A theoretical study of complexes formed between cations and curved aromatic systems: electrostatics does not always control cation-π interaction.

The present work studies the interaction of two extended curved π-systems (corannulene and sumanene) with various cations (sodium, potassium, ammonium, tetramethylammonium, guanidinium and imidazolium). Polyatomic cations are models of groups found in important biomolecules in which cation-π interaction plays a fundamental role. The results indicate an important size effect: with extended π systems and cations of the size of potassium and larger, dispersion is much more important than has been generally recognized for cation-π interactions. In most of the systems studied here, the stability of the cation-π complexes is the result of a balanced combination of electrostatic, induction and dispersion contributions. None of the systems studied here owes its stability to the electrostatic interaction more than 42%. Induction dominates stabilization in complexes with sodium, and in some of the potassium and ammonium complexes. In complexes with large cations and with flat cations dispersion is the major stabilizing contribution and can provide more than 50% of the stabilization energy. This implies that theoretical studies of the cation-π interaction involving large or even medium-size fragments require a level of calculation capable of properly modelling dispersion. The separation between the cation and the π system is another important factor to take into account, especially when the fragments of the cation-π complex are bound (for example, to a protein backbone) and cannot interact at the most favourable distance.

A theoretical study of ternary indole-cation-anion complexes.

The simultaneous interactions of an anion and a cation with a π system were investigated by MP2 and M06-2X theoretical calculations. Indole was chosen as a model π system for its relevance in biological environments. Two different orientations of the anion, interacting with the N-H and with the C-H groups of indole, were considered. The four cations (Na(+), NH4(+), C(NH2)3(+) and N(CH3)4(+)) and the four anions (Cl(-), NO3(-), HCOO(-) and BF4(-)) included in the study are of biological interest. The total interaction energy of the ternary complexes was calculated and separated into its two- and three-body components and all of them are further divided into their electrostatic, exchange, repulsion, polarization and dispersion contributions using the local molecular orbital-energy decomposition analysis (LMO-EDA) methodology. The binding energy of the indole-cation-anion complexes depends on both ions, with the cation having the strongest effect. The intense cation-anion attraction determines the geometric and energetic features in all ternary complexes. These structures, with both ions on the same side of the π system, show an anti-cooperative interaction. However, the interaction is not only determined by electrostatics, but also the polarization contribution is important. Specific interactions like the one established between the anion and the N-H group of indole or the proton transfer between an acidic cation and a basic anion play a significant role in the energetics and the structure of particular complexes. The presence of the polar solvent as modelled with the polarizable continuum model (PCM) does not seem to have a significant effect on the geometry of the ternary complexes, but drastically weakens the interaction energy. Also, the strength of the interaction is reduced at a faster rate when the anion is pushed away, compared to the results obtained in the gas phase. The combination of PCM with the addition of one water molecule indicates that the PCM method properly reproduces the main energetic and geometrical changes, even at the quantitative level, but the explicit hydration allows refining the solvent effect and detecting cases that do not follow the general trend.

Interaction of aromatic units of amino acids with guanidinium cation: The interplay of π···π, X-H···π, and M+ ···π contacts.

Complexes formed by guanidinium cation and a pair of aromatic molecules among benzene, phenol, or indole have been computationally studied to determine the characteristics of the cation···π interaction in ternary systems modeling amino acid side chains. Guanidinium coordinates to the aromatic units preferentially in the following order: indole, phenol, and benzene. Complexes containing two different aromatic units show an intermediate behavior between that observed for complexes with only one kind of aromatic unit. Most stable structures correspond to doubly-T shaped arrangements with the two aromatic units coordinating guanidinium by its NH2 groups. Other structures with only one aromatic unit coordinated to guanidinium, such as T-shaped or parallel-stacked ones, are less favorable but still showing significant stabilization. In indole and phenol complexes, the formation of hydrogen bonds between the aromatic molecules introduces extra stabilization in T-shaped structures. Three body effects are small and repulsive in doubly T-shaped minima. Only when hydrogen bonds involving the aromatic molecules are formed in T-shaped structures a cooperative effect can be observed. In most complexes the interaction is controlled by electrostatics, with induction and dispersion also contributing significantly depending on the nature and orientation of the aromatic species forming the complex. Although the stability in these systems is mainly controlled by the intensity of the interaction between guanidinium and the aromatic molecules coordinated to it, interactions between aromatic molecules can modulate the characteristics of the complex, especially when hydrogen bonds are formed.

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.

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.

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.