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 C(2)H(4)O···2HF, C(2)H(5)N···2HF and C(2)H(4)S···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.

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 C(2)H(4)O...HX and C(2)H(5)N... 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 upsilon((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.

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 C2H5...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 ppi...H hydrogen bonds. Finally, the possibility of an F...H(alpha) secondary interaction between the fluoride (F) of hydrogen fluoride and the axial hydrogen atoms (H(alpha)) 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.

The 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, (HF(D-R)...HF(D)). However, the existence of a tertiary interaction (F(lambda)...H(alpha)) 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.

Theoretical calculations of the molecular properties of maleimide and its dimer.

B3LYP theoretical calculations with 6-31++G(d,p) basis set have been performed to study the infrared spectrum of maleimide and its dimer. Our calculations have shown that the dimer formation leads to a binding energy of 44.0 kJ mol(-1) involving two intermolecular hydrogen bonds between the amide hydrogen and a carbonyl group of two neighboring maleimides. This value is essentially due to the electrostatic interaction term. Our calculations have also revealed the vibrational changes, in terms of frequencies and IR intensities, after dimer formation. The most affected modes are associated with the N-H stretching and in-plane bending bands. This behavior can be adequately interpreted by the hydrogen atomic charge and N-H charge-flux based on the modified charge-charge flux-overlap model for infrared intensities. The B3LYP frequency shifts are in very good agreement with the experimental ones.

An ab initio study of the C2H2-HF, C2H(CH3)-HF and C2(CH3)2-HF hydrogen-bonded complexes.

MP2/6-31++G** and B3LYP/6-31++G** ab initio molecular orbital calculations have been performed in order to obtain molecular geometries, binding energies and vibrational properties of the C2H2-HF, C2H(CH3)-HF and C2(CH3)2-HF H-bonded complexes. As expected, the more pronounced effects on the structural properties of the isolated molecules due to complexation was verified for the C[triple bond]C and H-F bond lengths, which are directly involved in the H-bond formation. These bond distances increased after complexation. BSSE uncorrected B3LYP binding energies are always lower than the corresponding MP2 values. However, the opposite trend has been verified after BSSE correction by the counterpoise method since it is much lower at B3LYP than at MP2 level. The binding energies for these complexes as well as for the HF acid submolecule modes (the HF stretching and vibrational frequency modes) showed an increasing hydrogen-bonding strength with increasing methyl substitution. The splitting in the HF in-plane and out-of-plane bending modes reflects the anisotropy in the hydrogen-bonding interaction with the pi system of the C[triple bond]C bond. The H-F stretching frequency is shifted downward after complexation and it increases with the methyl substitution. The IR intensities of the HF acid submolecule fundamentals are adequately interpreted through the atomic polar tensor of the hydrogen atom using the charge-charge flux-overlap model. The skeletal stretching modes of the Alkyne submolecule are decreased in the complex. The new vibrational modes arising from complexation show several interesting features.

A theoretical study of hydrogen complexes of the XH-pi type between propyne and HF, HCL or HCN.

The present manuscript reports a systematic investigation of the basis set dependence of some properties of hydrogen-bonded (pi type) complexes formed by propyne and a HX molecule, where X=F, Cl and CN. The calculations have been performed at Hartree-Fock, MP2 and B3LYP levels. Geometries, H-bond energies and vibrational have been considered. The more pronounced effects on the structural parameters of the isolated molecules, as a result of complexation, are verified on RCtriple bondC and HX bond lengths. As compared to double-zeta (6-31G**), triple-zeta (6-311G**) basis set leads to an increase of RCtriple bondC bond distance, at all three computational levels. In the case where diffuse functions are added to both hydrogen and 'heavy' atoms, the effect is more pronounced. The propyne-HX structural parameters are quite similar to the corresponding parameters of acetylene-HX complexes, at all levels. The largest difference is obtained for hydrogen bond distance, RH, with a smaller value for propyne-HX complex, indicating a stronger bond. Concerning the electronic properties, the results yield the following ordering for H-bond energies, DeltaE: propynecdots, three dots, centeredHF>propynecdots, three dots, centeredHCl>propynecdots, three dots, centeredHCN. It is also important to point out that the inclusion of BSSE and zero-point energies (ZPE) corrections cause significant changes on DeltaE. The smaller effect of ZPE is obtained for propynecdots, three dots, centeredHCN at HF/6-311++G** level, while the greatest difference is obtained at MP2/6-31G** level for propynecdots, three dots, centeredHF system. Concerning the IR vibrational it was obtained that larger shift can be associated with stronger hydrogen bonds. The more pronounced effect on the normal modes of the isolated molecule after the complexation is obtained for HX stretching frequency, which is shifted downward.