Ultraviolet and Visible Spectra
When electromagnetic radiation in the ultraviolet (UV) or visible region of the spectrum is absorbed by a molecule, π or non‐bonding (n) electrons are promoted into antibonding orbitals. Because the main electron transitions are π→π* transitions, the absorption of energy in the UV or visible range (200–700 nm) usually indicates the existence of π bond(s) and an unsaturated compound. Conversely, molecules that don't absorb in the UV or visible region don't contain an unsaturated system. The strength of the absorption, measured by its extinction coefficient, ε, is determined by the amount of conjugation in the system. The position of an absorption is determined to a large extent by the relative mobility of the π electrons.