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Outline
Start Chapter 18 Spectroscopy and Quantitative Analysis
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Electronic SpectroscopyUltraviolet and visible
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Where in the spectrum are these transitions?
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Review of properties of EM!
c= Where
c= speed of light = 3.00 x 108 m/s = wavelength in meters = frequency in sec-1
E=h or E=hc/
h=Planks Constant = 6.62606 x 1034 J.s
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Where in the spectrum are these transitions?
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Beer-Lambert Law
AKA - Beer’s LawAKA - Beer’s Law
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The Quantitative Picture Transmittance:
T = P/P0
b(path through sample)
P0
(power in)P
(power out) Absorbance:
A = -log10 T = log10 P0/P
The Beer-Lambert Law (a.k.a. Beer’s Law):A = bc
Where the absorbance A has no units, since A = log10 P0 / P
is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample in cmc is the concentration of the compound in solution, expressed in mol L-1 (or
M, molarity)
How do “we” select the How do “we” select the
wavelengthwavelength
to measure the absorbance?to measure the absorbance?
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Absorbance vs. Wavelength
A
420 440 460400380
Wavelength, nm
Why?Why?
1. Maximum Response for a given concentration
2. Small changes in Wavelength, result in small errors in Absorbance
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Red700 nm
Orange610 nm
Yellow570 nm
Green510 nm
Blue450 nm
Violet350nm
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KMnO4
400 500 600 700 800
Abs
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Limitations to Beer’s LawLimitations to Beer’s Law““Non-linear behavior”Non-linear behavior”
“Fundamental” “Experimental”
1. Concentration/Molecular Interactions2. Changes in Refractive Index
1. Not Using Peak wavelength
2. Colorimetric Reagent is limiting
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Interaction of Light and Interaction of Light and MatterMatter
Start with Atoms Finish with Molecules
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Consider Atoms - hydrogen
Energy
Very simple view of Energy states
n=1
n=2
n=3n=4n=5n=6
A
Wavelength, nm
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Molecular Spectroscopy
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Consider molecules With molecules, many energy levels.
Interactions between other molecules and with the solvent result in an increase in the width of the spectra.
Electronic States
Vibrational States
Rotational States
s1
s0
s2
s3
s4
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Electronic SpectrumA
bsor
banc
e
Wavelength, , generally in nanometers (nm)
0.0400 800
1.0
200
UV Visiblemaxwith certain extinction
Make solution of concentration low enough that A≤ 1
(Helps to Ensure Linear Beer’s law behavior)
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UV/Vis and UV/Vis and MolecularMolecular StructureStructure
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The UV Absorption process• * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis.
•n * transitions: non-bonding electrons (lone pairs), wavelength (max) in the 150-250 nm region.
•n * and * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm.
Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.
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What are the nature of these absorptions?
Example: * transitions responsible for ethylene UV absorption at ~170 nm calculated with semi-empirical excited-states methods (Gaussian 03W):
bonding molecular orbital antibonding molecular orbital
h 170nm photon
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Examples
Napthalene Absorbs in the UV
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Experimental details •What compounds show UV spectra?
•Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers.
•The NIST databases have UV spectra for many The NIST databases have UV spectra for many compoundscompounds You will find molar absorbtivities You will find molar absorbtivities in L•cm/mol, tabulated.in L•cm/mol, tabulated.
•Transition metal complexes, inorganics
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Notes on UV/Vis Qualitatively
Not too useful Band broadening
Quantitatively Quite Useful
Beer’s Law is obeyed through long range of concentrations
Thousands of methods Most commonly used Detection Limits ~ 10-4 – 10-6 M
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Notes on UV/Vis (cont’d)
Quant (cont’d) Cheap, inexpensive, can be relatively
fast Reasonably selective
Can find colorimetric method or use color of solution
Good accuracy ~1-5%
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What happens to the absorbed energy?
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