Anion-Anion Complexes Established between Aspartate Dimers.

Stable dimers aspartate-aspartate have been studied in aqueous and gas phase through theoretical simulations. The polarizable continuum model (PCM) has been applied to simulate the effect of the hydration on monomers and complexes. The quantum theory of atoms in molecules (QTAIM) and the interacting quantum atoms (IQA) scheme has been used to inquire into if, in the aqueous phase, individual hydrogen bonds have attractive electrostatic components. In all cases a spontaneous formation of the complexes in the aqueous phase are observed, while in the gas phase a considerable energy barrier must be overcome (between 100.8 to 263.2 kJ mol-1 ). The intermolecular distance at which this barrier is indicates when the hydrogen-bond interactions begin to take importance between the dimers and the corresponding molecular recognition among them. The IQA analysis shows that in aqueous phase, the hydrogen bonds N-H⋅⋅⋅O are mainly electrostatic in nature with a certain covalent character which increases linearly with the decrease of internuclear distances H⋅⋅⋅O. The H⋅⋅⋅H interactions observed are stabilizing and they are mainly quantum in nature.

Multicenter (FX)n/NH3 Halogen Bonds (X = Cl, Br and n = 1-5). QTAIM Descriptors of the Strength of the X∙∙∙N Interaction.

In the present work an in depth deep electronic study of multicenter XBs (FX)n/NH3 (X = Cl, Br and n = 1-5) is conducted. The ways in which X∙∙∙X lateral contacts affect the electrostatic or covalent nature of the X∙∙∙N interactions are explored at the CCSD(T)/aug-cc-pVTZ level and in the framework of the quantum theory of atoms in molecules (QTAIM). Calculations show that relatively strong XBs have been found with interaction energies lying between -41 and -90 kJ mol-1 for chlorine complexes, and between -56 and -113 kJ mol-1 for bromine complexes. QTAIM parameters reveal that in these complexes: (i) local (kinetics and potential) energy densities measure the ability that the system has to concentrate electron charge density at the intermolecular X∙∙∙N region; (ii) the delocalization indices [δ(A,B)] and the exchange contribution [VEX(X,N)] of the interacting quantum atoms (IQA) scheme, could constitute a quantitative measure of the covalence of these molecular interactions; (iii) both classical electrostatic and quantum exchange show high values, indicating that strong ionic and covalent contributions are not mutually exclusive.

Simultaneous Occurrence of Quadruple Lewis Acid-Base Interactions between Selenium Atoms in Selenocarbonyl Dimers.

High-level quantum chemical calculations are performed to investigate C=Se⋅⋅⋅Se=C interactions. Bounded structures are found with binding energies between -4 and -7 kJ mol-1 . An energy decomposition analysis shows that dispersion is the more attractive term, and in all cases save one, the electrostatic interaction is attractive despite each selenium atom having a positive σ-hole at the extension of the C=Se bond. The topological analysis of the molecular electrostatic potential and L(r)=-∇2 ρ(r) function, and natural bond orbital analysis reveal that these particular Se⋅⋅⋅Se contacts can be considered to be quadruple Lewis acid-base interactions.

Halogen bonding. The role of the polarizability of the electron-pair donor.

The nature of F-BrX-R interactions (with X = F, Cl, Br, I and R = -H, -F) has been investigated through theoretical calculation of molecular potential electrostatic (MEP), molecular polarizability, atoms in molecules (AIM) analysis and energetic decomposition analysis (EDA). A detailed analysis of the MEPs reveals that considering only the static electrostatic interactions is not sufficient to explain the nature of these interactions. The molecular polarizabilities of X-R molecules suggest that the deformation capacity of the electronic cloud of the lone pairs of the X atom plays an important role in the stability of these complexes. The topological analysis of the L(r) = -»∇(2)ρ(r) function and the detailed analysis of the atomic quadrupole moments reveal that the BrX interactions are electrostatic in nature. The electron acceptor Br atom causes a polarization of the electronic cloud (electronic induction) on the valence shell of the X atom. Finally, the electrostatic forces and charge transfer play an important role not only in the stabilization of the complex, but also in the determination of the molecular geometry of equilibrium. The dispersive and polarization forces do not influence the equilibrium molecular geometry.

Double Hole-Lump Interaction between Halogen Atoms.

In this paper a theoretical study has been carried out to investigate the nature of the unusual halogen-halogen contacts in the complexes R-X···X-R (with R = -H, -Cl, -F and X = Cl, Br, I). AIM, NBO, and MEP analyses have been used to characterize X···X interactions. Formation of the unusual X···X interactions leads to a significant increase of electron charge density in the bonding region between the two halogen atoms. The geometry and stability of these complexes is mainly due to electrostatic interactions lump(X1) → hole(X2) and lump(X2) → hole(X1) [or equivalently [VS,min(X1) → VS,max(X2) and VS,min(X2) → VS,max(X1)] and the charge transfers LP(X1) → σ*(R-X2) and LP(X2) → σ*(R-X1). In other words, these findings suggest that the electrostatic interactions and the charge transfer play a substantial role in determining the optimal geometry of these complexes, as in conventional halogen bonds, even though the dispersion term is the most important attractive term for all the complexes studied here, save one.

Physical meaning of the QTAIM topological parameters in hydrogen bonding.

This work examined the local topological parameters of charge density at the hydrogen bond (H-bond) critical points of a set of substituted formamide cyclic dimers and enolic tautomers. The analysis was performed not only on the total electron density of the hydrogen bonded complexes but also on the intermediate electron density differences derived from the Morokuma energy decomposition scheme. Through the connection between these intermediate electron density differences and the corresponding differences in topological parameters, the meaning of topological parameters variation due to hydrogen bonding (H-bonding) becomes evident. Thus, for example, we show in a plausible way that the potential energy density differences at the H-bond critical point properly describe the electrostatics of H-bonding, and local kinetic energy density differences account for the localization/delocalization degree of the electrons at that point. The results also support the idea that the total electronic energy density differences at the H-bond critical point describe the strength of the interaction rather than its covalent character as is commonly considered.

Computational study on the vinyl azide decomposition.

The decomposition mechanism of vinyl azide (CH2CHN3) has been studied by calculations of the electronic structure. In addition, a study based on the topology of the electron charge density distribution and its Laplacian function, within the Quantum Theory of Atoms in Molecules (QTAIM), has been carried out with the aim of comprehending the electron redistribution mechanisms that take place in the formation of vinyl nitrenes. The electronic structure calculations reveal that the decomposition of the s-cis conformer of vinyl azide leads to the formation of ketenimine through a single-step conversion, s-cis-CH2CHN3 → CH2CNH + N2, while the conversion of the s-trans conformer to acetonitrile occurs in two steps, s-trans-CH2CHN3 → cyc-CH2NCH + N2 → CH3CN + N2. The topological analysis of the L(r) function reveals that triplet vinyl nitrene has one lone pair on the valence shell charge concentration (VSCC) of nitrogen and thus could act as a monodentate Lewis base, while singlet vinyl nitrene has two lone pairs on the VSCC of nitrogen and thus could act as a bidentate Lewis base.

Structural, energetic, and UV-Vis spectral analysis of UVA filter 4-tert-butyl-4'-methoxydibenzoylmethane.

The growing awareness of the harmful effects of ultraviolet (UV) solar radiation has increased the production and consumption of sunscreen products, which contain organic and inorganic molecules named UV filters that absorb, reflect, or scatter UV radiation, thus minimizing negative human health effects. 4-tert-Butyl-4'-methoxydibenzoylmethane (BMDBM) is one of the few organic UVA filters and the most commonly used. BMDBM exists in sunscreens in the enol form which absorbs strongly in the UVA range. However, under sunlight irradiation tautomerization occurs to the keto form, resulting in the loss of UV protection. In this study we have performed quantum chemical calculations to study the excited-state molecular structure and excitation spectra of the enol and keto tautomers of BMDBM. This knowledge is of the utmost importance as the starting point for studies aiming at the understanding of its activity when applied on human skin and also its fate once released into the aquatic environment. The efficiency of excitation transitions was rationalized based on the concept of molecular orbital superposition. The loss of UV protection was attributed to the enol → keto phototautomerism and subsequent photodegradation. Although this process is not energetically favorable in the singlet bright state, photodegradation is possible because of intersystem crossing to the first two triplet states.

Is the decrease of the total electron energy density a covalence indicator in hydrogen and halogen bonds?

In this work, halogen bonding (XB) and hydrogen bonding (HB) complexes were studied with the aim of analyzing the variation of the total electronic energy density H(r b ) with the interaction strengthening. The calculations were performed at the MP2/6-311++G(2d,2p) level of approximation. To explain the nature of such interactions, the atoms in molecules theory (AIM) in conjunction with reduced variational space self-consistent field (RVS) energy decomposition analysis were carried out. Based on the local virial theorem, an equation to decompose the total electronic energy density H(r b ) in two energy densities, (-G(r b )) and 1/4∇(2)ρ(r b ), was derived. These energy densities were linked with the RVS interaction energy components. Through the connection between both decomposition schemes, it was possible to conclude that the decrease in H(r b ) with the interaction strengthening observed in the HB as well as the XB complexes, is mainly due to the increase in the attractive electrostatic part of the interaction energy and in lesser extent to the increase in its covalent character, as is commonly considered.

Nature of halogen bonding. A study based on the topological analysis of the Laplacian of the electron charge density and an energy decomposition analysis.

In this work we investigate the nature of the Cl···N interactions in complexes formed between substituted ammonium [NHn(X3-n) (with n = 0, 1, 2, 3 and X = -CH3, -F] as Lewis bases and F-Cl molecule as Lewis acid. They have been chosen as a study case due to the wide range of variation of their binding energies, BEs. Møller-Plesset [MP2/6-311++G(2d,2p)] calculations show that the BEs for this set of complexes lie in the range from 1.27 kcal/mol (in F-Cl···NF3) to 27.62 kcal/mol [in F-Cl···N(CH3)3]. The intermolecular distribution of the electronic charge density and their L(r) = -»∇(2)ρ(r) function have been investigated within the framework of the atoms in molecules (AIM) theory. The intermolecular interaction energy decomposition has also been analyzed using the reduced variational space (RVS) method. The topological analysis of the L(r) function reveals that the local topological properties measured at the (3,+1) critical point [in L(r) topology] are good descriptors of the strength of the halogen bonding interactions. The results obtained from energy decomposition analysis indicate that electrostatic interactions play a key role in these halogen bonding interactions. These results allow us to establish that, when the halogen atom is bonded to a group with high electron-withdrawing capacity, the electrostatic interaction between the electron cloud of the Lewis base and the halogen atom unprotected nucleus of the Lewis acid produces the formation and determines the geometry of the halogen bonded complexes. In addition, a good linear relationship has been established between: the natural logarithm of the BEs and the electrostatic interaction energy between electron charge distribution of N atom and nucleus of Cl atom, denoted as V e-n(N,Cl) within the AIM theory.

Topological analysis of aromatic halogen/hydrogen bonds by electron charge density and electrostatic potentials.

In this work, the intermolecular distribution of the electronic charge density in the aromatic hydrogen/halogen bonds is studied within the framework of the atoms in molecules (AIM) theory and the molecular electrostatic potentials (MEP) analysis. The study is carried out in nine complexes formed between benzene and simple lineal molecules, where hydrogen, fluorine and chlorine atoms act as bridge atoms. All the results are obtained at MP2 level theory using cc-pVTZ basis set. Attention is focused on topological features observed at the intermolecular region such as bond, ring and cage critical points of the electron density, as well as the bond path, the gradient of the density maps, molecular graphs and interatomic surfaces. The strength of the interaction increases in the following order: F[Symbol: see text]pi < Cl[Symbol: see text]pi < H[Symbol: see text]pi. Our results show that the fluorine atom has the capability to interact with the pi-cloud to form an aromatic halogen bond, as long as the donor group is highly electron withdrawing. The Laplacian topology allows us to state that the halogen atoms can act as nucleophiles as well as electrophiles, showing clearly their dual character.

Adsorption of alkenes on acidic zeolites. Theoretical study based on the electron charge density.

In the present work, experiments on electron density changes in the adsorption process of alkenes on acidic zeolites, in the framework of atoms in molecules theory (AIM), were carried out. Electron densities were obtained at MP2 and B3LYP levels using a 6-31++G(d,p) basis set. This study explores the energetic and the electron density redistributions associated with O-H...pi interactions. The main purpose of this work is to provide an answer to the following questions: (a) Which and how large are the changes induced on the molecular electron distribution by the formation of adsorbed alkenes? (b) Can a reasonable estimate of the adsorption energy of alkenes on the active site of zeolite be solely calculated from an analysis of the electron densities? We have used topological parameters to determine the strength and nature of the interactions in the active site of the zeolite. All the results derived from the electron density analysis show that the stabilization of the adsorbed alkenes follows the order isobutene > trans-2-butene congruent with 1-butene congruent with propene > ethene, reflecting the order of basicity of C=C bonds, i.e., (C(ter)=C(prim)) > (C(sec)=C(sec)) congruent with (C(prim)=C(sec)) > (C(prim)=C(prim)). In addition, we have found a useful set of topological parameters that are good for estimating the adsorption energy in adsorbed alkenes.