Enrique M. Cabaleiro-Lago

Computational study of the dissociation of H–X acids (X=F, Cl, Br, I) in water clusters

The ionic dissociation of H–X acids (X=F, Cl, Br, I) in water was examined by conducting a theoretical study on the properties of the clusters formed by the acids with up to five water molecules: X–H(H2O)n (n=1–5). Calculations were done using the DFT/B3LYP and MP2 methods in conjunction with the TZVP basis set and allowed the identification of several minima on the potential surfaces for the clusters. Based on the results, the MP2 method predicts a lower tendency to ionization than does the DFT/B3LYP method; however, both methods provide similar results. The dissociation characteristics of the acids were examined in terms of various parameters including the lengths of the bonds involved in the proton transfer and the frequencies associated with the X–H and O–H stretching modes in the bonds taking part in the proton transfer. The successive incorporation of water molecules to the cluster was found to lengthen X–H distances and simultaneously decrease O⋯H distances. In addition, the X–H stretching frequenc...

DFT conformational study of cysteine in gas phase and aqueous solution

Abstract Different conformers of cysteine in gas phase are investigated at the DFT B3LYP/6-31G∗ and B3LYP/6-311++G∗∗ levels. The effect of the solvent is simulated by using the Onsager and polarizable continuum (PCM) models within the self-consistent reaction field method (SCRF) at the B3LYP/6-31G∗ level. Specifically, five neutral forms, two anions and one zwitterion were analysed. Both, in gas phase and solution the most stable normal form has the carboxyl group directed toward the amino group. In accord with the experiment, the PCM model predicts that the most stable structure in solution is a zwitterion, a species that does not exist or has a very small stability in gas phase. A major stabilization in solution is also predicted for the zwitterionic form of anionic cysteine. Thus the PCM model renders correct stability order of the different conformers in solution, while the Onsager model does not, which is due to the underestimation of the electrostatic contributions to the solute–solvent interaction for the zwitterions.

DFT study of the interaction between alkaline cations and molecular bowls derived from fullerene.

A systematic study of the interaction of alkaline cations with curved π systems (molecular bowls) derived from fullerene (C(60)) shows the ability of these structures to form stable cation-π complexes with both of their sides: concave and convex. In all cases, complexes with the cation in the convex side are more stable than its corresponding partner inside the bowl. When forming the complexes with the alkaline cations, these bowls exhibit great stability but several descriptors that usually work in planar conjugate molecules (like the magnitude of the charge transferred to the cation or the cation-ligand distance) do not properly describe the trends observed. The present study shows that with these curved π systems inductive effects play a central role in the formation of complexes with alkaline cations. This conclusion contradicts the simplistic point of view on the dominant effect of the electrostatic term in the interaction between alkaline cations and bowls derived from fullerene. Additionally, steric effects can be relevant when bulky cations are placed in the concave side of the largest bowls.

Computational Study on the Characteristics of the Interaction in Naphthalene···(H2X)n=1,2 (X = O, S) Clusters

The characteristics of the interaction between the pi cloud of naphthalene and up to two H2O or H2S molecules were studied. Calculations show that clusters formed by naphthalene and one H2O or H2S molecule have similar geometric features, and also present similar interaction energies. Our best estimates for the interaction energy amount to -2.95 and -2.92 kcal/mol for H2O and H2S, respectively, as obtained with the CCSD(T) method. Calculations at the MP2 level employing large basis sets should be avoided because they produce highly overestimated interaction energies, especially for hydrogen sulfide complexes. The MPWB1K functional, however, provides values pretty similar to those obtained with the CCSD(T) method. Although the magnitude of the interaction is similar with both H2X molecules, its nature is somewhat different: the H2O complex presents electrostatic and dispersion contributions of similar magnitude, whereas for H2S the interaction is dominated by dispersion. In clusters containing two H2X molecules several minima were characterized. In water clusters, the total interaction energy is dominated by the presence of a O-H...O hydrogen bond and, as a consequence, structures where this contact is present are the most stable. However, clusters containing H2S show structures with no interaction between H2S moieties which are as stable as the hydrogen bonded ones, because they allow an optimal H2S...naphthalene interaction, which is stronger than the S-H...S contact. Therefore it is possible that in larger polycycles hydrogen sulfide molecules will be spread onto the surface maximizing S-H...pi interactions rather than aggregated, forming H2S clusters.

Ab initio study of interactions in methylamine clusters. The significance of cooperative effects

Methylamine clusters consisting of up to four molecules were studied by employing Hartree–Fock, density functional theory, and Moller–Plesset calculations with the 6-31+G* basis set. The dimer was found to exhibit two minima with similar interaction energies (−13 kJ/mol) and involving a hydrogen bond. The dipole moment for the dimer differs by up to 20% from the vector addition of the dipole moments for the individual molecules by effect of the interaction; also, the N–H bond distance in the group involved in the hydrogen bond is lengthened by up to 0.006 A as a result. The minima identified for the trimer and tetramer possess cyclic structures that differ in the position of the methyl groups with respect to the plane containing the hydrogen bonds. The contribution of nonadditivity to the interaction in these structures is quite significant (12%–18% of the overall interaction energy). N–H distances in the donor molecule are longer than in the dimer and increase with increasing cluster size. Likewise, the ...

Substituted corannulenes and sumanenes as fullerene receptors. A dispersion-corrected density functional theory study.

Stacking interactions between substituted buckybowls (corannulene and sumanene) with fullerenes (C60 and C70) were studied at the B97-D2/TZVP level of theory. Corannulene and sumanene monomers were substituted with five and six Br, Cl, CH3, C2H, or CN units, respectively. A comprehensive study was conducted, analyzing the interaction of corannulenes and sumanenes with several faces of both fullerenes. According to our results, in all cases substitution gave rise to larger interaction energies if compared with those of unsubstituted buckybowls. The increase of dispersion seems to be the main source of the enhanced binding, so an excellent correlation between the increase of interaction energy and the increase of dispersion contribution takes place. One of the noteworthy phenomena that appears is the so-called CH···π interaction, which is responsible for the strong interaction of sumanene complexes (if compared with that of corannulene complexes). This interaction also causes the substitution with CH3 groups (in which one of the H atoms points directly to the π cloud of fullerene) to be the most favorable case. This fact can be easily visualized by noncovalent interaction plots.

Ab initio study of M(CH3CN)n clusters (M=Li+, Na+, Mg2+) in the gas phase

Abstract Calculations for clusters consisting of an Li + , Na + or Mg 2+ ion and up to six acetonitrile molecules were carried out using the HF, DFT/B3LYP and MP2 methods with the 6-31+G * basis set. The three methods led to the same minima, which correspond to highly symmetric structures, where the dipole moment of acetonitrile points at the cation in order to facilitate a charge–dipole interaction. Intermolecular distances follow the sequence Na + >Mg 2+ >Li + with the three methods used; on the other hand, interaction energies decrease in the sequence Mg 2+ >Li + >Na + . Intermolecular distances increase gradually with increasing cluster size. By contrast, the energy change resulting from incorporation of an additional molecule into a given cluster decreases with increasing number of molecules. The coordination sphere of Li + saturates with five acetonitrile molecules, which reflects in increased distances and in a markedly decreased interaction energy for the cluster containing five acetonitrile molecules. The other two ions can easily accommodate up to six acetonitrile molecules. The interaction is essentially electrostatic, but ligand polarization contributions are significant.

Study of the interaction between water and hydrogen sulfide with polycyclic aromatic hydrocarbons

A computational study has been carried out for determining the characteristics of the interaction between one water and hydrogen sulfide molecule with a series of polycyclic aromatic hydrocarbons of increasing size, namely, benzene, anthracene, triphenylene, coronene, circumcoronene, and dicircumcoronene. Potential energy curves were calculated for structures where H(2)X (X=O,S) molecule is located over the central six-membered ring with its hydrogen atoms pointing toward to (mode A) or away from (mode B) the hydrocarbon. The accuracy of different methods has been tested against the results of coupled cluster calculations extrapolated to basis set limit for the smaller hydrocarbons. The spin component scaled MP2 (SCS-MP2) method and a density functional theory method empirically corrected for dispersion (DFT-D) reproduce fairly well the results of high level calculations and therefore were employed for studying the larger systems, though DFT-D seems to underestimate the interaction in hydrogen sulfide clusters. Water complexes in mode A have interaction energies that hardly change with the size of the hydrocarbon due to compensation between the increase in the correlation contribution to the interaction energy and the increase in the repulsive character of the Hartree-Fock energy. For all the other clusters studied, there is a continuous increase in the intensity of the interaction as the size of the hydrocarbon increases, suggesting already converged values for circumcoronene. The interaction energy for water clusters extrapolated to an infinite number of carbon atoms amounts to -13.0 and -15.8 kJ/mol with SCS-MP2 and DFT-D, respectively. Hydrogen sulfide interacts more strongly than water with the hydrocarbons studied, leading to a limiting value of -21.7 kJ/mol with the SCS-MP2 method. Also, complexes in mode B are less stable than the corresponding A structures, with interaction energies amounting to -8.2 and -18.2 kJ/mol for water and hydrogen sulfide, respectively. The DFT-D calculations give values of -16.2 and -9.3 kJ/mol for hydrogen sulfide complexes in modes A and B, less negative than those predicted by the SCS-MP2 method, probably indicating problems with sulfur dispersion parameters.

Ring-annelated corannulenes as fullerene receptors. A DFT-D study

Stacking π⋯π interactions of ring-annelated corannulenes with fullerenes C60 and C70 were studied at the B97-D level. For this purpose three-, four-, and five-membered rings (saturated and unsaturated) were annelated to the five rim bonds of corannulene. Substitution effects with NH, O and S units on each one of the five saturated five-membered rings annelated to the rim of corannulene were also evaluated. In all cases, complexation energies were larger than that for the parent compound, corannulene C20H10; in the best case an increase of almost 90% is found. According to our results, the increase of complexation energy is directly related to the increase of the dispersion contribution, where CH⋯π interactions play a very important role. This kind of interaction has special relevance for compounds with saturated annelated rings, where the spatial disposition favours the interaction between hydrogen atoms of CH2 groups and the π cloud of fullerene. For compounds with unsaturated annelated rings the increase of dispersion is not so pronounced; however, this is fairly offset by the increase of the electrostatic interaction. Finally, stacking interactions between ring-annelated corannulenes and fullerene C70 show complexation energy values quite similar to those obtained with fullerene C60.

A Computational Study of Anion-Modulated Cation−π Interactions

The interaction of anions with cation-π complexes formed by the guanidinium cation and benzene was thoroughly studied by means of computational methods. Potential energy surface scans were performed in order to evaluate the effect of the anion coming closer to the cation-π pair. Several structures of guanidinium-benzene complexes and anion approaching directions were examined. Supermolecule calculations were performed on ternary complexes formed by guanidinium, benzene, and one anion and the interaction energy was decomposed into its different two- and three-body contributions. The interaction energies were further dissected into their electrostatic, exchange, repulsion, polarization and dispersion contributions by means of local molecular orbital energy decomposition analysis. The results confirm that, besides the electrostatic cation-anion attraction, the effect of the anion over the cation-π interaction is mainly due to polarization and can be rationalized following the changes in the anion-π and the nonadditive (three-body) terms of the interaction. When the cation and the anion are on the same side of the π system, the three-body interaction is anticooperative, but when the anion and the cation are on opposite sides of the π system, the three-body interaction is cooperative. As far as we know, this is the first study where this kind of analysis is carried out with a structured cation as guanidinium with a significant biological interest.