V. Balachandran

Tautomeric purine forms of 2-amino-6-chloropurine (N9H10 and N7H10): Structures, vibrational assignments, NBO analysis, hyperpolarizability, HOMO–LUMO study using B3 based density functional calculations

Two purine tautomers of 2-amino-6-chloropurine (ACP), in labeled as N(9)H(10) and N(7)H(10), were investigated by vibrational spectroscopy and quantum chemical method. The FT-IR and FT-Raman spectra of ACP have been recorded in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. The measured spectra were interpreted by aid of a normal coordinate analysis following DFT full geometry optimization and vibrational frequency calculations at B3LYP/6-311++G(d,p) level. First-order hyperpolarizability, HOMO and LUMO energies were calculated at same level of theory. The calculated molecular geometry has been compared with the X-ray data. The observed and calculated frequencies were found in good agreement. The obtained NBO data and second-order perturbation energy values to elucidate the Lewis and non-Lewis types of bonding structures in the purine tautomer N(9)H(10), have indicated the presence of an intramolecular hyperconjucative interaction between lone pair N and N-C bond orbital.

Vibrational spectroscopic (FTIR and FT Raman) studies, first order hyperpolarizabilities and HOMO, LUMO analysis of p-toluenesulfonyl isocyanate using ab initio HF and DFT methods.

The Fourier transform infrared (FTIR) and FT Raman spectra of p-toluenesulfonyl isocyanate (p-tosyl isocyanate) have been measured. The molecular geometry, vibrational frequencies, infrared intensities, Raman activities and atomic charges have been calculated by using ab initio HF and density functional theory calculation (B3LYP) with 6-311+G(d,p) basis set. Complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT Raman data. The thermodynamic functions of the title compound were also performed with the aid of HF/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. Simulated FTIR and FT Raman spectra for p-tosyl isocyanate showed good agreement with the observed spectra. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The dipole moment (μ), polarizability (α) and the hyperpolarizability (β) values of the investigated molecule have been computed using HF and B3LYP methods.

Molecular structure, spectroscopic (FT-IR, FT-Raman) studies and first-order molecular hyperpolarizabilities, HOMO–LUMO, NBO analysis of 2-hydroxy-p-toluic acid

FT-Raman and FT-IR spectra for 2-hydroxy-p-toluic acid molecule had been recorded in the regions 3500-100 cm(-1) and 4000-400 cm(-1), respectively. Vibrational frequencies have been calculated in optimum state by employing density functional theory (DFT) and Hartree Fock (HF) methods with 6-311++G(d,p) basis set in monomeric form. SQM force fields have also been used to calculate potential energy distributions in order to make conspicuous vibrational assignments. Optimized geometries of the molecule had been interpreted and compared. The electric dipole moment and first hyperpolarizability values of the investigated molecule were computed using ab initio and DFT calculations. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. Stability of the molecule arising from hyper conjugative interactions, charge delocalization has been analyzed using natural bond analysis. The results show that charge in electron density (ED) in the σ(*) antibonding orbitals and E((2)) energies confirms the occurrence of ICT within the molecule.

Molecular structure, vibrational spectra (FTIR and FT Raman) and natural bond orbital analysis of 4-Aminomethylpiperidine: DFT study

The FT-IR and FT-Raman spectra of 4-Aminomethylpiperidine have been recorded using Perkin Elmer Spectrophotometer and Nexus 670 spectrophotometer. The equilibrium geometrical parameters, various bonding features, the vibrational wavenumbers, the infrared intensities and the Raman scattering activities were calculated using Hartree-Fock and density functional method (B3LYP) with 6-311+G(d,p) basis set. Detailed interpretations of the vibrational spectra have been carried out with the aid of the normal coordinate analysis. The spectroscopic and natural bonds orbital (NBO) analysis confirms the occurrence of intra molecular hydrogen bonds, electron delocalization and steric effects. The changes in electron density in the global minimum and in the energy of hyperconjugative interactions of 4-Aminomethylpiperidine (4AMP) were calculated. The theoretical UV-Visible spectrum of the compound was computed in the region 200-400nm by time-dependent TD-DFT approach. The calculated HOMO and LUMO energies show that charge transfer occur within the molecule. The dipole moment (μ) and polarizability (α), anisotropy polarizability (Δα) and hyperpolarizability (β) of the molecule have been reported.

Molecular structures, FT-IR and FT-Raman spectra, NBO analysis, NLO properties, reactive sites and quantum chemical calculations of keto–enol tautomerism (2-amino-4-pyrimidinol and 2-amino-pyrimidine-4(1H)-one)

The keto-enol tautomerism of 2-amino-4-pyrimidinol (APN) and 2-amino-pyrimidine-4(1H)-one (APO) are investigated by vibrational spectroscopy and quantum chemical method. FT-IR and FT-Raman spectra are recorded in the regions of 4000-400 cm(-1) and 3500-100 cm(-1), respectively for APN. Geometrical parameters, vibrational wavenumbers of APN and APO are predicted by density functional theory (DFT) employing B3LYP level with 6-311++G(d,p) and 6-311++G(2d,p) basis sets. The non-linear optical (NLO) properties of the title molecules are computed. The molecular electrostatic potential (MEP) surface maps are plotted and explained in detail. Natural bond orbital (NBO) analyses have been performed on APN molecule. The significant changes in occupancies and the energies of bonding and antibonding orbital have been explained in detail. Thermodynamic properties (heat capacities, entropies, and enthalpy) and their correlations with temperatures are also obtained from the calculated frequencies of the optimized structures. Reactivity descriptors, Fukui functions and electrophilic sites are found and discussed.

Theoretical investigations on molecular structure, vibrational spectra, HOMO, LUMO, NBO analysis and hyperpolarizability calculations of thiophene-2-carbohydrazide

The Fourier-Transform infrared and Fourier-Transform Raman spectra of thiophene-2-carbohydrazide (TCH) was recorded in the region 4000-400 cm(-1) and 3500-100 cm(-1). Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of TCH were carried out by DFT (B3LYP) method with 6-311++G(d,p) as basis set. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. Stability of the molecule arising from hyper conjugative interaction and charge delocalization has been analyzed using natural bond orbital (NBO) analysis. UV spectrum was measured in different solvent. The energy and oscillator strength are calculated by Time Dependant Density Functional Theory (TD-DFT) results. The calculated HOMO and LUMO energies also confirm that charge transfer occurs within the molecule. The complete assignments were performed on the basis of the potential energy distribution (PED) of vibrational modes, calculated with scaled quantum mechanics (SQM) method. Finally the theoretical FT-IR, FT-Raman, and UV spectra of the title molecule have also been constructed.

Vibrational spectroscopic (FT-IR and FT-Raman) studies, natural bond orbital analysis and molecular electrostatic potential surface of 3-hydroxy-6-methyl-2-nitropyridine

The optimized molecular structure and corresponding vibrational assignments of 3-hydroxy-6-methyl-2-nitropyridine have been investigated using density functional theory (DFT) B3LYP method with 6-311++G(d,p), 6-311++G(2d,2p) and 6-311++G(3d,3p) basis sets. Investigation of the relative orientation of the hydroxyl group with respect to the nitro group has shown that two conformers (O-cis) and (O-trans) exist. The vibrational analysis of the stable conformer of the title compound is performed by means of infrared absorption and Raman spectroscopy in combination with theoretical simulations. The molecular stability and bond strength were investigated by applying the natural bond orbital (NBO) analysis. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecule has been obtained by mapping electron density isosurface with electrostatic potential (ESP). The isotropic chemical shift computed by (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of the HMNP calculated using the gauge invariant atomic orbital (GIAO) method also shows good agreement with experimental observations.

FT-IR, FT-Raman spectra, NBO, HOMO-LUMO and thermodynamic functions of 4-chloro-3-nitrobenzaldehyde based on ab initio HF and DFT calculations.

FT-IR (4000-400 cm(-1)) and FT-Raman (3500-100 cm(-1)) spectral measurements of 4-chloro-3-nitrobenzaldehyde have been done. Ab initio (HF/6-311+G(d,p)) and DFT (B3LYP/6-311+G(d,p)) calculations have been performed giving energies, optimized structures, harmonic vibrational frequencies, infrared intensities and Raman activities. A detailed interpretation of the FT-IR and FT-Raman spectra of 4-chloro-3-nitrobenzaldehyde are reported on the basis of the calculated potential energy distribution. Stability of the molecule arising from hyper conjugative interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. The HOMO and LUMO energy gap reveals that the energy gap reflects the chemical activity of the molecule. The thermodynamic functions of the title compound have been performed by HF/6-311+G(d,p) and B3LYP/6-311+G(d,p). The observed and calculated wave numbers are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed spectra. Thermodynamic functions were calculated using vibrational wave numbers for different temperatures.

Theoretical investigations on the molecular structure, vibrational spectra, thermodynamics, HOMO–LUMO, NBO analyses and paramagnetic susceptibility properties of p-(p-hydroxyphenoxy)benzoic acid

The experimental FT-IR (4000-400 cm(-1)) and FT-Raman (3500-100 cm(-1)) spectra of p-(p-hydroxyphenoxy) benzoic acid have been recorded. Quantum chemical calculations of energies, geometries, and vibrational wavenumbers of p-(p-hydroxyphenoxy) benzoic acid (PPHPBA) are carried out using HF and DFT/B3LYP methods with 6-311G (d,p) basis set. The optimized geometrical parameters obtained by B3LYP method show a good agreement with experimental data. The difference between the observed and scaled wave number values of most of the fundamentals is very small. The complete assignments were performed on the basis of the potential energy distribution (PED) of the vibrational modes calculated with scaled quantum mechanical method. The calculated HOMO and LUMO energies allow the calculation of atomic and molecular properties and they also show that charge transfer occurs in the molecule. A detailed molecular picture of PPHPBA and its intermolecular interactions were obtained from NBO analysis. The temperature dependence of various thermodynamic parameters was also studied. The paramagnetic behavior of the molecule under consideration has been investigated and the variation of paramagnetic susceptibility with temperature has been studied.

Molecular structure, spectroscopic (FT-IR, FT-Raman), NBO and HOMO–LUMO analyses, computation of thermodynamic functions for various temperatures of 2, 6-dichloro-3-nitrobenzoic acid

The FT-IR and Raman spectra of 2, 6-dichloro-3-nitrobenzoic acid (DCNBA) have been recorded and analyzed. The equilibrium geometry, various bonding and harmonic vibrational wavenumbers have been calculated with the help of density functional theory (DFT/B3LYP/cc-pvdz/6-311++G(d,p)) method. The optimized geometrical parameters obtained by B3LYP/6-311++G(d,p) method show good agreement with experimental X-ray data. Most of the vibrational modes are observed in the expected range. The Mulliken population analysis shows the interactions of C-N-O···H-C and C-O···H-C. The most possible interaction is explained using natural bond orbital (NBO) analysis. The effects of molecular association through O-H···O hydrogen bonding have been described by the single dimer structure. The strengthening and polarization of the C=O bond increases due to the degree of conjugation. HOMO-LUMO energy and the thermodynamic parameters are also evaluated. The thermodynamic functions (heat capacity, internal heat energy, Gibbs energy and entropy) from spectroscopic data by statistical methods were obtained for the range of temperature 100-1000 K.