Presentation on theme: "Ultraviolet-Visible (UV-VIS) Spectroscopy"— Presentation transcript:
1 Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated p electron systems1
2 Transitions between electron energy states gaps between electron energy levels are greater than those between vibrational levelsgap corresponds to wavelengths between 200 and 800 nmDE = hn2
3 X-axis is wavelength in nm (high energy at left, low energy at right) Conventions in UV-VISX-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 systemY axis is a measure of absorption of electromagnetic radiation expressed as Absorbance or molar absorptivity (e)3
4 UV and Visible light cause only two kinds of electronic transitionsOnly organic compounds with p electrons can absorbenergy in the UV/Visible regionA visible spectrum is obtained if visible light is absorbedA UV spectrum is obtained if UV light is absorbed
5 pp* Transition in cis,trans-1,3-cyclooctadiene LUMOy3*DE = hny2HOMOy1Most stable p-electron configurationp-Electron configuration of excited state5
6 pp* Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bondas HOMO-LUMO energy difference decreases (smaller DE), lmax shifts to longer wavelengths5
7 Methyl groups on double bond cause lmax to shift to longer wavelengths Substituent EffectsMethyl groups on double bond cause lmax to shift to longer wavelengthsHHHCH3CCHCH3HHlmax 170 nmlmax 188 nm5
8 A chromophore is the part of a molecule which absorbs UV or visible light
13 The Beer–Lambert Law A = cle A = log(I / I0) c = concentration of substance in solutionl = length of the cell in cme = molar absorptivityThe molar absorptivity of a compound is a constant thatis characteristic of the compound at a particularwavelength
15 lmax 217 nm (conjugated diene) Substituent EffectsHClmax 217 nm (conjugated diene)CHCH3H3Clmax 263 nm conjugated triene plus two methyl groups5
16 Both the lmax and e increase as the number of conjugated double bonds increases
17 An auxochrome is a substituent in a chromphore that alters the lmax and the intensity of the absorption
18 Uses of UV/Vis Spectroscopy Measure the rates of a reactionDetermine the pKa of a compoundEstimate the nucleotide composition of DNA
19 Figure Number: 08-08Title: Figure 8.8Caption: Nitroethane anion formation can be monitored by UV at 240 nm.Notes: The rate of proton removal from nitroethane can be measured by monitoring the nitroethane anion formed by the reaction.
20 Figure Number: 08-09Title: Figure 8.9Caption: Pyruvate concentration can be monitored by UV at 340 nm.Notes: The rate of reduction of pyruvate by NADH can be measured by monitoring pyruvate disappearance at 340 nm.
21 Figure Number: 08-10Title: Figure 8.10Caption: The absorbance of an aqueous solution of phenol at 287 nm as a function of pH.Notes: The absorption at 287 nm is due to the presence of phenoxide ion (the conjugate base of phenol). When half of the phenol has been converted into phenoxide, the concentrations of phenol and phenoxide are equal. At this point, pH = pKa.
22 Figure Number: 08-11Title: Figure 8.11Caption: The absorbance of a solution of DNA at 260 nm as a function of the temperature of the solution.Notes: Single-stranded DNA absorbs at 260 nm because nonbonding electrons on nitrogen atoms, which are normally tied up in hydrogen bonding together complementary strands of DNA, are free to undergo n --> p* transitions in single-stranded DNA. When the temperature has risen to the point that half of the DNA is single-stranded, DNA is said to be at its "melting" temperature, Tm.
27 Vision: Receptive Field (RF) Definition: the area of the retina (or visual field) in which light signals evoke responsesIt’s a property of the cell, not a cell or a part of the cellIt depends largely on the synaptic inputs to the cell and to some degree the biophysical property of the cell itself
28 Projection from retina to LGN fixation pointNasal RGC: axons crossover, project to contralateral LGNTemporal RGC: axons stay on the same side (ipsilateral)Left visual field: right LGN, right V1Right visual field: left LGN, left V1fovea
31 Retinotopic map c b a a b c 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 areascProject orderly to LGN and V1baabcobjectretina
33 Different cells in the retina The Basic Retinal Circuit Back of eye6. Pigment cells1. Receptor Cells(Graded potential)(input)2. Bipolar Cells3. Ganglion Cells(action potential)(Output)4. Horozontal Cells(Graded potential)5. Amacrine Cells(Graded/action potential)Structure of the eyeFront of eye2
34 lightFovea: high spatial resolutionPeriphery: low spatial resolution
35 Direct pathway: Photo receptor Bipolar RGC +: excitatory synapse, preserve response direction-: inhibitory synapse, flip response directionDirect pathway is responsible to the RF center
36 Indirect pathwayIndirect pathway mediated by horizontal is responsible to the RF surroundThere are other indirect pathway mediated by amacrine cells
37 Summary of retinal circuit Direct pathwayIndirect pathwayPathways mediated by amacrine cells
38 The Nobel Prize in Physiology or Medicine 1981: Roger W.Sperry: for his discoveries concerning the functional specialization of the cerebral hemispheresDavid H. Hubel & Torsten N. Wiesel: for their discoveries concerning information processing in the visual system
39 Terpenes in Vision Can diet affect sight Terpenes in Vision Can diet affect sight?.....It might depend on what you’re looking at.Color vision vs. B&W: Rods vs. ConesThe Photochemical ProcessLycopenesCarotenesVitamin ARetinol / RetinalHumans only see in B&W at night, but some animals like the nocturnal hawkmoth see color. Nature, , 2002
40 orange-red pigment in tomatoes Lycopeneorange-red pigment in tomatoeslmax 505 nm5
61 Terpenes Class Number of carbon atoms Monoterpene 10 Sesquiterpene 15 Classification of TerpenesClass Number of carbon atomsMonoterpene 10Sesquiterpene 15Diterpene 20Sesterpene 25Triterpene 30Tetraterpene 40