What can we learn from two‐center three‐electron bonding with the topological analysis of ELF? (original) (raw)
Related papers
The Journal of Chemical Physics, 2014
Two-electron three-center bonding interactions in organic ions like methonium (CH + 5), ethonium (C 2 H + 7), and protonated alkanes n − C 4 H + 11 isomers (butonium cations) are described and characterized within the theoretical framework of the topological analysis of the electron density decomposition into its effectively paired and unpaired contributions. These interactions manifest in some of this type of systems as a concentration of unpaired electron cloud around the bond paths, in contrast to the well known paradigmatic boron hydrids in which it is not only concentrated close to the atomic nucleus and the bond paths but out of them and over the region defined by the involved atoms as a whole. This result permits to propose an attempt of classification for these interactions based in such manifestations. In the first type, it is called as interactions through bonds and in the second type as interactions through space type.
Toward a physical understanding of electron-sharing two-center bonds. I. General aspects
Journal of Computational Chemistry, 2006
In 1916, Lewis and Kossel laid the empirical ground for the electronic theory of valence, whose quantum theoretical foundation was uncovered only slowly. We can now base the classification of the various traditional chemical bond types in a threefold manner on the one-and two-electron terms of the quantum-physical Hamiltonian (kinetic, atomic core attraction, electron repulsion). Bond formation is explained by splitting up the real process into two physical steps: (i) interaction of undeformed atoms and (ii) relaxation of this nonstationary system. We aim at a flexible bond energy partitioning scheme that can avoid cancellation of large terms of opposite sign. The driving force of covalent bonding is a lowering of the quantum kinetic energy density by sharing. The driving force of heteropolar bonding is a lowering of potential energy density by charge rearrangement in the valence shell. Although both mechanisms are quantum mechanical in nature, we can easily visualize them, since they are of one-electron type. They are however tempered by two-electron correlations. The richness of chemistry, owing to the diversity of atomic cores and valence shells, becomes intuitively understandable with the help of effective core pseudopotentials for the valence shells. Common conceptual difficulties in understanding chemical bonds arise from quantum kinematic aspects as well as from paradoxical though classical relaxation phenomena. On this conceptual basis, a dozen different bond types in diatomic molecules will be analyzed in the following article. We can therefore examine common features as well as specific differences of various bonding mechanisms.
Topological Characterization of Three-Electron-Bonded Radical Anions
The Journal of Physical Chemistry A, 2002
The three-electron bond in radical anions of the H n XYH mtype, with X, Y) Cl, S, P, Si, F, O, N, C and n, m) 0-2, has been investigated from the topological analysis of the electron localization function (ELF) at the BH&HLYP level. It is shown that the topological modifications arising within the bonding region upon vertical electron attachment are of three different types, according to the vertical electron affinity (vEA) of the neutral compound: for vEA smaller than-16 kcal mol-1 the bonding population remains unchanged, as in H 4 P 2 , for negative vEA greater than-16 kcal mol-1 the bonding population decreases, as in H 2 S 2 , and for positive vEA the bonding population disappears, as in Cl 2. However, after relaxation of the geometry, the formation of the three-electron bond is accompanied in all cases by the disappearance of the X-Y bonding basin (which is the signature of the covalent bond in the neutral parent molecule). From a quantitative point of view, the topological approach also allows us to characterize the transfer of charge and spin densities that arises upon these processes toward the lone pairs basins of the X and Y atoms. Finally, to quantify the electron fluctuation between the two moieties, an index of delocalization has been defined from the analysis of the variance of the lone pairs population. This index increases approximately linearly with the dissociation energy D e of the radical anions, provided that they are separated into a group of weakly bonded ones (D e < 18 kcal mol-1) and a group of strongly 3e-bonded ones (D e > 18 kcal mol-1).
Topological Approaches of the Bonding in Conceptual Chemistry
Applications of Topological Methods in Molecular Chemistry, 2016
Though almost a century old, Lewis’s theory of chemical bonding remains at the heart of the understanding of chemical structure. In spite of their basic discrete nature, Lewis’s structures (topological 0-manifolds) continue to lend themselves to sophisticated treatments leading to valuable results in terms of topological analysis of chemical properties. The bonding topology is however not only defined, but also refined by direct consideration of the nuclear geometry, itself determined by the configuration of the embedding electron cloud. During the last century, the theory has thus been complemented by the mesomery concept, by the Linnett’s double quartet scheme and by the VSEPR/LCP models. These models rely on an assumed spatial disposition of the electrons which does not take the quantum mechanical aspects into account. These models are reexamined by investigation of the topological 1-manifolds generated by the gradient field of potential functions featuring the electron cloud configuration, such as the electron density or electron localization function (ELF). In this chapter, we reexamine these models in order to escape from the quantum mechanical dilemma and we show how topological analyzes enable to recover these models.
Theoretical Chemistry Accounts: Theory, Computation, and Modeling (Theoretica Chimica Acta), 2001
Theoretical calculations (B3LYP/6±311+ +G**) were performed on a series of formally hypervalent compounds showing linear three-center geometries. The bonding nature was analyzed by the electron density, q(r), and electron-localization function (ELF) topologies, including calculations of the AIM charges and NMR chemical shifts (GIAO method). In addition, a quantitative analysis was also performed of the localization and delocalization indexes, obtained from the electron-pair density in conjunction with the de-®nition of an atom in a molecule. Furthermore, the populations and¯uctuations in the ELF basins were also evaluated. The compounds studied presented linear (1± 5), T-shaped (6±9), and bipyramidal structures (10±15). Our results support the 3c-4e model for the linear (1±5) structures, but reveal for the T-shaped (6±9) structures only a small contribution from this model. In addition, there is no evidence to support the 3c-4e bond scheme for the bipyramidal compounds (10±15).
Analysis of the delocalization in the topological theory of chemical bond
Journal of Molecular Structure, 1998
The topological analysis of the gradient field of the electron localization function provides a convenient theoretical framework for the partition of the molecular space into basins of attractors having a clear chemical meaning. The basin populations are evaluated by integrating the one-electron density over the basins. The variance of the basin population provides a measure of the delocalization. The behavior of the core C(X) and protonated valence basins V(X, H) populations have been investigated. The analysis of the population variance in terms of cross-contributions is presented for aromatic and antiaromatic systems, hypervalent molecules and hydrogen-bonded complexes. For hypervalent molecules this analysis emphasizes the importance of the ionic resonance structures. ᭧ 1998 Elsevier Science B.V.
Journal of Physical Chemistry A, 2002
It is shown that the topological analysis of the electron localization function (ELF), a measure of the local Pauli repulsion, is a useful tool to describe the bonding nature of transition structures of simple pericyclic processes. In this work, we have revisited the [1 s ,3 a ]hydrogen, [1 a ,3 s ]methyl, and [1 a ,3 s ]fluorine simple sigmatropic rearrangements in the allyl system. Results based on the integrated densities over the ELF basins and their related properties at the B3LYP/6-311++G(d,p) level of theory showed explicitly a delocalized structure for the antarafacial (C s ) hydrogen rearrangement, a two radical interaction for the methyl suprafacial (C 2 ) migration, and a pair-ion interaction for the fluorine suprafacial (C s ) transfer. Results on these wellstudied systems confirm the topological analysis of the ELF as a useful descriptor for the study of bonding structure of pericyclic transition states. In this context, the ELF analysis is shown to be a complementary value to the Woodward-Hoffmann rules, which provide an orbital symmetry basis of understanding.