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I see them in the real world August 19, 2009

Posted by fetzthechemist in Uncategorized.

In chemistry, we learn a lot about and rely on a range of concepts that generally are just that, comcepts. Quantum behaviors and electron clouds and molecular orbitals are ideas we use to explain a lot of things. But we rarely, even those of us who spend time in labs, really see these concepts turned real by some behavior that we can see. I have been struck by a few of these recently and will pass them on for others to ponder. Maybe you’ll think of a few others.

C60 fullerene is purplish in solution. Depending on the solvent and concentrations, this can vary from a lavender-pink to a deep red-wine color. I saw the latter in a collaboration with the Huffman-Kratschmer group, purifying almost a hundred grams of it – Four four-liter beakers full of red-wine solutions in 1,2,4-trichlorobenxene.

The color should not be there, though. The main electronic transitions of C60 are at low wavelengths. The color arises from very low intensity bands at high wavelengths. These are symmetry-disallowed transitions that occur through quantum-tunneling. The orangy brown of C70, or the yellow, brown, or olive-green of other fullerenes arises similarly. That these transitions are broad in energy ranges also arises from this effect, so fullerenes absorbe at a long-wavelength range, and act as photon sponges because of it (the photon energy gets converted to extra motion in the fullerene sphere or ovoid, conversion to heat).

Another observed effect is seen with iodide ion. It is a rather large ion and electron shell theory says that it ought to be “:soft”, that its electrons – even in the ionic form, ought to be donated and shared in the Lewis acid-base model. Well, if you do ion chromatography of iodide ion, it behaves very diffeently that its cousin chloride. It has a mixed retention of ionic and reversed phase behavior. Those soft electrons make the ion very noticably more “non-polar” and iodide has weird retention.

The rare-earth elements, the lanthanides, are supposed to be odd-balls because their electron filling is in an inner f shell. Thus, they all behave similarly chemical, mostly as +2 ions, but sometimes as +1. They are difficult to separate because of this. But is there a real-world observation of those buried f electrons?  Yes. These elements might just be curiosity pieces except for those. The transtions of those f electrons is not in the outer shell region that interacts with ligands, solvents, or oxidation-state (+1 or _2 in this case). Those effects and interactions broaden the energy range of the electron in the outer shells, thus the broad, featureless UV-visible spectra for copper, iron, nickel, chromium, and other d shell ions.

The sheltered f orbital electrons have very narrow energy ranges. Their UV-visible bands are only 1 nm to 2 nm wide. But this gives them very specific and unalterable behaviors. These shielded electrons can phosphoresce and are the reason some of these elements are useful in optics.s



1. Will - August 20, 2009

Most lanthanides are trivalent. Sm, Eu, Tm, and Yb can do 2+ and Ce can do 4+, but most are still 3+.

2. fetzthechemist - August 20, 2009

y mistake for relying on memory of inorganic classes decades ago rather than checking. But the valencies are not determined by the f electrons that differentiate the lanthanides as atoms. So the point is that those buried electron shells do have some unique properties. The UV-visible spectra of them are amazing, especially when compared to the usual ones for metal ions.

3. Will - August 20, 2009

Yeah, that’s true, for some Ln complexes, the f-f transitions are really pronounced and interesting.

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