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In article <66fqjk$kmn$1@agate.berkeley.edu>, kevl@flash.net says...
> But from the quantum mechanicalperspective,
> why would certain molecules have electrons that absorb UV,but not IR?

>Is it the electron's tendancy to leap from energy
>level x - to - energy level y that determines its resonant frequency?

I will intriduce to you some qualitative story.( not quantitative, but "quntum".  :-) )

Energy of UV photon coressponds to gap of two energy level of electron of molecule.
Energy of IR photon coressponds to gap of two energy level of vibration of molecule. 
Energy order of UV photon may be greater than 10 times that of IR.


v2 ---
v1 ---
E1 ------


v2 ---
v1 ---
E0 ------

Fig.1 energy diagram of electron and vibration 

En are electronic level, and vn are vibration level.(n = 0,1,2,...)

E0 ->E1 transition corresponds to energy of one UV photon.
E0 ->E1v1 transition corresponds to energy of another UV photon.
We can think that E0 ->E1v1  transition corresponds to energy of

 one UV photon + one IR photon.

This is not good model  from selection rule of quantum mechanics, 
but some good ideal model to understand energy order of UV, IR, etc.
These energy levels depends on "each" molecular structure.

>2)  And water absorbs microwave frequencies, so why in the world does it
>also slow violet more than red? Shouldn't it be better at slowing the red
>because red is closer to the frequencies it absorbs?

r2 --
r1 --
v2 ---
r2 --
r1 --
v1 ---
E1 ------


r2 --
r1 --
v2 ---
r2 --
r1 --
v1 ---
E0 ------

Fig.2  energy diagram of electron and vibration and rotation

En are electronic level, and vn are vibration level, rn are rotaion level.(n = 0,1,2,...)

Energy of Microwave photon coressponds to gap of two energy level of rotation of molecule. 
Energy order of vibration may be greater than 10 times that of rotation.

If you want to know more information, see textbook of physical chemistry.

-- 
// Takayuki Nakamura
// arumakan@ca2.so-net.or.jp