Silvia Antonia Brandán

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.

A complete characterization of the vibrational spectra of sucrose

We combined experimental vibrational spectroscopy (FTIR-Raman) and ab-initio calculations based on density functional theory (DFT) to predict the structural and vibrational properties of sucrose in solid phase. The structural properties of sucrose, such as the bond order, possible charge-transfer, and the topological properties of the glucopyran and glucofuran rings were studied by means of the Natural Bond Orbital (NBO) and Atoms in Molecules theory (AIM) investigation. For a complete assignment of the infrared and Raman spectra, the density functional theory (DFT) calculations were combined with Pulays 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. A complete assignment of the 129 normal vibration modes for sucrose was performed. Five very intense characteristic bands in the infrared spectrum of sucrose at 3391, 3339, 1069, 1053, and 991 cm(-1) were assigned, the first two to the OH stretching modes while the other ones to C-O stretching modes.

Density Functional Theory Calculations of the Molecular Force Field of l-Ascorbic Acid, Vitamin C

We have studied L-ascorbic acid and characterized it by infrared spectroscopy in solid and aqueous solution phases. The density functional theory (DFT) method together with Poples basis set show that three stable molecules for the compound have been theoretically determined in the gas phase, and that an average of only two more stable conformations are present in the solid phase, as it was experimentally observed. The harmonic vibrational wavenumbers for the optimized geometries of both structures were calculated at B3LYP/6-31G*and B3LYP/6-311++G** levels at the proximity of the isolated molecule. For a complete assignment of the vibrational spectra in the compound solid and aqueous solution phases, DFT calculations were combined with Pulays scaled quantum mechanics force field methodology in order to fit the theoretical wavenumber values to the experimental ones. In this way, a complete assignment of all the observed bands in the infrared spectrum for l-ascorbic acid was performed. The natural bond orbital study reveals the characteristics of the electronic delocalization of the three structures while the corresponding topological properties of electronic charge density are analyzed by employing Baders atoms-in-molecules theory.

An experimental and theoretical study of L-Tryptophan in an aqueous solution, combining two-layered ONIOM and SCRF calculations.

ONIOM and SCRF calculations were performed to study the most stable theoretical structure of L-tryptophan in an aqueous solution phase and to observe the changes occurring in the structural and vibrational properties of L-tryptophan in the aqueous media. L-Tryptophan was characterized by infrared and Raman spectroscopies in the solid state and as an aqueous solution. Optimized geometries and relative stabilities for the L-tryptophan zwitterion were calculated while taking into account solvent effects using the self-consistent reaction field (SCRF) theory. The obtained results in the aqueous solution were compared with those calculated for the zwitterion in the gas phase by means of two-layered ONIOM calculations. For the complete assignments of the IR and Raman spectra of L-tryptophan in the aqueous solution phase, density functional theory (DFT) calculations were combined with Pulays scaled quantum mechanical force field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental values. Additionally, the roles of specific and bulk contributions from solvent effects on the properties of l-tryptophan were analyzed. Furthermore, bands corresponding to the normal modes of vibration were localized and assigned, and they served as the bases for the calculations of the corresponding force constants. Significant effects on the geometrical and vibrational frequencies were found for the studied zwitterion.

Experimental and theoretical vibrational investigation on the saccharinate ion in aqueous solution.

We combined experimental vibrational spectroscopy (FTIR-Raman) and ab-initio calculations based on the density functional theory (DFT) to predict the structural and vibrational properties of sodium saccharinate in the solid and aqueous solution phases. The structural properties for the saccharinate ion and its dimer, such as the bond order, possible charge-transfer and the topological properties for both rings in the two media were studied by means of the Natural Bond Orbital (NBO) and the Atoms in Molecules theory (AIM) investigation. For a complete assignment of the IR and Raman spectra, the density functional theory calculations were combined with Pulays 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. Four intense bands in the infrared spectrum characteristic of the dimeric species of the compound were detected.

Vibrational Study and Force Field of the Citric Acid Dimer Based on the SQM Methodology

We have carried out a structural and vibrational theoretical study for the citric acid dimer. The Density Functional Theory (DFT) method with the B3LYP/6-31G∗ and B3LYP/6-311

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 Pulays 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.

A complete assignment of the vibrational spectra of sucrose in aqueous medium based on the SQM methodology and SCRF calculations.

In the present study, a complete assignment of the vibrational spectra of sucrose in aqueous medium was performed combining Pulays Scaled Quantum Mechanics Force Field (SQMFF) methodology with self-consistent reaction field (SCRF) calculations. Aqueous saturated solutions of sucrose and solutions at different molar concentrations of sucrose in water were completely characterized by infrared, HATR, and Raman spectroscopies. In accordance with reported data of the literature for sucrose, the theoretical structures of sucrose penta and sucrose dihydrate were also optimized in gas and aqueous solution phases by using the density functional theory (DFT) calculations. The solvent effects for the three studied species were analyzed using the solvation PCM/SMD model and, then, their corresponding solvation energies were predicted. The presence of pure water, sucrose penta-hydrate, and sucrose dihydrate was confirmed by using theoretical calculations based on the hybrid B3LYP/6-31G(∗) method and the experimental vibrational spectra. The existence of both sucrose hydrate complexes in aqueous solution is evidenced in the IR and HATR spectra by means of the characteristic bands at 3388, 3337, 3132, 1648, 1375, 1241, 1163, 1141, 1001, 870, 851, 732, and 668cm(-1) while in the Raman spectrum, the groups of bands in the regions 3159-3053cm(-1), 2980, 2954, and 1749-1496cm(-1) characterize the vibration modes of those complexes. The inter and intra-molecular H bond formations in aqueous solution were studied by Natural Bond Orbital (NBO) and Atoms in Molecules theory (AIM) investigation.

Structural study, coordinated normal analysis and vibrational spectra of 4-hydroxy-3-(3-methyl-2-butenyl)acetophenone

Structural and vibrational properties of 4-hydroxy-3-(3-methyl-2-butenyl)acetophenone, isolated from Senecio nutans Sch. Bip. (Asteraceae) were studied by infrared and Raman spectroscopies in solid phase. The Density Functional Theory (DFT) method together with Poples basis set show seven stable conformers for the compound in the gas phase and that only two conformations are probably present in the solid phase. The harmonic vibrational wavenumbers for the optimized geometry were calculated at B3LYP/6-31G and B3LYP/6-311++G levels. For a complete assignment of the vibrational spectra, DFT calculations were combined with Pulay´s Scaled Quantum Mechanics Force Field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental ones. Then, a complete assignment of all the observed bands in the vibrational spectra was performed. The natural bond orbital (NBO) study reveals the characteristics of the electronic delocalization of the two stable structures, while the corresponding topological properties of electronic charge density were analyzed by employing Baders Atoms in the Molecules theory (AIM).

Theoretical study of the structure and vibrational spectra of VO2X2− (X=F, Cl, Br, I) anions

Abstract We have carried out a structural and vibrational theoretical study for the four members of the series VO 2 X 2 − (X=F, Cl, Br, I). In order to assess the behaviour of such theoretical calculations (ab initio and DFT) we have made a comparative work for the fluorine and chlorine derivatives, for which vibrational data exist, to evaluate not only the best level of theory but also the best basis set to be used to reproduce the experimental wavenumbers. The results were then used to predict the vibrational spectra and molecular geometry of the other members of the series, for which there are no experimental data. The theoretical vibrational calculations allowed us to obtain a set of scaled force constants fitting the observed wavenumbers.