Mozart N. Ramos

Infrared intensities: from intensity parameters to an overall understanding of the spectrum

Abstract The current state of the art in the field of vibrational intensity is reviewed. The parametrisation of the infrared intensities and the physical meaning of the parameters is discussed. A comparison is made of experimental infrared atomic charges with those calculated from quantum-chemical methods. The applications of intensity parameters in spectroscopy and in molecular sciences are presented and discussed.

An ab initio study of hydrogen complexes of the X-H … π type between acetylene and HF or HCl

Abstract Ab initio calculations are performed at the HF/6-311G, HF/6-311G∗∗, HF/6-311++G∗∗, MP2/6-311G∗∗ and MP2/6-311++G∗∗ theoretical levels to obtain geometries, H-bond energies and vibrational properties for the symmetrical T-shaped structures of hydrogen-bonded complexes involving acetylene (as the proton acceptor) and the halides HF and HCl (as the proton donors). The calculated H-bond lengths are a little overestimated at the Hartree-Fock level, whereas MP2 results are in very close agreement with the experimental values. For acetylene-HF the H-bond energy is predicted to be 11 kJ mol−1 at the HF level (including BSSE) and 19 kJ mol−1 at the MP2 level, indicating that the complex is more strongly bound than a simple van der Waals complex. The acetylene-HCl complex is less strongly bound than acetylene-HF, with a ΔE value of 5.8 kJ mol−1 at the HF/6-311++G∗∗ level (including BSSE) and of 13.2 kJ mol−1 at the MP2/6-311++G∗∗ level. The strengths of the H-bonds in both complexes can be interpreted in terms of intermolecular charge transfers by means of atomic charges derived from infrared intensity parameters. The intermolecular vibrational modes exhibit the usual effects of H-bond formation, which are an increased intensity in the X-H stretching of the proton-donor mode together with little change in the electron-donor spectrum. These normal modes are adequately interpreted in terms of the charge-charge flux-overlap modified model for infrared intensities. The new vibrational modes arising from complexation show several interesting features, especially the bending modes of the proton-donor molecule and the intermolecular stretching mode.

An ab initio study of the molecular properties of the acetylene-HX hydrogen complexes

Abstract MP2/6–311++G∗∗ ab initio molecular orbital calculations indicate that larger ΔQcorr intermolecular charge transfer values are associated with stronger hydrogen bonds in the acetylene-HX complexes where X is F, Cl, CN, NC or CCH. The MP2/6–311++G∗∗ H-bond lengths are in very good agreement with the corresponding experimental values. The HX stretching frequency is shifted downward upon H-bond formation. Its displacement shows an excellent linear correlation with the intermolecular charge transfer, in agreement with the experimental behaviour previously observed in such complexes. As expected, the more pronounced effect on the IR intensities occurs with the HX stretching intensity, and it is much enhanced after complexation owing to the charge-flux term. The new low-frequency vibrational modes arising from complexation show several interesting features and their normal modes are schematically described herein.

Rotational Isomerism in Acrylic Acid: A Combined Matrix-isolated IR, Raman and ab initio Molecular Orbital Study

The results of a combined vibrational and structural study of the acrylic acid monomer undertaken by matrix-isolated low-temperature IR spectroscopy and ab initio SCF-HF and MP2 MO calculations are presented. In addition, both Raman and IR spectra of liquid acrylic acid and the Raman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exists as a mixture of two conformers of similar energies, differing by the relative orientation of the CC—CO axis. Upon irradiation at λ= 243 nm by a xenon lamp, the s-cis form (CC—CO dihedral angle equal to 0 °), corresponding to the conformational ground state, converts to the s-trans form (CC—CO dihedral angle equal to 180 °). In the liquid phase, dimeric structures strongly predominate, but the existence in this phase of the two conformational states referred to above can also be inferred from the corresponding vibrational spectra. In turn, in the crystal only the thermodynamically most stable form (s-cis) exists. Results of ab initio SCF-HF and MP2 molecular orbital (MO) calculations, in particular optimised geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of acrylic acid are also presented and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the studied conformers. Finally, the calculated vibrational spectra and both the results of a normal-mode analysis based on the theoretical harmonic force fields and of IR intensity studies based on the charge–charge flux–overlap (CCFO) model were used to help interpret the experimental vibrational data, enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.

s-cis and s-trans Conformers of formic, thioformic and dithioformic acids. An ab initio study

Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6–31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the ‘charge’-‘charge flux’-‘overlap’(CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis(thiol) > s-trans(thiol) > s-cis(thione) > s-trans(thione)]. The success of these ab initio calculations should be partially ascribed to the inclusion of polarization functions on all non-hydrogen atoms.

The molecular properties of heterocyclic and homocyclic hydrogen-bonded complexes evaluated by DFT calculations and AIM densities.

This theoretical study presents a comparative analysis of the molecular properties of heterocyclic (C2H4O⋯HF and C2H5N⋯HF) and homocyclic (C3H6⋯HF) hydrogen-bonded complexes. Initially, the equilibrium geometries of these complexes were analyzed in detail at the B3LYP/6–311++G(d,p) level of theory. Subsequently, the interaction energies and polarizabilities were also evaluated, as well as the infrared stretch frequencies and absorption intensities. In addition, by combining intermolecular criteria and charge density concepts, calculations of Bader’s theory of atoms in molecules were used to determine the maxima and minima for electron density in order to measure the strength of the n⋯H and pπ⋯H hydrogen bonds. Finally, the possibility of an F⋯Hα secondary interaction between the fluoride (F) of hydrogen fluoride and the axial hydrogen atoms (Hα) of the C2H4O and C2H5N heterocyclic rings was explored.

The electronic structure of the C2H4O···2HF tri-molecular heterocyclic hydrogen-bonded complex: a theoretical study

AbstractThe geometries of three isomers of the C2H4O···2HF tri-molecular heterocyclic hydrogen-bonded complex were examined through B3LYP/aug-cc-pVDZ calculations. Analysis of structural parameters, determination of CHELPG (charge electrostatic potential grid) intermolecular charge transfer, interpretation of infrared stretching modes, and Bader’s atoms in molecules (AIM) theory calculations was carried out in order to characterize the hydrogen bonds in each isomer of the C2H4O···2HF complex. The most stable structure was determined through the identification of hydrogen bonds between C2H4O and HF, (O···H), as well as in the hydrofluoric acid dimer, (HFD–R···HFD). However, the existence of a tertiary interaction (Fλ···Hα) between the fluoride of the second hydrofluoric acid and the axial hydrogen atoms of C2H4O was decisive in the identification of the preferred configuration of the C2H4O···2HF system. FigureGeometries of three isomers of the C2H4O···2HF tri-molecular heterocyclic hydrogen-bonded complex

Um estudo teórico de propriedades moleculares em complexos de hidrogênio trimoleculares C2H4···2HF, C2H2···2HF e C3h6···2HF

We present a theoretical study of molecular properties in C2H4···2HF, C2H2···2HF and C3H6···2HF trimolecular hydrogen-bonded complexes. From B3LYP/6-311++G(d,p) calculations, the most important structural deformations are related to the C=C (C2H4), C≡C (C2H2), C-C (C3H6) and HF bond lengths. According to the Baders atoms in molecules and CHELPG calculations, it was identified a tertiary interaction between the fluorine atom of the second hydrofluoric acid molecule and hydrogen atoms of the ethylene and acetylene within the C2H4···2HF and C2H2···2HF complexes, respectively. Additionally, the evaluation of the infrared spectrum characterized the new vibrational modes and bathochromic effect of the HF molecules.

Synthesis and analgesic profile of novel N-containing heterocycle derivatives: arylidene 3-phenyl-1,2,4-oxadiazole-5-carbohydrazide.

This paper describes recent results of a research program aimed at the synthesis and pharmacological evaluation of new heterocyclic N-acylhydrazone (NAH) compounds, belonging to the arylidene (3-phenyl)-1,2,4-oxadiazolyl-5-carboxyhydrazide (8a-p) series. These compounds were structurally planned by applying the molecular hybridization strategy on previously described arylidene 1-phenylpyrazole-4-carbohydrazide (5) derivatives, considered as lead-compounds, which present potent analgesic properties. The analgesic profile of the title compounds 8a-p, evaluated in the model of abdominal constrictions induced by acetic acid, showed that the 4-methoxybenzylidene derivatives 8c and 8k were the most active ones, exhibiting a relative analgesic activity comparable with that of dipyrone 1 used as standard.

A chemometrical study of intermolecular properties of hydrogen-bonded complexes formed by C2H4O...HX and C2H5N...HX with X = F, CN, NC, and CCH.

We presents a chemometrical study of the intermolecular properties of the C2H4O⋅⋅⋅HX and C2H5N⋅⋅⋅HX hydrogen-bonded complexes with X = F, CN, NC, and CCH. Through the MP2 perturbation theory and B3LYP hybrid functional, as well as modifications on 6-31ijGk basis sets with i = triple-zeta, j = diffuse and k = polarization functions, systematic tendencies in the R(n⋅⋅⋅HX) hydrogen bond distances and υ(n⋅⋅⋅HX) stretch frequencies were determined by the hierarchical cluster analysis, two level factorial designs and principal component analysis. Based on well-fitted math models, not only because polarization functions provide a great variance on statistical analysis, but this basis set reproduces more efficiently the available experimental results. Moreover, independent of whether the quality on basis set is increased, the effects yielded by both DFT and MP2 were not considered important in the statistical analysis.