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UV-vis Absorption (Extinction) Spectroscopy. Single-Beam or Double-Beam Fixed l or Dispersive Common: Source – Tungsten Halogen Lamp (360-2000 nm) Sample – Liquid In Cuvette Dispersion – Spectrograph w/ Diffraction Grating Detector – CCD. Beer’s Law: A = e bc. Assumptions.

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slide1

UV-vis Absorption (Extinction) Spectroscopy

Single-Beam or Double-Beam

Fixed l or Dispersive

Common:

Source – Tungsten Halogen Lamp (360-2000 nm)

Sample – Liquid In Cuvette

Dispersion – Spectrograph w/ Diffraction Grating

Detector – CCD

Beer’s Law: A = ebc

assumptions
Assumptions

Ingle and Crouch, Spectrochemical Analysis

apparent deviations from beer s law
Apparent Deviations from Beer’s Law

Non-Zero Intercept

Improper blank measurement or correction.

Instrumental drift.

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

apparent deviations from beer s law4
Apparent Deviations from Beer’s Law

Non-Linear Calibration Plot

Chemical Equilibrium – if multiple chemical forms of analyte exist and only one absorbs

Other Chemical Effects – solute/solvent interactions, solute/solute interactions, H bonding at high concentrations

Using Polychromatic Radiation – non-optimum wavelengths are still transmitted and detected

Stray Light – causes measured transmittance to be larger than it should be

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

slide5

Absorbed/Emitted Colors

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

chromophores
Chromophores

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

Group will have a characteristic lmax and e.

Molecular structure or environment can influence lmax and e.

Shift to longer l bathochromic (red) shift.

Shift to shorter l  hypsochromic (blue) shift.

Increase in e  hyperchromic effect.

Decrease in e  hypochromic effect.

What effect does conjugation usually have?

hyperchromic effect / bathochromic shift

slide7

Characteristic Electronic Transitions

L mol-1 cm-1

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

slide8

Characteristic Electronic Transitions

L mol-1 cm-1

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

auxophore
Auxophore

Does not absorb

Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore (e.g. -OH, -Br, -NH2).

Solvent Effects

Hypsochromic shiftin n  p* transitions as solvent polarity increases. Solvation stabilizes the nonbonding pair.

Bathochromic shiftin p  p* transitions as solvent polarity increases. Solvation stabilizes p*, which is often more polar than p.

slide10

Conjugated

Alkenes

Woodward-Fieser

Rules

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

1 3 butadiene

*

1,3-butadiene

antibonding

bonding

uv absorption of conjugated alkenes
UV Absorption of Conjugated Alkenes

e units = L mole-1 cm-1

  • Increasing conjugation gives:
    • longer wavelength absorption
    • more intense absorption
b carotene
b-Carotene

11 double bonds

lmax = 460 nm (e = 139,000)

slide15

Fieser-Kuhn Rules

Systems with More than 4 Double Bonds

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

n = number of conjugated double bonds

M = number of alkyl or alkyl like substituents on the

conjugated system

Rendo = number of rings with endocyclic double bonds in the

conjugated system

Rexo = number of rings with exocyclic double bonds

are you getting the concept
Are you getting the concept?

Calculate the absorption maximum for lycopene: