# How to determine the number of π-electrons in a conjugated system from UV-Vis data?

Is there a way to determine the number of π-electrons in a conjugated system from UV-Vis data? I am modelling the electrons as particles in an infinite box of length $$L$$ equal to the length of the conjugated system. I am unsure how to interpret the spectra to obtain the number of π-electrons, including the systems with more than one peak.

• I don't know if you can determine the number of pi-electrons from UV-vis, because UV-vis is basically the HOMO-LUMO transition. In some cases, there are additional bands, but they might come from sigma or non-bonding electrons being excited. So, it's hard to say what the total number of pi electron is, even if the peak comes from pi->pi* excitation. Aside from that, particle in a box is very inaccurate quantitatively, so I don't know what you are trying to achieve with that. Mar 8 at 22:25
• Yes and no. I might come back to this but it depends on the level of prediction you have in mind and on the classes of compounds. In all cases a good amount of knowledge is required as starting point. The peak to look at would be that associated with the pi-pi* transition. You can right "when the particle in a box is somehow a decent model". Mar 9 at 9:14
• I think the point of the exercise I’m doing is to show how inaccurate this is. How would I go about finding the number of pi electrons? I just don’t see it with this theory Mar 9 at 21:03
• Not really. A uv-vis spectrum normally contains peaks due to vibrational transitions on top of an electronic transition, but your particle in a box calculation only produces electronic transitions and in any case is a very crude approximation. If you can see two or more electronic transitions and work out the transition energy that would be better. So you can only have an inspired guess at best, not a proper prediction. Apr 1 at 14:53