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Uv seminar ppt


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UV spectroscopy ppt

Uv seminar ppt

  2. 2. Contents1 INTRODUCTION To uv2 principle of uv-visible spectroscopy3 instrumentation of uv-visible4 Applications of uv-visible spectroscopy5 DERIVATIVE SPECTROSCOPY6 REFERENCES
  3. 3. “The study of interaction of electromagnetic radiation withmolecules/atoms ”. Types: 1)Absorption Spectroscopy: The study of absorbed radiation by molecule , in theform of spectra.Eg: UV, IR, NMR, colorimetry,Atomic absorption spectroscopy2)Emission Spectroscopy: The radiation emitted by molecules can also bestudied to reveal the structure of molecule.Eg:flame photometry, flourimetry
  4. 4. Study of spectroscopyAtomic spectroscopy: interaction of EMR+ATOMSChanges in energy take place at atomic levelEg: atomic absorption spectroscopy, flame photometryMolecular spectroscopy:Interaction of EMR + moleculesChanges in energy take place at molecular levelEg: UV, IR, colorimetryResults in transitions between vibrational,& rotational energy levels
  5. 5. The region beyond red is called infra-red while thatbeyond violet is called as ultra –violet.
  7. 7. .
  8. 8.  Ultraviolet: 190~400nm Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm
  9. 9. THEORY INVOLVED When a beam of light falls on a solution or homogenous media ,a portion of light is reflected ,from the surface of the media, a portion is absorbed within the medium and remaining is transmitted through the medium. Thus if I0 is the intensity of radiation falling on the media Ir is the amount of radiations reflected, Ia is the amount of radiation absorbed & It the amount of radiation transmitted then I0 = Ir + Ia + It
  10. 10. ABSORPTION LAWSLambert’s lawBeer’s lawBeer-lambert’s law
  11. 11. LAMBERT’S LAW When a beam of monochromatic light is passed through a homogenous absorbing medium, the rate of decrease of intensity of radiation with thickness of absorbing medium is proportional to the intensity of the incident light (radiation) . dI/dt = KI I= intensity of incident light of wavelength λ t= thickness of medium On integrating the equation & putting I=I0 We get In I0 / It =kt It = I0 e-kt I0 = denotes the intensity of incident light It =denotes the intensity of transmitted light K= constant which depend on λ & absorbing medium Convert the equation into natural logarithms i.e. lo base 10 It = I0 10-0.4343kt = I0 10-kt
  12. 12. BEER’S LAW Intensity of a beam of monochromatic light decreases exponentially with increase in conc. Of absorbing substance arithmetically. It = I0 e-kc It = I0 10-0.4343kc = I0 10-kc
  13. 13. BEER-LAMBERT’S LAW On combing the two laws, the beer-lambert law can be formulated as below It It log I0/I =€.c.l =A T %T x 100 Io Io light intensity (I) I0 =intensity of incident light I = intensity of transmitted light Io € =molar extinction co-efficient Io It C=conc. Of solution l L= path length of sample It cuvette l A = absorbance Sample depth
  14. 14. LIMITATIONS &DEVIATIONS keto-enol tautomers fluorescent compounds solute & solvent form complexesDeviations from beer-lambert’s law Real deviations Instrumental deviations Chemical deviations
  15. 15. UV-visible spectroscopy measurethe response of a sample to ultraviolet and visible range ofelectromagnetic radiation. Molecules have either n,π orElectrons.These electrons absorbUV radiation & undergoestransitions from ground state toexcited state.
  16. 16. The absorption of uv radiation brings about the promotionof an electron from bonding to antibonding orbital. The wavelength of radiation is slowly changed fromminimum to maximum in the given region, and theabsorbance at every wavelength is recorded. Then a plot ofenergy absorbed Vs wavelength is called absorptionspectrum. The significant features: λmax (wavelength at which there is a maximumabsorption) єmax (The intensity of maximum absorption) The UV spectrum depends on solvents concentration of solution
  17. 17. UV Spectroscopy Observed electronic transitions Here is a graphical representation Unoccupied levels Atomic orbital Atomic orbital Energy n Occupied levels Molecular orbitals 21
  18. 18. 1)σ- σ* Transition2) π-π* Transition3)n- σ* Transition4)n- π* Transition
  19. 19. Different types of Excitations
  20. 20. TYPES OF TRANSITIONS ALLOWED TRANSITIONS The transitions with the values of extinction co- efficient more than 104 are usually called allowed transitions. They generally arise due to π-π* Transition . Eg: In 1,3-butadiene molar extinction co-efficient is very high i.e.21000
  21. 21. TYPES OF TRANSITIONS2)FORBIDDEN TRANSITIONS: These transitions are as a result of the excitation of one electron from the lone pair present on the hetero atom to an anti bonding π* orbital. Eg: carbonyl compounds Molar extinction co-efficient value is 104
  22. 22. CHROMOPHORES Bathochromic shift (red shift) – a shift to longer wavelength; lower energy Hypsochromic shift (blue shift) – shift to shorter wavelength; higher energy Hyperchromic effect – an increase in intensity Hypochromic effect – a decrease in intensity
  23. 23. Chromophore Excitation max, nm Solvent C=C → * 171 hexane n→ * 290 hexane C=O → * 180 hexane n→ * 275 ethanol N=O → * 200 ethanol C-X n→ * 205 hexane X=Br, I n→ * 255 hexane
  24. 24. Solvent effect
  25. 25. WOODWARD-FEISER RULEWoodward (1941) : gave certain rules for correlating max with molecular structure Scott-Feiser (1959): modified rule with more experimental data, the modified rule is known as Woodward-Feiser rule used to calculate the position of max for a given structure by relating the position and degree of substitution of chromophore.
  26. 26. 1. Homoannular diene: cyclic diene having conjugated double bonds in the same ring. 2. Heteroannular diene: cyclic diene having conjugated double bonds in different ring
  27. 27. 2. Endocyclic double bond: double bond present in a ring3. Exocyclic double bond: double bond in which one of the doubly bonded atoms is a part of a ring system Ring A Ring B Ring A has one exocyclic and endocyclic double bond. Ring B has only one endocyclic double bond
  28. 28. Woodward-Feiser rule for conjugateddienes, triens, polyenes Each type of diene or triene system is having a certain fixed value at which absorption takes place; this constitutes the BASIC VALUE or PARENT VALUE The contribution made by various alkyl substituents or ring residue,double bonds extending conjugation and polar groups such as – Cl, -Br are added to the basic value to obtain max for a particular compound
  29. 29. Parent values and incriments for different substituent/groupsa) Parent valuei. Acyclic conjugated diene and : 215nm heteroannular conjugated dieneii. Homoannular conjugated diene : 253nmiii. Acyclic triene : 245nm
  30. 30. b) Increments i. Each alkyl substituents or ring residue : 5 nm ii. Exocyclic double bond : 5 nm iii. Double bonds extending conjugation : 30nm c) Auxochrome : -OR : 6 nm -SR : 30 nm -Cl, -Br : 5 nm -NR2 : 60nm -OCOCH3 : 0 nm
  31. 31. Calculate max for 1,4- dimethylcyclohex-1,3- diene H3C CH3 H3C CH3 Parent value for homoannular ring : 253 nm Two alkyl substituents : 2 * 5= 10 nm Two ring residue : 2 * 5= 10 nm calculated value : =273 nm observed value : = 263 nm
  32. 32. Calculate max Parent value for heteroannular diene : 215 nm Four ring residue : 4 * 5 = 20 nm calculated value : 235 nm observed value : 236 nm
  33. 33. Calculate max Parent value for heteroannular diene : = 215 nm Three ring residue : 3*5 = 15 nm One exocyclic double bond : = 5 nm Calculated value : = 235 nm Observed value : =
  34. 34. a) Parent valuesi. , -unsaturated acyclic or six membered ring : 215 nm ketoneii. , -unsaturated five – membered ring ketone : 202nmiii. , -unsaturated aldehyde : 207 nmb) Incrementsi. Each alkyl substituent or ring residue at position : 10
  35. 35. ii. Each exocyclic double bond : 5 nmiii.Double bond extending conjugation : 30 nmiv. Homoannular conjugated diene : 39 nmv. Auxochromes position -OH 35 30 50 -OR 35 30 17 -SR - 85 - -OCOCH3 6 6 6 -Cl 15 12 - -NR2 - 95 -
  36. 36. CALCULATE max  Parent value : 215 nm  One α ring residue : 10nm  One δ residue : 18nm  One double bond extending : 30nm conjugation • One homoannular conjugated diene : 39nm • One exocyclic double bond : 5nm • Calculated value : = 317nm • Observed value : = 319nm
  37. 37. A. 1. The construction of a traditional UV-VIS spectrometer is very similar to an IR, as similar functions – sample handling, irradiation, detection and output are required 2. Here is a simple schematic that covers most modern UV spectrometers: log(I0/I) = A I0 I sampleUV-VIS sources 200 700 , nm detector monochromator/ referenc beam splitter optics I0 I0 e 44
  38. 38. UV SpectroscopyII. Instrumentation and Spectra A. 3. Two sources are required to scan the entire UV-VIS band: • Deuterium lamp – covers the UV – 200-330 • Tungsten lamp – covers 330-700 4. As with the dispersive IR, the lamps illuminate the entire band of UV or visible light; the monochromator (grating or prism) gradually changes the small bands of radiation sent to the beam splitter 5. The beam splitter sends a separate band to a cell containing the sample solution and a reference solution 6. The detector measures the difference between the transmitted light through the sample (I) vs. the incident light (I0) and sends this information to the recorder
  39. 39. UV SpectroscopyII. Instrumentation and Spectra A. Instrumentation 7. As with dispersive IR, time is required to cover the entire UV-VIS band due to the mechanism of changing wavelengths 8. A recent improvement is the diode-array spectrophotometer - here a prism (dispersion device) breaks apart the full spectrum transmitted through the sample 9. Each individual band of UV is detected by a individual diodes on a silicon wafer simultaneously – the obvious limitation is the size of the diode, so some loss of resolution over traditional instruments is observed Diode array UV-VIS sources sample Polychromator – entrance slit and dispersion device 46
  40. 40. UV SpectroscopyII. Instrumentation and Spectra B. Instrumentation – Sample Handling 1. Virtually all UV spectra are recorded solution-phase 2. Cells can be made of plastic, glass or quartz 3. Only quartz is transparent in the full 200-700 nm range; plastic and glass are only suitable for visible spectra 4. Concentration (we will cover shortly) is empirically determined A typical sample cell (commonly called a cuvet):
  41. 41. UV SpectroscopyIII. Chromophores A. Definition 1. Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N 2. Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves 3. A functional group capable of having characteristic electronic transitions is called a chromophore (color loving) 4. Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions 48
  42. 42. UV SpectroscopyIII. Chromophores C. Substituent Effects General – from our brief study of these general chromophores, only the weak n  * transition occurs in the routinely observed UV The attachment of substituent groups (other than H) can shift the energy of the transition Substituents that increase the intensity and often wavelength of an absorption are called auxochromes Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens
  43. 43. UV SpectroscopyIII. Chromophores C. Substituent Effects General – Substituents may have any of four effects on a chromophore i. Bathochromic shift (red shift) – a shift to longer ; lower energy ii. Hypsochromic shift (blue shift) – shift to shorter ; higher energy iii. Hyperchromic effect – an increase in intensity iv. Hypochromic effect – a decrease in intensity Hyperchromic Hypsochromic Bathochromic Hypochromic 200 nm 700 nm
  44. 44. UV SpectroscopyIV. Structure Determination A. Dienes 2. Woodward-Fieser Rules - Dienes The rules begin with a base value for max of the chromophore being observed: acyclic butadiene = 217 nm The incremental contribution of substituents is added to this base value from the group tables: Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH3 +0 -OR +6 -SR +30 -Cl, -Br +5 -NR2 +60 51
  45. 45. UV SpectroscopyIV. Structure Determination A. Dienes 2. Woodward-Fieser Rules - Dienes For example: Isoprene - acyclic butadiene = 217 nm one alkyl subs. + 5 nm 222 nm Experimental value 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs. +10 nm 232 nm Experimental value 237 nm
  46. 46. UV SpectroscopyIV. Structure Determination B. Enones C C C C C C C C 2. Woodward-Fieser Rules - Enones O O Group Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue and higher 10, 12, 18 -OH and higher 35, 30, 18 -OR 35, 30, 17, 31 -O(C=O)R 6 -Cl 15, 12 -Br 25, 30 -NR2 95 Exocyclic double bond 5 Homocyclic diene component 39
  47. 47. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - Enones Aldehydes, esters and carboxylic acids have different base values than ketones Unsaturated system Base Value Aldehyde 208 With or alkyl groups 220 With or alkyl groups 230 With alkyl groups 242 Acid or ester With or alkyl groups 208 With or alkyl groups 217 Group value – exocyclic double bond +5 Group value – endocyclic bond in 5 +5 or 7 membered ring
  48. 48. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - Enones Unlike conjugated alkenes, solvent does have an effect on max These effects are also described by the Woodward-Fieser rules Solvent correction Increment Water +8 Ethanol, methanol 0 Chloroform -1 Dioxane -5 Ether -7 Hydrocarbon -11 55
  49. 49. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - Enones Some examples – keep in mind these are more complex than dienes cyclic enone = 215 nm O 2 x - alkyl subs. (2 x 12) +24 nm 239 nm Experimental value 238 nm R cyclic enone = 215 nm extended conj. +30 nm -ring residue +12 nm O -ring residue +18 nm exocyclic double bond + 5 nm 280 nm Experimental 280 nm 56
  50. 50. 1) Determination of structure of organic compound: Exam: Element, Functional group, etc.2) Determination of stereochemistry: Exam: Cis or Trans.3) Strength of Hydrogen bond: