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UV / visible Spectroscopy l Introduction l Identification of organic species l Quantitation of inorganic species Colorimetric analysis.

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Presentation on theme: "UV / visible Spectroscopy l Introduction l Identification of organic species l Quantitation of inorganic species Colorimetric analysis."— Presentation transcript:

1 UV / visible Spectroscopy l Introduction l Identification of organic species l Quantitation of inorganic species Colorimetric analysis

2 UV / visible Spectroscopy l The origin of the analytical signal l Excitation of an atom or molecule by ultraviolet or visible radiation. l nm

3 UV / visible Spectroscopy l The radiation which is absorbed has an energy which exactly matches the energy difference between the ground state and the excited state. l These absorptions correspond to electronic transitions.

4 UV / visible Spectroscopy / nm Abs / nm Abs

5 UV / visible Spectroscopy

6 l Electronic transitions involve the promotion of electrons from an occupied orbital to an unoccupied orbital. l Energy differences of kJ/mole.

7 UV / visible Spectroscopy l Beer-Lambert Law A = log(I O /I) =  cl

8 UV / visible Spectroscopy A = log(I O /I) =  cl v A = Absorbance (optical density) v I O = Intensity of light on the sample cell v I = Intensity of light leaving the sample cell v c = molar concentration of solute v l = length of sample cell (cm)   = molar absorptivity (molar extinction coefficient)

9 UV / visible Spectroscopy l The Beer-Lambert Law is rigorously obeyed when a single species is present at relatively low concentrations.

10 UV / visible Spectroscopy l The Beer-Lambert Law is not obeyed: v High concentrations v Solute and solvent form complexes v Thermal equilibria exist between the ground state and the excited state v Fluorescent compounds are present in solution

11 UV / visible Spectroscopy l The size of the absorbing system and the probability that the transition will take place control the absorptivity (  ). l Values above 10 4 are termed high intensity absorptions. l Values below 1000 indicate low intensity absorptions which are forbidden transitions.

12 UV / visible Spectroscopy l Organic Spectroscopy l Transitions between MOLECULAR ORBITALS

13 UV / visible Spectroscopy l Highest occupied molecular orbital HOMO l Lowest unoccupied molecular orbital LUMO

14 UV / visible Spectroscopy

15 l Not all transitions are observed l There are restrictions called Selection Rules l This results in Forbidden Transitions

16 UV / visible Spectroscopy l The characteristic energy of a transition and the wavelength of radiation absorbed are properties of a group of atoms rather than of electrons themselves. l The group of atoms producing such an absorption is called a CHROMOPHORE

17 UV / visible Spectroscopy

18

19 l It is often difficult to extract a great deal of information from a UV spectrum by itself. l Generally you can only pick out conjugated systems.

20 UV / visible Spectroscopy

21 ALWAYS use in conjunction with nmr and infrared spectra.

22 UV / visible Spectroscopy l As structural changes occur in a chromophore it is difficult to predict exact energy and intensity changes. l Use empirical rules. Woodward-Fieser Rules for dienes Woodward’s Rules for enones

23 UV / visible Spectroscopy 1. Bathochromic shift (red shift) v lower energy, longer wavelength v CONJUGATION. 2. Hypsochromic shift (blue shift) v higher energy, shorter wavelength. 3. Hyperchromic effect v increase in intensity 4. Hypochromic effect v decrease in intensity


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