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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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