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Spectroscopic Methods the Electromagnetic Spectrum Lecture 11

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Spectroscopic Methods the Electromagnetic Spectrum Lecture 11 12/05/2019 Spectroscopic methods The Electromagnetic Spectrum Lecture 11 1 2 Frequency, wavelength, energy Velocity of light, 3 . 108 m s-1 Wavelength (m) • Wavelength is defined as the distance between adjacent maximal values of waves and may be presented in meters, centimeters or frequency energy nanometers (10-9 meters). Planck’s constant, 6.6 .10-34 J s • Frequency is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz (Hz). 3 4 1 12/05/2019 Quiz 1. The shortest length has: a) gamma rays; b) X-rays; c) UV; d) VIS; e) IR; f) microwave; f) radio wave 2. The smallest energy exhibits…. The energy associated with a given segment of the spectrum is proportional to its frequency. 5 6 Quiz Quiz Calculate the energy of UV rays with λ = 340nm. What is the wavelength of rays with energy 3.2 .10-16 J. A) 6.19 .10-20m; b) 6.19 .10-10m; c) 61.9 .nm; d) 619 nm ΔE = 5.82 . 10-19 J 7 8 2 12/05/2019 decrease Photon e nergy nergy UV-Vis spectroscopy Physical Chemistry; Understanding our Chemical World. Paul Monk. Manchester Metropolitan University, UK; John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England 9 10 The energies of the UV - VIS range of spectrum are sufficient to promote or excite a molecular electron to a higher energy orbital. Consequently, absorption spectroscopy carried out Theoretical background in this region is sometimes called "electronic spectroscopy". 11 12 3 12/05/2019 LUMO HOMO Comparison of the transitions met most frequently with simple organic compounds. The four types of transition are united on a single energy diagram in order to situate them with respect to each other and to correlate them with the corresponding spectral ranges concerned. 13 14 The graph of absorbance (A) versus wavelength the isoprene spectrum . Since isoprene is colorless, it does not absorb in the visible part of the spectrum and this region is not displayed on the graph. 16 4 12/05/2019 Chromophore Example Excitation λmax, nm ε Solvent Chromophores C=C Ethene π __> π* 171 15,000 hexane C≡C 1-Hexyne π __> π* 180 10,000 hexane The functional groups of organic compounds n __> π* 290 15 Hexane C=O Ethanal (ketones, amines, nitrogen derivatives, etc.), π __> π* 180 10,000 hexane responsible for absorption in UV/Vis range of the spectrum are called chromophores. n __> π* 275 17 Ethanol N=O Nitromethane π __> π* 200 5,000 ethanol Methyl bromide C-X X=Br n __> σ* 205 200 Hexane Methyl X=I n __> σ* 255 360 hexane Iodide 17 18 Isolated chromophores - do not interact with each other because they are separated by at least two single bonds in the skeleton, then the overlapping of the effects of each individual chromophore is observed. Conjugated chromophore systems - interact with each other and this process cause displacement of the absorption spectrum towards longer wavelengths (bathochromic effect) and additionally enhancement of the absorption intensity (hyperchromic effect). 19 20 5 12/05/2019 Terminology for Absorption Shifts Bathochromic and hyperchromic effects Nature of Shift Descriptive Term Change of To Longer Wavelength Bathochromic wavelength To Shorter Wavelength Hypsochromic Hyperchromic Hyperchromic To Greater Absorbance Hyperchromic Change of light intensity To Lower Absorbance Hypochromic 21 bathochromic 22 The comparison of systems conjugated chromophores in naphthalene, anthracene and tetracene. Visible light Hyperchromic Hyperchromic bathochromic 23 24 6 12/05/2019 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 Chlorophyll A ROY G BIV. Chlorophyll B carotenoids 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 http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm#uv2 25 26 Food colouring- Red3; Because the λmax of 524 nm (the green region of the spectrum) the compound appears red to our eyes 27 28 7 12/05/2019 Factors affecting the absorption •pH A common feature of all these coloured compounds, displayed below, is a system of extensively conjugated pi-electrons (chromophores).31 32 The pH of the solvent in which the solute is dissolved can have an important effect on its spectrum. Amongst the compounds that present this effect in a spectacular fashion are chemical indicator strips, whose change in color is used during acidimetric measurements. 33 34 8 12/05/2019 Hydrangea in acidic soil Hydrangea in alkaline soil. Absorption spectra of bromocresol green at different stages of protonation 35 36 Red cabbage juice as the pH indicator 1 2 3 4 5 6 Solvent type 1- high pH value (strong base), 2 – base; 3- neutral pH; 4- weak acid; 5 – medium acidic; 6- strong acid (low pH) 37 38 9 12/05/2019 • Different compounds may have very different absorption maxima and absorbances. Intensely absorbing compounds must be examined in dilute solution, so that significant light energy is received by the detector, and this requires the use of completely transparent (non- absorbing) solvents. • The most common solvents used in UV-VIS range of spectrum are: water, ethanol, hexane and cyclohexane. • Solvents having double or triple bonds, or heavy atoms (e.g. S, Br & I) are generally avoided. 39 40 Vis spectrum of 1,2,4,5 tetrazine. Spectrum of solute in a) gaseous state b) in hexane (nonpolar solvent) c) in water (polar solvent) From S.F. Mason, J. Chem Soc., 1959, 1265 41 42 10 12/05/2019 In spectroscopy, the transmittance T is a measure of the attenuation of a beam of monochromatic light. It is based upon the comparison between the intensities of the transmitted light (I) and the incident light I0. T is expressed as a fraction or a percentage: T = I/I0 or %T = I/I0 ×100 The absorbance (old name optical density) is defined by: Transmittance vs. absorbance A=2−log %T ; A = log T 43 44 • Absorption may be presented as transmittance or absorbance. If the sample Quiz compound does not absorb light of a given wavelength, I = I0. Thus no Calculate the absorbance of the compound when its absorption has occurred, T = 1.0 and A= 0. transmittance is equal 50%. • However, if the sample compound absorbs light then I is less than I0 and the transmittance or absorbance is observed. (T < 1; A > 0) • Most spectrometers display absorbance on the vertical axis, and the commonly observed range is from 0 (100% transmittance) to 2 (1% transmittance). 45 46 11 12/05/2019 UV-VIS spectroscopy equipment 47 48 • Light source. More than one type of source can be used in the same instrument which automatically swaps lamps when scanning between the UV and visible regions: • for the visible region of the spectrum, an incandescent lamp; • for the UV region a deuterium arc lamp (<350nm); • a xenon arc lamp can be used for routine apparatuses. http://devarchive.cnx.org/contents/02e7b3d6-cf47-4c92-a380-d011ce5658b1@1/basics-of-uv-visible-spectroscopy 49 50 12 12/05/2019 As the monochromator gratings the following tools may be used: (a) single concave spherical mirror Photodetectors (b) two spherical concave mirrors system (c) the concave grating • Photodiodes • Phototransistors • Semiconductors • photomultiplier 51 52 Quantitative analysis • The measurements are based upon the Lambert–Beer law which, under certain conditions, links the absorption of the light to the concentration of a compound in solution. UV –Vis analysis 53 54 13 12/05/2019 A= l C ε • A is an absorbance, • l is the thickness (in cm) of the solution through which the incident light is passed (optical path), • C the molar concentration of solution Absorption of the light by a homogeneous material and representation −1 −1 • ε the molar absorption coefficient (molar absorptivity) L mol cm at of percentage transmittance as a function of the material’s thickness. The light reaching the sample can be reflected, diffused, transmitted wavelength , at which the measurement is made. It characterizes the or absorbed. Here only this last fraction is taken into account. compound being analyzed. 55 56 Rose Bengal (4,5,6,7- tetrachloro-2',4',5',7'- tetraiodofluorescein) UV-vis spectra of different concentrations of Rose Bengal Calibration curve of Rose Bengal. Equation of line: y = 0.0977x – Illustration of the Lambert–Beer law. Spectra of aqueous solutions of 0.1492 (R2 = 0.996) increasing concentration in potassium permanganate. Graph of the corresponding absorbances measured at 525nm showing the linear growth of this parameter. 57 http://devarchive.cnx.org/contents/02e7b3d6-cf47-4c92-a380-d011ce5658b1@1/basics-of-uv-visible-spectroscopy 58 14 12/05/2019 This law applies when the following conditions are fulfilled: Quiz • the light used must be monochromatic Calculate the absorbance of an organic dye C =7×10−4 mol L−1, knowing that the • the concentrations must be low molar absorptivity ε= 650 L mol −1 cm−1 and that the length of the optical path of the cell used is 1cm. • the solution must be neither fluorescent or heterogeneous What would happen to the absorbance if the cell used was of triple its present thickness? • the solute must not undergo to photochemical transformations • the solute must not undertake variable associations with the solvent. 59 60 For all wavelengths, the absorbance of a mixture is equal to the sum of the absorbances of each component within the mixture (assuming the Additivity of absorbances same molar concentrations in the two experiments).
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