uv / visible spectroscopy l introduction l identification of organic species l quantitation of...

UV / visible SpectroscopyUV / visible Spectroscopy

IntroductionIntroduction

Identification of organic speciesIdentification of organic species

Quantitation of inorganic speciesQuantitation of inorganic species

Colorimetric analysisColorimetric analysis


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.


/ nm

Abs

/ nm

Abs


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.


Beer-Lambert Law Beer-Lambert Law

A = log(IA = log(IOO/I) = /I) = clcl


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)


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.


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


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.


Organic Spectroscopy Organic Spectroscopy

Transitions betweenTransitions between

MOLECULAR ORBITALSMOLECULAR ORBITALS


Highest occupied molecular orbitalHighest occupied molecular orbital

HOMOHOMO

Lowest unoccupied molecular orbitalLowest unoccupied molecular orbital

LUMOLUMO


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


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


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.


ALWAYSALWAYSuse in conjunction withuse in conjunction with

nmr and infrared spectra.nmr and infrared spectra.


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


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

uv / visible spectroscopy l introduction l identification of organic species l quantitation of...

Documents

uv visible spectroscopy

nm slide

chromophore slide

enones slide

solution slide

visible radiation

organic spectroscopy

uv spectrum