Antônio B. Carvalho

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.

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.

Um estudo teórico relativo à não-linearidade da ligação de hidrogênio em sistemas heterocíclicos C2H4O-C2H2 e C2H4S-C2H2

B3LYP/6-31G(d,p) calculations were used to determine the optimized geometries of the C2H4O-C2H2 and C2H4S-C2H2 heterocyclic hydrogen-bonded complexes. Results of structural, rotational, electronic and vibrational parameters indicate that the hydrogen bonding is non-linear due to the p bond of the acetylene interacting with the hydrogen atoms of the methyl groups of the three-membered rings. Moreover, the theoretical investigation showed that the non-linearity is much more intriguing, since there is a structural disjunction on the acetylene within the heterocyclic system.

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.

Hydrogen bonds determine the structures of the ternary heterocyclic complexes C2H4O···2HF, C2H5N···2HF and C2H4S···2HF: density functional theory and topological calculations

A theoretical study of structural, electronic, topological and vibrational parameters of the ternary hydrogen-bonded complexes C2H4O···2HF, C2H5N···2HF and C2H4S···2HF is presented here. Different from binary systems with a single proton donor, the tricomplexes have the property of forming multiple hydrogen bonds, which are analyzed from a structural and vibrational point of view, but verified only by means of the quantum theory of atoms in molecules (QTAIM). As traditionally done in the hydrogen bond theory, the charge transfer between proton donors and acceptors was computed using the CHELPG calculations, which also revealed agreement with dipole moment variation and a cooperative effect on the tricomplexes. Furthermore, redshift events on proton donor bonds were satisfactorily identified, although, in this case, an absence of experimental data led to the use of a theoretical argument to interpret these spectroscopic shifts. It was therefore the use of the QTAIM parameters that enabled all intermolecular vibrational modes to be validated. The most stable tricomplex in terms of energy was identified via the strength of the hydrogen bonds, which were modeled as directional and bifurcated.

HETEROCYCLIC HYDROGEN BONDED COMPLEXES INVOLVING THREE AND FIVE MEMBERS: A QUANTUM CHEMICAL STUDY ABOUT THE CASE OF THE NONLINEARITY ON THE HYDROGEN BONDING

B3LYP/6-311++G(d,p) and B3LYP/6-31G(d,p) theoretical levels combined with topological calculations based on the theory of atoms in molecules (AIM) were used to study the formation of intermolecular interactions on heterocyclic hydrogen-bonded complexes formed by ethylene oxide, sulfide oxide, 2,5-dihydrofuran, thiophene, and the hydrofluoric acid. As observed, the electrophilic attack of the hydrofluoric acid occur aligned to the n lone pair of the heteroatom of the cyclic systems, the modeled structure of the hydrogen-bonded complexes is interpreted in terms of the intermolecular parameters of the (n ⋯ HF) hydrogen bond, such as its R(n ⋯ HF) distance, ΔEC binding energy, and the new υ(n ⋯ HF) stretching vibrational mode. From these criteria, a relationship between the strength (R, ΔEC and υ) of the hydrogen bonds and its nonlinearity deviation was verified, which is formed by a secondary interaction between the hydrofluoric acid and axial hydrogen atoms of the cyclic structure. Moreover, even though the Baders electronic density partitioning has been projected with low dependence from ab initio wave functions, it is shown here that the 6-31G(d,p) basis sets characterized the secondary interaction in geometrical point of view, as well as by means of the AIM protocol through the specific calculation of bond critical points between the fluoride and the axial hydrogen atoms.

Preparation, characterization, electrochemical studies and crystal structure determination of salicylaldehyde–aroylhydrazone, ferrocenyl–aroylhydrazone and salicylaldehyde–ferrocenoylhydrazone complexes of indium

The reaction of triethylindium with salicylaldehyde–aroylhydrazone, C 6 H 4 ( o -OH)CHNNHC(O)C 6 H 4 X (X= p -H, p -OCH 3 , p -NO 2 , m -NO 2 , o -OH), yields the tetrahydrofuran (THF) adduct of (C 14 H 9 N 2 O 2 X) 2 InC 2 H 5 . With formylferrocenyl–aroylhydrazones C 5 H 5 FeC 5 H 4 CHNNHC(O)C 6 H 4 X (X= p -H, p -OCH 3 , p -NO 2 , m -NO 2 , p -Cl, o -Cl), the diethylindium–hydrazone complex, (C 18 H 14 N 2 OFeX)In(C 2 H 5 ) 2 , was formed. With salicylaldehyde–ferrocenoylhydrazone, C 6 H 4 ( o -OH)CHNNHC(O)C 5 H 4 FeC 5 H 5 , the monohydrazone complex (C 18 H 15 N 2 O 2 Fe)In(C 2 H 5 ) 2 tetrahydrofuran solvate was obtained. The crystal structures of the p -OCH 3 derivatives of the first two series of complexes and of the ferrocenoylhydrazone derivative were determined. In the solid state, the three complexes are dimers, containing a set of seven, three and five fused rings, respectively. In every case, it was observed a symmetric anti arrangement of fused rings in relation to a central In 2 O 2 four-membered ring, resulting in a configuration that resembles a chair. Cyclic voltammetric studies on the ferrocenylaroylhydrazone complexes revealed electron transference from the ferrocene group towards the indium atom, while for the salicilaldehyde–ferrocenoyl complex this was not observed.

O ensino de reações orgânicas usando química computacional: I. reações de adição eletrofílica a alquenos

Basic concepts that play an important role in some organic reactions are revisited in this paper, which reports a pedagogical experience involving undergraduate and graduate students. A systematic procedure has been applied in order to use widespread available computational tools. This paper aims to discuss the use of computers in teaching electrophilic addition reactions to alkenes. Two classical examples have been investigated: addition to non-conjugated alkenes and addition to conjugated dienes. The results were compared with those normally discussed in organic textbooks. Several important concepts, such as conformational analysis and energy control (kinetic and thermodynamic) involved in reaction mechanisms can be taught more efficiently if one connects theoretical and practical tools.