibrational bands of paraldehyde show a significant change in their peak position in going from neat prldh to the complexes. One peculiar feature in this study is that the solvent band which occurs at ∼1034 cm−1 shows significant red shift of >4 cm−1 at higher mole fractions of prldh. This red shift is caused due to the hydrogen bonding between the O atom of the prldh ring and H atom of the methanol molecules. The Raman band at ∼1097 cm−1 band shows a blue shift of ∼5 cm−1 upon dilution and its linewidth also shows an increase of ∼5 cm−1 in going from neat prldh to extreme dilution. The line broadening upon dilution clearly indicates that the diffusion mechanism plays a dominant role in the dephasing of this vibrational mode. Density functional theoretic (DFT) calculations were performed employing B3LYP functional and high level basis set, 6-311++G(d,p) to obtain the ground state geometry of neat prldh and its hydrogen bonded complexes with methanol in gas phase. Basis set superimpose error (BSSE) correction was also introduced using the counterpoise method. In order to realize a condition quite close to the experiment, the polarizable continuum model (PCM, specific plus bulk solvation) calculations were also performed. For a detailed vibrational assignment of the normal modes, potential energy distribution (PED) calculation was also performed.

Crystal-smectic G transformation investigated by temperature-dependent Raman study

We report on a temperature-dependent Raman study of terephthalidine-bis-butylaniline (TBBA) and terephthalidine-bis-heptylaniline (TB7A) in the temperature range 10–406 K and in the wavenumber regions 1120–1240 cm−1 and 1500–1700 cm−1. The variations of peak positions and linewidths of several Raman marker bands with temperature, which clearly show crystal–SmG transition at ∼334 K, have been used to discuss the dynamics of this phase transition. The temperature-dependent Raman study revealed that the increased vibration of the long alkyl tail and the rotation around the long molecular axis are the two primary phenomena responsible for the crystal–SmG transition. Density functional theory (DFT) at the B3LPY/6-31G(d) level was employed to obtain the optimized geometry and the harmonic vibrational wavenumbers of TBBA. The eigenvectors of the modes corresponding to these marker bands were also calculated.