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Principles of Spectroscopy Interaction of Radiation and Matter If Matter Is Exposed to Electromagnetic Radiation, E.G

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Principles of Spectroscopy Interaction of Radiation and Matter If Matter Is Exposed to Electromagnetic Radiation, E.G Principles of Spectroscopy Interaction of radiation and matter If matter is exposed to electromagnetic radiation, e.g. infrared light, the radiation can be absorbed, transmitted, reflected, scattered or undergo photoluminescence. Photoluminescence is a term used to designate a number of effects, including fluorescence, phosphorescence, and Raman scattering. Matter Photoluminescence Incident light beam Absorption Transmission Reflection Scattering Electromagnetic Spectrum Type of Radiation Frequency Wavelength Range Type of Transition Range (Hz) Gamma-rays 1020-1024 <10-12 m nuclear X-rays 1017-1020 1 nm-1 pm inner electron Ultraviolet 1015-1017 400 nm-1 nm outer electron Visible 4-7.5x1014 750 nm-400 nm outer electron Near-infrared 1x1014-4x1014 2.5 mm-750 nm outer electron molecular vibrations Infrared 1013-1014 25 mm-2.5 mm molecular vibrations Microwaves 3x1011-1013 1 mm-25 mm molecular rotations, electron spin flips* Radio waves <3x1011 >1 mm >1 mm The complement of the absorbed light gets transmitted. The color of an object we see is due to the wavelengths transmitted or reflected. Other wavelengths are absorbed. The more absorbed, the darker the color (the more concentrated the solution). In spectrochemical methods, we measure the absorbed radiation. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) The distance of one cycle is the wavelength (l). The frequency (n) is the number of cycles passing a fixed point per unit time. l = c/n (c = velocity of light, 3 x 1010 cm s-1). The shorter the wavelength, the higher the energy: E = hn This is why UV radiation from the sun burns you. Fig. 16.1. Wave motion of electromagnetic radiation. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) We see only a very small portion of the electromagnetic spectrum . In spectrochemical methods, we measure the absorption of UV to far IR radiation. UV = 200-380 nm VIS = 280-780 nm IR = 0.78 mm-300 mm Visible Fig. 16.2. Electromagnetic spectrum. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) TYPES OF OPTICAL INSTRUMENTS • Spectroscope: uses human eye as a detector • Spectrograph: photographic emulsion used as detector • Spectrometer: has photoelectric readout 1. Monochromator: one exit slit, Greek for "one color" 2. Polychromator: multiple exit slits • Spectrophotometer:electronics takes ratio of two beams (%T), may be at same or different wavelengths, may be single beam or double beam Atomic versus Molecular Transitions Various Relaxation (de-excitation) Modes • Relaxation by emission of the same wavelength – atomic – refer back to the emission spectra of brine • Non-radiative – molecular usually • Fluorescence – molecular usually • Phosphorescence – molecular usuall Phosphorescence • A molecule is excited by EM radiation • A transition takes place from some state (usually ground) to an excited state • Relaxation back to that ground state takes place over relatively long periods – The excited state is actually a metastable state, meaning that it is more stable than an excited state but still less stable (thermodynamically) than the ground state – E-5 seconds to minutes or hours after excitation • Chemiluminescence – light sticks, etc…... Fluorescence and Phosphorescence Instruments….. Excitation Beam Emitted Beam (usually @ < E, > wavelength) Detector Fluorescence • Resonance Fluorescence – Usually atomic – Emitted light has same E as excitation light – Simpler, atomic systems with fewer energy states (vs molecules) undergo resonance fluorescence • Not as widely used in analytical chemistry as non-resonance fluorescence – Hg analysis is one example Excitation Beam Emission (identical) Non-resonance Fluorescence • Typical of molecular fluorescence • Large number of excited states – rotational – vibrational – etc.. • Molecules relax by ‘stepping’ from one state to another • Resulting emitted light “shifts” to lower energies – longer wavelengths – Stokes Shift Excitation Beam Emission (lower E shift) Some Basic Concepts…... • Why are even “line” spectra not truly lines? – They are really broad distributions that are just over a range of about 1 nm or less. • Some of this (especially with respect to lines) is due to the uncertainty principle! • Remember, than an atom or molecule does not go from one distinct energy state to another – it goes from some “high probability’ state to another “high probability” state – we can never know the exact energy – limited by h/t – Heisenberg’s Uncertainty Principle in action! Absorption of Light by a Sample in UV-Vis and IR Spectroscopy Incident Transmitted beam I or P o o beam I or P Quantitative Relationships for Optical Spectroscopy • Beer’s Law (you should know) • Definitions: P0 = incident light intensity, P = transmitted light intensity • Transmittance: I T %T 100 x T • Absorbance Io – A = abc “c” in gm/l – A= εbc “c” in moles/l • bC = cm*mol/1000 cm3 = mol/1000 cm2 • a units cm2/gm ε unit = cm2/mol • (old literature often dm2/gm) I A - log T log o bc I Limitations of the Beer-Lambert law The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of nonlinearity include: • deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity • Interaction with solvent: hydrogen bonding • scattering of light due to particulates in the sample • fluoresecence or phosphorescence- a positive deviation in % T and negative deviation for A • changes in refractive index at high analyte concentration • shifts in chemical equilibria as a function of concentration • non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band • stray light Chromophores and Auxophores Group ν (10 cm-1) λ (nm) ε (L mol-1cm-1) C=C 55 182 250 57.3 174 16,000 58.6 170 16,500 62 162 10,000 58 172 2,500 C=O 34 295 10 54 185 Strong C=S 22 460 Weak 36 277 10 -NO2 47.5 210 10,000 -N=N- 28.8 347 15 >38.5 <260 Strong C6H5 39 255 200 50 200 6,300 55.5 180 100,000 Energy Levels in UV-Vis Molecular Spectroscopy Electronic Transitions in UV Region Wavelength Functional Group Transition 177 nm -C=C- pi -----> pi* 178 C=C p i-----> pi* 280 -C=O n ----->sigma *, n-----> pi * 204 -COOH n -----> pi * 214 -CNO (amide) n -----> pi * 339 -N=N- n -----> pi * 280 -NO2 n -----> pi * 270 -NO3 n -----> pi * Chromophores and auxophores Group ν (10 cm-1) λ (nm) ε (L mol-1cm-1) -Cl 58 172 - -Br 49 204 1800 -I 38.8 258 - 49.7 201 1200 -OH 55 183 200 67 150 1900 -SH 43 232 160 -NH2 46.5 215 580 52.5 190 3200 -S- 44 228 620 46.5 215 700 49.3 203 2300 These groups absorb in the UV or visible regions. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Absorption Characteristics of Aromatic Compounds Compound E2 Band B Band lmax (nm) emax lmax(nm) emax Benzene C6H6 204 7,900 256 200 Toluene C6H5CH3 207 7,000 261 300 M-Xylene C6H4(CH3)2 ------ ------ 263 300 Chlorobenzene C6H5Cl 210 7,600 265 240 Phenol C6H5OH 211 6,200 270 1,450 - Phenolate ion C6H5O 235 9,400 287 2,600 Aniline C6H5NH2 230 8,600 280 1,430 + Anilinium ion C6H5NH3 203 7,500 254 160 Thiophenol C6H5SH 236 10,000 269 700 Naphthalene C10H8 286 9,300 312 289 Styrene C6H5CH==CH2 244 12,000 282 450 Effect of Ligands on Absorption Maxima Associated with dd Transitions Central Ion lmax(nm) for the Indicated Ligands Increasing Ligand Field Strength - - 6Cl 6H2O 6NH3 3en 6CN Cr(III) 736 573 462 456 380 Co(III) ---- 538 534 428 294 Co(II) ---- 1345 980 909 ----- Ni(II) 1370 1279 925 863 ----- Cu(II) ---- 794 663 610 ----- PMT: Photomultiplier Tubes Single Beam Double Beam Absorption Measurements • Procedure • Problems 1) Set 0 % T to record 1) Scattering dark current---- block light path 2) Reflection 2) Set 100 % T --- 3) Inhomogeneities record pure solvent 4) Stray light 3) Measure sample signal --- determine T or % T or A Theory of Vibrational Spectroscopy The model of molecular vibrations is given by the anharmonic oscillator. The potential energy is then calculated by the Morse equation, and is asymmetric. The energy levels are no longer equally spaced, and are given by: 2 Ev=(v + ½) h - (v + ½) xGl h where xGl is the anharmonicity constant. The anharmonic oscillator model allows for two important effects: 1) As two atoms approach each other, the repulsion will increase very rapidly. 2) If a sufficiently large vibrational energy is reached the molecule will dissociate (break apart). This is called the dissociation energy. Potential energy curve for an anharmonic oscillator In the case of the anharmonic oscillator, the vibrational transitions no longer only obey the selection rule v = 1. This type of vibrational transition is called fundamental vibration. Vibrational transitions with v = 1, 2, 3, ... are also possible, and are termed overtones. Infrared Spectrometer Designs Dispersive IR (top) Michelson Interferometer For FTIR (bottom) Origin of the interferogram Nine wavelengths Since spectrometers are equipped with a polychromatic light source (i.e. many wavelengths) the interference already mentioned occurs at each wavelength, as shown in the upper figure on the right. The interference patterns produced by each Optical retardation wavelength are summed to get the resulting interferogram, as Resulting detector signal: shown in the second figure. At the zero path difference of the moving mirror (Dx=0) both paths all wavelengths have a phase difference of zero, and therefore undergo constructive interference. The intensity is therefore a maximum value. As the optical retardation increases, each wavelength undergoes constructive and destructive interference at different mirror positions. Optical retardation The third figure shows the intensity as a function of frequency (I.e. the spectrum), and we now have nine lines. Spectrum consisting of 9 single frequencies Frequency Origin of the interferogram Spectrometers are equipped with a broadband light source, which yields a continuous, infinite number, of wavelengths, as shown in the figure on the left.
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