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Chapter 14

Chapter 14. Applications of Ultraviolet-Visible Molecular Absorption Spectrometry. Absorption by Organic Compounds. Electronic Transitions Between Molecular Orbitals. Constructive and destructive interference of electron waves. Transitions Between Molecular Orbitals. CHROMOPHORE.

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Chapter 14

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  1. Chapter 14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry

  2. Absorption by Organic Compounds Electronic Transitions Between Molecular Orbitals Constructive and destructive interference of electron waves.

  3. Transitions Between Molecular Orbitals

  4. CHROMOPHORE A center of light absorption in a molecule.

  5. "Allowed" Transitions Angular Momentum of the Electron is Conserved * *

  6. "Allowed" Transitions Angular Momentum of the Electron is Conserved n* "Forbidden" Transitions Angular Momentum of the Electron is Not Conserved n*

  7. “allowed” “forbidden”

  8. General Absorbance Spectrum alkenes, alkynes, aromatics cmpd’s with heteroatoms alkanes carbonyls *(allowed) n*(forbidden) n*(allowed) * (allowed) 105 104 103   102 10 150 300 possible transitions?  (nm)

  9. Spectral Details are Lost in the Liquid Phase 1,2,4,5-tetrazine

  10. Qualitative Analysis • Solvent Shifts – is a peak a * or an n* transition? • Effect of ring substituents on the absorbance spectrum of cpmd’s with aromatic rings • Effect of pH on absorbance spectra, e.g. indicators HIn  H+ + In-

  11. Solvent Shifts * "red shift“ in a polar solvent

  12. * energy level is polar and stabilized by a polar solvent

  13. n*“blue shift“ in a polar solvent

  14. Aromatic Rings electron donating (stabilizes ring, lowers energy, red shift): methyl, phenol, R2N -, RO- electron withdrawing (destabilizes ring, raises energy, blue shift):: nitro, carboxylic, ester, ketones, aldehydes

  15. Effect of pH, e.g. Indicators phenolphthalein crystal violet phenol red

  16. Quantitative Analysis • Mixture Analysis • Sample “matrix” – Standard Addition methods • Derivative Spectroscopy – enhancing weak spectral details • Enzyme Kinetics

  17. Beer's Law for Mixtures is Additive 1. Mixture Analysis A = AM + AN e.g. determination of caffeine + sodium benzoate in soft drinks (Ch312)

  18. Standard Addition Methods (Ch 1, p. 13-17) 2. Can be used with any instrumental technique; useful if there is an irreproducible sample “matrix” = all the impurities in the sample, e.g. microorganisms, dissolved metals and organics, particulate matter, humics, etc. Signal → add increasing volumes of known concentration (Vspike) to fixed volume of sample Vspike →

  19. "spike" = added analyte of known concentration and volume Vx = volume unknown Cx = concentration unknown Vs = volume "spike" Cs = concentration "spike" Vt = total volume dilute toVt Vs, Cs Vx, Cx A = Ax + As = bCdx + bCds Plot A vs. Vs m = b = Cx =

  20. Example 1-1 10 mL aliquots of a natural water sample were pipetted into 50.00 mL volumetric flasks. Exactly 0.00, 5.00, 10.00, 15.00 and 20.00 mL of a standard solution containing 11.1 ppm of Fe3+ was added to each, followed by an excess of SCN- ion to give the red complex Fe(SCN)2+. After dilution to volume, the instrument response S for each of the 5 solutions, measured with a colorimeter, was found to be 0.240, 0.437, 0.621, 0.809, and 1.009 respectively. What was the concentration of Fe3+ in the water sample?

  21. Derivative Spectra 3. Derivative Spectroscopy - used to enhance weak spectral features

  22. k1 k2 E + S ES  P + E k-1 Spectrophotmetric Kinetics Methods 4. Enzyme Kinetics – “Michaelis-Menten” Mechanism

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