Orbital nonbonding

The allyl free radical with 3 electrons, 2 in the bunding orbital and I in the nonbonding orbital, has... 

Its charge density distribution is like that of the cation (with sign reversal) because the added electron goes into the nonbonded orbital with a node at the central carbon atom. The probability of finding that electron precisely at the central carbon atom is zero. 

Sr) and the nonbonding orbital formed by the Oxygen 2s and 2pz orbitals eombining to form the "lone pair" orbital direeted along the z-axis away from the two Hydrogen atoms). 

This is first illustrated for the two nonbonding -type orbitals n, and n2 of para-benzyne and pyrazine (Fig. 31). These nonbonding orbitals are derived from outer (2s, 2p) sp2 type hybrids which have not been used in any bonding interaction. Although the overlap between n, and n2 is zero each one overlaps with the central CC bond orbitals. All told, there will arise two distinct molecular orbitals in which nj and n2 enter as combinations (symmetric or antisymmetric) and which have different energies, because of selective interactions with the central bonds. 

An s-orbital and a p-orbital on different atoms may overlap to form molecular orbitals. One of these interactions forms a bonding o-orbital and the other is nonbonding. Draw diagrams to represent the two types of orbital overlap that give rise to tr-bonding and nonbonding orbitals. 

FIGURE 3 In an n-to-ir transition of a carbonyl group, an electron in a nonbonding orbital (one localized wholly on the oxygen atom) is excited into an antibonding TT -orbital spread over both atoms. 

FIGURE 2.8 Energy levels for the benzyl cation, free radical, and carbanion. Since a is the energy of a p orbital (p. 35) the nonbonding orbital has no bonding energy. 

The rule is applicable to all but S3 in Table 1 [7], The most stable is thiozone (C J, whereas Sej has symmetry [14], The lone pair repulsion may destabilize the S3 ring and is weaker in the Se3 ring due to the smaller overlap between the nonbonding orbitals. 

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