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Regional electrophilicity at the active sites of the reagents involved in polar Diels−Alder processes may be described on a quantitative basis using an extension of the global electrophilicity index recently introduced by Parr et al. (J. Am. Chem. Soc. 1999, 121, 1922). The local or regional electrophilicity provides useful clues about the expected regioselectivity of the products on Diels−Alder reactions showing significant polar character. The local (regional) electrophilicity index shows significant advantages over other unnormalized definitions of relative electrophilicity proposed in the literature in the sense that it clearly identifies the relevant electrophilic sites in a molecule without restricting the search to those sites having comparable values of regional electrophilic/nucleophilic softness.

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.

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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.

The quantum mechanical SMD continuum universal solvation model can be applied to predict the free energy of solvation of any solute in any solvent following specification of various macroscopic solvent parameters. For three ionic liquids where these descriptors are readily available, the SMD solvation model exhibits a mean unsigned error of 0.48 kcal/mol for 93 solvation free energies of neutral solutes and a mean unsigned error of 1.10 kcal/mol for 148 water-to-IL transfer free energies. Because the necessary solvent parameters are not always available for a given ionic liquid, we determine average values for a set of ionic liquids over which measurements have been made in order to define a generic ionic liquid solvation model, SMD-GIL. Considering 11 different ionic liquids, the SMD-GIL solvation model exhibits a mean unsigned error of 0.43 kcal/mol for 344 solvation free energies of neutral solutes and a mean unsigned error of 0.61 kcal/mol for 431 water-to-IL transfer free energies. As these errors are similar in magnitude to those typically observed when applying continuum solvation models to ordinary liquids, we conclude that the SMD universal solvation model may be applied to ionic liquids as well as ordinary liquids.

The transition structures (TSs) for a series of related Diels-Alder reactions between cyclopentadiene and mono-, di-, tri-, and tetracyanoethylene derivatives have been studied with use of DFT methods at the B3LYP/6-31G computational level. The increase of the electron-withdrawing substitution on ethylene increases the rate of these polar cycloadditions. However, the symmetric arrangement of cis and trans 1,2-di- and tetracyanoethylenes decreases the effectiveness of the substitution, which can be related to the symmetry found at the corresponding TSs. A DFT analysis of the global and local electrophilicity power of these series of cyano ethylenes provides a sound explanation about the nature of these synchronous processes. The present theoretical study is in agreement with the experimental outcomes.