Electron pairing and chemical bonds. On the accuracy of the electron pair model of chemical bond (original) (raw)
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Electron Pairing and Chemical Bonds
ChemPlusChem, 1994
The recently proposed population analysis of pair densities is applied to the investigation of molecular structure of several simple molecules. The values of pairon populations straightforwardly reproduce the classical structural formula including the multiplicity of the bonds and provide thus the so far missing link between quantum chemical and Lewis's classical picture of bonding. As demonstrated, the formalism of the proposed approach provides strong theoretical evidence for the frequently expected but so far elusive role of electron pairing in chemical bonding.
Bond electron pair: Its relevance and analysis from the quantum chemistry point of view
Journal of Computational Chemistry, 2007
This paper first comments on the surprisingly poor status that Quantum Chemistry has offered to the fantastic intuition of Lewis concerning the distribution of the electrons in the molecule. Then, it advocates in favor of a hierarchical description of the molecular wave-function, distinguishing the physics taking place in the valence space (in the bond and between the bonds), and the dynamical correlation effects. It is argued that the clearest pictures of the valence electronic population combine two localized views, namely the bond (and lone pair) Molecular Orbitals and the Valence Bond decomposition of the wave-function, preferably in the orthogonal version directly accessible from the complete active space self consistent field method. Such a reading of the wave function enables one to understand the work of the nondynamical correlation as an enhancement of the weight of the low-energy VB components, i.e. as a better compromise between the electronic delocalization and the energetic preferences of the atoms. It is suggested that regarding the bond building, the leading dynamical correlation effect may be the dynamical polarization phenomenon. It is shown that most correlation effects do not destroy the bond electron pairs and remain compatible with Lewis' vision. A certain number of free epistemological considerations have been introduced in the development of the argument. q
International Journal of Quantum Chemistry, 1998
The role of electron pairing in chemical bonding stressed by the Lewis electron-pair model of the chemical bond is analyzed and discussed from the point of view of the proposal that chemical bonds are the regions of space populated roughly by two electrons and which at the same time exhibit low fluctuation of an electron pair. Based on this assumption, we have been able to introduce a new localization procedure, Ž. the output of which are just the orbitals chemical bonds satisfying the criterion of minimum pair fluctuation. It has been shown that these orbitals remarkably well display the most important attributes of chemical bonds, namely, the localization in the regions where classical bonds are expected and there is very high transferability from one molecule to another. The applicability of this procedure as a new means of the analysis and the visualization of the molecular structure is also discussed.
On the electron-pair nature of the hydrogen bond in the framework of the atoms in molecules theory
2003
Delocalization indices, as defined in the atoms in molecules theory, have been calculated between hydrogen-bonded atoms in 20 molecular complexes that are formed between several H-donor and acceptor molecules. In general, the delocalization index associated to an intermolecular hydrogen bond depends on the interaction energy of the complex, but also on the nature of the H-donor and acceptor atoms.
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).
Bond! Chemical Bond: Electronic Structure Methods at Work
Structural Chemistry, 2018
This chapter plunges into applied quantum chemistry, with various examples, ranging from elementary notions, up to rather advanced tricks of know-how and non-routine procedures of control and analysis. In the first section, the first-principles power of the ab initio techniques is illustrated by a simple example of geometry optimization, starting from random atoms, ending with a structure close to the experimental data, within various computational settings (HF, MP2, CCSD, DFT with different functionals). Besides assessing the performances of the different methods, in mutual respects and facing the experiment, we emphasize the fact that the experimental data are affected themselves by limitations, which should be judged with critical caution. The ab initio outputs offer inner consistency of datasets, sometimes superior to the available experimental information, in areas affected by instrumental margins. In general, the calculations can retrieve the experimental data only with semi-quantitative or qualitative accuracy, but this is yet sufficient for meaningful insight in underlying mechanisms, guidelines to the interpretation of experiment, and even predictive prospection in the quest of properties design. The second section focuses on HF and DFT calculations on the water molecule example, revealing the relationship with ionization potentials, electronegativity, and chemical hardness (electrorigidity) and hinting at non-routine input controls, such as the fractional tuning of populations in DFT (with the ADF code) or orbital reordering trick in HF (with the GAMESS program). Keeping the H 2 O as play pool, the orbital shapes are discussed, first in the simple conjuncture of the Kohn-Sham outcome, followed by rather advanced technicalities in handling localized orbital bases, in a Valence Bond (VB) calculation, serving to extract a heuristic perspective on the hybridization scheme. In a third section, the H 2 example forms the background for discussing the bond as spin-coupling phenomenology, constructing the Heisenberg-Dirac-van Vleck (HDvV) effective spin Hamiltonian. In continuation, other calculation procedures, such as Complete Active Space Self-Consistent Field (CASSCF) versus Broken-Symmetry (BS) approach, are illustrated, in a hands-on style, with specific input examples, interpreting the results in terms of the HDvV model parameters, mining for physical meaning in the depths of methodologies.
Electron Pairing and Chemical Bonds. Physical Meaning of Effective Pairs
ChemPlusChem, 1994
The physical meaning of the so-called effective pairs which have been introduced recently within the formalism of pair population analysis is discussed using the analysis of conditional probabilities of electron density distribution for electron 1 with the reference electron fixed in a certain point 2. It is demonstrated that from the point of view of the mutual coupling of electron motions, the effective pairs behave analogously to singlet pairs. Based on this finding, effective pairs can be interpreted as the fraction of singlet pairs that is directly involved in bonding.