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©2015 Gregory R. Cook, NDSU
UV-Vis SpectroscopyChem 744Fall 2015
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The EM Spectrum
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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
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
http://chemwiki.ucdavis.edu
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Electronic Transitions
http://chemwiki.ucdavis.edu
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Electronic Transitions
http://chemwiki.ucdavis.edu
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Electronic Transitions
http://chemwiki.ucdavis.edu
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Electronic Transitions
http://chemwiki.ucdavis.edu
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Beer-Lambert Law
A = log ( I0 / I1 ) = ε l c
‣ 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
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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.
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
http://chemwiki.ucdavis.edu
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Electronic Transitions
http://chemwiki.ucdavis.edu
3
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Electronic Transitions
http://chemwiki.ucdavis.edu
Blue Dye #1
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Solvents‣ Measuring UV-Vis spectra <200 nm is
impractical
‣ Quartz glass cutoff 210 nm
‣ fused synthetic silica can get down to 190 nm
‣ Solvent Cutoffs
18
solvent cut-off (nm)water 205
acetonitrile 210
cyclohexane 210
diethyl ether 210
ethanol 210
methanol 210
dioxane 220
solvent cut-off (nm)THF 220
dichloromethane 235
chloroform 245
carbon tet 265
benzene 280
acetone 300
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Solvent effects on spectra
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 vibrationscan disrupt thesymmetry
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Transitions
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Nature of absorption
hν
‣ Ethane - λmax = 135 nm
C C
H
H H
H
HH
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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)O
O
O
O
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Tetraphenyldicyclopentadienone
O
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Electronic Transitions
http://chemwiki.ucdavis.edu
NADH / NAD+
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Ethylene
‣ 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
‣ 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
‣ 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)
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Conjugation
‣ 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)
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Conjugation
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
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
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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