2UV SpectroscopyIntroductionThe Spectroscopic ProcessIn UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiationIf a particular electronic transition matches the energy of a certain band of UV, it will be absorbedThe remaining UV light passes through the sample and is observedFrom this residual radiation a spectrum is obtained with “gaps” at these discrete energies – this is called an absorption spectrum
3The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition which can be considered;1. Transitions involving σ,π and n electrons2. Transitions involving charge-transfer electrons3. Transitions involving d and f electrons
4s* p* Energy n p s Observed electronic transitions Here is a graphical representations*Unoccupied levelsp*Atomic orbitalAtomic orbitalEnergynOccupied levelspsMolecular orbitals
5s* p* Energy n p s Observed electronic transitions From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy:s*spns*p*alkanescarbonylsunsaturated cmpds.O, N, S, halogensp*Energynps
7Transitions σ σ * Transitions An electron in a bonding σ orbital is excited to the corresponding antibonding orbital. The energy required is large.For example, methane (which has only C-H bonds, and can only undergo σ σ * transitions) shows an absorbance maximum at 125 nm.Absorption maxima due to σ σ * transitions are not seen in typical UV-Vis. spectra ( nm)
8n -> σ * TransitionsSaturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n -> σ * transitions.These transitions usually need less energy than σ σ * transitions.They can be initiated by light whose wavelength is in the range nm.The number of organic functional groups with n -> σ * peaks in the UV region is small.
9n ->π* and π -> π* Transitions Most absorption spectroscopy of organic compounds is based on transitions of n or π electrons to the π * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum ( nm).These transitions need an unsaturated group in the molecule to provide the π electrons.