[Charles Furman]

What does it mean Ring Residue in UV spectroscopy? and How can I know it?

[Corribus]

It's occasionally encountered in the context of empirical methods to predict the absorption properties, particularly the maximum absorbance wavelength, of conjugated organic molecules. Essentially, it refers to an alkyl or other similar substituent that impacts the UV-Vis spectrum of a molecule. I imagine there's some historical reason why it's called a residue, but someone else will have to enlighten us on that score.