Onkar Prasad

Raman, FT-IR spectroscopic analysis and first-order hyperpolarisability of 3-benzoyl-5-chlorouracil by first principles

3-Benzoyl-5-chlorouracil (3B5CU), a biologically active synthetic molecule, has been analysed at DFT/6-311+ + G(d,p) level and reported for the first time as a potential candidate for nonlinear optical (NLO) applications. The optimised skeleton of 3B5CU molecule is non-planar. The frontier orbital energy gap, dipole moment, polarisability and first static hyperpolarisability have been calculated. The first static hyperpolarisability is found to be almost 15 times higher than that of urea. The high value of first static hyperpolarisability (2.930 × 10− 30 e.s.u.) due to the intra-molecular charge transfer in 3B5CU has been discussed using first principles. A complete vibrational analysis of the molecule has been performed by combining the experimental Raman, FT-IR spectral data and the quantum chemical calculations. In general, a good agreement of calculated modes with the experimental ones has been obtained. The strong vibrational modes contributing towards NLO activity, involving the whole charge transfer path, have been identified and analysed.

The spectroscopic (FT-Raman, FT-IR, UV and NMR), molecular electrostatic potential, polarizability and hyperpolarizability, NBO and HOMO–LUMO analysis of monomeric and dimeric structures of 4-chloro-3,5-dinitrobenzoic acid

Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of 4-chloro-3,5-dinitrobenzoic acid (CDNBA) were carried out by using density functional (DFT/B3LYP) method with 6-311++G(d,p) as basis set. To determine lowest-energy molecular conformation of the title molecule, the selected torsion angle is varied every 10° and molecular energy profile is calculated from 0° to 360°. The optimized geometrical parameters obtained by DFT calculations are in good conformity with single crystal XRD data. The FT-IR and FT-Raman spectra were measured in the condensed state. The fundamental vibrational wavenumbers as well as their intensities were calculated and a good agreement between observed and scaled calculated wavenumbers has been achieved. The electric dipole moment, polarizability and the first hyperpolarizability values of the CDNBA have been calculated at the same level of theory and basis set. The calculation results also show that the CDNBA molecule may has nonlinear optical (NLO) behavior with non-zero values. Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using natural bond orbital analysis. Ultraviolet-visible (UV) spectrum of the title molecule has also been calculated using CIS and TD-DFT methods. The calculated energy and oscillator strength almost exactly reproduce the experimental data. The (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by the gauge independent atomic orbital (GIAO) method and compared with experimental results. The thermodynamic properties of the studied compound at different temperatures were calculated.

Experimental (FT-IR, FT-Raman, UV and NMR) and quantum chemical studies on molecular structure, spectroscopic analysis, NLO, NBO and reactivity descriptors of 3,5-Difluoroaniline.

Comprehensive investigation of geometrical and electronic structure in ground as well as the first excited state of 3,5-Difluoroaniline (C6H5NF2) was carried out. The experimentally observed spectral data (FT-TR and FT-Raman) of the title compound was compared with the spectral data obtained by DFT/B3LYP method using 6-311++G(d,p) basis set. The molecular properties like dipole moment, polarizability, first static hyperpolarizability, molecular electrostatic potential surface (MEPs), and contour map were calculated to get a better insight of the properties of the title molecule. Natural bond orbital (NBO) analysis was applied to study stability of the molecule arising from charge delocalization. UV-Vis spectrum of the title compound was also recorded and the electronic properties, such as Frontier orbitals and band gap energies were measured by TD-DFT approach. Total and partial density of state (TDOS and PDOS) and also overlap population density of state (OPDOS) diagrams analysis were presented. Global and local reactivity descriptors were computed to predict reactivity and reactive sites on the molecule. (1)H and (13)C NMR spectra by using gauge including atomic orbital (GIAO) method of studied compound were compared with experimental data obtained. Moreover, the thermodynamic properties were evaluated.

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

An experimental and theoretical investigation of Acenaphthene-5-boronic acid: conformational study, NBO and NLO analysis, molecular structure and FT-IR, FT-Raman, NMR and UV spectra.

The solid state Fourier transform infrared (FT-IR) and FT-Raman spectra of Acenaphthene-5-boronic acid (AN-5-BA), have been recorded in the range 4000-400cm(-1) and 4000-10cm(-1), respectively. Density functional theory (DFT), with the B3LYP functional was used for the optimization of the ground state geometry and simulation of the infrared and Raman spectra of the molecule. The vibrational wave numbers and their assignments were examined theoretically using the Gaussian 09 set of quantum chemistry codes and the normal modes were assigned by a scaled quantum mechanical (SQM) force field approach. Hydrogen-bonded dimer of AN-5-BA, optimized by counterpoise correction, has also been studied by B3LYP at the 6-311++G(d,p) level and the effects of molecular association through O-H⋯O hydrogen bonding have been discussed. The (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by Gauge-Including Atomic Orbital (GIAO) method. Natural bond orbital (NBO) analysis has been applied to study stability of the molecule arising from charge delocalization. UV spectrum of the title compound was also recorded and the electronic properties, such as frontier orbitals, and band gap energies were measured by TD-DFT approach. The first order hyperpolarizability 〈β〉, its components and associated properties such as average polarizability and anisotropy of the polarizability (α and Δα) of AN-5-BA was calculated using the finite-field approach.

Molecular structure, electronic properties, NLO, NBO analysis and spectroscopic characterization of Gabapentin with experimental (FT-IR and FT-Raman) techniques and quantum chemical calculations

Gabapentin (GP), structurally related to the neurotransmitter GABA (gamma-aminobutyric acid), mimics the activity of GABA and is also widely used in neurology for the treatment of peripheral neuropathic pain. It exists in zwitterionic form in solid state. The present communication deals with the quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of GP using density functional (DFT/B3LYP) method with 6-311++G(d,p) basis set. In view of the fact that amino acids exist as zwitterions as well as in the neutral form depending on the environment (solvent, pH, etc.), molecular properties of both the zwitterionic and neutral form of GP have been analyzed. The fundamental vibrational wavenumbers as well as their intensities were calculated and compared with experimental FT-IR and FT-Raman spectra. The fundamental assignments were done on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanical (SQM) method. The electric dipole moment, polarizability and the first hyperpolarizability values of the GP have been calculated at the same level of theory and basis set. The nonlinear optical (NLO) behavior of zwitterionic and neutral form has been compared. Stability of the molecule arising from hyper-conjugative interactions and charge delocalization has been analyzed using natural bond orbital analysis. Ultraviolet-visible (UV-Vis) spectrum of the title molecule has also been calculated using TD-DFT method. The thermodynamic properties of both the zwitterionic and neutral form of GP at different temperatures have been calculated.

Heat capacity and vibrational dynamics of α poly(β‐benzyl‐L‐aspartate)

SYNOPSIS Poly(P-benzyl-L-aspartate) (PBLA) is an unusual polypeptide, which is capable of going into four different conformations, namely, left-handed a helix, right-handed a helix, o helix, and fi pleated sheet. The present work is a complete study of normal modes and their dispersion in the unusual left-handed a form. A special feature of some of the dispersion curves is their tendency to bunch in the neighborhood of helix angle. This is attributed to the presence of strong intramolecular interactions. Crossing and repulsion between the dispersion curves is also observed. The N-deuterated analogue of PBLA has been studied to check the validity of assignments and force field (Urey Bradley). Specific heat has been obtained from dispersion curves via density of states. A comparative study of left-handed and right-handed forms is presented. 0 1996 John Wiley & Sons, Inc.

Structural and spectroscopic characterization of a novel potential anti-inflammatory agent 3-(adamantan-1-yl)-4-ethyl-1H-1,2,4-triazole-5(4H)thione by first principle calculations.

A comprehensive investigation on the molecular structure, electronic properties and vibrational spectra of the 3-(adamantan-1-yl)-4-ethyl-1H-1,2,4-triazole-5(4H)thione, a novel potential anti-inflammatory agent has been done with the hope that the results of present study may be helpful in the prediction of its mechanism of biological activity. The experimentally observed spectral data (FT-IR and FT-Raman) of the title compound was compared with the spectral data obtained by DFT/B3LYP method. The (1)H nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by the Gauge Including Atomic Orbital method and compared with experimental results. The molecular properties like dipole moment, polarizability, first static hyperpolarizability, the molecular electrostatic potential surface, contour map have been calculated to get a better insight of the properties of the title molecule. Natural bond orbital (NBO) analysis has been applied to study stability of the molecule arising from charge delocalization. UV-Vis spectrum of the title compound was also recorded and the electronic properties, such as Frontier orbitals and band gap energies were calculated by TD-DFT approach. Global and local reactivity descriptors have been computed to predict reactivity and reactive sites on the molecule.

Phonon dispersion in poly(α-aminoisobutyric acid)

Abstract The present work is a study of phonons and their dispersion in poly(α-aminoisobutyric acid) (PAIB), which is an example of a poly(amino acid) having a 3 10 helical conformation. PAIB has been used as a model to carry out conformational studies on proteins and polypeptides. Dispersion curves show two interesting opposite features: (1) a tendency of some curves to crowd or close in near δ = ϕ , which is indicative of coupling between various modes, and (2) a divergence of some dispersion curves belonging to species of the same symmetry. The specific heat is obtained from the dispersion curves via the density of states.

Quantum-chemical (DFT, MP2) and spectroscopic studies (FT-IR and UV) of monomeric and dimeric structures of 2(3H)-Benzothiazolone.

Molecular geometry and vibrational wavenumbers of 2(3H)-Benzothiazolone (C7H5NSO, HBT) was investigated using density functional (DFT/B3LYP) method with 6-311+G(d,p) basis set. The vibrational wavenumbers are found to be in good agreement with experimental FT-IR spectra. Hydrogen-bonded dimer of HBT, optimized by counterpoise correction, was studied by MP2 and DFT/B3LYP at the 6-311+G(d,p) level and the effects of molecular association through NH---O hydrogen bonding were discussed. A detailed analysis of the nature of the hydrogen bonding, using topological parameters, such as electronic charge density, Laplacian, kinetic and potential energy density evaluated at bond critical points (BCP) has also been presented. The UV absorption spectra of the compound dissolved in ethanol and chloroform solutions were recorded in the range of 200-600 nm. The UV-vis spectrum of the title molecule was also calculated using TD-DFT method. The calculated energy and oscillator strength almost exactly reproduce the experimental data. Total and partial density of state (TDOS, PDOS) of the HBT in terms of HOMOs and LUMOs and molecular electrostatic potential (MEP) were calculated and analyzed. The electric dipole moment, polarizability and the first static hyper-polarizability values for HBT were calculated at the DFT/B3LYP with 6-311+G(d,p) basis set. The results also show that the HBT molecule may have nonlinear optical (NLO) comportment with non-zero values. Stability of the molecule arising from hyper-conjugative interactions and charge delocalization was analyzed using natural bond orbital (NBO) analysis.