Resumo A proposição de uma dimensionalidade atômica moderna consagra cientificamente o físico-matemático Richard Bader como um verdadeiro pioneiro. Considerando a variável mecânico quântica densidade eletrônica, Bader formulou um protocolo matemático para modelar os fluxos de densidade de carga nas superfícies moleculares. Para este fim, ele assumiu o ambiente atômico como sendo um espaço aberto e adequado para a troca de carga e momentum entre os átomos vizinhos. A integração da superfície molecular é governada pelo teorema virial mediante as contribuições das densidades de energia cinética e potencial, em que, ambos são representados pelo campo Laplaciano da densidade eletrônica.

Abstract The proposition of a modern atomic dimensionality should be laureate to the pioneering spirit of the theoretician in physics and chemistry, Richard Bader. By taking into account the electronic-density as a pure and physical variable, Bader has formulated a mathematical protocol for modeling the charge density fluxes on the molecular surfaces. To this end, he also assumed the atomic environment as being an open space suitable for exchange charge and momentum with the neighbors around it. The integration of the molecular surface is ruled by the virial theorem embodied by the contributions of the kinetic and potential energy densities, wherein, both of them are represented by the Laplacian field of the electronic density.

The DFT, QTAIM, NBO and ChelpG methods were used in a theoretical study of structural parameters, vibration stretch modes and absorption intensities, electronic properties, molecular orbital analyses and molecular topography of the heptamer C2H2···6(HF) complex. One of the great aims concerns to the studying of the proton donor structure, which is considered as two 3(HF) cyclic moieties. The structures of the HF trimer interact with acetylene in an opened configuration because the energies of the F···HF hydrogen bonds are enhanced from 34.45 to 58.43 as well as 61.48 kJ.mol-1. Moreover, instead of the unusual π····H hydrogen bonds formed on the middle of the π bond of acetylene, the more uncommon C···H contact was certified. The red-shifts on the stretch frequencies of HF bonds were evaluated aiming to unveil the covalent character in the F···HF hydrogen bonds.

In this work, a theoretical study on the basis of structural, vibrational, electronic and topological parameters of the C2H2‧‧‧(HF), C2H2‧‧‧2(HF) and C2H2‧‧‧3(HF) complexes concerning the formation of π‧‧‧H, F‧‧‧H and C‧‧‧H hydrogen bonds is presented. The main difference among these complexes is not properly the interaction strength, but the hydrogen bond type whose benchmark is ruled justly by the structure. Meanwhile, the occurrence of π‧‧‧H hydrogen bonds was unveiled in both C2H2‧‧‧(HF) dimer and C2H2‧‧‧3(HF) tetramer, although in latter, this interaction is stronger than C‧‧‧H of the C2H2‧‧‧2(HF) trimer. However, the F‧‧‧H hydrogen bonds within the subunits of hydrofluoric acid are the strongest ones, reaching a partial covalent limit, and thereby contribute decisively to the stabilization of the tetramer structure. In line with this, the largest red-shifts were observed on the hydrofluoric acid trimer of the C2H2‧‧‧3(HF) complex.

Along the historical background of science, the hydrogen bond became widely known as the universal interaction, thus playing a key role in many molecular processes. Through the available theoretical approaches, many of these processes can be unveiled on the basis of the molecular parameters of the subject intermolecular system, such as the variation of bond length and mainly the frequency shift observed in the proton donor. Supported by the natural bond analysis (NBO) with the quantification of the hybridization contributions, the structural deformations and vibrational effects cited above are also attributed to the outcome of the intermolecular interaction strength, which consequently can be estimated by means of the quantum theory of atoms in molecules (QTAIM) as well as evaluated by the symmetry-adapted perturbation theory (SAPT). Moreover, to identify the preferential interaction sites for proton donors and acceptors, the molecular electrostatic potential (MEP) is useful in this regard.

Theoretical analysis at the BHandHLYP/6-311++G(d,p) level of theory with the support of QTAIM calculations was used to reinvestigate the structure of an intermolecular system formed between ethylene oxide (C2H4O) and formic acid (HCO2H) after the epoxidation known as the Prileschajew reaction. Geometric and infrared vibration results revealed that HO2CH forms a strong hydrogen bond with C2H4O followed by a larger red-shift of the H−O bond. NBO analysis was applied to justify this frequency shift. Finally, QTAIM calculations identified the formation of two hydrogen bonds, namely O···H−O and H···O=C.

This article presents a theoretical study of the molecular properties of trimolecular clusters of CnHm∙∙∙HCN∙∙∙HX formed by the ϖ∙∙∙H and n∙∙∙H hydrogen bonds. The interaction strengths of these interactions are in line with the variations in s-character, and independently, the red-shift rise whether stronger or weaker bound systems are carried out. This behavior was justified via NBO analysis and supported by Bent´s rule, wherein the greater variations in s-character of X are in good agreement with larger red-shifts and vice-versa. To conclude, the refinement of the supermolecule approach and NBO binding energies also corroborate in this regard.

It is through the application of an electronic partition approach called Symmetry-Adapted Perturbation Theory (SAPT) that the nature of hydrogen bonds and van der Waals interactions can be unveiled according to the contribution of electrostatic, charge transfer, exchange repulsion, polarization, and dispersion terms. Among these, electrostatic partition governs the formation of the hydrogen bonds, whose energies are arguably high. However, the weakness of the interaction strength is caused by dispersion forces, whose contribution decisively lead to the stabilization of complexes formed via van der Waals interactions.

Neste artigo reportamos um estudo teórico objetivando revisitar a origem da conformação π-π stacking do (+)-acrilato de 8-fenilmentila (2) ser a preferencial. Para isso, calculamos geometrias de mínimo local, freqüências vibracionais, propriedades termoquímicas e deslocamentos químicos de RMN de ¹H para 2 e para o composto modelo acrilato de 3-fenilpropila (3) usando vários métodos ab initio e de DFT. Observamos que o funcional MPW1B95 foi o método mais apropriado em explicar os dados experimentais de RMN de ¹H nos quais mostram que a conformação stacking de 2 (2S) é mais estável que a conformação trans (2T) e a conformação stacking de 3 (3S) é menos estável que a conformação trans (3T) . Após isso, detalhes geométricos e energéticos do complexo intermolecular benzeno...metilacrilato (4) também foram estudados usando o método MPW1B95. A partir dos nossos resultados percebemos que ambos os efeitos, estérico e de dispersão, desempenham papéis chave no equilíbrio conformacional de 2.

In this paper we report a theoretical study aiming to revisit the origin of the preferential π-π stacking conformation of the (+)-8-phenylmentyl acrylate (2). For this, we have applied several DFT and ab initio methods to calculate local minimum geometries, vibrational frequencies, thermochemical properties and ¹H NMR chemical shifts for 2 and the model compound 3-phenylpropyl acrylate (3). We have observed that the MPW1B95 functional was the most suitable method to explain ¹H NMR experimental data which show the stacking conformation of 2 (2S) is more stable that trans conformation (2T) and the stacking conformation of 3 (3S) is less stable that trans conformation (3T). After that, geometrical and energetic features of the intermolecular complex benzene...methylacrylate (4) have also been studied using MPW1B95 method. From our results, we have noticed that both steric and dispersion effects play a key role in the conformational equilibrium of 2.

Hydrogen bonds formed through the interaction between a high electronic density center (lone electron pairs, π or pseudo-π bonds) and proton donors cause important electronic and vibrational phenomena in many systems. However, it was demonstrated that proton donors interact with hydrides, such as alkali and alkaline earth metals (BeH2, MgH2, LiH and NaH), what yields a new type of interaction so-called dihydrogen bonds. The characterization of these interactions has been performed at light of the Quantum Theory of Atoms in Molecules (QTAIM), by which the electronic densities ρ are quantified and the intermolecular regions are characterized as closed-shell interactions through the analysis of the Laplacian field ∇2ρ.

We present a theoretical study of solvent effect on C2H5N···HF hydrogen-bonded complex through the application of the AGOA methodology. By using the TIP4P model to orientate the configuration of water molecules, the hydration clusters generated by AGOA were obtained through the analysis of the molecular electrostatic potential (MEP) of solute (C2H5N···HF). Thereby, it was calculated the hydration energies on positive and negative MEP fields, which are maxima (PEMmax) and minima (PEMmin) when represent the -CH2- methylene groups and hydrofluoric acid, respectively. By taking into account the higher and lower hydration energy values of -370.6 kJ mol-1 and -74.3 kJ mol-1 for PEMmax and PEMmin of the C2H5N···HF, our analysis shows that these results corroborate the open ring reaction of aziridine, in which the preferential attack of water molecules occurs at the methylene groups of this heterocyclic.

The optimized geometries of the C2H3Cl···HX vinylchloride complexes with X = Cl, CN, NC and CCH were obtained at B3LYP/6-311++G(d,p) level of theory. In a first analysis, the (Cl···HX) hydrogen bonds were examined by taking into account their distances, intermolecular interaction energies and CHELPG charge transfer, as well as charge densities obtained through the calculations of theory of Atoms In Molecules (AIM). Moreover, a (X···H) secondary interaction between the hydrogen atoms of vinylchloride and fluorine of HF, nitrogen of both HCN and HNC molecules, and p bond of HCCH was also a target of investigation. This secondary interaction was studied not only in terms of topological criteria or by results of structural and electronic parameters, but it was also analyzed the stretch modes in the infrared spectrum. At last, as far as the vibrational red-shifts of the HX molecules were characterized, the stretch frequencies and absorption intensities of the (X···H) secondary interactions were also identified, but only for the C2H3Cl···HCCH complex.

Las geometrías del C2H3Cl···HX vinylchloride complejos con X = Cl, CN, NC y CCH fueron obtenidas en el nivel de teoría B3LYP/6-311++G(d,p). En un primer análisis, el ligaciones de hidrógeno (Cl···HX) fueron examinadas teniendo en cuenta sus distancias, energías de interacción intermoleculares y transferencia de carga CHELPG, así como las densidad de electróns obtenidas por los cálculos de la teoría de Atoms in Moleculres (AIM). Además, una interacción secundaria (X···H) entre los átomos de hidrógeno de vinylchloride y el flúor de HF, nitrógeno de HCN como de las moléculas de HNC, y la ligacion p de HCCH fueron también un objetivo de investigación. Esta interacción secundaria fue estudiada no sólo en términos de criterios topológicos o por resultados de parámetros estructurales y electrónicos, pero también fue analizado los modos de extensión en el espectro infrarrojo. Por fin, por lo que los cambios rojos vibrational de las moléculas HX fueron caracterizados, las frecuencias y intensidades de absorción del las interacciones secundarias (X•••H) también fueron identificadas, pero sólo para el complejo C2H3Cl ••• HCCH.

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 Bader's 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.

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 pi 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.

Hydrogen-bonded complexes formed by the interaction of the heterocyclic molecules C2H4O and C2H5N with HF, HCN, HNC and C2H2 have been studied using density functional theory. The hydrogen bond strength has been analyzed through electron density charge transfer from the proton acceptor to the proton donor. The density charge transfer has been estimated using different methods such as Mulliken population analysis, CHELPG, GAPT and AIM. It has been shown that AIM-estimated charge transfer correlates very well with the hydrogen bond energy and the infrared bathochromic effect of the proton donor stretching frequencies.