2 UV 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
3 The 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
4 s* p* Energy n p s Observed electronic transitions Here is a graphical representations*Unoccupied levelsp*Atomic orbitalAtomic orbitalEnergynOccupied levelspsMolecular orbitals
5 s* 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
7 Transitions 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)
8 n -> 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.
9 n ->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.