Dong - Sun Lee / CAT - Lab / SWU http://mail.swu.ac.kr/~cat 2010 - Fall version Chapter 24 Spectrochemical methods
Richard P. Feynman (1918~1988) was one of the most well-known and renowned scientists of the 20th century. He was awarded the Nobel Prize in Physics in 1965. This composite image of sunspot group was collected with the Dunn solar telescope at the Sacramento Peak Observatory in New Mexico on Mar. 29, 2001. The lower portion, consisting of four frames, was collected at a wavelength of 293.4 nm. The upper portion was collected at 430.4 nm. The lower image represents calcium ion concentration, with the intensity of color proportional to the amount of calcium ion in the sunspot. The upper image shows the presence of the CH molecule.
Spectrophotometry Spectroscopy : the science that deals with the interaction of electromagnetic radiation with matter. Spectrometry : a more restrictive term, denotes the quantitative measurement of the intensity of electromagnetic radiation at one or more wavelengths with photoelectric detector. Spectrum (pl. spectra) : a display of the intensity of radiation emitted, absorbed, or scattered by a sample versus a quantity related to photon energy(E), such as wave length() or frequency(). Intensity wave length(, nm) or frequency(, cm–1). Spectrum
30 max=524nm 25 20 15 10 5 ppm CAT-Lab/SWU UV-visible absorption spectra of cefazolin antibiotics. Absorption spectra of Fe(III)-salicylic acid complex.
Plane-polarized electromagnetic radiation showing the electric field, and the direction of propagation. Electric field component of plane-polarized electromagnetic radiation.
Properties of light : Electromagnetic radiation ; EM wave ; radiation ; radient ray ; ray ; light Duality ; 1) Wave theory ------ Huygens = c wavelength (cm/cycle) × frequency (cycles/sec) = velocity (cm/sec) where wavelength, , is the length per unit cycle. Frequency, , is the number of cycles per unit time. C = 2.99792458 × 108 m/s is speed of light 2) Particle (energy packets ; photon) theory --- Newton E = h = hc / where E is the energy in joules (J) h is Plancks constant (6.62608 × 10 – 34 J s) 1 erg = 10 –7 J 1 eV = 1.6021 × 10 –19 J
Wave number, , is the number of cycles per unit length, cm. = 1 / = cm – 1 (reciprocal centimeter ; Kayser) = / c = E / hc Ex. 400 nmxeV ? E = h = hc / 6.63 10 – 34 J s 3.00 108m s –1 = 400 10 – 9 m 1.6 10 – 19 J/eV = 3.1 eV
Change in wavelength as radiation passes from air into a dense glass and back to air. Note that the wavelength shortens by nearly 200 nm, or more than 30%, as it passes into glass; a reverse change occurs as the radiation again enters air.
Regions of EM spectrum Designation Wavelength Energy or Transition range wave number Cosmic ray -ray X-ray Vacuum UV near UV Visible Near IR Middle IR Far IR Microwave Radio wave 10 – 12m 10 – 11m >2.5 105 eV 10 – 8m 124 eV 180 10 – 9m 7 eV 380 10 – 9m 3.3 eV 780 10 – 9m 1.6 eV 2500 10 – 9m 4000 cm– 1 50 10 – 6m 200 cm– 1 10 – 3m 10 cm– 1 0.3 m Nuclear K,L shell electron Middle shell Valence electron Molecular electron Molecular vibration Molecular vibration Molecular rotation Molecular rotation Electron, & nuclear spin
The visible spectrum Wavelength Color absorbed Color observed (nm) (complement) 380-420 Violet Green-yellow 420-440 Violet-blue Yellow 440-470 Blue Orange 470-500 Blue-green Red 500-520 Green Purple 520-550 Yellow-green Violet 550-580 Yellow Violet-blue 580-620 Orange Blue 620-680 Red Blue-green 680-780 Purple Green ROYG RIV Red, Orange, Yellow, Green, Blue, Indigo, Violet
The electromagnetic spectrum showing the colors of the visible spectrum.
Types of interaction between radiation and matter 1. Reflection & scattering 2. Refraction & dispersion 3. Absorption & transition 4. Luminescence & emission Emission or chemiluminescence Refraction Sample Sample Reflection Scattering and photoluminescence A B Absorption along radiation beam Transmission C Sample Types of interaction between radiation and matter.
Several spectroscopic phenomena 1) depend on transition between energy states of particular chemical species E* higher energy (excited state) E applied energy E olowest energy (ground state) 2) depend on the changes in the optical properties of EM radiation that occur when it interacts with the sample or analyte or on photon-induced changes in chemical form (e.g. ionization or photochemical reactions) Emission or Absorption Photoluminescence chemiluminescence A B C Antistokes Stokes Combination of nonradiative transition transition and radiative deactivation D E F Common types of optical transition. non-radiative process Radiative process non radiative
Absorption methods. Photoluminescence methods.
Absorption of EM radiation Sun Eye Visual center C P0 P b Source Monochromator Cuvet Detector P P – dP Incident light Emerging light db b = b Molar concentration [C] b = 0 b Absorption of EM radiation
Color of a solution. White light from a lamp or the sun strikes the solution of Fe(SCN)2+. The fairly broad absorption spectrum shows a maximum absorbance in the 460 to 500 nm range. The complementary red color is transmitted.
Reflection and scattering losses with a solution contained in a typical glass cell. Attenuation of a beam of radiation by an absorbing solution.
2 E2 = h2 = hc/2 1 Sample Incident radiation 0 Transmitted radiation E1 = h1 = hc/1 0 (a) (b) A 0 2 1 (c) Absorption methods. Radiation of incident power 0 can be absorbed by the analyte producing a beam of diminished transmitted power (a) if the frequency of the incident beam, 2 corresponds to energy difference, E1 or E2 (b). The spectrum is shown in (c).
Lambert Beer’s law Transmittance T = P / P0 %T = (P / P0) 100 Absorbance (A, O.D., E, As) A = log T = log P/ P0 Lambert’s law Lambert and Bouger found that the intensity of the transmitted energy decrease exponentially as the depth (b ; path length of the beam through the sample) increases. dP = k P db dP/P = k db dP/P = k db ln P/P0 = k b log P/P0 = (k/2.303) b A = log P/P0 = (/2.303) b T A Path length Path length Effect of path length on transmittance and absorbance of light.
Beer’s law Beer in 1852 found that concentration (C) is a reciprocal exponential function of transmittance and absorbance is directly proportional to the concentration. dP = P dC dP/P = dC dP/P = dC ln P/P0 = C log P/P0 = (/2.303) C A = log P/P0 = (/2.303) C Lambert - Beer’s law A = bC where is molar absorptivity A log T [C] [C] Effect of concentration of analyte on transmittance and absorbance of light.
Limitation Beer’s law 1. Concentration deviation ; A = log T = log P/P0 = bC (Eq 1) (0.434 / T) dT = b dC (Eq 2) Eq 2 ÷ Eq 1 (0.434 / T) dT log T dC / C = ÷ b b = (0.434 / T log T) dT C/C = (0.434 / T log T) T 4 C/C A 2 1 [C] T = 36.8 % A = 0.434 normal working range15%T(0.824A)~80%T(0.097A) Twyman Lothian curve
2. Refractive index deviation A = bC [ n / (n2 + 2)2] where n is refractive index 3. Instrumental deviation ; difficult to select single wavelength beam max The effect of polychromatic radiation on Beer’s law.
Choosing wavelength and monochromator band width.Increasing the monochromator bandwidth broadens the bands and decreases the apparent absorbance.
Deviation from Beer’s law caused by various levels of stray light. Absorbance error introduced by different levels of stray light.
4. Chemical deviation ; dissociation or reaction with solvent ex. Acidic form intermediate form basic form Chemical deviation from Beer’s law for unbuffered solution of the Indicator HIn. 5. Solvent deviation T = tsolution / tsolvent 6. Temperature ; narrower spectrum band at below 50C 7. Pressure ; gas phase sample
Typical visible absorption spectra of 1,2,4,5-tetrazine in different solvent. Errors in spectrophotometric measurements due to instrumental electrical noise and cell positioning imprecision.
CAT-Lab/SWU Absorption spectra of KMnO4
Energy level diagram showing some of the energy changes that occur during absorption of IR, VIS, UV radiation by a molecular species. Partial energy level diagram for sodium, showing the transitions resulting from absorbtion at 590, 330, and 285 nm
Electronic transitions • The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition which can be considered; • Transitions involving p, s, and n electrons • Transitions involving charge-transfer electrons • Transitions involving d and f electrons (not covered in this Unit) • When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each other. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level.
Types of the electronic transition Unoccupied levels(antibonding) * * LUMO Frontier orbital E n HOMO non-bonding bonding Occupied level Characteristics of electronic transitions. Transition Wavelength (nm) log Examples * < 200 >3 Saturated hydrocarbon n * 160~260 2~3 Alkenes, alkynes, aromatics E * 200~500 ~ 4 H2O,CH3OH, CH3Cl CH3NH2 n * 250-600 1~2 Carbonyl, nitro, nitrate, carboxyl (note) forbidden transition ; * , * James D. Ingle, Jr., Stanley R. Crouch, Spectrochemical Analysis, Prentice-Hall, NJ,1988, p. 335.
Absorbing species containing p, s, and n electrons Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy. The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band. Possible electronic transitions of p, s, and n electrons are;
* Transitions An electron in a bonding s orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo * transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to * transitions are not seen in typical UV-Vis. spectra (200 - 700 nm) n* Transitions Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n * transitions. These transitions usually need less energy than * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n * peaks in the UV region is small.
n* and * Transitions Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the p electrons. Molar absorbtivities from n* transitions are relatively low, and range from 10 to100 L mol-1 cm-1 . * transitions normally give molar absorbtivities between 1000 and 10,000 L mol-1 cm-1 .
The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species. Peaks resulting from n* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital. Often (but not always), the reverse (i.e. red shift) is seen for * transitions. This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced - resulting in a small red shift. This effect also influences n* transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.
Charge - Transfer Absorption Many inorganic species show charge-transfer absorption and are called charge-transfer complexes. For a complex to demonstrate charge-transfer behaviour, one of its components must have electron donating properties and another component must be able to accept electrons. Absorption of radiation then involves the transfer of an electron from the donor to an orbital associated with the acceptor. Molar absorbtivities from charge-transfer absorption are large (greater that 10,000 L mol-1 cm-1).
Energy level diagram for formaldehyde. Bonding in formaldehyde.
s * s * p * 2 p 2 p 2 sp n 2 sp p 1s s s 2 H CH C C=O O atoms fragment atom fragment atom 2 2 2 2 2 4 C 1 s 2 s 2 p O 1 s 2 s 2 p Energy level diagram for formaldehyde.
General guideline to the use of UV data (nm) Number of band Intensity(log ) Transition <270 Single band 2~4 n * Amines, alcohols, ethers, thiols <2 n * C N 250~360 Single band & 1~2 n * 200~250 no major absorption C=O, C=N, N=N, NO2, COOR, COOH, CONH >200 Two bands 3~4 Aromatic system >210 Bands 4 , -unsaturated ketone or diene, polyene (cf. Woodward-Fieser rule or Fieser-Kuhn rule) >300 Two absorptions low n * <250 high * Simple ketones, acids, esters, amides, other and n electrones (cf. Woodward rule or Nielson rule) Visible Highly colored compounds Long chain conjugated(4~5) chromophores Polycyclic aromatic chromophores Simple nitro, azo, nitroso, -diketo, polybromo, polyiodo, quinoid
Selected electronic transitions in organic molecules Absorption Molar maximum, absorptivity, Electronic max Compound transition (nm) (1 mol1 cm 1) Ethane * 135 Water n * 167 7000 Methanol n * 183 500 Ethane * 165 16500 Acetone * 150 n * 188 1860 n * 279 15 Benzene * 180 60000 * 200 8000 * 255 215 Phenol * 210 6200 * 270 1400
Common solvents used in UV and their transparencies Minimum wavelength Approximate for 10 nm cell transparency Solvent (nm) region (nm) Water 190 180-200 Cyclohexane 195 210-400 Hexane 200 205-400 Methanol 200 205-400 Ethanol 200 210-400 Dichloromethane 220 210-400 Chloroform 240 250-400 Dioxane 190 220-400
Absorption characteristics of saturated compounds with hetero atoms (n*) transmission Absorption maximum Molar absorptivity Compound max (nm) (1mol-1cm-1) Solvent Chloromethane 173 200 Hexane Methanol 177 200 Hexane Di-n-butyl 210 1200 Ethanol sulphide Trimethyl lamine 199 3950 Hexane Methyl iodide 259 400 Hexane Diethyl ether 188 1995 Gas phase 171 3982 Gas phase
Absorption data for conjugated alkenes ( transition) Absorption maximum Molar absorptivity Compound max (nm) (1mol-1cm-1) Sorvent 1,3-Butadiene 217 21000 Hexane 1,3,5-Hexatriene 253 -50000 Isooctane 263 52500 Isooctane 274 -50000 Isooctane 1,3-Cyclohexadiene 256 8000 Hexane 1,3-Cyclopentadiene 239 3400 Hexane