ultraviolet visible uv vis spectroscopy n.
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Ultraviolet-Visible (UV-VIS) Spectroscopy

Ultraviolet-Visible (UV-VIS) Spectroscopy

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Ultraviolet-Visible (UV-VIS) Spectroscopy

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  1. Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated p electron systems

  2. Transitions between electron energy states gaps between electron energy levels are greater than thosebetween vibrational levels gap corresponds to wavelengthsbetween 200 and 800 nm DE = hn

  3. Conventions in UV-VIS X-axis is wavelength in nm (high energy at left, low energy at right) lmax is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially p electron system Y axis is a measure of absorption of electromagnetic radiation expressed as Absorbance or molar absorptivity (e)

  4. Only organic compounds with p electrons can absorb • energy in the UV/Visible region • A visible spectrum is obtained if visible light is absorbed • A UV spectrum is obtained if UV light is absorbed UV and Visible light cause only two kinds of electronic transitions

  5. y4* y3* y2 y1 pp* Transition in cis,trans-1,3-cyclooctadiene y4* y3* LUMO DE = hn y2 HOMO y1 Most stable p-electron configuration p-Electron configuration of excited state

  6. pp* Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond as HOMO-LUMO energy difference decreases (smaller DE), lmax shifts to longer wavelengths

  7. C C C C Substituent Effects Methyl groups on double bond cause lmax to shift to longer wavelengths H H CH3 H H CH3 H H lmax 170 nm lmax 188 nm

  8. A chromophore is the part of a molecule which absorbs UV or visible light

  9. Effect of Conjugation onlmax

  10. C C C C C C Substituent Effects Extending conjugation has a larger effect on lmax; shift is again to longer wavelengths H H H H H H H H H H lmax 170 nm lmax 217 nm

  11. 236 nm What is carvone’s max?

  12. The Beer–Lambert Law A = cle • A = log(I / I0) • c = concentration of substance in solution • l = length of the cell in cm • e = molar absorptivity The molar absorptivity of a compound is a constant that is characteristic of the compound at a particular wavelength

  13. UV Spectrum of cis,trans-1,3-cyclooctadiene 2000 Molarabsorptivity (e) lmax 230 nm emax 2630 1000 200 220 240 260 280 Wavelength, nm

  14. H H C C H H C C H H H3C H C C H H H C C H C C CH3 H Substituent Effects lmax 217 nm(conjugated diene) lmax 263 nmconjugated triene plus two methyl groups

  15. Both the lmax and e increase as the number of conjugated double bonds increases

  16. An auxochrome is a substituent in a chromphore that alters the lmax and the intensity of the absorption

  17. Measure the rates of a reaction • Determine the pKa of a compound • Estimate the nucleotide composition of DNA Uses of UV/Vis Spectroscopy

  18. The Visible Spectrum and Color

  19. Vision Biological / Physiological response to light stimuli

  20. The Eye and Vision

  21. The Eye and its Neurological Wiring

  22. Vision: Receptive Field (RF) • Definition: the area of the retina (or visual field) in which light signals evoke responses • It’s a property of the cell, not a cell or a part of the cell • It depends largely on the synaptic inputs to the cell and to some degree the biophysical property of the cell itself

  23. Projection from retina to LGN fixation point • Nasal RGC: axons crossover, project to contralateral LGN • Temporal RGC: axons stay on the same side (ipsilateral) • Left visual field: right LGN, right V1 • Right visual field: left LGN, left V1 fovea

  24. Visual pathway from retina to V1

  25. Optical imaging of orientation map

  26. Retinotopic map neighboring cells have neighboring RF retinotopic map is true in the retina, LGN and V1, but it gets fuzzy as you move on to higher visual areas c Project orderly to LGN and V1 b a a b c object retina

  27. The Retina & Photoreceptive Cells

  28. Different cells in the retina The Basic Retinal Circuit Back of eye 6. Pigment cells 1. Receptor Cells (Graded potential) (input) 2. Bipolar Cells (Graded potential) 3. Ganglion Cells (action potential) (Output) 4. Horozontal Cells (Graded potential) 5. Amacrine Cells (Graded/action potential) Structure of the eye Front of eye 2

  29. light Fovea: high spatial resolution Periphery: low spatial resolution

  30. Direct pathway: Photo receptor  Bipolar  RGC +: excitatory synapse, preserve response direction -: inhibitory synapse, flip response direction Direct pathway is responsible to the RF center

  31. Indirect pathway Indirect pathway mediated by horizontal is responsible to the RF surround There are other indirect pathway mediated by amacrine cells

  32. Summary of retinal circuit Direct pathway Indirect pathway Pathways mediated by amacrine cells

  33. The Nobel Prize in Physiology or Medicine 1981: Roger W.Sperry: for his discoveries concerning the functional specialization of the cerebral hemispheres David H. Hubel & Torsten N. Wiesel: for their discoveries concerning information processing in the visual system

  34. Terpenes in VisionCan diet affect sight?.....It might depend on what you’re looking at. • Color vision vs. B&W: Rodsvs.Cones • The Photochemical Process • Lycopenes • Carotenes • Vitamin A • Retinol / Retinal • Humans only see in B&W at night, but some animals like the nocturnal hawkmoth see color. Nature, 922-25, 2002

  35. Lycopene orange-red pigment in tomatoes lmax 505 nm

  36. Cis-Trans Isomerization & Vision

  37. B&W ChemistryStep One: Oxidation

  38. B&W ChemistryStep Two: Trans -> Cis Isomerization

  39. B&W ChemistryStep Three: “Hooked on Opsin”

  40. B&W ChemistryStep Four: “Flash”