Absorption Spectroscopy 3

8/11/2019 Absorption Spectroscopy 3

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Absorption Spectroscopy

Electronic structures-Organic compounds

Halan Prakash


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Light Phenomenon

light consisted of particles By 1900 most scientists believed that

light behaved as a wave.

Light we perceive is a part of the electromagnetic radiation/spectrum


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The Spectrum of Visible Light

The visible part of the spectrum may be further subdivided according to color, withred at the long wavelength end and violet at the short wavelength end,

Interaction of UV and visible light with matter- UV-Visible absorption spectroscopyElectronic redistribution in molecules

http://scienceblogs.com/startswithabang/2011/09/14/the-color-of-space/
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When white light passes through or is reflected by a colored substance, a characteristicportion of the mixed wavelengths is absorbed. The remaining light will then assume thecomplementary color to the wavelength(s) absorbed.

Complementary colors are diametrically opposite each other.

Absorption of 420-430 nm light (VIOLET) renders a substance (YELLOW),and absorptionof 500-520 nm light makes it red . Green is unique in that it can be created by absorption

close to 400 nm as well as absorption near 800 nm.

White light is a mixture of various colors


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Novalek - Malachite Green

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Example of an Absorptionspectrum- Malachite Green

Journal of Hazardous Materials 213 214 (2012) 19 27Degradation of Malachite green studied by usingabsorption spectrometer- IMA Lab
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Malachite Green

Methyl Orange

Distinguish the structure


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Absorption SpectrumAbsorbance Vs Wavelength, nm

Enhanced Absorbanceand red shift conjuagted carbonylcompounds ,can be distinguished fromnonconjugated

The number of double bonds inconjugation can be determined

Extent of aromatic system canbe distinguished


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Types of electronic transitions


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Molecular Orbital Description of 1,3-Butadiene

The bonding -orbitals are made from 4 porbitals that provide greater delocalizationand lower energy than in isolated C=C


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Rhe single bond between the two double bonds isstrengthened by overlap of p orbitals

In summary, we say electrons in 1,3-butadiene are delocalizedover the bond system

Delocalization leads to stabilization


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The single bond between theconjugated double bonds is shorter andstronger than sp 3 in butane


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http://www.chem.umn.edu/groups/taton/chem2302/handouts/9_10.pdf

165 nm


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1240 nm


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I = Io10 cl T = I/I o = 10

cl

Iog T = log I/I o = cl

log I o /I = A = cl


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Relationship between %Transmittance and lightpath length and concentration

100

80

60

40

20

0 length

%T

100

80

60

40

20

0

%T

concentration

Transmittance = I T/I 0 = e - cl where

a is an extinction constant, c isconcentration and l is light path length


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Transmittance (%) = I T /I 0*100 Absorbance = log 10(I0 /I T )

Converting Transmittance to Absorbance %T/100 = I T/I0 100/%T = I 0/ITTaking logs on both sides Log 100 log %T = log I0/IT 2 log%T = Absorbance

I = Io10 cl T = I/I o = 10

cl

Iog T = log I/I o = cl

log I o /I = A = cl


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Absorption is additive


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Absorption band resolved into small absorption bands


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Franck-Condon principle- Because the nuclei are so much more massive thanElectrons, an electronic transition takes place faster than the nuclei can respond


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Predicting absorption maximum- empirical relationshipsNoble Laureate R.B. Woodward rules - You dont have to remember all the values!


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Absorption Spectroscopy 3

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