I follow that organic conjugated systems absorb in the UV-Vis range. As the electrons in the system are promoted to a more energetic state they absorb at a specific wavelength.
Why does the absorbance spectrum show a fairly broad peak around the maxima? Why not very specific and sharp (like seen in atomic absorption spectroscopy).
For that matter, why does IR spectroscopy show such broadening too?
In AA, the atoms are independent (the flame is a atomiser for a reason), and electrons are jumping from one precise (quantised) energy level to another and absorbing/emitting a specific amount of energy - thus a spectra that is very precise.
In organic molecules, the bonds are less precise - the two bonded atoms move relative to each other, increasing and decreasing the bond length. The bond itself has a resonant frequency that is being excited by the IR/Vis/UV light. The groups on the ends of the bond change the response of the bond - large or long chains tangle and resist the motion, and smear the absorption frequency. Small unreactive groups mean more precise peaks. Hydrogen bonds between molecules change the motion.
Organic rings have some specific modes of vibration that are less affected by attachments, giving more specific signatures.
Si - (dredging up unused Analytical Chem from many, many years back)
In multi-atom molecules, electronic excitations are accompanied by vibrational and rotational transitions. The vibrational and rotaional transitions are much smaller than the electronic ones and it is usually not possible to resolve them. In IR spectra, there is no electronic excitation, but the vibrational and rotational transitions still are not usually resolved.
As has been stated, there are very many overlaping vibrational and rotational modes in the UV spectrum that are not resolvable. Typically, the more bonds that are conjugated in a row, the lower the absorption band will be. Think of this as a “particle in a box” experiment with electrons. As the box gets bigger the wavelength gets longer. Since the box is flexible, it is constantly changing sizes. Additionally there are charge transfer absorptions which could simply be affected by the surrounding solvent that is constantly reorganizing. In a vacuum many of these peaks would sharpen up somewhat.
I would dissagree with Roogs point about IR however. While rotational transitions are almost always indistinguishable, vibrational transitions give very specific peaks in the IR.
You are right. It was poor wording. What I meant to say was that the rotational transitions are indistinguishable in the presence of the strong vibrational transitions.