©2019 Gregory R. Cook, NDSU

UV-Vis SpectroscopyChem 744

Spring 2019

©2019 Gregory R. Cook, NDSU

The EM Spectrum

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©2019 Gregory R. Cook, NDSU

UV-Vis Spectroscopy

200 300 400

ultraviolet

‣ Every organic molecule absorbs UV-visible light

‣ Energy of electronic transitions

‣ saturated functionality not in region that is easily accessible (obscured by solvent and atmosphere)

‣ Conjugation


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Basic Instrument Design

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Electronic Transitions

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σ

π

n

π∗

σ∗

σ σ∗

σ π∗

π π∗

n σ∗

n π∗

possible electronic transitions

alkanes

carbonyls

alkenes, carbonyls, alkynes, etc.

heteroatoms - O, N, S, X, etc.

carbonyls

E

ΔE = [Eexcited - Eground] = hν

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Electronic Transitions

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Electronic Transitions

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http://chemwiki.ucdavis.edu

http://chemwiki.ucdavis.edu

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Electronic Transitions

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http://chemwiki.ucdavis.edu

http://chemwiki.ucdavis.edu

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Electronic Transitions

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Electronic Transitions

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Beer-Lambert Law

A = log ( I0 / I1 ) = ε l c

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‣ A is absorbance (no units)‣ ε is the molar absorptivity or 


extinction coefficient (L mol-1 cm-1) 
(how strongly it absorbs - intrinsic)

‣ l is the path length of the sample (cm)‣ c is the concentration of the comppound (mol L-1)

‣ I0 is the intensity of the incident light

‣ I1 is the intensity of the transmitted light

©2019 Gregory R. Cook, NDSU

Organic Molecules UV-Vis Characteristics

‣ Most organic molecules absorb in UV region unless highly conjugated

‣ Most common detector for HPLC‣ best to have conjugated chromophore‣ Spectra are broad (why?) making it useful for

qualitative identification

‣ Can quantitate using Beer’s law analysis

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Presentation of Spectra

‣ many vibrational bands - many slightly different absorbances‣ Absorption of light occurs in 10-15 s, faster than vibrational changes‣ Franck-Codon principle - absorption occurs via a vertical transition 


- all bond lengths, angles, conformations and solvation are conserved in the transition.

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200 nm 800 nm

λmax

λmax

hyperchromic

bathochromichypsochromic

hypochromic

(blue) (red)A

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Presentation of Spectra

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Electronic Transitions

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Electronic Transitions

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http://chemwiki.ucdavis.edu

3

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Electronic Transitions

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http://chemwiki.ucdavis.edu

Blue Dye #1

http://chemwiki.ucdavis.eduhttp://chemwiki.ucdavis.edu

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Solvents

‣ Measuring UV-Vis spectra

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Solvent effects on spectra

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OH

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Selection Rules

σ

π

n

π∗

σ∗

possible electronic transitions

E

‣ Not all transitions are observed‣ Depends on symmetry and multiplicity

‣ “Forbidden Transitions” (e.g. n-π*) can be seen but are weak

‣ Molecular vibrations
can disrupt the
symmetry


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Transitions

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Nature of absorption

hν

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C C

H

H H

H

HH

‣ Ethane - λmax = 135 nm

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Nature of absorption

‣ Acetone - λmax = ~166 nm (n-σ* ; ε = >10000) 
 λmax = 188 nm (π-π* ; ε = 1860) 
 λmax = 279 nm (n-π* ; ε = 15)

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O

O

O

O

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Tetraphenyldicyclopentadienone

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O

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Electronic Transitions

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http://chemwiki.ucdavis.edu

NADH / NAD+

http://chemwiki.ucdavis.edu

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Ethylene

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‣ Ethylene π-π* λmax = 165 nm (n-σ* ; ε = 16,000)

‣ Substitution with an atom containing non-bonding electrons (–OH, OR, –NH2, –NHR, –SH, –SR, –Hal) results in a bathochromic shift –> the non-bonding electrons interact with the π-orbitals of the double bond

‣ the energy difference between the HOMO and LUMO decreases

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Conjugation

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Conjugation

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‣ Conjugation of two or more double bonds results in decreasing energy difference between the HOMO and LUMO

λmax 217 253 220 227 227 256 263

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Conjugation

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Conjugation

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‣ Woodward-Fieser Rules work well up to 4 double bonds

base 2172 alkyls 10exo bond 5total 232 (actual 237)

base 2143 alkyls 15exo bond 5total 234 (actual 235)

for more than 4 conjugated double bonds: Fieser-Kuhn Rules

λmax = 114 + 5(# alkyl substituents) + n(48 - 1.7n) - 16.5(# endo) - 10(# exo)

©2019 Gregory R. Cook, NDSU

Conjugation

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‣ Lycopene and beta-carotene

λmax = 114 + 5(8) + 11(48 - 1.7•11) - 0 - 0 = 476 nm

λmax (actual) = 474

λmax = 114 + 5(10) + 11(48 - 1.7•11) - 16.5(2) - 0 = 453.3 nm

λmax (actual) = 452

λmax = 114 + 5(# alkyl substituents) + n(48 - 1.7n) - 16.5(# endo) - 10(# exo)

©2019 Gregory R. Cook, NDSU

Conjugation

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lycopene (red color of tomatos)

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Benzene

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Aromatic Substituent Effects

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Aromatic Substituent Effects

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Aromatic Dyes

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O

R3'

R4'

R5'

R3

R5R6

R7

anthocyaninsR = H, OH, OCH3

O

OHR

R

OROH

HO NaOH O

OR

R

OROH

O

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Polyaromatic Systems

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Carbonyl Compounds

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Carbonyl Compounds

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C O aldehyde or ketone n -> σ* 166 nm ε = 16,000π -> π* 189 nm ε = 900n -> π* 166 nm ε = 10-20

O279

O279

O288

O295

O299

O285

H

O290

H

O292

O290

OH

O204

OEt

O207

NH2

O220

Cl

O235

O225

H

O

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Carbonyl Compounds

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