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The charm of conjugated systems: how do they improve molecular stability?
In theoretical chemistry, a conjugated system is a molecular system containing linked p orbitals and delocalized electrons, which generally lowers the overall energy and increases molecular stability. The traditional representation of such systems is alternating single and multiple bond combinations. Lone pairs of electrons, free radicals or carbocations can all be part of this system, forming either cyclic or acyclic, linear or mixed structures.
The appeal of conjugated systems lies in their ability to promote the delocalization of π electrons through overlap of p orbitals, thereby improving the stability of the molecule.
The term was coined in 1899 by the German chemist Johannes Thiele. The phenomenon of conjugation refers to the overlap of one p orbital with another adjacent σ bond. In this system, the pi electrons no longer belong to a single bond or atom, but are shared by multiple atoms. Molecules that contain a conjugated system are called conjugated molecules, such as 1,3-butadiene, benzene, and the allyl cation.
Chemical bonds in conjugated systems
Conjugation can be achieved by alternating single and double bonds, with each atom providing a p orbital perpendicular to the plane of the molecule. However, this is not the only way to achieve conjugation. As long as each consecutive atom has an available p orbital, the system can be considered conjugated. For example, furan is a five-membered ring with two alternating double bonds on its edge and the oxygen atom has two lone pairs of electrons, one of which participates in conjugation and overlaps with the p orbital of the adjacent carbon atom. The other lone pair remains in the plane and therefore does not participate in the conjugation.
In order to achieve conjugation, the first condition is that the orbitals overlap. Therefore, the conjugated system needs to maintain a planar structure.
Stabilization Energy
The stability of conjugated systems is of great importance, especially in cationic systems. Although the stabilizing effect on neutral systems is relatively small, the aromatic stabilizing effect is quite significant. Taking benzene as an example, the resonance energy of its conjugated system is estimated to be between 36 and 73 kcal/mol, showing strong thermodynamic and kinetic stability.
The resonance energy is the energy difference between the real chemical species and its localized π bond.
Conjugated cyclic compounds
Cyclic compounds may be partially or fully conjugated. Completely conjugated monocyclic hydrocarbons can exhibit aromatic, nonaromatic, or antiaromatic properties. If the conjugated system is planar and exhibits a number of (4n + 2) π electrons, the compound is called aromatic and has extraordinary stability.
Color and Light Absorption
In a conjugated π system, electrons are able to capture specific photons, similar to how a radio antenna detects photons along its length. Compounds with a sufficient number of conjugated bonds can absorb light in the visible range. Therefore, they often appear in rich colors. As the system becomes longer, it can absorb longer wavelengths of light compared to short conjugated systems. For example, the long conjugated carbon-hydrogen chain of beta-carotene gives it its intense orange color.
The absorption capacity of visible light is proportional to the number of double bonds in the conjugated system, and the color ranges from yellow to red as the number of conjugated double bonds increases.
In addition, phthalocyanine-like compounds are also widely used in synthetic pigments. Not only do they have low-energy excitation in the visible light range, they can also easily accept or donate electrons. But will these pigments continue to gain popularity in the future?
