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In this publication, we propose a new set of reactivity/selectivity descriptors, derived within a Rayleigh–Schrödinger perturbation theory framework, for chemical systems undergoing an electrostatic (point-charge) perturbation. From the electron density polarization at first order, qualitative insight on reactivity is retrieved, while more quantitative information (noteworthy selectivity) can be obtained from either the second-order energy response or the number of shifted electrons under perturbation. Noteworthily, only a small number of excitations contribute significantly to the overall responses to perturbation, suggesting chemical reactivity could be foreseen by a careful scrutiny of the electron density reorganization upon excitation.

A new selection rule for transmission under small voltage for molecular electronic devices consisting of alternant hydrocarbons is established, based on curly arrow drawings of the displacement of electrons through the conjugated system of the molecule. If the displacement of electrons from one contact atom to the other can be drawn, transmission is possible. If not, then no transmission will take place. This simple, back of the envelope ansatz allows determination of quantum interferences for a wide range of experimentally studied molecules (linear polyenes, benzenoids, cross-conjugated molecules, ...) avoiding any calculation. This new selection rule bridges the conceptual gap between traditional reactivity theory and molecular electronics.

The fundamental principles of pericyclic reactions are governed by the Woodward–Hoffmann rules, which state that these reactions can only take place if the symmetries of the reactants’ molecular orbitals and the products’ molecular orbitals are the same. As such, these rules rely on the nodal structure of either the wave function or the frontier molecular orbitals, so it is unclear how these rules can be recovered in the density functional reactivity theory (or “conceptual DFT”), where the basic quantity is the strictly positive electron density. A third, nonsymmetry based approach to predict the outcome of pericyclic reactions is due to Zimmerman which uses the concept of the aromatic transition states: allowed reactions possess aromatic transition states, while forbidden reactions possess antiaromatic transition states. Based on our recent work on cycloadditions, we investigate the initial response of the chemical hardness, a central DFT based reactivity index, along the reaction profiles of a series of electrocyclizations. For a number of cases, we also compute complete initial reaction coordinate (IRC) paths and hardness profiles. We find that the hardness response is always higher for the allowed modes than for the forbidden modes. This suggests that the initial hardness response along the IRC is the key for casting the Woodward–Hoffmann rules into conceptual DFT.

Abstract: Regioselectivity of the photochemical [2 + 2] cyclo-addition of triplet carbonyl compounds with a series of ground state electron-rich and electron-poor alkenes, the Paterno-Büchi reaction, is studied. Activation barriers for the first step of the triplet reaction are computed in the case of the O-attack. Next, the observed regioselectivity is explained using a series of DFT-based reactivity indices. In the first step, we use the local softness and the local HSAB principle within a softness matching approach, and explain the relative activation barriers of the addition step. In the final step, the regioselectivity is assessed within the framework of spin-polarized conceptual density functional theory, considering response functions of the system’s external potential v, number of electronsN and spin numberN s , being the difference between the number ofα andβ electrons in the spin-polarized system. Although the concept of local spin philicity, introduced recently within this theory, appears less suited to predict the regioselectivity in this reaction, the correct regioselectivity emerges from considering an interaction between the largest values of the generalized Fukui functionsf ss on both interacting molecules.

Aromatic stabilisation energies using the recently introduced isomerisation method of Schleyer and Pühlhofer Org. Lett.2002, 4, 2873–2876 are computed for a series of five-membered C4H4X compounds ranging from anti-aromatic to highly aromatic. These energies are correlated with the classical estimate from a homodesmotic reaction. Moreover, this isomerisation method is also used to compute relative magnetic susceptibilities and hardnesses. The latter is shown to be linearly related to other aromaticity estimates, such as the aromatic stabilisation energy and Nucleus Independent Chemical Shifts. Finally, this methodology is applied to a study of the aromaticity of a recently synthesised platinabenzene compound.

Abstract: An analysis of the global and local aromaticity was made for a series of hydrogenated fullerenes of the type C60H2n (n=1–6) at the ab initio HF(Hartree-Fock)/3–21G level of theory, the isomers considered being obtained with an octahedral addition pattern. The relation between this addition pattern and the magnetic properties was established, showing low aromatic regions to be preferred for addition. These results show that local aromaticity, as shown in the nucleus-independent chemical shift, can be used to predict addition sites in these systems.

A study is presented concerning the performance and basis set dependence of density functional methods in the calculation of dipole and quadrupole moments. A set of some small molecules was thereby used, together with seven commonly used exchange correlation functionals and Dunnings correlation consistent aug-cc-pVDZ, aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets. The results were compared with experiment and previously published values obtained with traditional ab initio molecular orbital methods. The best performance in the calculation of dipole and quadrupole moments is due to the hybrid functionals B3LYP and B3PW91. Moreover, the results seem to be essentially converged at the aug-cc-pVTZ level. Comparison of the LDA/aug-cc-pV5Z results with the numerical LDA results of Dickson and Becke, reveals that both sets of values are identical for most cases.

The reactivity of acetaldehyde and some aromatic aldehydes towards acid‐catalysed oxygen‐18 exchange reactions with H2O18was studied using the density functional theory (DFT)‐based reactivity descriptors local softness and local hardness. Local softness is used to predict the preferable reactive sites within a given molecule, whereas local hardness reproduces the experimental intermolecular reactivity trends. A new concept, intrinsic global hardness, obtained via filtering out the volume effect of the global softness, shows an excellent correlation with the degree of aromaticity of the compounds. Copyright © 1999 John Wiley & Sons, Ltd.

A study is presented on the basicities of alkyl-substituted amines both in the gas phase and in solution. In the first step, calculated group hardnesses are used to clarify the origin of the inversion of the alkyl amine basicity when passing from the gas phase to solution for a series (CH3)nNH3−nwith n=0, 1, 2 and 3. The basicity in the gas phase is primarily determined by the intramolecular stabilization of the positive charge in the conjugate acid whereas, in aqueous solution, the solvent accounts for this effect. Next, Sanderson's electronegativity equalization principle is shown to be a useful tool in describing the charge distribution in the molecules RNH2(R=CH3, CH2CH3and CH(CH3)3) and their conjugate acids in solution. These calculated charge distributions are then used in a study on the inversion of the alkyl amine basicities from the gas phase to aqueous solution for these molecules.

The Henry and separation constants and heat of adsorption of N2 and O2 in faujasite are calculated in a non-empirical way; they show fair agreement with experiment taking into account the extreme sensitivity of the configuration integral to small variations in interaction energy values.

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