Computational Physics

In this work, the experimental and theoretical spectra of 2-(trifluoroacetyle)acetophenone (2TFAAP) are studied and FT-IR and FT-Raman spectra of title molecule have been recorded in the region 4000–400cm?1 and 3500–¬100cm?1, respectively. The structural and spectroscopic data of the molecule in the ground state have been calculated by using Hartree-Fock and density functional theory method with the B3LYP/6-31G basis sets. The vibrational frequencies are calculated and scaled values are compared with the experimental FT-IR and FT-Raman spectra. The DFT (B3LYP/6-31G) calculations are more reliable than the ab initio HF/6-31G calculations for the vibrational study of (2TFAAP). The optimized geometric parameters (bond lengths and bond angles) are compared with experimental values of the molecule. Stability of the molecule was analyzed using NBO analysis. Thermal properties of the molecule have been calculated for various range of temperature and HOMO-LUHO energies also calculated, it shows that the charge transfer occurs within the molecule.

The solid phase FTIR and FT-Raman spectra of 2-chloro-5-nitrobenzaldehyde (CNB) 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 CNB in two conformational states of C₁ and C₂ obtained by the ab initio HF and DFT levels of theory using B3LYP/6-31G** 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 with in the molecule. The observed and calculated frequencies are found to be in good agreement.

The understanding of optimized molecular geometry, vibrational analysis of the heterocyclic organic compounds plays a vital role in the process of drug discovery. The present work provides geometrical parameters, vibrational assignments for pharmaceutical intermediate N-(Methyl)phthalimide (NMP). Moreover, the present study aims to illustrate how intramolecular interactions appear within the molecule on account of HOMO-LUMO studies. In addition to these, Mullikan’s Atomic charges associated with each atom of the stable conformer are also reported. Entire vibrational, geometrical parameters, Mullikan’s Atomic charges and HOMO-LUMO Energy gap of NMP were predicted with the aid of B3LYP level of theory with 6-311++G(d,p) basis set on a quantum chemical software Gaussian 03W. In view of visual inspection, 51 normal modes of vibrations contributed to NMP were found out. HOMO-LUMO studies provided information about occupied and unoccupied molecular orbitals and intramolecular interactions of NMP. Mullikan’s Atomic charge on each atom of NMP shows Charge-stability relations.

The FTIR and FT-Raman spectra of the o-nitrobenzaldehyde oxime (ONBO) have been recorded in the regions 4000-400 and 3500-100 cm^-1 respectively. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimization and force field calculations based on density functional theory (DFT) using standard B3LYP/6-31+G(d,p) and B3LYP/6-31++G(d,p) method and basis set combination. The geometries and normal modes of vibration obtained from the DFT method are in good agreement with the experimental data. The total energy distribution (TED) values obtained reflect the correctness of the vibrational assignments.

The infrared and Raman spectra of L-cysteine molecule has been recorded in the nujol mull as well as in the aqueous solution in the range of 4000-400cm-1. The observed infrared and Raman frequencies are assigned on the basis of the theoretically calculated vibrational frequencies for the most stable structure of zwitterionic cysteine hydrated with six water molecules at B3LYP/6-311G* level. The electronic absorption spectrum of the cysteine molecule has been measured in solution at different concentrations and at different pH in the range of 400- 200 nm. It is observed that the electronic absorption band shifts with the change of pH as well as the concentrations. The TDDFT calculations for the same molecule (zwitterionic cysteine + six water molecules) has also been carried out at B3LYP/6-311G* level. The optimized bond lengths and bond angles for the most stable structure of cysteine hydrated with six water molecules were compared with those reported for X-ray data for cysteine.

A combined experimental and theoretical studies were conducted on the molecular structure and vibrational, spectra of 3-methyl-2-nitro anisole (MNA). The FT-IR and FT-Raman spectra of (MNA) were recorded in the solid phase. The molecular geometry and vibrational frequencies of MNA in the ground state have been calculated by using the ab-initio HF (Hartree-Fock) and density functional methods (B3LYP) invoking 6-31+G (d,p) basis set. The optimized geometric bond lengths and bond angles obtained by HF method shows best agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of MNA with calculated results by HF and density functional methods indicates that B3LYP is superior to the scaled HF approach for molecular vibrational problems. The difference between the observed and scaled wave number values of most of the fundamental is very small. The thermodynamic functions and atomic change of the title compound was also performed at HF/B3LYP/6-31+G(d,p) level of theories. A detailed interpretation of the NBO, NMR spectra of MNA was also reported. The thermodynamic function of the title compound was also performed at HF/6-31+G (d,p) and B3LYP/6-31+G (d,p) level of theories. Natural bond orbital analysis has been carried out to explain the change transfer or delocalization of change due to the intra-molecular interactions. Energy of the highest occupied molecular (HOMO) orbital and lowest unoccupied (LUMO) molecular orbital have been predicted.

The FT-IR and FT-Raman spectra of 4-nitro-o-toluidine (4NOT) were recorded in the region 4000–400 cm?1 and 4000–100 cm?1 respectively. Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of 4NOT were carried out by density functional theory (DFT/B3LYP) method with 6-31G* and 6-311+G** basis sets. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. The values of the total dipole moment (?) and the first order hyperpolarizability (?) of the compound were investigated. The calculated results show that 4NOT have non-linear optical (NLO) behavior. A detailed interpretation of Infrared and Raman spectra of 4NOT is also reported. The calculated HOMO–LUMO energy gap shows that charge transfer occurs within the molecule. Electronic excitation energies, oscillator strength and nature of the respective excited states were calculated by the closed-shell singlet calculation method for the molecule.

In the present study, the FT-IR and FT-Raman spectra of 6-amino-2,3-dihydro-1,4-phthalazinedione (ADHPD) have been recorded in the region 4000-400 cm-1 and 3500-50 cm-1, respectively. The fundamental modes of vibrational frequencies of ADHPD are assigned. Theoretical information on the optimized geometry, harmonic vibrational frequencies, infrared and Raman intensities were obtained by means of Hartree-Fock (HF) and density functional theory (DFT) gradient calculations with complete relaxation in the potential energy surface using 6-311+G(d,p) basis set. The vibrational frequencies which were determined experimentally from the spectral data are compared with those obtained theoretically from ab intio HF and DFT/B3LYP calculations. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors. The predicted first hyperpolarizability also shows that the molecule might have a reasonably good non-linear optical (NLO) behaviour. The calculated HOMO-LUMO energy gap reveals that charge transfer occurs within the molecule. Unambiguous vibrational assignment of all the fundamentals was made using the total energy distribution (TED).

The Fourier transform infrared (FTIR) and FT - Raman spectra of 2, 6-dichlorophenyl isocyanate have been recorded in the liquid state. The molecular geometry, vibrational frequencies, and atomic charges have been calculated by using density functional theory (DFT) calculation (B3LYP) with 6-311+G(d,p) and 6-311++G(d,p) basis sets. Simulated FTIR and FT - Raman spectra for 2, 6-dichlorophenyl isocyanate showed good agreement with the observed spectra. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The molecular stability and bond strength were investigated by applying the natural bond orbital (NBO) analysis and electrostatic potential (ESP). The isotropic chemical shift computed by 1H and 13C nuclear magnetic resonance (NMR) chemical shifts of the 2, 6-dichlorophenyl isocyanate calculated using the gauge invariant atomic orbital (GIAO) method also shows good agreement with experimental observations.

In this work, the experimental and theoretical vibrational spectra of 3'-nitroacetophenone [3?NAP] were studied. FTIR and FT Raman spectra were recorded in the region 4000–400 cm?1 and 4000–50 cm?1, respectively. The structural and spectroscopic data of the molecule in the ground state were calculated by using ab initio Hartree–Fock and density functional method (B3LYP) with the 6-311++G(d,p) basis set. The vibrational frequencies were calculated and scaled values were compared with experimental FTIR and FT Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The optimized geometric parameters were calculated. The predicted first hyperpolarizability also shows that the molecule might have a reasonably good nonlinear optical (NLO) behaviour. The calculated HOMO–LUMO energy gap reveals that the charge transfer occurs within the molecule besides, molecular electrostatic potential (MEP),