UV-vis Absorption (Extinction) UV-vis Absorption (Extinction) SpectroscopySpectroscopy

Single-Beam or Double-Beam

Fixed or Dispersive

Common:Source – Tungsten Halogen Lamp (360-2000 nm)Sample – Liquid In CuvetteDispersion – Spectrograph w/ Diffraction GratingDetector – CCD

Beer’s Law: A = bc

Assumptions

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Apparent Deviations from Beer’s Law

Non-Zero InterceptNon-Zero Intercept

Improper blank Improper blank measurement or measurement or correction.correction.

Instrumental drift.Instrumental drift.

Skoog, Hollar, Nieman, Skoog, Hollar, Nieman, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis, Saunders , Saunders College Publishing, Philadelphia, 1998.College Publishing, Philadelphia, 1998.

Apparent Deviations from Beer’s Law

Non-Linear Calibration PlotNon-Linear Calibration Plot

Chemical Equilibrium – Chemical Equilibrium – if multiple chemical forms of analyte if multiple chemical forms of analyte exist and only one absorbsexist and only one absorbsOther Chemical Effects – Other Chemical Effects – solute/solvent interactions, solute/solvent interactions, solute/solute interactions, H bonding at high concentrationssolute/solute interactions, H bonding at high concentrationsUsing Polychromatic Radiation – Using Polychromatic Radiation – non-optimum wavelengths non-optimum wavelengths are not optimally absorbed, calibration curve slope is too loware not optimally absorbed, calibration curve slope is too lowStray Light – Stray Light – causes measured transmittance to be larger causes measured transmittance to be larger than it should bethan it should be

Skoog, Hollar, Nieman, Skoog, Hollar, Nieman, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis, Saunders , Saunders College Publishing, Philadelphia, 1998.College Publishing, Philadelphia, 1998.

Absorbed/Emitted ColorsAbsorbed/Emitted Colors

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

ChromophoresOften transitions are localized in specific bonds or Often transitions are localized in specific bonds or functional groups within a molecule.functional groups within a molecule.

Group will have a characteristic Group will have a characteristic maxmax and and ..

Molecular structure or environment can affect Molecular structure or environment can affect maxmax and and ..

Shift to longer Shift to longer bathochromic (red) shift. bathochromic (red) shift.Shift to shorter Shift to shorter hypsochromic (blue) shift. hypsochromic (blue) shift.

Increase in Increase in hyperchromic effect. hyperchromic effect.Decrease in Decrease in hypochromic effect. hypochromic effect.

What effect does conjugation usually have?What effect does conjugation usually have?

hyperchromic effect / bathochromic shift

Characteristic Electronic TransitionsCharacteristic Electronic Transitions

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

L mol-1 cm-1

Characteristic Electronic TransitionsCharacteristic Electronic Transitions

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

L mol-1 cm-1

AuxophoreDoes not absorbDoes not absorb

Induces a bathochromic shift and hyperchromic effect when Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore (e.g. -OH, -Br, -NHconjugated to a chromophore (e.g. -OH, -Br, -NH22).).

Solvent EffectsSolvent Effects

Hypsochromic shiftHypsochromic shift in n in n * transitions as solvent polarity * transitions as solvent polarity increases. Solvation stabilizes the nonbonding pair.increases. Solvation stabilizes the nonbonding pair.

Bathochromic shiftBathochromic shift in in * transitions as solvent polarity * transitions as solvent polarity increases. Solvation stabilizes increases. Solvation stabilizes *, which is often more polar *, which is often more polar than than ..

Conjugated Conjugated AlkenesAlkenes

Woodward-FieserWoodward-FieserRulesRules

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

1,3-butadiene

bonding

antibonding

*

Conjugated DienesConjugated Dienes

UV Absorption of Conjugated Alkenes

*

max 175 217 258

15,000 21,000 35,000

E

Increasing conjugation gives:• longer wavelength absorption• more intense absorption

-Carotene

11 double bonds

max = 460 nm ( = 139,000)

Systems with More than 4 Double BondsSystems with More than 4 Double Bonds

max (nm) = 114 + 5M + n(48.0-1.7n) – 16.5Rendo – 10Rexo

n = number of conjugated double bondsM = number of alkyl or alkyl like substituents on the

conjugated systemRendo = number of rings with endocyclic double bonds in the

conjugated systemRexo = number of rings with exocyclic double bonds

Fieser-Kuhn Rules

Are you getting the concept?

H3C

CH3 CH3 CH3

CH3CH3 CH3

CH3

CH3

CH3

Calculate the absorption maximum for lycopene:

Substituted BenzenesSubstituted Benzenes

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

Polyaromatics

Substituent Effects on Aromatic Absorption

255 nm band is sensitive to electron density of aromatic ring

OH OCH3

270 nm ( 1,450) 287 nm ( 2,600)

OC

O

CH3

258 nm ( 250)

PHENOL ANISOLE PHENYL ACETATE

Electron densityRed = highest Green = moderate

un

its =

L m

ole-1

cm

-1

pH Effects on Aromatic Absorption

OH Obase

270 nm ( 1,450) 287 nm ( 2,600)

280 nm ( 1,430) 254 nm ( 160)

NH2 NH3

Phenoxide ionelectrostatic potential map

Anilinium ionelectrostatic potential map

un

its =

L m

ole-1

cm

-1

AromaticAromaticCarbonylCarbonyl

CompoundsCompounds

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of OrganicStructure Determination of OrganicCompoundsCompounds

Woodward Rules for Enones

• Base values– acyclic or 6-membered ring 215 nm– 5-membered ring 202 nm

• Additions for– double bond extending conjugation 30 nm– alkyl group or ring residue 10 nm

12 nm

18 nm

exocyclic double bond 5 nm

O

~

O

Are you getting the concept?Predict the absorption max for cholesta-1,4-dien-3-one andthe enol of 1,2-cyclopentanedione.

HO

O

Common SolventsCommon Solvents

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds