Patricio Fuentealba

A direct evaluation of regional Fukui functions in molecules

Abstract A simple formalism to obtain regional Fukui functions is presented. The model is based on an exact relationship between this local reactivity descriptor and the frontier molecular orbitals. Within this approach, it becomes possible to define an orbital Fukui function that directly yields the condensed-to-atom quantity. By this procedure, we avoid additional calculations of the anion or cation associated with the molecule, thereby maintaining the spin multiplicity of the system. It is shown that our proposed definition of the Fukui function mainly contains symmetry information about the molecular system. The proposed methodology is tested against several benchmark model reactions that are well documented in the experimental and theoretical literature.

On the condensed Fukui function

A critical comparison among recently proposed methods for evaluating the condensed Fukui function neglecting relaxation effects is presented. The sign of the condensed Fukui function is discussed and arguments for a positive definite condensed Fukui function are given. Our numerical calculations in two series of molecules show that: (i) the condensed Fukui function can give, in general, valuable information about the site selectivity in chemical reactions and systematization in a family of molecules. In particular, it has been shown that the selectivity towards protonation in anilines and derivatives molecules can be correctly assessed by the electrophilic Fukui function described in this paper. Within this approach non-negative values for the condensed Fukui function are obtained for the relevant protonation sites in these polyfunctional systems; and (ii) the solvent effects on the condensed Fukui function are negligible, confirming a recently presented theoretical prediction.

Variation of the Electrophilicity Index along the Reaction Path

Some exact conditions for the extremals of the electrophilicity index, ω = μ(2)/2η (Parr, R. G.; von Szentpály, L.; Liu, S. J. Am. Chem. Soc. 1999, 121, 1922), along an arbitrary reaction coordinate, have been carefully examined. Implications within the widely used finite difference approximation for the density-functional based reactivity descriptors, their relationship with the maximum hardness principle, and the reliability of the general relationships have been tested in the framework of computational evidence for some simple systems of chemical interest.

Sigma–pi separation of the electron localization function and aromaticity

The electron localization function (ELF) has been separated in its sigma and pi components. The topological analysis of the new ELFsigma and ELFpi functions has been used to quantify the concept of resonance. The highest bifurcation values of these functions describe in a correct way the aromaticity of classical ring molecules and some new aromatic compounds as B6CO6, Al4(2-), and N5-. In the case of Al4(2-), an important sigma delocalization contribution has been found, which is in agreement with previous interpretation.

Chemical Reactivity Descriptors for Ambiphilic Reagents: Dual Descriptor, Local Hypersoftness, and Electrostatic Potential

The second-order response of the electron density with respect to changes in electron number, known as the dual descriptor, has been established as a key reactivity indicator for reactions like pericyclic reactions, where reagents accept and donate electrons concurrently. Here we establish that the dual descriptor is also the key reactivity indicator for ambiphilic reagents: reagents that can act either as electrophiles or as nucleophiles, depending on the reaction partner. Specifically, we study dual atoms (which are proposed to act, simultaneously, as an electron acceptor and an electron donor), dual molecules (which react with both electrophiles and nucleophiles, generally at different sites), and dual ion-molecule complexes (which react with both cations and anions). On the basis of our analysis, the dual atom (an Al(I) that has been purported to be dual in the literature) is actually pseudodual in the sense that it does not truly accept electrons from a nucleophiles; rather, it serves as a conduit through which an electrophile can donate electrons to the attached aromatic ring. For understanding dual ion-molecule complexes, it helps to understand that the dual descriptor makes a key contribution to the long-range portion of the quadratic hyperpolarization. In all cases, a complete description of the reactivity of the ambiphilic reagent requires considering both an orbital-based descriptor of electron transfer (the dual descriptor or the local hypersoftness) and the electrostatic potential. The local hypersoftness strongly resembles the dual descriptor.

An aromaticity scale based on the topological analysis of the electron localization function including σ and π contributions

In this work, the average bifurcation value of the electron localization function (ELF) of both σ (ELFσ) and π (ELFπ) contributions was used to construct an aromaticity scale for chemical compounds. We have validated the scale with a series of well-known molecules and then used it to evaluate global aromaticity on aluminum based clusters, which present σ aromaticity and π antiaromaticity. The proposed scaled predicts an overall antiaromatic character for the Al4(4)(-) moiety.

Ground state properties of alkali and alkaline–earth hydrides

The ground state potential energy curves of alkali (LiH to CsH) and alkaline–earth monohydrides (BeH to BaH) have been calculated. A pseudopotential formalism including a core‐polarization potential has been used. For the valence correlation energy, two different methods, the local spin‐density functional and the configuration interaction with single and double excitations, have been employed. Dissociation energies, bond lengths, vibrational frequencies, anharmonicity constants, and dipole moments are reported. The agreement with experimental values, where available, is very good. A discussion and a comparison with other theoretical values, at different levels of approximation, are also included.

Stochastic Search of the Quantum Conformational Space of Small Lithium and Bimetallic Lithium-Sodium Clusters

In this paper we report the results obtained by an implementation and application of the simulated annealing optimization procedure to the exploration of the conformational space of small neutral and charged lithium clusters (Li(n)(q), n = 5, 6, 7; q = 0, +/-1) and of the bimetallic lithium/sodium clusters (Li5Na) in their lowest spin states. Our methodology eliminates the structure guessing procedure in the process of generating cluster configurations. We evaluate the quantum energy, typically with the Hartree-Fock Hamiltonian, of randomly generated points in the conformational space and use a modified Metropolis test in the annealing algorithm to generate candidate structures for atomic clusters. The structures are further optimized by analytical methods (gradient following) at the Møller-Plesset second order perturbation theory level (MP2), in conjunction with basis sets including polarization functions with and without diffuse functions. High accuracy ab initio energies at the coupled clusters level, with single, double, and triple substitutions from the Hartree-Fock determinant (CCSD(T)), on the MP2 geometries were calculated and used to establish the relative stability of the isomers within each potential energy surface. Various cluster properties were computed and compared to existing values in order to validate our methods. Our results show excellent agreement with previous experimental and theoretical reports. Even at these small sizes, evidence for 10 new structures never reported before for the lithium clusters and four new structures for the bimetallic clusters is presented.

A local model for the hardness kernel and related reactivity parameters in density functional theory

The local density approximation is used to construct a model for the reactivity parameters defined in density functional theory. Starting from a model for the hardness kernel, equations are derived to compute the softness kernel, local softness and local hardness, global softness and global hardness, the Fukui function and also the electric dipole polarizability. A very simple functional is used to test the equations in some atomic systems. The results are promising. Extensions and applications of the model are discussed.

Topological Analysis of the Fukui Function

In this work an alternative to the analysis of the Fukui function will be presented and compared with the traditional condensed function. The topological analysis allows us to define basins corresponding to different regions of the space, and the numerical integration of the density over those volumes gives a number amenable of a chemical interpretation in line with the Fukui function applications. Various examples are shown, a series of small molecules, a couple of clusters, and aromatic molecules. They are discussed in comparison with other methodologies and with the experimental evidence.