[chynagrl]

Hi everyone,

I have no idea how to predict molecular orbital patterns. I mean like for example, if we have a benzene ring there are orbitals, I think there are supposed to be six hybrid pi orbitals (in the conjugated system) with the first system being like ++++++, the second ++00-- and blah blah blah, till the sixth is +-+-+-. I'm sorry if this is really confusing--like I mean how the orbitals are lined up, like + next to a - means they cannot have overlapping regions, whereas 0 are nodes, and the like. How can I predict the pattern in which ++++++ goes to +-+-+-, meaning that for the six orbitals, how can I predict how the pi orbitals are oriented in each overall orbital level?

I'm really sorry if this does not make sense. I just really need to learn it for a test. Please e-mail me if you have any questions whatsoever.

Please see the attached photo (.bmp) for more info. I think the stuff on there explains my question more clearly.

Any help will be welcome! Thank you so much!

[Squirmy]

There's a lot of good of information on the link below. If you scroll down to the picture with a bunch of MO's, they have all the linear systems from 2-6 p-orbitals. Do you see any patterns there?

http://www.meta-synthesis.com/webbook/44_pi-systems/pi_systems.html

[lucas89]

I think I know what you're getting at here. A great way (in which I've been taught in three of my undergraduate classes that I've taken thus far) to go about this is to know that as you increase in energy to the next orbital, the number of nodes increases.

Well, let me explain more. Lets take benzene as you've already explained. There are six orbitals, as you've already mentioned, one low energy one being completely bonding (++++++), and one high energy one being completely antibonding (+ - + - + -). There are also four orbitals in between which are a combination of bonding and antibonding.

Constructing the MO first: a good rule of thumb, which has yet to fail me from what I've seen this far - as you go up in energy (ie. go to the next molecular orbital) , the number of nodes in that orbital increase. As we can see, in the lowest energy orbital, everything was bonding. Now, if we were to go to the next orbital above that, there'd be one node ( +++ ---). And with the orbital above this one, we need to add another node (++ -- ++). The pattern continues like this until you've done all the molecular orbitals.

From here, fill in the electrons for the MO. The orbital with the highest energy electrons is the HOMO level. And the molecular orbital directly above this, ie. the first orbital lacking electrons is the LUMO level.

I hope that helps!