Vibrational study of tolazoline hydrochloride by using FTIR-Raman and DFT calculations.

Quantum mechanical (QM) calculations have been carried out in order to study the tolazoline hydrochloride theoretical structure and vibrational properties. This compound was characterized by infrared and Raman spectroscopies in the solid phase. For a complete assignment of the IR and Raman spectra, the density functional theory (DFT) calculations were combined with Pulay's Scaled Quantum Mechanics Force Field (SQMFF) methodology in order to fit the theoretical frequency values to the experimental ones. An agreement between theoretical and available experimental results was found. Three intense bands in the infrared spectrum characteristic of the protonated species of the compound were detected. Also, the possible charge-transfer and the topological properties for both benzyl and imidazoline rings were studied by means of Natural Bond Orbital (NBO) and Atoms in Molecules theory (AIM) investigation.

Theoretical and experimental vibrational spectrum study of 4-hydroxybenzoic acid as monomer and dimer.

Theoretical calculations on the molecular geometry and the vibrational spectrum of 4-hydroxybenzoic acid were carried out by the Density Functional Theory (DFT/B3LYP) method. In addition, IR and Raman spectra of the 4-hydroxybenzoic acid in solid phase were newly recorded using them in conjunction the experimental and theoretical data (including SQM calculations), a vibrational analysis of this molecular specie was accomplished and a reassignment of the normal modes corresponding to some spectral bands was proposed. The geometries of monomers and dimers in gas phase were optimized using the DFT B3LYP method with the 6-31G*, D95** and 6-311++G** basis sets. Also, both the vibrational spectra recorded and the results of the theoretical calculations show the presence of one stable conformer for the 4-hydroxybenzoic acid cyclic dimer. The B3LYP/6-31G* method was used to study the structure for cyclic dimer of 4-hydroxybenzoic acid and for a complete assignment our results were compared with results of the cyclic dimer of benzoic acid. A scaled quantum mechanical analysis was carried out to yield the best set of harmonic force constants. The formation of the hydrogen bond was investigated in terms of the charge density by the AIM program and by the NBO calculations.

DFT calculation of the chromyl nitrate, CrO2(NO3)2 The molecular force field.

We have carried out a structural and vibrational theoretical study for chromyl nitrate. The density functional theory has been used to study its structure and vibrational properties. The geometries were fully optimised at the B3LYP/Lanl2DZ, B3LYP/6-31G* and B3LYP/6-311++G levels of theory and the harmonic vibrational frequencies were evaluated at the same levels. The calculated harmonic vibrational frequencies for chromyl nitrate are consistent with the experimental IR and Raman spectra in the solid and liquid phases. These calculations gave us a precise knowledge of the normal modes of vibration taking into account the type of coordination adopted by nitrate groups of this compound as monodentate and bidentate. We have also made the assignment of all the observed bands in the vibrational spectra for chromyl nitrate. The nature of the Cr-O and Cr<--O bonds in the compound were quantitatively investigated by means of Natural Bond Order (NBO) analysis. The topological properties of electronic charge density are analysed employing Bader's Atoms in Molecules theory (AIM).

Experimental and theoretical study of the hydration of phosphate groups in esters of biological interest.

We have studied the influence of different groups esterified to phosphates on the strength of the interaction of the PO bond with one water molecule. Experimental vibrational spectra of PO(4)3-, HPO4(2-), H2PO4-, phosphoenolpiruvate (PEP) and ortho-phosphocholamine (o-PC) were obtained by means of FTIR spectroscopy. Geometry calculations were performed using standard gradient techniques and the default convergence criteria as implemented in GAUSSIAN 98 Program. In order to assess the behaviour of such DFT theoretical calculations using B3LYP with 6-31G* and 6-311++G** basis sets, we carried out a comparative work for those compounds. The results were then used to predict the principal bands of the vibrational spectra and molecular parameters (geometrical parameters, stabilisation energies, electronic density). In this work, the relative stability and the nature of the PO bond in those compounds were systematically and quantitatively investigated by means of Natural Bond Order (NBO) analysis. The topological properties of electronic charge density are analysed employing Bader's Atoms in Molecules theory (AIM). The hydrogen bonding of phosphate groups with water is highly stable and the PO bond wavenumbers are shifted to lower experimental and calculated values (with the DFT/6-311++G** basis set). Accordingly, the predicted order of the relative stability of the hydrogen bonding of the water molecule to the PO bond of the investigated compounds is: PO(4)3->HPO4(2-)>H2PO4->phosphoenolpiruvate>phosphocholamine for the two basis sets used.

Hydration of inorganic phosphates in crystal lattices and in aqueous solution. An experimental and theoretical study.

B3LYP/6-31G* and 6-311++G** calculations have been carried out in order to study the hydration of phosphates in aqueous media. Optimized geometries and relative stabilities for PO4(-3), HPO(4)-2, H2PO4(-1) have been calculated considering the interaction with one, two, three, four and five discrete water molecules and taking into account the solvent effect by using the self-consistent reaction field theory (Onsager and PCM methods). The role of specific and bulk contributions of solvent effect on the observable properties of phosphate compounds is analysed. Good agreement between theoretical and available experimental results of harmonic vibration frequencies is found. Significant effects on the geometrical and vibrational frequencies are found for those studied phosphate anions. The results presented here provide a first step toward the understanding of the phosphate group as a hydration sensor in lipid bilayers.