Molecular bonds/orbitals alternation

This reaction proceeds, according to perturbation theory, by electrons moving from occupied bonding, or non-bonding, orbitals into empty, anti-bonding orbitals. Alternatively the anti-bonding orbitals are mixed with the bonding orbitals to form new molecular orbitals. 

Molecular orbital theory is an alternative approach to bonding than valence bond theory. According to this theory, molecular orbitals form when orbitals overlap. This overlap gives rise to bonding orbitals and antibonding orbitals. 

V-UV Application First Excited State of Linear Polyenes. The first electronic absorption band of perfect linear aromatic polyenes (CH)X, or perfect polyacetylene shifts to the red (to lower energies) as the molecule becomes longer, and the bond length alternation (BLA) would be zero. This was discussed as the free-electron molecular orbital theory (FEMO) in Section 3.3. If this particle-in-a-box analysis were correct, then as x > oo, the energy-level difference between ground and first excited state would go to zero. This does not happen, however first, because BLA V 0, next, because these linear polyenes do not remain linear, but are distorted from planarity and linearity for x > 6. 

Each Be atom is connected to four H atoms, and each H atom is connected to two Be atoms. Since Be has two valence electrons, and H only one, obviously there are insufficient electrons to fulfill the traditional organic two-center, two-electron bonding description. Alternatively, three-center interactions persist, whereby banana-shaped molecular orbitals (a three-center, two-electron bond, see Three-center Bond) connect the Be-H-Be atoms, each containing two electrons (see Figure 2 for the molecular orbital scheme see Molecular Orbitals) for this type of Delocalized... 

An alternative approach is to first master the VSEPR theory and the related topic of molecular polarity for different structures, and then learn how the VB theory describes the overlap of bonding orbitals in these structures. If your instructor takes this approach, you should study this chapter in the following order ... 

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