[Kalibasa]

I think I understand the basics of MO's pretty well, but I'm having trouble recognizing them when they are drawn out. I thought that formaldehyde would have eight MO's and two nonbonding orbitals in this order:

σ*
σ*
σ*
π*
n
n
π
σ
σ
σ

I thought that there would be twelve electrons, with n as the HOMO and π* as the LUMO. Is this right? If it is I don't understand this diagram our professor gave us. It's supposed to show only the lowest seven MO's, but it looks to me like the two in the upper left are the pi and pi* orbitals. If this is the case, the order I have can't be right.  What orbital corresponds to each picture?

Thanks!

[Ligander]

It is obvious(!) that 1st(from bottom) left is bonding sigma, 3rd left is bonding pi and 4th left is antibonding pi. All are orbitals that formed from atomic ones of C and O.

2nd left figure seems like 2 bonding sigmas between H and C(I'm not sure).

 So... three right are antibonding sigmas.

 

I'm having trouble recognizing them when they are drawn out

 If there is a "cloud" that belongs to two atoms simultaneously("connected space" in terms of topology, see Wikipedia) so it is bonding orbital. If you see two diconnected "clouds" near two different atoms - it is antibonding orbital.

==================================

 You can play with Google Image typing something like "antibonding pi orbital" and maybe you'll find how do different MO look like.

[Kalibasa]

If that's the scheme though (and that's what I thought when I looked at the pictures), I still don't understand the order. What is the energy scheme for the MO's supposed to be?

[jinclean]

hello!Kalibasa,i am confused in the MO method ,in my contry the MO theory is ignored in many class just like organic or basic chemistry ,so we have to use the hybird orbital to analyse the structure.but the MO method is so useful! i am wondering how to study it?must i read some math book to finish the mathmatic and physical basic knowledge?what kind of books should i read for the study?thank you

[Synth]

If you're not into large amounts of math...

Dougherty and Anslyn's Modern Physical Organic Chemistry book gives a pretty solid introduction on MO theory.
Additionally, the new edition of Fleming's Molecular Orbitals and Organic Chemical Reactions book is fantastic!

[Ligander]

I still don't understand the order. What is the energy scheme for the MO's supposed to be?

Emm, you've written the order in the first post of the thread

 The lowest energy is for bonding sigmas and there are first and second on the figure, then bonding pi it is the third on the tfigure then antibonding pi and it is fourth in the left column, then you have three antibonding sigma orbitals in the right column.

 The thumb rule : the lower bonding MO the higher is corresponding antibonding MO.

 Or you know all this and I haven't understood your question?

[Kalibasa]

If anyone else was curious, I checked with my professor and figured it out. The generic scheme I listed can only be used to identify the HOMO and LUMO; it can't be used to actually identify all the orbitals properly, which is where I got mixed up. So the proper list of orbitals (as shown in the picture) is four sigma MO's, one pi, one nonbonding, one pi*, and three sigma*.

Thanks though!

[Ligander]

four sigma MO's...and three sigma*.

Thanks though!

 Between which atoms the fourth sigma?

 4 sigma bonding and 3 sigma antibonding... doesn't it contradict to the Energy Conservation Law ?