18-19_UV-Vis Molecular Absorption Spectroscopy.pptx

Uv vis

1. Chem 550 1 Figure 6.1  Basic schematic of a UV-vis spectrophotometer. UV-Vis Molecular Absorption Spectrometry UV-vis spectrum

2. Chem 550 2 Measurement of Transmittance and Absorbance Skoog Figure 13-1 To compensate attenuation effects, the transmittance of the analyte solution is compared with that of solvent in an identical cell.   The power of radiation passed through the sample The power of radiation passed through the blank, which is a sample that does not have any of the absorbing species you wish to measure)

3. Measurement of Transmittance and Absorbance   For non-interacting multiple component systems Beer’s law Chem 550 3 What is the factors that influence the absorbance of the sample? Is each factor directly or inversely proportional to the absorbance? Concentration Pathlength Molar absorptivity or molar extinction coefficient

4. Using Beer’s Law to calculate the concentration If you know the extinction coefficient, Chem 550 4 Figure 6.12  UV-vis spectrum of 1-10-phenanthroline-5,6-dione platinum(IV) chloride [Pt( dione )Cl 4 ] in dry acetonitrile. Absorbance A = 0.145 **290.5 nm seem to be a typo. It should be ~315 nm. What is the concentration of the compound? The path length of the cell is 1 cm.

5. Chem 550 5 Using Beer’s Law to calculate the concentration If you do not know the extinction coefficient, If you do not know the molar extinction coefficient of your compound, you can determine the concentration by drawing a standard curve. The slope is e b (recall A = e bc ). If it is measured with a cell with 1-cm-path length, the slope is the molar absorptivity. Measure your unknown. Calculate the concentration using the molar absorptivity determined above. Alternatively, you can calculate the concentration with the equation for the standard curve (recall excel basics in 457). Do not extrapolate the lower or higher concentration range where no standard concentration was measured! It could be nonlinear.

6. Chem 550 6 Figure 6.13  A representation of the linearity of Beer’s law as a function of concentration. Beer’s law is generally linear below one absorbance. Above one absorbance, the slope of the Abs. vs. Conc. line usually bends towards zero. 1. Internal Screening from too high absorbance > 1 The concentration is so high that some of the absorbing species lie within the “ shadow ” of other absorbing species and are hidden from radiant source. Figure 6.14  A representation of internal screening. Solution: Keep the absorbance below 1. Deviation from Beer’s law

7. Deviation from Beer’s law Chem 550 7 If molar absorptivity, Beer’s law deviates at high concentration. interaction between solutes change the Beer’s law describes the absorption behavior of media at low analyte concentration (<10 mM). Refractive index change of a solution. The absorptivity depends on the index of refraction (n). e ’ = e n /(n 2 +2) 2 2. Concentration limit

8. UV-Vis Molecular Absorption Spectrometry Limitations to Beer’s law Chem 550 8 2. Apparent Chemical Deviations Apparent deviations from Beer’s law arise when an analyte dissociates, associates, or reacts with a solvent to produce a product with a different absorption spectrum than the analyte. See the example 13-1. Color-changing acid indicator. HIn H + + In -

9. Chem 550 9 3. Instrumental Deviations due to Polychromatic Radiation Beer’s law strictly applies only when measurements are made with monochromatic source radiation. It is not possible to get purely monochromatic radiation using a dispersing element with a slit. The sample has a slightly different molar absorptivity for each wavelength of radiation. When the molar absorptivities are not the same for composing wavelengths, the total absorbance added over all the different wavelengths is no longer linear with concentration.     When Deviation from Beer’s law

10. Deviation from Beer’s law Chem 550 10 3. Instrumental Deviations due to Polychromatic Radiation Skoog Fig 13-4 Deviations from Beer’s law with Polychromatic radiation. Absorber has the indicated molar absorptivities at the two wavelengths l’ and l”.

11. Deviation from Beer’s law Chem 550 11 3. Instrumental Deviations due to Polychromatic Radiation Select the band of wavelengths selected for spectrophotometeric measurements such that the molar absorptivities are essentially constant. Select a wavelength band near the wavelength of maximum absorption. Monochromator or filter ∆ eff < 1/10 of the absorption band of full width at half maximum Fig 13-4 Skoog Fig 13-5 Polychromatic band A causes less band Beer’s law deviation because of less ∆ ℇ within band A. Make a question in Wenzel text figure 1.6.

12. Deviation from Beer’s law Chem 550 12 Grainger Figure 6.15  UV-vis spectrum of mavidin 3-O-glucoside. At λ max a bandwidth of D λ = 6nm gives nearly the same absorption value (the top of the peak is almost flat) but at 560 nm a bandwidth of D λ = 6nm gives us a range of absorption values of D A = (4.5 × 10 –3 – 2.3 × 10 –3 ) = 2.2 × 10 –3 Abs. 16% error

13. Instrumental Analysis Granger, Yochum, Granger, and Sienerth Copyright © 2017 Oxford University Press Grainger Figure 6.19  Schematic representation of a single-beam scanning UV-vis spectrometer. The source carousel is used to select between a tungsten–halogen lamp (visible) and a D 2 lamp (ultraviolet). Instrument: single-beam UV-Vis spectrometer

14. Instrumental Analysis Granger, Yochum, Granger, and Sienerth Copyright © 2017 Oxford University Press Grainger Figure 6.20  A schematic of a dual-beam spectrophotometer. From the source to the beam splitter, the dual-beam spectrometer is conceptually identical to the single-beam spectrometer. Instrument: double-beam UV-Vis spectrometer

15. UV-Vis Molecular Absorption Spectrometry Limitations to Beer’s law Chem 550 15 4. Instrumental Deviations in the Presence of Stray Radiation Stray beam: Outside the nominal wavelength band Scattering, reflection off the surfaces of grating, lenses or mirrors, filters, and windows. Always negative absorption error

16. Instrument Component: Source Chem 550 16 To measure absorption spectrum, the UV-Vis spectrometer requires continuum source whose radiant power does not sharply change over a considerable range of wavelengths. Deuterium and Hydrogen Lamps: UV region (190-400 nm) Electrical discharge lamp. Formation of excited molecular deuterium followed by dissociation into atomic deuterium produces various wavelengths of UV light.

17. Chem 550 17 Tungsten Filament Lamps: Visible region (350-2500 nm) Spectrum follows black-body radiation Tungsten filament lamps. Operated at 2870K. Tungsten-halogen lamps (Quartz-halogen lamp). Operated at 3500K. Enhanced lifetime because sublimated W forms a volatile product WI 2 that allow W to be redeposited to the filament. LED: White LED (400-800 nm) or semimonochromatic Xenon Arc Lamps: 200-1000 nm. Instrument Component: Source

18. Instrumental Analysis Granger, Yochum, Granger, and Sienerth Copyright © 2017 Oxford University Press Figure 6.27  (left) A schematic of an eight-stage PMT and (right) a photograph of a PMT. The incident photon dislodges an electron from the scintillator, which initiates a cascade of ejected electrons as they proceed toward the collector. Instrument Component: Detector Photomultiplier tube

19. Chem 550 19 Figure 6.29  An MOS semiconductor used in a CCD detector. Instrument Component: Detector Charge Coupled Device (CCD) CCD is used in an array format. No exit slit in monochromator.

20. Chem 550 20

21. Supplementary discussion of electromagnetic radiation

22. Interaction with light and matter n = 1 n > 1 n = 1 Light travels more slowly in matter. The ratio of the speed of light in vacuum (c) to the speed of light in a material ( c material ) is called the index of refraction (n). n = c/ c material Air Dielectric medium Air Consequences: l n = l /n c n = c /n Frequency v remains the same

23. Reflected wave  r Refraction by an Interface Refractive index n 1 = 1 Speed = c Refractive index n 2 Speed = c / n Incident wave  1 Refracted wave  2 Snell’s law:   / n Mirror law:  r =  1 n 1 Sin(  1 ) = n 2 Sin(  2 )

24. Refraction by an Interface Refractive index n 1 = 1 Refractive index n 2 =1.45 Incident wave  1 = 45   Refracted wave  2   / n Q. What is the angle refracted wave?

25. UV-Visible Spectroscopy

26. Absorption of UV-Vis radiation by atoms vs molecules Chem 550 26 Emission spectrum of helium shows line (or peak) nature of an atomic spectrum.

27. Absorption of UV-Vis radiation by atoms vs molecules Chem 550 27 Atomic absorption Molecular absorption Molecular energy levels

28. Chem 550 28 Molecular absorption shows continuum spectrum due to collisional broadening Liquid at room temp Crystalline solid and low temp

29. Possible electronic transitions in organic compounds Chem 550 29 The  σ  to σ*   transition requires an absorption of a photon with a wavelength which does not fall in the UV-vis range (~ < 130 nm). Thus, only  π  to  π*   and  n to  π * transitions occur in the UV-vis region are observed (>160 nm).

30. The effect of conjugation in UV-Visible absorption Chem 550 30 Anti-bonding bonding D E Non conjugated 176 nm

31. Chem 550 31 Conjugated The effect of conjugation in UV-Visible absorption The energy diagram for the orbitals when two adjacent p and p *-orbitals are conjugated                 The possible alignments of the signs of the wave functions of the p-orbitals in 1,3-butadiene

32. The effect of conjugation in UV-Visible absorption Chem 550 32 292 nm

33. Structure and color in conjugated systems Chem 550 33 Figure 6.5  UV-vis spectra of anthracene and tetracene . The energy level in a ”Particle in a box” h = Planck’s constant n = positive whole number representing the quantum state m = mass L = length of the “box” L increases as the degree of conjugation increases. In other words, the energy gap between p and p * decreases.

34. Effect of non-bonding electrons

35. Chem 550 35