Understanding Resonance Structures: Tips and Guidelines

[member 392791]

Hello,

I am having difficulty discerning which resonance structures are considerable enough to include and which are so unimportant that they should be discarded. There seems to be no general rule on this that I can find, so I was wondering if such a rule existed or if someone can lead me in the right direction.

 

[Ygggdrasil]

http://www.chem.ucla.edu/harding/tutorials/resonance/imp_res_str.html

 

[member 392791]

Thanks, that cleared a lot of things up (though they need to spell check and grammar check that article again)!

 

[DrDu]

http://www.chem.ucla.edu/harding/tutorials/resonance/imp_res_str.html

Rule number 5 is debatable. In general, two one electron bonds are more stable than one two-electron bond due to electronic repulsion.
Especially, what do they mean with their example?
How can structure 1 have 3 unpaired electrons? Either the electrons combine to total spin 1/2, then the molecule is already described by structures II and III, or the electrons combine to spin 3/2, then structure I corresponds to an excited state of the molecule which won't mix in structures II and III.
In the case of oxygen you also don't need molecular orbital considerations to show that triplet oxygen is more stable than singlet oxygen. Especially two three electron bonds are more stable than one two electron bond and one four electron bond. This was already shown in 1937 by Wheland and Lennard-Jones http://pubs.rsc.org/en/content/articlepdf/1937/tf/tf9373301499