A theoretical study of dihydrogen bonds in small protonated rings: aziridine and azetidine cations.

B3LYP/6-311++G(d,p) calculations were used to predict some molecular properties of the C2H6N+...BeH2, C2H6N...MgH2, C3H8N...BeH2 and C3H8N+...MgH2 dihydrogen-bonded complexes. In these systems, it was demonstrated that the C2H6N+ and C3H8N+ protonated rings are potential candidates to bind with protonic hydrogens derived from alkaline earth metal compounds, BeH2 and MgH2. In terms of structural parameters and quantification of the dihydrogen bond energies, we should mention that the C2H6N+ three-membered ring provides the formation of stronger bound systems, which are 4.0 kJ mol-1 more stables than C3H8N+ four-membered ones. As complement, the analysis of the infrared spectrum indicated that red-shifts and blue-shifts are occurring in the N-H bonds of both C2H6N+ and C3H8N+ cationic rings. However, these two vibrational shifts were also verified on BeH2 and MgH2, what lead us to affirm that cationic compounds derived from small nitrogen rings and earth alkaline molecules are able to form unusual dihydrogen-bonded complexes by means of distinct spectroscopic phenomena, the red-shits and blue-shifts.

Matrix isolation FTIR spectroscopic and theoretical study of 3,3-dichloro-1,1,1-trifluoropropane (HCFC-243).

The molecular structure and infrared spectrum of the atmospheric pollutant 3,3-dichloro-1,1,1-trifluoropropane (HCFC-243) were characterized experimentally and theoretically. The theoretical calculations show the existence of two conformers, with the gauche (G) and trans (T) orientation around the HCCC dihedral angle. Conformer G was calculated to be more stable than form T by more than 10 kJ mol (-1). In consonance with the large predicted relative energy of conformer T, only the G form was identified spectroscopically in cryogenic argon (10 K) and xenon (20 K) matrices prepared from room-temperature equilibrium vapor of the compound. The observed infrared spectra of the matrix-isolated HCFC-243 were interpreted with the aid of high-level density functional theory calculations and normal coordinate analysis. For experimental identification of the weakest IR absorption bands, the spectrum of HCFC-243 in the neat solid state at 145 K was obtained. This spectrum also confirmed the sole presence of the G conformer in the sample. Natural bond orbital and atomic charge analyses were carried out for the two conformers to shed light on the most important intramolecular interactions in the two conformers, in particular those responsible for their relative stability.

DFT calculations on the cyclic ethers hydrogen-bonded complexes: molecular parameters and the non-linearity of the hydrogen bond.

B3LYP/6-311+G(d,p) calculations were used to explore the geometry, intermolecular energy and the vibrational harmonic spectrum of heterocyclic complexes formed between 2,5-dihydrofuran and thiophene cyclic ethers and the HCl and HF acids. The simulated structures of these hydrogen complexes are discussed in terms of the linearity deviation of the n...HX hydrogen bond. Theoretical results are satisfactory as compared to the experimental equilibrium structure. The energies of the hydrogen bonds were determinate through the difference between the complex and its correspondent isolated monomers. Moreover, to obtain the correct energies of the hydrogen bonds, it was included the values of the zero point vibrational energy and the basis set superposition error. The infrared spectra reveal the direct relationship between the distance of the hydrogen bond and its stretching frequencies, as well as a good interpretation of the bathochromic effect of the HCl and HF stretching modes from intermolecular charge transfer.

A theoretical prediction of stability in hydrogen-bonded complexes formed between oxirane and oxetane rings with HX (X=F and Cl).

The optimised geometries of heterocyclic hydrogen-bonded complexes, C2H4O...HX and C3H6O...HX, where X=F or Cl, were determined at DFT/B3LYP/6-311++G(d,p) computational level. Structural, electronic and vibrational properties of these complexes are used in order to compare the strained ring, which confer the great reactivity of these heterocyclic rings with monoprotic acids, forming a primary hydrogen bond. A secondary hydrogen bond between the hydrogen atoms of the CH2 groups and the halide species also takes place, thus causing a nonlinearity (characterized by the theta angle), in the primary hydrogen bond.

A theoretical study of nonlinearity in heterocyclic hydrogen-bonded complexes.

Ab initio calculations are performed at the MP2/6-311++G(d,p) and DFT/B3LYP/6-311++G(d,p) theoretical levels to obtain geometries, H-bond energies and harmonic infrared vibrational properties for the Cs symmetry structures of heterocyclic hydrogen-bonded complexes, CnHmY-HX. The H-bond lengths in DFT/B3LYP calculation level are in better agreement with the experimental values than the MP2 results. The geometry optimization are interpreted in terms of hydrogen bond nonlinearity represented by theta; and phi angles, once the hydrogen bond is formed among n-electrons pairs of the heteroatom in heterocyclic and the hydrogen atom in HX. The hydrogen bond energy after of the zero-point vibrational energy (ZPE) and basis set superposition error (BSSE) corrections are overestimated at DFT/B3LYP, whereas the MP2 BSSE corrections are very large than corresponding DFT/B3LYP. For example, the BSSE corrections for the C2H4S-HNC complex are 7.60 and 0.09 kJ mol(-1) in MP2 and DFT/B3LYP calculations levels, respectively. The new vibrational modes in infrared harmonic spectrum arising from complexation show several interesting features, especially the intermolecular stretching mode.

An MP2 and DFT study of heterocyclic hydrogen complexes CnHmY-HX with n=2, m=4 or 5, Y=O, S or N and X=F or Cl.

We present a theoretical study through MP2 ab initio molecular orbital calculations and B3LYP density functional theory with the 6-311++G(d,p) basis set of the heterocyclic hydrogen complexes, CnHmY-HX, where CnHmY = C2H4O, C2H5N and C2H4S, and X=F or Cl. This study aided in the elucidation the main changes in the structural, electronic and the vibrational properties in isolated species, due the hydrogen complexes formation, CnHmY-HX, revealing systematic tendencies in these chemical systems studied. The complexes has CS symmetry, with the HX subunit lying in the plane perpendicular to that of CYC nuclei of heterocyclic and acting as proton donor in forming a hydrogen bond to the heteroatom, Y. A weak secondary interaction between the CH2 groups of heterocyclics and the X atoms in HX causes a significant nonlinearity of the primary hydrogen bond. The hydrogen bond linearity deviations in these complexes due to secondary interactions are represented by theta angle. The MP2 intermolecular distances of complexes C2H5N-HF, C2H4O-HF and C2H4S-HF correspond the 1.652, 1.671 and 2.164 A, respectively, these results are in excellent agreement with experimental results of 1.700 and 2.193 A found for the last two complexes. In the same way, the MP2 values to theta angle, 14.7, 19.1 and 16.8 degrees, has a better reproduction in the experimental results of 16.5, 21.0 and 16.8 degrees, get to the C2H4O-HCl, C2H4S-HCl and C2H4S-HF complexes, respectively.

An ab-initio study of the C3H6-HX, C2H4-HX and C2H2-HX hydrogen-bonded complexes with X=F or Cl.

MP2/6-31G** ab-initio molecular orbital calculations have been performed to obtain geometries, H-bond energies and vibrational properties of the C3H6-HX, C2H4-HX and C2H2-HX H-bonded complexes with X=F or Cl. The more pronounced effects on the structural parameters of the isolated molecules due to complexation are verified to the CC and HX bond lengths, which are directly involved in the H-bond formation. They are increased after complexation. The calculated H-bond lengths for the hydrogen complexes for X=F are shorter than those for x-Cl by about 0.55 A, whereas the corresponding experimental value is 0.58 A. The H-bond energies are essentially determined by the nature of the proton donor molecule. For X=F, the AE mean value is 20 kJ/mol, whereas it is approximately 14.5 kJ/mol for X-Cl. The H-bond energies including zero-point corrections show a good correlation with the H-bond lengths. The more pronounced effect on the normal modes of the isolated molecules after complexation occurs to the H-X stretching mode. The H-X stretching frequency is shifted downward, whereas its IR intensity is much enhanced upon H-bond formation. The new vibrational modes arising from complexation show several interesting features.

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.

[QSAR analysis of the toxicity of a series of pyridines related to the mushroom toxin orellanine: comparison of the results obtained with physico-chemical, steric and quantal parameters]. / Analyse QSAR de la toxicité d'une série pyridinique apparentée à la toxine fongique orellanine: comparaison des résultats obtenus à l'aide de paramètres physico-chimiques, stériques et quantiques.

To complete a preceding work, we have made a QSAR study of a series of mono- or bi-pyridines, aminoxidized or not, wearing diverse substituents and related to the mushroom toxin orellanine. For the description of these molecules, we tried several sterical parameters which we have recently defined (sigma D and sigma S) or which we define here (D3) as well as physical and quantal parameters calculated with the CNDO/2 method, besides the parameter of hydrophobicity log P. Among the regression equations obtained, three have a correlation coefficient greater than 0.96, which is very high, taking into account the nature of the biological response (LD50). The importance of the hydrophobicity and of the size of the molecules in the expression of their toxicity is confirmed, as well as the particularity of orellanine in the series studied.

[Fine-needle aspiration cytology. An exploratory study]. / Citología por aspiración con aguja fina. Un estudio exploratorio.

This paper shows result from fine-needle aspiration biopsies done at the University Hospital and Falcon Clinic in Maracaibo, Venezuela, between December 1985 and October 1986. The study was conducted in order to determine factibility of fine-needle aspiration biopsies in our environment. Results are satisfactory and show that the method is a good one to diagnosis and prevent cancer, it is cheap and easy to use in large populations with quick diagnosis.