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Uv spectroscopy.

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Presentation on theme: "Uv spectroscopy."— Presentation transcript:

1 Uv spectroscopy

2 UV Spectroscopy Introduction The Spectroscopic Process In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed The remaining UV light passes through the sample and is observed From 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 electrons 2. Transitions involving charge-transfer electrons 3. Transitions involving d and f electrons

4 s* p* Energy n p s Observed electronic transitions
Here is a graphical representation s* Unoccupied levels p* Atomic orbital Atomic orbital Energy n Occupied levels p s Molecular 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* s p n s* p* alkanes carbonyls unsaturated cmpds. O, N, S, halogens p* Energy n p s

6 Transitions involving σ,π and n electrons

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 * Transitions Saturated 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.

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