João Bosco P. da Silva

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.

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.

Hydrogen bonds between pyrazine and HX linear acids (X = F, NC, CN and CCH): a theoretical study

Estudos teoricos usando os niveis de calculo B3LYP e MP2 com um conjunto de base 6-31++G** foram empregados para caracterizar complexos de hidrogenio envolvendo pirazina e acidos lineares HX com X= F, NC, CN e CCH. As propriedades moleculares destes complexos e as mudancas estruturais, eletronicas e vibracionais que ocorrem nas especies isoladas devido a formacao da ligacao de hidrogenio foram calculadas e comparadas com aquelas anteriormente estimadas em complexos do tipo-T envolvendo hidrocarbonetos insaturados simples como doadores de eletrons. Nossos calculos mostraram que estas mudancas nos complexos de Pyz HX sao muito mais pronunciadas do que aquelas verificadas nos complexos do tipo p...HX, assim como suas energias de ligacao-H. Correcoes BSSE para as energias da ligacao-H estimadas pelos calculos B3LYP sao aproximadamente tres vezes menores que aquelas obtidas atraves dos calculos MP2. Nossos calculos tambem mostraram que ligacoes de hidrogenio mais fortes nos complexos Pyz...HX estao associadas com menores comprimentos de ligacao-H, maiores valores de transferencia de carga intermolecular e maiores deslocamentos para baixo na frequencia de estiramento H-X. A forca dessas ligacoes de hidrogenio segue a ordem: X = F > X = NC > X = CN > X = CCH. Como esperado, a intensidade de estiramento H-X foi aumentada substancialmente apos a formacao da ligacao. Finalmente, os novos modos vibracionais que surgem em decorrencia da complexacao foram tambem analisados.

Chemometric study of liquid water simulations. I. The parameters of the TIP4P model potential

The multivariate chemometric techniques two level factorial design (TLFD) and principal component analysis (PCA) were used to investigate the TIP4P model potential behavior with respect to perturbations on all intermolecular interaction parameters. The effects of these perturbations were calculated for the enthalpy of vaporization, the density, the first maximum of the radial distribution functions of the OH and OO pairs, and the second maximum of the radial distribution function of the OH pair obtained from Monte Carlo simulations of liquid water at 25°C. The principal effects were quantified and rationalized in terms of the pair‐wise interaction potential of the TIP4P model. They also corroborate previously published sensitivity analysis results using molecular dynamics and other model potentials. In addition, significant interaction effects between some parameters of the TIP4P model potential were observed and quantified, which hardly could be obtained without such a statistic approach. These interaction effects are very regular and systematic, and their behavior has not been encountered in other chemometric studies and cannot be rationalized in terms of the functional form of the pair‐wise potential.

Estrutura, reatividade e propriedades biológicas de hidantoínas

Hydantoin (imidazolidine-2,4-dione) is a 2,4-diketotetrahydroimidazole discovered by Baeyer in 1861. Thiohydantoins and derivatives were prepared, having chemical properties similar to the corresponding carbonyl compounds. Some biological activities (antimicrobial, anticonvulsant, schistosomicidal) are attributed to the chemical reactivity and consequent affinity of hydantoinic rings towards biomacromolecules. Therefore, knowledge about the chemistry of hydantoins has increased enormously. In this review, we present important aspects such as reactivity of hydantoins, acidity of hydantoins, spectroscopy and cristallographic properties, and biological activities of hydantoin and its derivatives.

An application of chemometric techniques to the study of ab initio rotational constants of linear molecules

Abstract The chemometric techniques Principal Component Analysis (PCA) and two-level Factorial Design (FD) were employed to analyze the effects of wave function modifications on calculated rotational constants for a set of linear molecules. These multivariate investigations led to the selection of the MP2/6-311G** wave function for a calibrational investigation to predict rotational constants. The rotational constants predicted by the MP2/6-311G** wave functions for the nine training set molecules are 1.3% higher than the experimental values. Using an appropriate scaling factor the predicted rotational constant B 0 value for FC 7 N is 360.1 MHz. The quality of this prediction is supported by estimates of B 0 for FC 5 N, HC 7 N and HC 9 N, which were not used in the calibration set. Predicted B 0 values, 754.1, 564.0 and 290.0 MHz, are in excellent agreement with the experimental ones, 755.4, 564.0 and 290.5 MHz. On the other hand, PLS results using theoretical values for eight wave function predict values of 768.8, 577.7 and 372.3 MHz for the FC 5 N, HC 7 N and FC 7 N molecules.

Determination of metabolite profiles in tropical wines by 1H NMR spectroscopy and chemometrics

Traditionally, wines are produced in temperate climate zones, with one harvest per year. Tropical wines are a new concept of vitiviniculture that is being developed, principally in Brazil. The new Brazilian frontier is located in the northeast region (São Francisco River Valley) in Pernambuco State, close to the equator, between 8 and 9°S. Compared with other Brazilian and worldwide vineyards, the grapes of this region possess peculiar characteristics. The aim of this work is a preliminary study of commercial São Francisco River Valley wines, analyzing their metabolite profiles by 1H NMR and chemometric methods. Copyright

AGOA: A Hydration Procedure and Its Application to the 1-Phenyl-beta-Carboline Molecule

A new procedure, named AGOA, has been developed and implemented in a computer program written in FORTRAN 77 to explore the hydration structures of polar solutes using its molecular electrostatic potential (MEP). This procedure can be generalized to polar solvents other than water. It has been tested for several small molecules, and applied to complex molecules of pharmacological interest, such as the b-carbolinic systems derived from indole. This is a stringent, but not general, test of the AGOA procedure and shows its robustness, flexibility and low computational costs, since either semiempirical or ab initio wavefunctions can be employed. The comparisons with procedures based upon the geometry optimization of the solute-water complex show the superior performance of the AGOA procedure for the anti and syn b-carboline conformers, reassuring its use to comprehend and to quantify the specific interactions involved in solvent effects.

Accuracy of Faecal Calprotectin and Neutrophil Gelatinase B-associated Lipocalin in Evaluating Subclinical Inflammation in UlceRaTIVE Colitis-the ACERTIVE study.

Background and Aims Mucosal healing and histological remission are different targets for patients with ulcerative colitis, but both rely on an invasive endoscopic procedure. This study aimed to assess faecal calprotectin and neutrophil gelatinase B-associated lipocalin as biomarkers for disease activity in asymptomatic ulcerative colitis patients. Methods This was a multicentric cross-sectional study including 371 patients, who were classified according to their endoscopic and histological scores. These results were evaluated alongside the faecal levels of both biomarkers. Results Macroscopic lesions [i.e. endoscopic Mayo score ≥1] were present in 28% of the patients, and 9% had active disease according to fht Ulcerative Colitis Endoscopic Index of Severity. Moreover, 21% presented with histological inflammation according to the Geboes index, whereas 15% and 5% presented with focal and diffuse basal plasmacytosis, respectively. The faecal levels of calprotectin and neutrophil gelatinase B-associated lipocalin were statistically higher for patients with endoscopic lesions and histological activity. A receiver operating characteristic-based analysis revealed that both biomarkers were able to indicate mucosal healing and histological remission with an acceptable probability, and cut-off levels of 150-250 μg/g for faecal calprotectin and 12 μg/g for neutrophil gelatinase B-associated lipocalin were proposed. Conclusions Faecal calprotectin and neutrophil gelatinase B-associated lipocalin levels are a valuable addition for assessment of disease activity in asymptomatic ulcerative colitis patients. Biological levels of the analysed biomarkers below the proposed thresholds can rule out the presence of macroscopic and microscopic lesions with a probability of 75-93%. However, caution should be applied whenever interpreting positive results, as these biomarkers present consistently low positive predictive values.

Adenine formation without HCN.

From a historic point of view adenine was always presumed to be the product of HCN pentamerization. In this work a new mechanism for adenine synthesis in the gas phase without HCN is proposed. The concept of retrosynthetic analysis was employed to create a tautomer of adenine, which can be reached from previously observed interstellar molecules C3NH and HNCNH and its isomer H2NCN. MP2/6-311++G(2d,2p) calculations were performed to calculate the Gibbs free energy of the minimum and the transition state (TS) structures involved in the six step mechanism. This new mechanism requires a smaller number of steps, the reaction energy is twice as exergonic, and the rate determining TS is lower in energy than the corresponding ones proposed elsewhere in the literature.