Question

describe what molecular characteristics allow a molecule to absorb Ultraviolet light (and be observed by UV spectroscopy). What causes an absorption at increased wavelength?

Answer 1

When light passes through the compound, energy from the light is used to promote an electron from a bonding or non-bonding orbital into one of the empty anti-bonding orbitals.

An absorption spectrometer works in a range from about 200 nm (in the near ultra-violet) to about 800 nm (in the very near infra-red). Only a limited number of the possible electron jumps absorb light in that region.

To absorb in UV a big jump is required, from sigma bonding to pi anti bonding or pi bonding to sigma antibonding.