Molecular structure and vibrational spectra of N-acetylglycine oligomers and polyglycine I using DFT approach.

We have determined the geometric, vibrational, and electronic properties of N-acetylglycine oligomers by performing density functional theory quantum chemical calculations. The normal mode analysis was performed and the potential energy distribution was calculated among the internal coordinates. The optically active vibrational modes of PGI have been determined by selecting the modes from the calculated results of the pentamer and the observed vibrational spectra of PGI have been explained. The molecular electrostatic potential surface of N-acetylglycine pentamer reveals the sites of electrophilic attack and also provides clues for the role of electrostatic interactions involved in the reactivity. Natural bond orbital analysis has been performed to understand the charge transfer and various hyperconjugative interactions in the molecular system. The electronic properties of the oligomers have been discussed by calculating the transitions with the help of time dependent density functional theory method. The global reactivity descriptors such as hardness, chemical potential, and electrophilicity index have also been calculated.

Quantum chemical study on influence of intermolecular hydrogen bonding on the geometry, the atomic charges and the vibrational dynamics of 2,6-dichlorobenzonitrile.

FT-IR (4000-400 cm(-1)) and FT-Raman (4000-200 cm(-1)) spectral measurements on solid 2,6-dichlorobenzonitrile (2,6-DCBN) have been done. The molecular geometry, harmonic vibrational frequencies and bonding features in the ground state have been calculated by density functional theory at the B3LYP/6-311++G (d,p) level. A comparison between the calculated and the experimental results covering the molecular structure has been made. The assignments of the fundamental vibrational modes have been done on the basis of the potential energy distribution (PED). To investigate the influence of intermolecular hydrogen bonding on the geometry, the charge distribution and the vibrational spectrum of 2,6-DCBN; calculations have been done for the monomer as well as the tetramer. The intermolecular interaction energies corrected for basis set superposition error (BSSE) have been calculated using counterpoise method. Based on these results, the correlations between the vibrational modes and the structure of the tetramer have been discussed. Molecular electrostatic potential (MEP) contour map has been plotted in order to predict how different geometries could interact. The Natural Bond Orbital (NBO) analysis has been done for the chemical interpretation of hyperconjugative interactions and electron density transfer between occupied (bonding or lone pair) orbitals to unoccupied (antibonding or Rydberg) orbitals. UV spectrum was measured in methanol solution. The energies and oscillator strengths were calculated by Time Dependent Density Functional Theory (TD-DFT) and matched to the experimental findings. TD-DFT method has also been used for theoretically studying the hydrogen bonding dynamics by monitoring the spectral shifts of some characteristic vibrational modes involved in the formation of hydrogen bonds in the ground and the first excited state. The (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. Standard thermodynamic functions have been obtained and changes in thermodynamic properties on going from monomer to tetramer have been presented.

Use of vibrational spectroscopy to study 2-[4-(N-dodecanoylamino)phenyl]-5-(4-nitrophenyl)-1,3,4-oxadiazole: a combined theoretical and experimental approach.

Quantum chemical calculations of geometric structure and vibrational wavenumbers of 2-[4-(N-dodecanoylamino)phenyl]-5-(4-nitrophenyl)-1,3,4-oxadiazole (AF51) were carried out by using density functional theory (DFT/B3LYP/6-311G(d,p) method. The fundamental vibrational modes were characterized depending on their potential energy distribution (PED). In order to predict the reactive sites for electrophilic and nucleophilic attacks of the title molecule, electrostatic potential surface has been plotted. The UV absorption spectrum was examined in chloroform solvent and compared with the calculated one in gas phase as well as in solvent environment using TD-DFT/ PCM approach. The (1)H NMR spectra was recorded. Comparison between the experimental and the theoretical results is satisfactory. The thermodynamic properties of the title compound at different temperatures have been calculated. A relationship between molecular structural features, non-linear responses and hyperpolarizability of AF51 has been established using vibrational spectra with emphasis on the role of intramolecular charge transfer mechanism in such organic NLO materials.