Eduard Matito

Electron delocalization and aromaticity in low-lying excited states of archetypal organic compounds

Aromaticity is a property usually linked to the ground state of stable molecules. Although it is well-known that certain excited states are unquestionably aromatic, the aromaticity of excited states remains rather unexplored. To move one step forward in the comprehension of aromaticity in excited states, in this work we analyze the electron delocalization and aromaticity of a series of low-lying excited states of cyclobutadiene, benzene, and cyclooctatetraene with different multiplicities at the CASSCF level by means of electron delocalization measures. While our results are in agreement with Bairds rule for the aromaticity of the lowest-lying triplet excited state in annulenes having 4nπ-electrons, they do not support Soncini and Fowlers generalization of Bairds rule pointing out that the lowest-lying quintet state of benzene and septet state of cyclooctatetraene are not aromatic.

Scalar and spin-orbit relativistic corrections to the NICS and the induced magnetic field: The case of the E12 2- spherenes (E = Ge, Sn, Pb)

Can relativistic effects modify the NICS and the B(ind) values? In this manuscript we evaluate the relativistic corrections incorporated via the zeroth-order regular approximation to the calculations of nucleus-independent chemical shifts and the induced magnetic field (B(ind)) in the E12(2-) spherenes (E = Ge, Sn, Pb). We found that both electron delocalization descriptors are strongly affected by the relativistic corrections. For instance, for plumbaspherene, the difference in values from the nonrelativistic to the relativity-included calculation is almost 40 ppm! Our results show that the changes observed in the NICS and B(ind) values in the title cages are a consequence of the treatment of the relativistic effects. If these effects are included as scalar or spin-orbit calculations, then we can establish the next trend: Ge12(2-) is a nonaromatic species, Sn12(2-) is a low aromatic species, and Pb12(2-) is strongly aromatic, according to calculated NICS and B(ind) values. Thus, any prediction of electron delocalization in molecules containing heavy elements without considering an adequate treatment for relativistic effects may lead to an erroneous chemical interpretation.

OO Bond Formation Mediated by a Hexanuclear Iron Complex Supported on a Stannoxane Core

In recent years, much attention has been focused on the incorporation of redox-active transition-metal complexes into the dendrimer structure owing to their potential applications in various fields. Also, the antenna-like structure of the dendrimers, in many cases, was found to provide an ideal organization for these chromophores and redox centers to work in synergistic ways in carrying out a number of important transformations. For example, an extensive cooperative effect between the Cu centers was observed during the cleavage of supercoiled DNA catalyzed by a hexanuclear Cu-porphyrin complex, supported on a stannoxane core. The above-mentioned hexaporphyrin assembly was synthesized in high yields and in a single step utilizing the orACHTUNGTRENNUNGganostannoxane approach, whereby n-butyl stannoic acid was made to react with the corresponding porphyrin carboxylic acid in 1:1 stoichiometry in benzene; the molecular structure of the ligand was established on the basis of Sn NMR and DFT calculations. In the present paper we report the synthesis of a non-heme hexanucleating ligand (1) supported on a drum-like stannoxane central core utilizing the same organostannoxane approach (Scheme 1). 1 is characterized by X-ray diffraction, Sn NMR, and infrared methods. Most importantly, we show that the Fe-metalated hexa non-heme assembly, 2, in the presence of 2-(tert-butylsulfonyl)-iodosylbenzene (PhIO), performs a rare O O bond formation reaction, thereby generating a Fe-(O2 · ) Fe superoxo unit. Such a metal mediated O O bond formation step is considered to be the most critical part of dioxygen evolution in photosystem II and hence plays a vital role in the context of attaining a clean renewable energy source. The condensation reaction (Scheme 1) of equimolar amounts of n-butyl stannoic acid and 4-(1,3-bis(2-pyridylmethyl)-2-imidazolidinyl)benzoic acid (L1) in toluene afforded a pale yellow solid 1, whose molecular structure (Scheme 1) shows a giant-wheel arrangement of the six non-heme ligand units with a drum-like stannoxane central core serving as the structural support for the hexanucleating assembly. The general features of the stannoxane framework are found to be similar to the other structurally characterized drum-shaped molecules and have a crystallographic S6 symmetry, so that six tin atoms are crystallographically and chemically equivalent. Sn NMR spectrum of 1 exhibits a sharp singlet at 482.4 ppm (Figure S1 top), which is the characteristic signature for a hexameric organostannoxane [a] S. Kundu, F. F. Pfaff, F. Heims, B. R bay, Dr. B. Braun, Dr. K. Ray Humboldt-Universit t zu Berlin, Institut f r Chemie Brook-Taylor-Strasse 2, 12489 Berlin (Germany) Fax: (+49) 3020937387 E-mail : kallol.ray@chemie.hu-berlin.de [b] Dr. E. Matito Institute of Physics, University of Szczecin Wielkopolska 15, 70451 Szczecin (Poland) [c] Dr. S. Walleck, Prof. Dr. T. Glaser Lehrstuhl f r Anorganische Chemie I Fakult t f r Chemie, Universit t Bielefeld Universit tstr. 25, 33615 Bielefeld (Germany) [d] Dr. J. M. Luis Institut de Qu mica Computacional Department de Qu mica, Facultat de Ci ncies Universitat de Girona, 17071 Girona (Spain) [e] Dr. A. Company Institute of Chemistry: Metalorganic materials Technische Universit t Berlin Strasse des 17. Juni 135, 10623 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102326. Scheme 1. Scheme showing the synthesis of the complexes. Hydrogen atoms and the n-butyl groups on tin have been omitted for clarity. Molecular structures of the hexanuclear ligand 1 and the complex [Fe(L2)ACHTUNGTRENNUNG(CH3CN)2]2+ are determined by X-ray crystallography. Structure of 2 is proposed based on ICP-MS, Sn-NMR, F NMR, IR, Mçssbauer and DFT methods (see the Supporting Information for a color version of Scheme 1).

All‐Metal Antiaromaticity in Sb4‐Type Lanthanocene Anions

Antiaromaticity, as introduced in 1965, usually refers to monocyclic systems with 4n πu2005electrons. This concept was extended to all-metal molecules after the observation of Li3 Al4 (-) in the gas phase. However, the solid-phase counterparts have not been documented to date. Herein, we describe a series of all-metal antiaromatic anions, [Ln(η(4) -Sb4 )3 ](3-) (Ln=La, Y, Ho, Er, Lu), which were isolated as the K([2.2.2]crypt) salts and identified by single-crystal X-ray diffraction. Based on the results obtained from the chemical bonding analysis, multicenter indices, and the electron-counting rule, we conclude that the core [Ln(η(4) -Sb4 )3 ](3-) fragment of the crystal has three locally π-antiaromatic Sb4 fragments. This complex represents the first locally π-antiaromatic all-metal system in the solid state, which is stabilized by interactions of the three π-antiaromatic units with the central metal atom.

Calculation of local spins for correlated wave functions

The recent formula for decomposing the expectation value of the total spin operator for general (correlated) wave functions has been rewritten in terms of the cumulant and is realized numerically for the first time. The results confirm its conformity with the physical expectations.

Robust validation of approximate 1-matrix functionals with few-electron harmonium atoms

A simple comparison between the exact and approximate correlation components U of the electron-electron repulsion energy of several states of few-electron harmonium atoms with varying confinement strengths provides a stringent validation tool for 1-matrix functionals. The robustness of this tool is clearly demonstrated in a survey of 14 known functionals, which reveals their substandard performance within different electron correlation regimes. Unlike spot-testing that employs dissociation curves of diatomic molecules or more extensive benchmarking against experimental atomization energies of molecules comprising some standard set, the present approach not only uncovers the flaws and patent failures of the functionals but, even more importantly, also allows for pinpointing their root causes. Since the approximate values of U are computed at exact 1-densities, the testing requires minimal programming and thus is particularly suitable for rapid screening of new functionals.

The Electronic Structure of the Al3− Anion: Is it Aromatic?

Multiconfigurational high-level electronic structure calculations show that the Al3(-) ring-like cluster anion has three close low-lying electronic states of different spin, all of them having strong multiconfigurational character. The aromaticity of the cluster has, therefore, been studied by means of total electron delocalization and normalized multicenter electron delocalization indices evaluated from the multiconfigurational wave functions of each state. The lowest-lying singlet and triplet states are found to be highly aromatic, whereas the next lowest-lying state, the quintet state, has much less, though non-negligible, aromatic character.

Peculiar All-Metal σ-Aromaticity of the [Au2Sb16]4− Anion in the Solid State

Gas-phase clusters are deemed to be σ-aromatic when they satisfy the 4n+2 rule of aromaticity for delocalized σu2005electrons and fulfill other requirements known for aromatic systems. While the range of nu2005values was shown to be quite broad when applied to short-lived clusters found in molecular-beam experiments, stability of all-metal cluster-like fragments isolated in condensed phase was previously shown to be mainly ascribed to two electrons (n=0). In this work, the applicability of this concept is extended towards solid-state compounds by demonstrating a unique example of a storable compound, which was isolated as a stable [K([2.2.2]crypt)]+ salt, featuring a [Au2 Sb16 ]4- cluster core possessing two all-metal aromatic AuSb4 fragments with six delocalized σu2005electrons each (n=1). This discovery pushes the boundaries of the original idea of Kekulé and firmly establishes the usefulness of the σ-aromaticity concept as a general idea for both small clusters and solid-state compounds.

Note: The weak-correlation limit of the three-electron harmonium atom.

Asymptotic energy expressions for the weak-correlation limits of the two lowest energy states of the three-electron harmonium atom are obtained in closed forms. When combined with the known results for the strong-correlation limit, these expressions, which are correct through the second order of perturbation theory, yield robust Padé approximants that allow accurate estimation of energies in question for all magnitudes of the confinement strength.

Bonding description of the Harpoon mechanism

Abstract The lowest lying states of LiH have been widely used to develop and calibrate many different methods in quantum mechanics. In this paper, we show that the electron-transfer processes occurring in these two states are a difficult test for chemical bonding descriptors and can be used to assess new bonding descriptors on its ability to recognise the harpoon mechanism. To this aim, we study the bond formation mechanism in a series of diatomic molecules. In all studied electron reorganisation mechanisms, the maximal electron-transfer variation point along the bond formation path occurs when about half electron has been transferred from one atom to another. If the process takes place through a harpoon mechanism, this point of the reaction path coincides with the avoided crossing. The electron sharing indices and one-dimensional plots of the electron localisation function and the Laplacian of the electron density along the molecular axis can be used to monitor the bond formation in diatomics and provide a distinction between the harpoon mechanism and a regular electron reorganisation process.