Nonbonding molecular orbitals

Ibis hindered rotation of >f2-H2 is governed by various forces, which can be divided into bonded (electronic) and nonbonded interactions ( steric effects). The direct electronic interaction between M and H2 results from overlap of the appropriate molecular orbitals. Nonbonded interactions such as van der Waals forces between the q2-H2 atoms and the other atoms on the molecule may vary as 2- 2 rotates. Intermolecular interactions should not contribute much to the barrier to rotation of q2-H2, as the metals are far apart. However, they may have a minor effect on the coordination geometry about M, which could in turn affect M-H2 binding. 

The lowest-unoccupied molecular orbital is called the LUMO (+0.1 au). The LUMO has nonbonding character, and looks like a 2p atomic orbital on carbon. If this molecule... 

Backside attack may be favored in order to facilitate transfer of nonbonding electrons from the nucleophile into the electrophile s lowest-unoccupied molecular orbital (LUMO). Efficient electron transfer requires maximal overlap of the LUMO and the donor orbital (usually a nonbonded electron pair on the nucleophile). Examine the LUMO of methyl bromide. How would a nucleophile have to approach in order to obtain the best overlap Is your answer more consistent with preferential backside or frontside attack ... 

Lone Pair. A pair of electrons contained in a Nonbonded Molecular Orbital. 

Nonbonded Molecular Orbital. A molecular orbital that does not show any significant bonding or antibonding characteristics. Nonbonded molecular orbitals often correspond to Lone Pairs. 

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. 

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