Poonam Tandon

Structural and spectroscopic studies on 2-pyranones.

Experimental methods of infrared, Raman and electronic absorption spectroscopy and DFT calculations using B3LYP functionals and 6-31G** and 6-311++G** basis sets have been used to understand the structural and spectral characteristics of 2-pyranones, 6-phenyl-4-methylsulfanyl-2-oxo-2H-pyran and 6-phenyl-4-methylsulfanyl-2-oxo-2H-pyran-3-carbonitrile in the electronic ground (S(0)) and first excited (S(1)) states. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). Based on TD-DFT calculations using 6-31+G**5D basis set, an assignment of absorption peaks in the UV-VIS region has been suggested. The S(1) state is found to be a (1)(pi,pi*) state. A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman spectra and calculated frequency and intensity of the vibrational bands and potential energy distribution over the internal coordinates. Characteristic vibrational bands of the 2-pyranone ring and methylsulfanyl and carbonyl groups have been identified.

Experimental and theoretical (FT-IR, FT-Raman, UV-vis, NMR) spectroscopic analysis and first order hyperpolarizability studies of non-linear optical material: (2E)-3-[4-(methylsulfanyl) phenyl]-1-(4-nitrophenyl) prop-2-en-1-one using density functional theory.

A combined experimental and theoretical investigation on FT-IR, FT-Raman, NMR, UV-vis spectra of a chalcone derivative (2E)-3-[4-(methylsulfanyl) phenyl]-1-(4-nitrophenyl) prop-2-en-1-one (4N4MSP) has been reported. 4N4MSP has two planar rings connected through conjugated double bond and it provides a necessary configuration to show non-linear optical (NLO) response. The molecular structure, fundamental vibrational frequencies and intensity of the vibrational bands are interpreted with the aid of structure optimizations and normal coordinate force field calculations based on density functional theory (DFT) with B3LYP functional and 6-311++G(d,p) basis set combination. The analysis of the fundamental modes was made with the help of potential energy distribution (PED). Molecular electrostatic potential (MEP) surface was plotted over the geometry primarily for predicting sites and relative reactivities towards electrophilic and nucleophilic attack. The delocalization of electron density of various constituents of the molecule has been discussed with the aid of NBO analysis. The electronic properties, such as excitation energies, oscillator strength, wavelengths, HOMO and LUMO energies, were calculated by time-dependent density functional theory (TD-DFT) and the results complement the experimental findings. The recorded and calculated 1H chemical shifts in gas phase and MeOD solution are gathered for reliable calculations of magnetic properties. Thermodynamic properties like heat capacity (C°p,m), entropy (S°m), enthalpy (H°m) have been calculated for the molecule at the different temperatures. Based on the finite-field approach, the non-linear optical (NLO) parameters such as dipole moment, mean polarizability, anisotropy of polarizability and first order hyperpolarizability of 4N4MSP molecule are calculated. The predicted first hyperpolarizability shows that the molecule has a reasonably good nonlinear optical (NLO) behavior.

Vibrational dynamics and heat capacity of β-poly(L-serine)

Abstract Poly( l -serine) is a poly(amino acid) having a side group (CH2OH). It is found to exist in an anti-parallel β sheet structure. A study of complete normal modes and their dispersion has been carried out using Higgs modification of Wilsons GF matrix method and the Urey-Bradley force field. The constants of the field have been best fitted to the Fourier transform infra-red and Raman frequencies. Repulsion and exchange of character between various pairs of modes have been observed. Heat capacity as a function of temperature, obtained from the dispersion curves via density-of-states, is in good agreement with the measurements of Roles et al. (Biopolymers, 1993, 33, 753).

Heat Capacity and Phonon Dispersion in Poly( L-Methionine)

Poly( L-methionine) (PMet) is one of the two sulfur containing polyamino acids. Raman, FTIR spectra, and heat capacity measurements of PMet have been well interpreted through the normal mode analysis and the density of states derived there- from. Earlier interpretation of heat capacity data is limited because it is based on the Tarasov model, wherein the concept of group frequency and skeletal similarity are used. A special feature of some dispersion curves is their tendency to bunch in the neighborhood of the helix angle. This has been attributed to the presence of strong intramolecular interactions. Repulsion between the dispersion curves is also observed. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2281-2292, 1997

Conformational analysis and vibrational study of daidzein by using FT-IR and FT-Raman spectroscopies and DFT calculations.

Daidzein (C15H10O4) is a type of isoflavone. It was isolated from Butea monosperma that belongs to the Fabaceae family. Soybeans and soy products are the abundant source of daidzein. It is the subject of investigation for many reasons, as it has got wide applications, such as anti-tumor, anti-estrogen, weak pro-estrogen and anti-cancer activities. In the present study, a complete vibrational assignment is provided for the observed IR and Raman spectra of daidzein. Electronic properties have been analyzed using TD-DFT method for both gaseous and solvent phase. The optimized geometry, total energy, potential energy surface and vibrational wavenumbers of daidzein have been determined using density functional theory (DFT/B3LYP) method with 6-311++G(d,p) basis set and a good correlation was found between observed and calculated values. The double well potential energy curve of the molecule about three bonds, has been plotted, as obtained from DFT/6-31G basis. The HOMO-LUMO energy gap of possible conformers has been calculated for comparing their chemical activity. Global reactivity descriptors have been calculated for predicting the chemical reactivity and the stability of chemical systems. Electrostatic potential surface has been plotted for predicting the structure activity relationship. NBO analysis has also been performed to study the stability of the molecule. NLO study reveals the nonlinear properties of the molecule. 1H and 13C NMR spectra have also been studied. Finally, the calculated results were used to simulate infrared and Raman spectra of the title compound which showed a good agreement with the observed spectra.

A Comparative Ab Initio and DFT Study of Polyaniline Leucoemeraldine Base and Its Oligomers

Ab initio Hartree-Fock (HF) and density functional theory (DFT) calculations are being performed to investigate the geometric, vibrational, and electronic properties of the polyaniline leucoemeraldine base (PANI-LB). Vibrational spectra of PANI-LB have been analyzed using the DFT oligomer approach, and complete assignments are being reported. Lower region spectral assignments of the PANI-LB which were not being reported earlier are being done in the present work. DFT calculations with the 6-31G** basis set produce very good results of not only vibrational modes but also of energy band gap.

Vibrational analysis and chemical activity of paracetamol-oxalic acid cocrystal based on monomer and dimer calculations : DFT and AIM approach

The study of structural and spectral characteristics of a paracetamol–oxalic acid (PRA–OXA) cocrystal has been carried out using two models (monomer and dimer), with the aim to understand the supramolecular structure and intramolecular interactions within the cocrystal. The cocrystal has been characterized by infrared and Raman spectroscopy combined with quantum chemical calculations molecular electrostatic potential surface (MEPS), frontier orbital analysis and electronic reactivity descriptors were used to understand the role of interactions involved in affecting the chemical reactivity of individual molecules in the cocrystal. It is observed that the CO, N–H and O–H groups of paracetamol are involved in hydrogen bonds to form cocrystals. NBO analysis suggests that the two types of interactions LP(1)(N8) → π*(C9–O10) and LP(2)(O10) → σ*(O25–H28) are responsible for the stability of the molecule. AIM analysis suggested that the non-covalent interactions are moderate in nature. The calculated HOMO–LUMO energies reveal that the charge transfer occurs within the cocrystal. Chemical reactivity parameters show that the cocrystal is more active than paracetamol.

An ab initio and DFT study of structure and vibrational spectra of γ form of Oleic acid: Comparison to experimental data

Oleic acid (cis-9-octadecenoic acid) is the most abundant cis-unsaturated fatty acid in nature; it is distributed in almost all organisms. In this work, we present a detailed vibrational spectroscopy investigation of Oleic acid by using infrared and Raman spectroscopies. These data are supported by quantum mechanical calculations, which allow us to characterize completely the vibrational spectra of this compound. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and activities of Raman scattering were calculated by ab initio Hartree-Fock (HF) and density functional theory (DFT) employing B3LYP with complete relaxation in the potential energy surface using 6-311G(d, p) basis set. After a proper scaling the calculated wavenumbers show a very good agreement with the observed values. A complete vibrational assignment is provided for the observed Raman and infrared spectra of Oleic acid. In this work, we also investigate the deviation of vibrational wavenumbers computed with two quantum chemical methods (HF and B3LYP).

DFT Study and Heat Capacity of Polyaniline Pernigraniline Base

Polyaniline (PANI) is a very important polymer owing to its diversified chemistry and interesting physical properties. To investigate the structural parameters of polyaniline pernigraniline base (PANI PNB), DFT geometry optimization using B3LYP/6-31G** basis set was carried out on its two well-known model compounds. Electrostatic potential surfaces have been mapped over the electron density isosurfaces to obtain information about size, shape, charge distribution, and chemical reactivity of these molecules. The heat capacity of PANI PNB in the temperature range 10-300 K is being reported for the first time.

Molecular structure, spectral analysis and hydrogen bonding analysis of ampicillin trihydrate: a combined DFT and AIM approach

Ampicillin trihydrate chemically associated with the empirical formula C16H19N3O4S·3H2O, is a semi-synthetic amino-penicillin derived from the elementary penicillin nucleus, 6-aminopenicillanic acid. It is a very common antibiotic that is active against an extensive range of Gram-positive and Gram-negative organisms. It is used to treat certain varieties of bacterial infections, like gonorrhea and infections of the urinary, intestinal and respiratory tracts. In the present effort, quantum chemical calculations of molecular geometries (bond lengths and bond angles) and bonding features of the monomer and dimer of ampicillin trihydrate in the ground state have been carried out due to its biological and industrial importance. The optimized geometry and wavenumber of the vibrational bands of the molecule have been calculated by ab initio density functional theory (DFT) using Beckes three-parameter hybrid functional (B3LYP) with a 6-311++G(d,p) basis set. Vibrational wavenumbers were compared with the observed FT-Raman and FT-IR spectra. Molecular electrostatic potential (MEP) has also been plotted for predicting the molecule reactivity towards positively or negatively charged reactants and it shows that electropositive potential is visualized in the vicinity of the –NH3+ group and the electropositive region is found near the H2O molecule in both monomer and dimer. HOMO–LUMO analysis has been done to describe the way the molecule interacts with other species. Natural bond orbital (NBO) analysis has been carried out to inspect the intra- and inter-molecular hydrogen-bonding, conjugative and hyperconjugative interactions and their second order stabilization energy E(2). Nonlinear optical (NLO) analysis has also been performed to study the non-linear optical properties of the molecule by computing the first hyperpolarizability (β0). The variation of thermodynamic properties with temperature has been studied. Topological parameters at bond critical points (BCP) have been evaluated by ‘Quantum theory of atoms in molecules’ (QTAIM).