Luís Santos

Transkript

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2 Solar Radiation

3 UV-VIS Spectroscopy Near UV: nm Vis: nm ( ev) These wavelengths are most widely used for routine analytical work since they give little structural information. Chromophore: group of atoms responsible for an electronic absorption. Ex: alkenes, carbonyl, carboxyl The Beer-Lambert law applies and is used for determining the concentration of the unknown. Sometimes a standard is used and a standard curve is constructed. Sometimes we have no standard, but the molar absorptivity is known. A = ε c l

4 Absorption Beer`s law (I) Transmission 100% 50% C 100% 25% 2C 100% 100% 50% 25% 3C 12.5% 100% 4C 6.75% 100% 5C 3.125% Concentration

5 Absorption Beer`s law (II) T = I/I 0 = e -αl A = log 1/T = log I o /I = εcl α absorption coefficient [cm -1 ] ε molar absortivity [dm 3 mol -1 cm -1 ] ε = f (substance, temperature, wavelength, solvent) c concentration of absorbent species [mol dm -3 ] optic path [cm]

6 Absorption Beer`s law (III)

7 Calibration curve A Absorvância da amostra Concentração da amostra Concentração

8 Two-Component Mixture Example of a two-component mixture with little spectral overlap

9 Two-Component Mixture Example of a two-component mixture with significant spectral overlap

10 Beer`s law The absorbance at any λ of a mixture, is the sum of the absorbencies of each component at that λ. Usually one measure the absorbencies at a number of λ = to the number of components, at the λ of maximum absorbance of each component. Ex: λ A 1 λ x y A 1 λ x A 1 λ λ 1 ( + ) = + y x x y =ε 1c l +ε cy l λ A 2 λ x y A 2 λ x A 2 λ λ 2 ( + ) = + y x x y =ε 2 c l +ε cy l After calibration with concentration standards of the pure components, at the selected λ, one can determine c x e c y.

11 UV-VIS Spectroscopy Electronic Transitions photon absorption promotes an electron from a bonding to an antibonding orbital Selection Rules 1) no change in spin orientation. 2) the change in the quantum number of the angular momentum must be 0 or ±1.

12 H 2 Molecule

13 H 2 Molecule

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15 UV/Vis absorptions

16 Energy Levels and Transitions E n e rg y E n e rg y σ π n π σ σ σ π π n σ n π e le c tro n ic e n e rg y le v e ls p o s s ib le e le c tro n ic tra n s itio n s

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18 How to Approach UV-Vis Data UV-Vis data alone gives little structural information UV-Vis data more useful when general idea of structural features are known; then empirical rules can be applied. 1) single band low intensity (ε 100 to 10,000) λ max < 220 nm usually n σ* possible groups: alcohols, ethers, amines, thiols, etc. 2) single band low intensity (ε 10 to 100) 250 nm < λ max < 360 nm usually n π* without absorption at lower wavelength simple chromophore

19 How to Approach UV-Vis Data 3) two bands medium intensity (ε 1,000 to 10,000) both λ max > 200 nm usually π π* of aromatic system look for fine structure in longer wavelength band 4) bands of high intensity (ε 10,000 to 20,000) λ max > 220 nm usually conjugated π system check dienes and α,β-unsaturated carbonyls

20 How to Approach UV-Vis Data 5) band of low intensity λ max > 300 nm (n π*) and band of high intensity λ max < 250 nm (π π*) unconjugated ketones, esters, acids, etc. 6) organic compounds that are highly colored are likely to have a long conjugated system or a polynuclear aromatic core. Some benzenoid compounds are colored if they have enough conjugating substituents.

21 How to Approach UV-Vis Data 7) Used for routine analysis. Ex: Hemoglobin concentration in blood. 8) UV/Vis can be a complement to IR. 9) Determination of the electronic gap.

22 Dispersive Spectrometer

23 Monochromator = Dispersion element plus slit system Acronym: ROYGBIV

24 Monochromator = Grating Radiation Source Exit slit

25 COLOUR The visible range in the electromagnetic spectrum is defined in terms of the waveleghts that the human eye is sensitive to. This range goes from ~ 380 nm (violet) to ~ 780 nm (red). Violeta: nm Indigo: nm Azul: nm Verde: nm Amarelo: nm Cor de Laranja: nm Vermelho: nm

26 COLOUR When the white light passes or is reflected by a substance, part is absorved. The remaining light will assume the complementary color of the absorved wavelenghts.

27 Transparency range

28 Transparency range Sílica Used in windows, lenses, fibres and prisms. Transparent in the UV, Vis and IR. Transmission range: 185 ~ 2500 nm Saphire Al 2 O 3 crystal with good transmission in the UV, Vis and IR. Transmission range: 180 ~ 4500 nm CaF 2 Crystal used in windows, lenses and prisms. Transparent in the UV, Vis and IR. Transmission range: 170 ~ 7800 nm

29 Filters A band-pass filter is a device where frequencies within a certain range go through and rejects (attenuates) frequencies outside that range. A band-stop filter or band-rejection filter is the opposite. A notch filter is a band-stop filter with a narrow band, the stopband

30 Edge pass filter

31 Edge pass filters (high-pass filters and low-pass filters) Edge pass filters provide a sharp cut-off either above or below a particular wavelength. They are used to pass, or transmit, a range of wavelengths and to block, or reflect, other wavelengths. High-pass filter: filter that transmits high frequencies but attenuates the frequencies lower than the filter's cutoff frequency. It cuts the frequencies lower than the cutoff frequency. Low-pass filter: filter that transmits low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. It cuts the frequencies higher than the cutoff frequency. 3% These filters are specified by the frequency at which 50% is transmitted and the transition range is typically lower than 6% of the cutting frequency.

32 Application Example Photonic Crystals are artificially made structures produced in an optical material (crystal or amorphous) with variations of its index of refraction at a periodicity on a wavelength scale. There is an analogy between electrons in a semiconductor and photons in a photonic crystal, i.e. at a certain range of frequencies photons are forbidden to propagate in 1, 2 or 3 directions. Like in semiconductors, there is a photonic band gap. By introducing "photonic defects" into the regular lattice of a photonic crystal the optical properties may be tailor-made, leading to innovative optical devices in the up-coming future.

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35 Photonic Crystals Transmittance Wavelength (nm)

36 Photonic materials - Visible Transmittance Wavelength (nm)

37 Absorção no UV/Vis (UV cut-off)

38 UV-Vis spectra Absorbance % Er 1% Er 2% Er 5% Er 6% Er Comprimento de onda (nm)

39 Rapid Determination of Phenolic Compounds in Red Wine Fermentations

40 Phenolic Compounds in Red Wine O-R 2 HO O O O H R 1 H H Anthocyanins H HO HO O O O-Galla te Tannin R 3 O H HO O HO HO O HO O-GLU COS IDE O O Polymeric Pigment LPP / SPP Non-Tannin Phenols Diverse class of compounds HO O-Galla te O Dihydroxy functionality

41 UV-visible spectra of phenolic compounds Hydroxycinnamic acids Flavonols Benzoic acids Anthocyanins

42 Conclusion Analyze individual phenolic compounds in red wine Collect data and & use calibration curves Predict anthocyanins, pigmented polymers, tannins, and non-tannin phenols. Effective for juices, fermenting musts and wine.