Delokal bonding found in compounds containing unsaturated one or more bonding orbitals are not limited to the two atoms, but spread up to three atoms or more. Such a bond is called bond delokal. In the valence bond method, to describe the actual structure of Such molecules, several Lewis structures (canonical form) is written, then taken their average weight. The weight of an average of two or more forms called resonance.
If the wave equation solved, then by calculating the participation form 1 and form 2 is balanced then obtained a form of energy lower than the energy form 1 and form 2. It should be noted that in summing the wave function in the equation above, which may be summed is the canonical form of the wave function of the same energy level or at least almost the same. Energy difference between the actual molecular structure with energy Lewis is the lowest energy is called resonance energy.
Qualitatively, it can be seen that each carbon atom in the benzene linked to three other atoms by using sp2 orbital to form sigma bonds, so that there are twelve atoms located in one area. Each carbon atoms still have the remaining p orbitals, which these p orbitals together with two neighboring carbon atoms p orbitals overlap to produce six new orbitals, three of which are bonding orbitals (called orbital p), which occupies the same space. One of them has a lower energy than two bonding orbital of the other, although they are degenerate orbitals. Each orbital has the ring plane as a node, so that they each is in two parts, one part on top of the field and one part in below the plane. Two other high-energy orbitals also have other nodes. The six electrons occupy the torus-shaped cloud of electrons is called sekstet aromatic. Carbon-carbon bond order in benzene which has been calculated by molecular orbital method is 1.667.
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