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Papers by Shabbir Ahmad

Research paper thumbnail of Computational and Vibrational Spectroscopy Study of 5Nitro6-Methyluracil by Density Functional Theory

Vibrational spectra of 5-nitro-6-methyluracil have been investigated using quantum chemical densi... more Vibrational spectra of 5-nitro-6-methyluracil have been investigated using quantum chemical density functional calculations, FT-IR (400-4000 cm-1) and Raman (50-4000 cm-1) spectra. Harmonic vibrational frequencies were calculated by DFT (B3LYP) method using the basis sets 6-31G and 6-311G and compared with the experimental ones. The vibrational band compositions of various modes are reported.

Research paper thumbnail of Anharmonic vibrational analysis of uracil by ab initio Hartree–Fock and density functional theory calculations

Journal of Molecular Structure-theochem, 2009

The molecular structure and vibrational spectra of uracil have been investigated by Hartree-Fock ... more The molecular structure and vibrational spectra of uracil have been investigated by Hartree-Fock and density functional (B3LYP) methods with the 6-31G(d,p) basis set. The optimized geometric bond lengths and bond angles obtained by computations show good agreement with experimental data. Comparison of the experimental spectra with anharmonic vibrational frequencies indicates that B3LYP results are more accurate than the HF approach for the molecule.

Research paper thumbnail of Computational studies of vibrational spectra and molecular properties of 6-methyluracil using HF, DFT and MP2 methods

Indian Journal of Physics, 2011

Theoretical investigations of atomic charges, conformers, frontier molecular orbitals, molecular ... more Theoretical investigations of atomic charges, conformers, frontier molecular orbitals, molecular geometries, thermodynamic properties, hyperpolarizabilities and harmonic vibrational frequencies of 6-methyluracil (6MU) have been carried out using ab initio Hartree-Fock (HF), density functional theory (DFT) and second order Møller-Plesset (MP2) methods. All calculations were performed using the GAMESS-US program package with the basis sets 6-31G(d,p) and 6-311G(d,p). FT-IR and Raman spectra of 6MU were recorded in the regions 50–4000 cm−1 and 60–4000 cm−1 respectively. Optimized geometries were obtained using the global optimization procedure. The calculated structural parameters for two conformers of 6MU have been compared with experimentally observed values. The energy barrier (ΔE=ELUMO-EHOMO) between the HOMO and LUMO is predicted on the basis of theoretical calculations. The simulated TD-DFT spectrum has been compared with experimental electronic spectrum for 6MU. The calculated potential energy distribution (PED) values have been utilized to perform vibrational assignment of the infrared and Raman spectra.

Research paper thumbnail of Computational and Vibrational Spectroscopy Study of 5Nitro6-Methyluracil by Density Functional Theory

Vibrational spectra of 5-nitro-6-methyluracil have been investigated using quantum chemical densi... more Vibrational spectra of 5-nitro-6-methyluracil have been investigated using quantum chemical density functional calculations, FT-IR (400-4000 cm-1) and Raman (50-4000 cm-1) spectra. Harmonic vibrational frequencies were calculated by DFT (B3LYP) method using the basis sets 6-31G and 6-311G and compared with the experimental ones. The vibrational band compositions of various modes are reported.

Research paper thumbnail of Anharmonic vibrational analysis of uracil by ab initio Hartree–Fock and density functional theory calculations

Journal of Molecular Structure-theochem, 2009

The molecular structure and vibrational spectra of uracil have been investigated by Hartree-Fock ... more The molecular structure and vibrational spectra of uracil have been investigated by Hartree-Fock and density functional (B3LYP) methods with the 6-31G(d,p) basis set. The optimized geometric bond lengths and bond angles obtained by computations show good agreement with experimental data. Comparison of the experimental spectra with anharmonic vibrational frequencies indicates that B3LYP results are more accurate than the HF approach for the molecule.

Research paper thumbnail of Computational studies of vibrational spectra and molecular properties of 6-methyluracil using HF, DFT and MP2 methods

Indian Journal of Physics, 2011

Theoretical investigations of atomic charges, conformers, frontier molecular orbitals, molecular ... more Theoretical investigations of atomic charges, conformers, frontier molecular orbitals, molecular geometries, thermodynamic properties, hyperpolarizabilities and harmonic vibrational frequencies of 6-methyluracil (6MU) have been carried out using ab initio Hartree-Fock (HF), density functional theory (DFT) and second order Møller-Plesset (MP2) methods. All calculations were performed using the GAMESS-US program package with the basis sets 6-31G(d,p) and 6-311G(d,p). FT-IR and Raman spectra of 6MU were recorded in the regions 50–4000 cm−1 and 60–4000 cm−1 respectively. Optimized geometries were obtained using the global optimization procedure. The calculated structural parameters for two conformers of 6MU have been compared with experimentally observed values. The energy barrier (ΔE=ELUMO-EHOMO) between the HOMO and LUMO is predicted on the basis of theoretical calculations. The simulated TD-DFT spectrum has been compared with experimental electronic spectrum for 6MU. The calculated potential energy distribution (PED) values have been utilized to perform vibrational assignment of the infrared and Raman spectra.

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