SAPT: ligação de hidrogênio ou interação de van der Waals?

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

Relação entre transferência de carga e as interações intermoleculares em complexos de hidrogênio heterocíclicos

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

Um estudo teórico relativo à não-linearidade da ligação de hidrogênio em sistemas heterocíclicos C2H4O-C2H2 e C2H4S-C2H2 A theoretical study about the non-linearity of the hydrogen bonding in the C2H4O-C2H2 and C2H4S-C2H2 heterocyclic systems

<abstract language="eng">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

Metodologia AGOA: a modelagem de clusters de hidratação no complexo aziridina···ácido fluorídrico

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

Dissociation of ground and nσ* states of CF3Cl using multireference configuration interaction with singles and doubles and with multireference average quadratic coupled cluster extensivity corrections

Extended complete active space self-consistent field (CASSCF), multireference configuration interaction with singles and doubles (MR-CISD), and multireference average quadratic coupled cluster (MR-AQCC) calculations have been performed on the ground (S0) and first excited (nσ*,S1) states of the CF3Cl molecule. Full geometry optimizations have been carried out for S0 as well as “relaxed” potential energy calculations for both states, along the C–Cl bond distance. Vertical excitation energies (ΔEvertical), dissociation energies (ΔEdiss), dissociation enthalpies (ΔHdiss), and the oscillator strength (f) have also been computed. Basis set effects, basis set superposition error (BSSE), and spin-orbit and size-extensivity corrections have also been considered. The general agreement between theoretical and available experimental results is very good. The best results for the equilibrium geometrical parameters of S0 (at MR-AQCC/aug-cc-pVTZ+d level) are 1.762 and 1.323 Å, for the C–Cl and C–F bond distances, respectively, while the corresponding experimental values are 1.751 and 1.328 Å. The ∠ClCF and ∠FCF bond angles are in excellent agreement with the corresponding experimental values (110.3° and 108.6°). The best calculated values for ΔEvertical, ΔHdiss, and f are 7.63 eV [at the MR-AQCC/aug-cc-pV(T+d)Z level], 3.59 eV[MR-AQCC/aug-cc-pV(T+d)Z level+spin-orbit and BSSE corrections], and 2.74×10−3 (MR-CISD/cc-pVTZ), in comparison with the corresponding experimental values of 7.7±0.1 eV, 3.68 eV, and 3.12×10−3±2.50×10−4. The results concerning the potential energy curves for S0 and S1 show a tendency toward the nonoccurrence of crossing between these two states (in the intermediate region along the C–Cl coordinate), as the basis set size increases. Such tendency is accompanied by a decreasing well depth for the S1 state. Dynamic electronic correlation (especially at the MR-AQCC level) is also an important factor toward an absence of crossing along the C–Cl coordinate. Further investigations of a possible crossing using gradient driven techniques (at CASSCF and MR-CISD levels) seem to confirm its absence

Um estudo teórico de propriedades moleculares em complexos de hidrogênio trimoleculares C2H4···2HF, C2H2···2HF e C3h6···2HF A theoretical study of molecular properties of C2H4···2HF, C2H2···2HF AND C3H6···2HF trimolecular hydrogen-bonded complexes

<abstract language="eng">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&#8801;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