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Molecular Orbital energy levels

by Duane
Tags: energy, levels, molecular, orbital
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Duane
#1
Dec11-11, 07:01 AM
P: 8
Hi everyone
I'm currently studying the MO theory and ran into some trouble understanding orbital energy levels.

As I have previously read, σ bonds are stronger than ∏ bonds, therefore are lower in orbital energy. However, from the MO theory, the σ2p orbital is actually higher in energy than the ∏2p orbital in molecules like B2 and N2. But in molecules like O2 and F2, this sequence is reversed, with the ∏2p orbital having the higher orbital energy.

Can anyone tell me why?
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DrDu
#2
Dec12-11, 05:56 AM
Sci Advisor
P: 3,633
a) I think you are referring to the anti-bonding orbitals, not the bonding ones.
b) the sigma bonds are not formed from pure p orbitals but from a superposition of 2s and 2p orbitals. The energetic distance of the 2s and 2p orbitals increases from left to right in a period, which explains the interchange of the energetic ordering of the molecular orbitals.


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