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

Assumptions Ingle and Crouch, Spectrochemical Analysis

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 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 not optimally absorbed, calibration curve slope is too low Stray Light – causes measured transmittance to be larger than it should be Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

Absorbed/Emitted Colors Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

Chromophores Often transitions are localized in specific bonds or functional groups within a molecule. Group will have a characteristic max and . Molecular structure or environment can affect max and . Shift to longer  bathochromic (red) shift. Shift to shorter  hypsochromic (blue) shift. Increase in   hyperchromic effect. Decrease in   hypochromic effect. What effect does conjugation usually have? hyperchromic effect / bathochromic shift

Characteristic Electronic Transitions Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds L mol -1 cm -1

Characteristic Electronic Transitions Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds L mol -1 cm -1

Auxophore Does not absorb Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore (e.g. -OH, -Br, -NH 2 ). Solvent Effects Hypsochromic shift in n   * transitions as solvent polarity increases. Solvation stabilizes the nonbonding pair. Bathochromic shift in    * transitions as solvent polarity increases. Solvation stabilizes  *, which is often more polar than .

ConjugatedAlkenesWoodward-FieserRules Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

1,3-butadiene bonding antibonding  **

Conjugated Dienes

UV Absorption of Conjugated Alkenes Increasing conjugation gives: longer wavelength absorption more intense absorption

 -Carotene 11 double bonds max = 460 nm (  = 139,000)

Systems with More than 4 Double Bonds max (nm) = M + n( n) – 16.5R endo – 10R exo n = number of conjugated double bonds M = number of alkyl or alkyl like substituents on the conjugated system R endo = number of rings with endocyclic double bonds in the conjugated system R exo = number of rings with exocyclic double bonds Fieser-Kuhn Rules

Are you getting the concept? Calculate the absorption maximum for lycopene:

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

Polyaromatics

Substituent Effects on Aromatic Absorption 255 nm band is sensitive to electron density of aromatic ring Electron density Red = highest Green = moderate  units = L mole -1 cm -1

pH Effects on Aromatic Absorption Phenoxide ion electrostatic potential map Anilinium ion electrostatic potential map  units = L mole -1 cm -1

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

Woodward Rules for Enones Base values –acyclic or 6-membered ring215 nm –5-membered ring202 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

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

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