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The Amazing World of Molecular Orbitals: Do You Know How They Affect Chemical Reactions?
In the world of chemistry, molecular orbitals are like harmonies in music, fully demonstrating the interactions and relationships between elements. Among them, linear combination atomic orbitals (LCAO) is a very attractive concept. Through the quantum superposition of atomic orbitals, it provides us with a powerful tool to analyze molecular orbitals. So, how does such a wave function affect chemical reactions?
Under linear combination, atomic orbitals will combine to form new molecular orbitals, a process that is crucial to understanding chemical reactions.
In quantum mechanics, the electron configuration of an atom is described as a wave function. These wave functions are expressed mathematically to form a set of basis functions that describe the electrons of a particular atom. Especially during chemical reactions, the wave functions of electron orbitals change depending on the types of atoms involved in covalent bonds, which in turn affects the shape of the molecular structure.
The concept of LCAO was first proposed by British scientist John Leonard-Jones in 1929, who used this method to describe diatomic molecules in the first main group of the periodic table. Linus Paul had already applied a similar technique to molecular hydrogen cations (H2+). The core of this method is that n atomic orbitals will combine to form n molecular orbitals, and not all orbitals are necessarily the same.
“Molecular orbitals can be viewed as a reorganization of b, which in turn comes from the calculated atomic orbitals.”
Based on this assumption, we can express the i-th molecular orbital as a set of linear expansions in the following form:
ϕ_i = c_{1i}χ_1 + c_{2i}χ_2 + c_{3i}χ_3 + ... + c_{ni}χ_n
Among them, ϕ_i represents molecular orbital, χ_r represents atomic orbital, and c_{ri} is the contribution weight of each atomic orbital to the molecular orbital. Using the Hartley-Fock method, we can calculate these weights and deduce the shapes and energies of the molecular orbitals.
With the development of computational chemistry, the LCAO method is no longer just a mathematical optimization description, but is used for qualitative analysis, which makes it an extremely useful tool to predict and rationalize the results obtained by more modern techniques. result. In this process, the shapes and energies of molecular orbitals are often predicted based on the energy differences of atomic orbitals, using basic concepts such as energy level repulsion.
"The shape and energy of a molecular orbital reflect the relative positions and energies of the atomic orbitals involved in bonding."
In order to clearly present this process, scientists often use "correlation diagrams" to help understand. The energy of atomic orbitals can be calculated using Koopmans' theorem and the symmetry of the molecule and the orbitals to establish a nanobottom integration. The first step of this method is to assign a point group to the molecule, and then apply operations on the point group to analyze the characteristics of the molecule.
Molecular orbital diagrams provide a simple qualitative LCAO treatment in addition to quantitative theories such as the Hückel method, the extended Hückel method, and the Pariser-Parr-Pople method, which are used for more in-depth analysis.
In short, molecular orbitals are not only key players in chemical reactions, but also reveal the mysteries of interactions between atoms. Whether from a quantitative or qualitative perspective, LCAO provides us with a window to observe chemical changes. This got us thinking, when we gain a deeper understanding of the properties of these orbits, will we be able to predict more unknown chemical reactions in the future?
