Valence shell electron pair repulsion theory, or VSEPR theory (/ˈvɛspər, vəˈsɛpər/ VESP-ər,[1]: 410 və-SEP-ər[2]), is a modelused in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms.[3] It is also named the Gillespie-Nyholm theory after its two main developers, Ronald Gillespie and Ronald Nyholm.
The premise of VSEPR is that the valence electron pairs surrounding an atom tend to repel each other and will, therefore, adopt an arrangement that minimizes this repulsion. This in turn decreases the molecule's energy and increases its stability, which determines the molecular geometry. Gillespie has emphasized that the electron-electron repulsion due to the Pauli exclusion principle is more important in determining molecular geometry than the electrostatic repulsion.[4]
The insights of VSEPR theory are derived from topological analysis of the electron density of molecules. Such quantum chemical topology (QCT) methods include the electron localization function (ELF) and the quantum theory of atoms in molecules (AIM or QTAIM).[4][5] Hence, VSEPR is unrelated to wave function based methods such as orbital hybridisation in valence bond theory.[6]
https://en.wikipedia.org/wiki/VSEPR_theory
In chemistry, orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory. For example, in a carbon atom which forms four single bonds the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp3 mixtures which are arranged in a tetrahedralarrangement around the carbon to bond to four different atoms. Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties and are symmetrically disposed in space. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies.[1]
https://en.wikipedia.org/wiki/Orbital_hybridisation
The quantum theory of atoms in molecules (QTAIM) is a model of molecular and condensed matter electronic systems (such as crystals) in which the principal objects of molecular structure - atoms and bonds - are natural expressions of a system's observable electron density distribution function. An electron density distribution of a molecule is a probability distribution that describes the average manner in which the electronic charge is distributed throughout real space in the attractive field exerted by the nuclei. According to QTAIM, molecular structure is revealed by the stationary points of the electron density together with the gradient paths of the electron density that originate and terminate at these points. QTAIM was primarily developed by Professor Richard Bader and his research group at McMaster University over the course of decades, beginning with analyses of theoretically calculated electron densities of simple molecules in the early 1960s and culminating with analyses of both theoretically and experimentally measured electron densities of crystals in the 90s. The development of QTAIM was driven by the assumption that, since the concepts of atoms and bonds have been and continue to be so ubiquitously useful in interpreting, classifying, predicting and communicating chemistry, they should have a well-defined physical basis.
QTAIM recovers the central operational concepts of the molecular structure hypothesis, that of a functional grouping of atoms with an additive and characteristic set of properties, together with a definition of the bonds that link the atoms and impart the structure. QTAIM defines chemical bonding and structure of a chemical system based on the topology of the electron density. In addition to bonding, QTAIM allows the calculation of certain physical properties on a per-atom basis, by dividing space up into atomic volumes containing exactly one nucleus, which acts as a local attractor of the electron density. In QTAIM an atom is defined as a proper open system, i.e. a system that can share energy and electron density which is localized in the 3D space. The mathematical study of these features is usually referred to in the literature as charge density topology.
QTAIM rests on the fact that the dominant topological property of the vast majority of electron density distributions is the presence of strong maxima that occur exclusively at the nuclei, certain pairs of which are linked together by ridges of electron density. In terms of an electron density distribution's gradient vector field, this corresponds to a complete, non-overlapping partitioning of a molecule into three-dimensional basins (atoms) that are linked together by shared two-dimensional separatrices (interatomic surfaces). Within each interatomic surface, the electron density is a maximum at the corresponding internuclear saddle point, which also lies at the minimum of the ridge between corresponding pair of nuclei, the ridge being defined by the pair of gradient trajectories (bond path) originating at the saddle point and terminating at the nuclei. Because QTAIM atoms are always bounded by surfaces having zero flux in the gradient vector field of the electron density, they have some unique quantum mechanical properties compared to other subsystem definitions, including unique electronic kinetic energy the satisfaction of an electronic virial theorem analogous to the molecular electronic virial theorem and some interesting variational properties. QTAIM has gradually become a method for addressing possible questions regarding chemical systems, in a variety of situations hardly handled before by any other model or theory in chemistry.[1][2][3][4]
https://en.wikipedia.org/wiki/Atoms_in_molecules
In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed. In contrast, molecular orbital theory has orbitals that cover the whole molecule.[1]
https://en.wikipedia.org/wiki/Valence_bond_theory
In quantum chemistry, the electron localization function (ELF) is a measure of the likelihood of finding an electron in the neighborhood space of a reference electron located at a given point and with the same spin. Physically, this measures the extent of spatial localization of the reference electron and provides a method for the mapping of electron pair probability in multielectronic systems.
ELF's usefulness stems from the observation that it allows electron localization to be analyzed in a chemically intuitive way. For example, the shell structure of heavy atoms is obvious when plotting ELF against the radial distance from the nucleus; the ELF for radon, for example, has six clear maxima, whereas the electronic density decreases monotonically and the radially weighted density fails to show all shells. When applied to molecules, an analysis of the ELF shows a clear separation between the core and valence electron, and also shows covalent bonds and lone pairs, in what has been called "a faithful visualization of VSEPR theory in action". Another feature of the ELF is that it is invariant concerning the transformation of the molecular orbitals.
https://en.wikipedia.org/wiki/Electron_localization_function
In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century.
In molecular orbital theory, electrons in a molecule are not assigned to individual chemical bonds between atoms, but are treated as moving under the influence of the atomic nuclei in the whole molecule.[1] Quantum mechanics describes the spatial and energetic properties of electrons as molecular orbitals that surround two or more atoms in a molecule and contain valence electrons between atoms.
Molecular orbital theory revolutionized the study of chemical bonding by approximating the states of bonded electrons—the molecular orbitals—as linear combinations of atomic orbitals (LCAO). These approximations are made by applying the density functional theory (DFT) or Hartree–Fock (HF) models to the Schrödinger equation.
Molecular orbital theory and valence bond theory are the foundational theories of quantum chemistry.
https://en.wikipedia.org/wiki/Molecular_orbital_theory
https://en.wikipedia.org/wiki/Category:Chemical_bonding
https://en.wikipedia.org/wiki/Space_complexity
https://en.wikipedia.org/wiki/Molecular_term_symbol
https://en.wikipedia.org/wiki/Main-group_element
https://en.wikipedia.org/wiki/Substituent
https://en.wikipedia.org/wiki/Molecular_geometry
https://en.wikipedia.org/wiki/Sigma_bond
https://en.wikipedia.org/wiki/Electron_pair
https://en.wikipedia.org/wiki/Cooper_pair
https://en.wikipedia.org/wiki/Radical_(chemistry)
https://en.wikipedia.org/wiki/Electron_density
https://en.wikipedia.org/wiki/Jellium
https://en.wikipedia.org/wiki/Covalent_bond
https://en.wikipedia.org/wiki/Molecular_orbital
https://en.wikipedia.org/wiki/Atomic_orbital
https://en.wikipedia.org/wiki/Valence_electron
https://en.wikipedia.org/wiki/Electron_configuration
https://en.wikipedia.org/wiki/Lone_pair
https://en.wikipedia.org/wiki/Valence_electron
https://en.wikipedia.org/wiki/Lewis_structure
https://en.wikipedia.org/wiki/Coordination_number
https://en.wikipedia.org/wiki/Ligand
https://en.wikipedia.org/wiki/Resonance_(chemistry)
https://en.wikipedia.org/wiki/Electron_density#Spin_density
https://en.wikipedia.org/wiki/Difference_density_map
https://en.wikipedia.org/wiki/Resolution_(electron_density)
https://en.wikipedia.org/wiki/Charge_density
https://en.wikipedia.org/wiki/Density_functional_theory
https://en.wikipedia.org/wiki/Pauli_exclusion_principle
https://en.wikipedia.org/wiki/Probability_current
https://en.wikipedia.org/wiki/Dimensionless_quantity
https://en.wikipedia.org/wiki/Hartree–Fock_method
In quantum chemistry, electronic structure is the state of motion of electrons in an electrostatic field created by stationary nuclei.[1] The term encompasses both the wave functions of the electrons and the energies associated with them. Electronic structure is obtained by solving quantum mechanical equations for the aforementioned clamped-nuclei problem.
Electronic structure problems arise from the Born–Oppenheimer approximation. Along with nuclear dynamics, the electronic structure problem is one of the two steps in studying the quantum mechanical motion of a molecular system. Except for a small number of simple problems such as hydrogen-like atoms, the solution of electronic structure problems require modern computers.
Electronic structure problem is routinely solved with quantum chemistry computer programs. Electronic structure calculations rank among the most computationally intensive tasks in all scientific calculations. For this reason, quantum chemistry calculations take up significant shares on many scientific supercomputer facilities.
A number of methods to obtain electronic structures exist and their applicability varies from case to case.[2]
https://en.wikipedia.org/wiki/Electronic_structure
No comments:
Post a Comment