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The effect of the recently uncovered nonuniqueness of the external magnetic field B(r[accent:_right_hook_over]) corresponding to a given pair of density n(r[accent:_right_hook_over]) and spin density ns(r[accent:_right_hook_over]) on the derivative of the energy functional of spin-polarized density functional theory, and its implications for the definition of chemical reactivity descriptors, is examined. For ground states, the nonuniqueness of B(r[accent:_right_hook_over]) implies the nondifferentiability of the energy functional Ev,B[n,ns] with respect to ns(r[accent:_right_hook_over]). It is shown, on the other hand, that this nonuniqueness allows the existence of the one-sided derivatives of Ev,B[n,ns] with respect to ns(r[accent:_right_hook_over]). Although the N-electron ground state can always be obtained from the minimization of Ev,B[n,ns] without any constraint on the spin number Ns=∫ns(r[accent:_right_hook_over])dr[accent:_right_hook_over], the Lagrange multiplier μs associated with the fixation of Ns does not vanish even for ground states. μs is identified as the left- or right-side derivative of the total energy with respect to Ns, which justifies the interpretation of μs as a (spin) chemical potential. This is relevant not only for the spin-polarized generalization of conceptual density functional theory, the spin chemical potential being one of the elementary reactivity descriptors, but also for the extension of the thermodynamical analogy of density functional theory for the spin-polarized case. For higher-order reactivity indices, B(r[accent:_right_hook_over])’s nonuniqueness has similar implications as for μs, leading to a split of the indices with respect to Ns into one-sided reactivity descriptors. [ABSTRACT FROM AUTHOR]

Spin-polarized density functional theory (SP-DFT) allows both the analysis of charge-transfer (e.g., electrophilic and nucleophilic reactivity) and of spin-polarization processes (e.g., photophysical changes arising from electron transitions). In analogy with the dual descriptor introduced by Morell et al. [J. Phys. Chem. A 109, 205 (2005)], we introduce new dual descriptors intended to simultaneously give information of the molecular regions where the spin-polarization process linking states of different multiplicity will drive electron density and spin density changes. The electronic charge and spin rearrangement in the spin forbidden radiative transitions S0→T(n,π*) and S0→T(π,π*) in formaldehyde and ethylene, respectively, have been used as benchmark examples illustrating the usefulness of the new spin-polarization dual descriptors. These quantities indicate those regions where spin-orbit coupling effects are at work in such processes. Additionally, the qualitative relationship between the topology of the spin-polarization dual descriptors and the vertical singlet triplet energy gap in simple substituted carbene series has been also discussed. It is shown that the electron density and spin density rearrangements arise in agreement with spectroscopic experimental evidence and other theoretical results on the selected target systems. [ABSTRACT FROM AUTHOR]

The electrophilicity index is analyzed within the framework of spin-polarized density-functional theory. In this context, constrained philicities, ωN≡(μN)2/(2ηNN), are introduced in order to define the capability of a system to acquire or donate electrons in a process at constant spin number. The spin-philicity/spin-donicity indices, ωS±≡(μS±)2/(2ηSS), are examined and rationalized here as the philicity of a given system to change its spin-polarization state, as being defined through the spin potential μS and spin hardness ηSS for a process at constant number of electrons. The local extension of these indices has been also outlined and numerical results have been discussed on the analysis of the electrophilic nature of some simple carbene systems both in the singlet and triplet states. [ABSTRACT FROM AUTHOR]

The central role of the shape function σ(r_) from the density functional theory (DFT), the ratio of the electron density ρ(r_) and the number of electrons N of the system (density per particle), is investigated. Moreover, its relationship with DFT based reactivity indices is established. In the first part, it is shown that an estimate for the chemical hardness can be obtained from the long range behavior of the shape function and its derivative with respect to the number of electrons at a fixed external potential. Next, the energy of the system is minimized with the constraint that the shape function should integrate to unity; the associated Lagrange multiplier is shown to be related to the electronic chemical potential μ of the system. Finally, the importance of the shape function for both molecular structure, reactivity, and similarity is outlined. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photonucleophilic aromatic substitution reactions of nitrobenzene derivatives Were studied by ab initio and Density Functional Theory methods. The photohydrolysis is shown to proceed via an addition—elimination mechanism with two intermediates, except in the case of a chlorine leaving group. Depending on the substituents, the addition step, the elimination step, or the radiationless transition is the rate-determining process. The solvent effect on the SN2 Ar* reactions was evaluated by a continuum model. Next, the regioselectivity of the addition step is investigated within the framework of the so-called spin-polarized conceptual density functional theory. It is shown that the preference observed for the meta or para (with respect to the NO2 group) pathways in the addition step can be predicted by using the spin- polarized Fukui functions applied for the prereactive π-complex. [ABSTRACT FROM AUTHOR]

The regioselectivity of ring-forming radical reactions is investigated within the framework of the so-called spin-polarized conceptual density functional theory. Two different types of cyclizations were studied. First, a series of model reactions of alkyl- and acyl-substituted radicals were investigated. Next, attention was focused on the radical cascade cyclizations of N-alkenyl-2-aziridinylmethyl radicals (a three-step mechanism). In both of these reactions, the approaching radical (carbon or nitrogen centered) adds to a carbon—carbon double bond within the same molecule to form a radical ring compound. In this process, the number of electrons is changing from a local point of view (a charge transfer occurs from one part of the molecule to another one) at constant global spin number Ns (both the reactant and the product ring compound are in the doublet state). It is shown that the experimentally observed regioselectivities for these ring-closure steps can be predicted using the spin-polarized Fukui functions for radical attack, ƒNN0(r). [ABSTRACT FROM AUTHOR]

Quantum molecular similarity (QMS), for which the first measures were introduced by Carbó in the early 1980s, has been shown to be an important concept when comparing properties and reactivity of different, albeit mostly related, molecules. In this article we investigate how various subfields of Density Functional Theory (DFT) contribute in their own way to the evaluation and the understanding of QMS, reflecting the combined interest of our group in both domains in recent years. The contribution of computational DFT is easy to pinpoint, as DFT can now be used in the evaluation of electron densities of ever-increasing quality. When establishing a link between the fundamental(s) of DFT and QMS the role of the shape function turns out to be predominant and an extension of Mezey's Holographic Electron Density Theorem for p(r) to the shape function σ(r) is presented. On this basis, similarity in shape is the fundamental issue to be looked at, both globally and locally. As an application, global and local similarity measures were evaluated using the Hirshfeld partitioning technique for the two enantiomers of CHFClBr, giving numerical evidence for the Holographic Electron Density Theorem. As an application of conceptual DFT, similarity indices constructed via DFT based local reactivity descriptors (e.g., local softness) are used to probe the ‘similarity of reactivity’ of a series of peptide isosteres. The use of the autocorrelation function for condensed indices turns out to be a valuable technique to circumvent the orientation-translation dependence of similarity indices. [ABSTRACT FROM AUTHOR]

Examines the influence of density functional theory on the evolution of quantum chemistry. Basis for the development of computational strategies; Accounts on the different types of spectroscopy; Role of quantum chemistry on chemical research.

Abstract: The existence of stable V2O5 compounds as gas-phase cluster, single crystal or as supported catalyst has motivated the present work aiming at characterizing the chemical reactivity of the different aggregated moieties. We apply the concepts of electrostatic potential and Fukui function, based on periodic calculations, to explain the different chemical reactivity found for oxygen sites in V2O5 systems. Qualitative and quantitative parameters are derived for such analysis. It is found that the so-called active site is not unique, reconciling the hypotheses proposed in the literature. Coverage and support effects strongly modify the reactive regions. [Copyright &y& Elsevier]