UV-Vis Spectroscopy - Absorbance Maxima?

The lowest energy (longest wavelength) absorption in the UV-Vis spectrum of a cmpd represents the lowest energy electronic transition (the H*OMO-LUMO gap). For organic molecules this is usually a

π → π* transition. Benzene has a humungous absorption just outside the visible due to the

π2 → π3* transition. Acetone is similar but is due to an n → π* transition (n = lone pair, nonbonding). In TM cmplxs (e.g. blue copper sulfate solutions) the transition is a d(t2g)→ d(eg) transition in the red (you observe the complementary color of pale blue). The bands are invariably broad (cf the sharp lines in atomic emission spectra). This is because the molecules wag up and down with thermal energies and with the surrounding solvent that changes the H*OMO-LuMO gap and spreads out the allowable possible energies. Nevertheless, the wavelength at the point of maximum absorption is almost characteristic of a cmpd and can be equated with the H*OMO-LUMO gap (have to keep the theoreticians off the street); the intensity of the band gives an indication of the type of transition. TM d-d transitions are weak

(ε= 10-500).

The important point is that the absorptions obey Beer's Law Abs = εbc so you can easily measure concs of unknown solns if you have previously measured standards. ε for some cmpds are as high as 50,000 so method is very sensitive. You can then measure the change in conc for a rxn with time and voilà you can do kinetic studies!

We won't get into fluorescence today. :)

PS: Phew - I deserve an ice cream!

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