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UV / visible SpectroscopyUV / visible Spectroscopy
IntroductionIntroduction
Identification of organic speciesIdentification of organic species
Quantitation of inorganic speciesQuantitation of inorganic species
Colorimetric analysisColorimetric analysis
UV / visible SpectroscopyUV / visible Spectroscopy
The origin of the analytical signalThe origin of the analytical signal
Excitation of an atom or molecule by Excitation of an atom or molecule by ultraviolet or visible radiation.ultraviolet or visible radiation.
190 - 900nm190 - 900nm
UV / visible SpectroscopyUV / visible Spectroscopy The radiation which is absorbed has an energy The radiation which is absorbed has an energy
which exactly matches the energy difference which exactly matches the energy difference between the ground state and the excited state.between the ground state and the excited state.
These absorptions correspond to electronic These absorptions correspond to electronic transitions.transitions.
UV / visible SpectroscopyUV / visible Spectroscopy
Electronic transitions involve the promotion Electronic transitions involve the promotion of electrons from an occupied orbital to an of electrons from an occupied orbital to an
unoccupied orbital.unoccupied orbital.
Energy differences of 125 - 650 kJ/mole.Energy differences of 125 - 650 kJ/mole.
UV / visible SpectroscopyUV / visible Spectroscopy
Beer-Lambert Law Beer-Lambert Law
A = log(IA = log(IOO/I) = /I) = clcl
UV / visible SpectroscopyUV / visible Spectroscopy
A = log(IA = log(IOO/I) = /I) = clcl
A = Absorbance (optical density)A = Absorbance (optical density) IIOO = Intensity of light on the sample cell = Intensity of light on the sample cell
I = Intensity of light leaving the sample cellI = Intensity of light leaving the sample cell c = molar concentration of solutec = molar concentration of solute l = length of sample cell (cm)l = length of sample cell (cm) = molar absorptivity (molar extinction coefficient)= molar absorptivity (molar extinction coefficient)
UV / visible SpectroscopyUV / visible Spectroscopy
The Beer-Lambert Law is rigorously obeyed The Beer-Lambert Law is rigorously obeyed when a single species is present at relatively when a single species is present at relatively
low concentrations.low concentrations.
UV / visible SpectroscopyUV / visible Spectroscopy
The Beer-Lambert Law is not obeyed:The Beer-Lambert Law is not obeyed:
High concentrationsHigh concentrations
Solute and solvent form complexesSolute and solvent form complexes
Thermal equilibria exist between the ground state Thermal equilibria exist between the ground state and the excited stateand the excited state
Fluorescent compounds are present in solutionFluorescent compounds are present in solution
UV / visible SpectroscopyUV / visible Spectroscopy
The size of the absorbing system and the The size of the absorbing system and the probability that the transition will take place probability that the transition will take place
control the absorptivity (control the absorptivity ().).
Values above 10Values above 1044 are termed high intensity are termed high intensity absorptions.absorptions.
Values below 1000 indicate low intensity Values below 1000 indicate low intensity absorptions which are forbidden transitions.absorptions which are forbidden transitions.
UV / visible SpectroscopyUV / visible Spectroscopy
Organic Spectroscopy Organic Spectroscopy
Transitions betweenTransitions between
MOLECULAR ORBITALSMOLECULAR ORBITALS
UV / visible SpectroscopyUV / visible Spectroscopy
Highest occupied molecular orbitalHighest occupied molecular orbital
HOMOHOMO
Lowest unoccupied molecular orbitalLowest unoccupied molecular orbital
LUMOLUMO
UV / visible SpectroscopyUV / visible Spectroscopy
Not all transitions are observedNot all transitions are observed
There are restrictions calledThere are restrictions called
Selection RulesSelection Rules
This results inThis results in
Forbidden TransitionsForbidden Transitions
UV / visible SpectroscopyUV / visible Spectroscopy
The characteristic energy of a transition and the The characteristic energy of a transition and the wavelength of radiation absorbed are properties wavelength of radiation absorbed are properties
of a group of atoms rather than of electrons of a group of atoms rather than of electrons themselves.themselves.
The group of atoms producing such an The group of atoms producing such an absorption is called aabsorption is called a
CHROMOPHORECHROMOPHORE
UV / visible SpectroscopyUV / visible Spectroscopy
It is often difficult to extract a great deal It is often difficult to extract a great deal of information from a UV spectrum by of information from a UV spectrum by
itself.itself.
Generally you can only pick out Generally you can only pick out conjugated systems.conjugated systems.
UV / visible SpectroscopyUV / visible Spectroscopy
ALWAYSALWAYSuse in conjunction withuse in conjunction with
nmr and infrared spectra.nmr and infrared spectra.
UV / visible SpectroscopyUV / visible Spectroscopy
As structural changes occur in a As structural changes occur in a chromophore it is difficult to predict chromophore it is difficult to predict exact energy and intensity changes.exact energy and intensity changes.
Use empirical rules.Use empirical rules.
Woodward-Fieser Rules for dienesWoodward-Fieser Rules for dienes
Woodward’s Rules for enonesWoodward’s Rules for enones
UV / visible SpectroscopyUV / visible Spectroscopy
1. 1. Bathochromic shiftBathochromic shift (red shift) (red shift) lower energy, longer wavelengthlower energy, longer wavelength
CONJUGATION.CONJUGATION.
2. 2. Hypsochromic shiftHypsochromic shift (blue shift) (blue shift) higher energy, shorter wavelength.higher energy, shorter wavelength.
3. 3. Hyperchromic effectHyperchromic effect increase in intensityincrease in intensity
4. 4. Hypochromic effectHypochromic effect decrease in intensitydecrease in intensity