Well, I'm still looking and thinking....
See some surfings at:
http://www.scienceagogo.com/forum/ubbthreads.php?ubb=showflat&Number=26349#Post26349...there's lots of stuff about 4.5 microns (so maybe I wasn't wrong after all?)
Meanwhile....
...as you say, a greenhouse is trapped air (it also involves glass for it's effect, unlike our atmosphere). I don't think we can answer these CO2 questions "starting at" a real greenhouse; we need to look at studies of real gasses.
That "biocab" page on CO2 Heat_Storage looks like an introductory course in physical chemistry, so it should provide some insights; and the rest of the site is quite a trip too.
I'm hoping you'll be interested in a site I found, and let me know what you think. I'm just starting to explore it "now."
http://www.docstoc.com/docs/529696/Physical-basis-of-the--Greenhouse-Effect--The-%E2%80%9Cwavelength-shift%E2%80%9D-
Physical basis of the Greenhouse Effect -The “wavelength shift”-
-& scroll down for many other papers linked to this topic--
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btw... feel free to "fisk" this, from post #26318 above:
"There's no reason that the warmer CO2 can't also warm up the surrounding oxygen and nitrogen molecules (through contact... -loss of vibrational energy), is there?"
