[johnj7]

hello,

from my MCAT review book, while reading the section on UV spec, I came across this picture explaining how carbonyls absorb UV light.  In the attached pdf file, it shows the structures in my book.  They just looked like weird / incorrect lewis structures, and i didn't understand the caption.  Could someone provide a good explanation?

thank you!

[nj_bartel]

That's a weird drawing.  If you want to get a firm grasp on what's going on, read up on molecular orbital theory.  I'm having trouble finding a link that's easily understood.  The basic idea is that there are different energy levels an electron is allowed to fall into, but only some of them (bonding orbitals) are occupied under normal conditions.  When you irradiate a compound with a specific uv wavelength that matches the energy input required to match a specific electron, that electron jumps up in energy into what's known as an antibonding orbital.  The electron absorbs the uv light and transfer it into going to a higher energy level.  A machine is able to detect which wavelengths a specific compound absorbs the light at, and this in turn can be used to identify the molecule.

[typhoon2028]

Please go to this link

http://www.chem.ucalgary.ca/courses/351/Carey/Ch13/ch13-uvvis.html#basics

It references a carbonyl bond.

I believe the picture you attached is trying to illustrate how a lone pair of electrons is excited into a pi anti-bond when hit with uv radiation.