UV spectroscopy is most useful for studying compounds containing double bonds or
heteroatoms with lone pairs that create conjugated systems. For the MCAT, that is all you
need to know.CONJUGATED SYSTEMS
Conjugated molecules, or molecules with unhybridized p-orbitals, can also be excited by ultraviolet
light. Conjugation shifts the absorption spectrum, resulting in higher maximum wavelengths (lower
frequencies). For example, benzene has three broad absorbances, which mark the energy level
transitions; these are found at 180, 200, and 255 nm wavelengths. Larger conjugated molecules may
even absorb light in the visible range, leading to color. Because the technique for UV spectroscopy
can also be used at visible wavelengths, it is sometimes called UV–Vis spectroscopy.
MCAT Concept Check 11.2:
Before you move on, assess your understanding of the material with these questions.What types of molecules can be detected by UV spectroscopy?In UV spectroscopy, what is the HOMO? What is the LUMO? How are they related to
absorption wavelength?HOMO: