Computational Chemistry

FT-IR and FT-Raman spectra of 2-amino-4-pyrimidinol (2A4P) have been recorded. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimizations and force field calculations based on density functional theory (DFT) using standard B3LYP/6-311+G** method. Normal coordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies. Further, density functional theory (DFT) combined with quantum chemical calculations to determine the first-order hyperpolarizability.

The geometries of various tautomers of different ring size lactams have been studied by ab initio Hatree–Fock and Density Functional Theory calculations at B3LYP level with different basis sets in the gas phase. Optimized geometries and total energies for 12 possible tautomers of studied lactams were determined. Thermodynamic properties and tautomeric equilibria between different tautomers were calculated using the frequency calculations. The results of calculations are applied to the bond lengths of ?- lactam which showed a good agreement with experimentally determined data. In the gas phase, the amino tautomers are computed to be more stable than the lactim tautomers. Equilibrium constant results suggested that the lactam form is a more dominant tautomer for all cases and the lactim forms are not present in detectable amounts. 

The Fourier transform infrared (FT-IR) spectra and FT-Raman spectra of N –acetyl thiourea (NATU) was recorded in the regions 4000-400 cm-1 respectively, in the solid phase. Molecular electronic energy, geometrical structure, harmonic vibrational frequencies and bonding features of the title compound were computed three parameter hybrid functional Lee-Yang -–Parr/6-31G(d,p) and 6-311++G(d,p) levels of theory. The vibrational studies were interpreted in terms of potential energy distribution. The results were compared with experimental values with the help of harmonic vibrational spectra. Infrared intensities and Raman scattering activities, highest occupied molecular orbital, lowest unoccupied molecular orbital energy, and energy gaps were computed above method. The observed wave number in FTIR spectra was analyzed and assigned to different normal modes of the molecule. Most of the modes have wave numbers in the expected range and are in good agreement with computed values. The first order hyperpolarizability of this molecular system and related properties (?, ?, ? and ??) are calculated using B3LYP method based on the finite-field approach. Stability of the molecule arising from hyper conjugative interactions, charge delocalization and intermolecular hydrogen bond-like weak interaction has been analyzed using natural bond orbital (NBO) analysis by using B3LYP method. The results show that electron density (ED) in the ?* and ?*antibonding orbital’s and second-order delocalization energies E(2) confirm the occurrence of intra-molecular charge transfer (ICT) within the molecule.

Density functional theory (DFT) studies H₂ adsorption on transition element (TE) doped single walled carbon nanotubes (SWCNTs) using Dmol³ software. We consider transition element as Ti doped various sites on the surface of armchair (5,5) SWCNT. The simulation results shows that binding energy of H₂ adsorption on Ti doped SWCNT is much higher than pure SWCNT. We calculate the density of states for doped and pure SWCNTs, which is a show dramatic change in their structure on adsorption of H₂ molecules. This study shows that the transition element doped SWCNTs can be used as a good H₂ storage at various sites.

A Schiff base namely 3-{4-[4-(Benzylideneamino)benzenesulfonyl]-phenyl}-2-phenylquinazolin-4(3H)-one has been considered here. Theoretical characterization was done by Gaussian-03. The AM1 route has been used. The various parameters such as bond length, bond angle, atomic charge distribution and vibrational frequency analysis have been discussed. The theoretically observed IR values are found to be in accordance with the experimentally obtained values.

The mixed-ligand complex of [Cu(bipy)2S8(ClO4)2 was synthesized and characterized by employing elemental analyses, metal analyses, infrared and UV-Vis spectroscopy, room temperature magnetic susceptibility measurements, conductivity measurements and theoretical studies. The elemental and metal analyses show good agreement which support the formulation for the complex. The Cu-S band in the infrared spectrum at 465 cm -1, indicate the formation of the complex. The UV-Vis spectra shows different intraligand transitions at 49,505 cm-1 , 33,445 cm-1, 32,467 cm-1 and the bands at 13,661 cm-1 with a shoulder at 11,820 cm-1 in the visible region supported the CHN data for the formation of the five-coordinate complex. The magnetic moment value at 1.40 B.M indicates the presence of a strong antiferromagnetic properties of the complex. A 1:2 electrolyte nature of the complex was revealed in the conductivity value of 179 mho cm2 mol-1. The theoretical calculations predicted a distorted square pyramidal with S8 in the apical position for the complex with Cu(II)-S bond distance calculated to be 2.874 Å and 2.199 Å for PM3 and DFT methods respectively. The low negative value of S.E (–1.58 kcal/mol) calculated for [Cu(bipy)2S8 ]2+ ion suggested that the complex is more likely to exist either in solution or as an amorphous compound.

The solid phase FTIR and FT-Raman spectra of 1-Benzylimidazole (BI) have been recorded in the regions 4000–400 cm-1 and 3500–100 cm-1, respectively. The optimized geometry, frequency and intensity of the vibrational bands, NBO analysis, HOMO- LUMO study of BI in DFT levels of theory using B3LYP/6-31G and B3LYP/6-311+G basis set. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FTIR and FT-Raman spectra. A detailed interpretation of the vibrational spectra of the title compound has been made on the basis of the calculated potential energy distribution (PED). Stability of the molecule arising from hyperconjucative interactions leading to its bioactivity, charge delocalization have been analyzed using natural bond orbital (NBO) analysis. The calculated HOMO-LUMO energies shows that charge transfer occur within the molecule. The observed and calculated frequencies are found to be in good agreement.

The FT-IR and FT-Raman spectra of 2,3,6-trimethylphenol (236TMP) were recorded in the regions 4000-400 cm^-1 and 4000-100 cm^-1. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) and standard B3LYP/6-311+G** basis set combination. The vibrational spectra were interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical (SQM) force field. The Infrared and Raman spectra were also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. Unambiguous vibrational assignment of all the fundamentals was made using the total energy distribution (TED). Further, density functional theory (DFT) combined with quantum chemical calculations to determine the first-order hyperpolarizability. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. Electronic excitation energies, oscillator strength and nature of the respective excited states were calculated by the closed-shell singlet calculation method were also calculated for the molecule.

The optimized molecular structure and corresponding vibrational assignments of 2,4-Difluoroanisole (24DFA) have been investigated using density functional theory (DFT)/B3LYP with 6-311+G** basis sets. The Fourier transform infrared and Fourier transform Raman spectra of title molecules were recorded. A detailed interpretation of the FT-IR and FT-Raman spectra of 24DFA are reported on the basis of the calculated potential energy distribution. The theoretically calculated harmonic frequencies are scaled by common scale factor. The observed and the calculated frequencies are found to be in good agreement. Frontier Molecular Orbitals (FMOs) have been constructed at B3LYP/6-311+G** level to understand the electronic properties. The first hyperpolarizability (?0) of this noval molecular system and related properties (?, ?0, and ??) of 24DFA are calculated using B3LYP/6-311+G** method on the finite-field approach. Finally the calculations results were applied to simulated infrared and Raman spectra of the title compound which show good agreement with observed spectra. The chemical parameters were calculated from the HOMO and LUMO values. Electronic excitation energies, oscillator strength and nature of the respective excited states were calculated by the closed-shell singlet calculation method were also calculated for the molecule.