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As part of our continuing interest in the connection between chemical reactivity and the HSAB principle, both globally and locally, we explored via 3-21 G calculations on local and global softness the statement made by Gazquez and Mendez. Their statement, claiming that two chemical species should react through atoms showing equal softness, has been precised to rationalize the regioselectivity of Normal Electron Demand Diels−Alder reactions between terminally monosubstituted 1,3-butadienes and monosubstituted ethenes. In the presence of this substitution pattern, a head-to-head mode of cyclization is largely experimentally observed, also known as “ortho rule”. The closest values of the condensed local softness were found for the unsubstituted termini, thereby suggesting that these cycloadditions would proceed through an asynchronous pathway, as supported by transition state geometries reported by Houk et al.

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The electronic structures of sixteen carbenes and four alkenes calculated at the B3LYP/6-31G* level were used to carry out a hard−soft acid−base investigation of carbene singlet−triplet gaps and of the addition of carbenes to alkenes. The carbenes chosen include examples where the ground state varies from triplet to singlet state and examples that have been characterized as nucleophilic, electrophilic, or ambiphilic based on their reactivity with alkenes having varying electron demands. We confirmed that singlet−triplet gaps calculated by DFT methods with the B3LYP functional correlate well with experimentally known values of carbenes spanning over 70 kcal/mol. Correlations between electron-density-based parameters and singlet−triplet gaps, or with carbene−alkene reactivity, were analyzed to gain insight into the electronic structure of the carbenes and their acid−base tendencies. A good correlation was found between the singlet−triplet energy gap and the condensed softness for electrophilic attack at the triplet carbene carbon [ (T)], which reflects the level of carbene stability upon electron donation. Trends in electron donation between carbenes and alkenes with different electrophilicities and nucleophilicities analyzed by the electronegativity equalization principle correlate well with Moss' reactivity indices. We also found that interaction energies calculated between alkenes and carbenes are most favorable with parameters that reflect mutual electron donation, reflecting the simultaneous acidity and basicity of carbenes and alkenes.

We present a review of the methods most frequently used for the synthesis of fullerenes and the changes that these methods have experienced since 1985 when Kroto and co-workers discovered C 60 . We also analyze the most important models that explain the mechanism of the formation of fullerenes in carbon soot, as well as the new methodologies that lead to the rational chemical synthesis of fullerenes and of fullerene fragments as precursors.

We evaluate herein the impact of positive charge distribution on the in vitro and in vivo properties of Mn porphyrins as redox modulators possessing the same overall 5+ charge and of minimal stericity demand: Mn(III) meso-tetrakis(trimethylanilinium-4-yl)porphyrin (MnTTriMAP(5+)), Mn(III) meso-tetrakis(N,N'-dimethylpyrazolium-4-yl)porphyrin (MnTDM-4-PzP(5+)), Mn(III) meso-tetrakis(N,N'-dimethylimidazolium-2-yl)porphyrin (MnTDM-2-ImP(5+)), and the ortho and para methylpyridinium complexes Mn(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MnTM-4-PyP(5+)) and Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin (MnTM-2-PyP(5+)). Both Mn(III)/Mn(II) reduction potential and SOD activity within the series follow the order: MnTTriMAP(5+)

The bonding of hydrogen in the Al13H aggregate is analyzed in the framework of density functional theory using the local density approximation. The interaction between the H-1s orbital and only certain molecular orbitals of Al13 is responsible for the binding. Different measures of the charge transfer give consistent results and predict the stabilization of a sizable amount of electronic charge, about two electrons, around the proton site. The state of the H atom can be described as a negatively charged impurity screened by the surrounding electron gas, similarly to a H impurity embedded in a vacancy in metallic aluminum. Friedel-type oscillations can be appreciated in the screening charge. Local Fukui functions and condensed Fukui indexes associated to the ground state of the cluster Al13 are used as indicators of molecular reactivity. Those indices allow to predict and understand the equilibrium location of H found in the total energy calculations for Al13H.