V.K. Rastogi

Quantum chemical predictions of the vibrational spectra of polyatomic molecules. The uracil molecule and two derivatives

This work describes the different scaling procedures used to correct the quantum-chemical theoretical predictions of the IR and Raman vibrational wavenumbers. Examples of each case are shown, with special attention to the uracil molecule and some derivatives. The results obtained with different semiempirical and ab initio methods, and basis sets, are compared and discussed. A comprehensive compendium of the main scale factors and scaling equations available to obtain the scaled wavenumbers is also shown.

3,5-Difluorobenzonitrile: ab initio calculations, FTIR and Raman spectra

Geometry, vibrational wavenumbers and several thermodynamic parameters were calculated using ab initio quantum chemical methods for the 3,5-difluorobenzonitrile molecule. The results were compared with the experimental values. With the help of three specific scaling procedures, the observed vibrational wavenumbers in FTIR and Raman spectra were analysed and assigned to different normal modes of the molecule. Most of the modes have wavenumbers in the expected range and the error obtained was in general very low. Using PEDs the contributions were determined for the different modes to each wavenumber. From the PED, it is apparent that the frequency corresponding to C[triple bond]N stretching contains 87% contribution from the C[triple bond]N stretching force constant and it mixes with the C-CN stretching mode 13 to the extent of 12%. Other general conclusions were also deduced.

Vibrational frequencies and structure of 2-thiouracil by Hartree/Fock, post-Hartree/Fock and density functional methods

Vibrational study of the biomolecule 2-thiouracil was carried out. Ab initio and density functional calculations were performed to assign the experimental spectra. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted frequencies of 2-thiouracil. Several scaling procedures were used. The geometry structure of the molecule was determined. The effect of sulfur substitution at C2 position in the uracil molecule, on the N1-H and N3-H frequencies and intensities reflects changes in proton donor abilities of these groups. Calculations with the 6-31 G** basis set with HF and DFT methods appear in general to be useful for interpretation of the general features of the IR and Raman spectra of the molecule. Using specific scale factors a very small error was obtained. The use of these specific scale factors resolve and correct some of the controversial assignments in the literature.

Relationships observed in the structure and spectra of uracil and its 5-substituted derivatives.

The effects on the geometry structure, atomic charges and vibrational wavenumbers of the main different substituents in the 5th position of the uracil ring were analysed, and relationships were established. The 5-monosubstituted derivatives studied were 5-XU (X=F, Cl, Br, I, CH(3), NH(2), NO(2)). The geometry and vibrational wavenumbers were determined in these molecules. The FT-IR and Raman spectra were studied with the support of B3LYP calculations using several basis sets. Several general conclusions were underlined.

Density functional theory calculations and vibrational spectral analysis of 3,5-(dinitrobenzoic acid)

The FT-IR and Raman spectra of 3,5-dinitrobenzoic acid (DNBA) 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) method. Most of the vibrational modes are observed in the expected range. Mulliken population analysis shows the interactions C-N-O⋯H-C and C-O⋯H-C. The most possible interaction is explained using natural bond orbital (NBO) analysis. The strengthening and polarization of the CO bond increases due to the degree of conjugation. HOMO-LUMO energy and the thermodynamic properties are also evaluated.

FTIR and FT-Raman spectra of 5-methyluracil (thymine)

FTIR and FT-Raman spectra of 5-methyluracil (thymine) were recorded in the regions 400–4000 cm−1 and 50–4000 cm−1, respectively. The observed wavenumbers were analysed and assigned to different normal modes of vibration of the molecule. Ab initio and density functional calculations were performed to support the assignments of the observed wavenumbers. The geometry structure was determined and a comparison with the uracil molecule was made. Good reproducibility of the experimental wavenumbers was obtained at the B3LYP level and the percentage error was very small, less than 1% in most cases. Copyright

Vibrational spectra, tautomerism and thermodynamics of anticarcinogenic drug: 5-fluorouracil.

The FT-IR and FT-Raman spectra of 5-Fluorouracil were recorded in the solid phase in the regions 400-4000 cm(-1) and 50-4000 cm(-1), respectively. The vibrational spectra were analysed and the observed fundamentals were assigned to different normal modes of vibration. The experimental wavenumbers were compared with the scaled vibrational values using DFT methods: the Ar matrix data were related to gas phase calculations, while the values of the solid state spectra were compared to those with dimer simulations. The study indicates that some features that are characteristic of vibrational spectra of uracil and its derivatives are retained in the spectrum of 5-fluorouracil and it exists in ketonic form in the solid phase. The tautomerism was also studied and the spectra of the two most stable forms were simulated. The calculated wavenumbers have been employed to yield thermodynamic properties.

Ab initio calculations, FTIR and Raman spectra of 2,3-difluorobenzonitrile

Geometry, vibrational frequencies, atomic charges and several thermodynamic parameters (the total energy, the zero point energy, the rotational constants and the room temperature entropy) were calculated using ab initio quantum chemical methods for 2,3-difluorobenzonitrile molecule. The results were compared with experimental values. With the help of two specific scaling procedures, observed FTIR and Raman vibrational frequencies were analysed and assigned to different normal modes of the molecule. The error obtained was in general very low. Other general conclusions have also been deduced.

Quantum Chemical Scaling and Its Importance: The Infrared and Raman Spectra of 5-Bromouracil

ABSTRACT This work describes the interest and necessity of scaling to correct the deficiencies in the calculation of the harmonic vibrational wave numbers. The use of adequate quantum-chemical methods and scaling procedures reduces the risk in the assignment and can also accurately determine the contribution of the different modes in an observed band. As an example, the IR and laser-Raman spectra of the 5-bromouracil biomolecule are shown.

Molecular structure and vibrational spectral investigation of charge transfer NLO crystal Naphthalene Picrate for THz application

Molecular structure and the vibrational spectra of Naphthalene Picrate have been calculated using density functional theoretical computation and compared with the experimental. The dipole moments (μ), polarizability (α), first hyperpolarizabilities (β) second hyperpolarizability (γ) and frontier molecular orbital energies are computed at the DFT level. The frontier molecular orbital calculation shows the inverse relationship of HOMO-LUMO gap with the total static hyperpolarizability. The hyperpolarizability value reveals that these classes of organic compounds show very large non-linear optical properties. Natural Bond Orbital analysis confirms the presence of intramolecular charge transfer and the hydrogen bonding interaction. Terahertz time-domain spectroscopy has been used to detect the absorption spectra in the frequency range from 0.2 to 1.5 THz.