Determination of Chemical Composition and Molecular Microstructures: Infrared Spectroscopy

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1 Determination of Chemical Composition and Molecular Microstructures: Infrared Spectroscopy 1

2 Infrared Spectroscopy Infrared spectrometry is applied to the qualitative and quantitative determination of molecular species of all types. The most widely used region is the mid-infrared that extends from about 200 to 4000 cm -1 (2.5 to 50 m). The near-infrared region from 4000 to 12,800 cm -1 (0.78 to 2.5 m) also finds considerable use for the routine quantitative determination. The far-infrared region has been for the determination of the structures of inorganic and metal-organic species wavenumber (in cm ) (in m) 2

3 Infrared Spectroscopy (IR) Infrared absorption spectrum of a thin polystyrene film recorded with a modern infrared spectrophotometer. Note that the abscissa scale changes at 2000 cm -1. Usually wavenumber is used: directly proportional to energy; 3

4 Bond length and strength vs Stretching frequency Bond C-H =C-H -C-H Length Strenght 506 kj 444 kj 422 kj IR freq cm cm cm -1 4

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6 Infrared Spectroscopy Position, Intensity and Shape of bands gives clues on Structure of molecules Modern IR uses Michelson Interferometer => involves computer, and Fourier Transform Sampling => plates, polished windows, Films Must be transparent in IR NaCl, KCl : Cheap, easy to polish NaCl transparent to cm -1 KCl transparent to cm -1 KBr transparent to cm -1 6

7 Comparison Between Dispersion Spectrometer and FTIR To separate IR light, a grating is used. Grating Slit Light source Sample To select the specified IR light, A slit is used. Detector Dispersion Spectrometer In order to measure an IR spectrum, the dispersion Spectrometer takes several minutes. Also the detector receives only a few % of the energy of original light source. Fixed CCM B.S. Moving CCM Light source An interferogram is first made by the interferometer using IR light. Sample Detector FTIR The interferogram is calculated and transformed into a spectrum using a Fourier Transform (FT). In order to measure an IR spectrum, FTIR takes only a few seconds. Moreover, the detector receives up to 50% of the energy of original light source. (much larger than the dispersion spectrometer.) 7

8 Sample Handling INFRARED SPECTROMETRY No good solvents exist that are transparent throughout the region of interest. As a consequence, sample handling is frequently the most difficult and time-consuming part of an infrared spectrometric analysis. Gases: The spectrum of a low-boiling liquid or gas can be obtained by permitting the sample to expand into an evacuated cylindrical cell equipped with suitable windows. Solutions: A convenient way of obtaining infrared spectra is on solutions prepared to contain a known concentration of sample. This technique is somewhat limited in its applications, however, by the availability of solvents that are transparent over significant regions in the infrared. Solvents: No single solvents is transparent throughout the entire mid-infrared region. Water and alcohols are seldom employed, not only because they absorb strongly, but also because they attack alkali-metal halides, the most common materials used for cell windows. 8

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10 Sample Handling-cont. Cells: Sodium chloride windows are most commonly employed; even with care, however, their surfaces eventually become fogged due to absorption of moisture. Polishing with a buffing powder returns them to their original condition. Liquids: When the amount of liquid sample is small or when a suitable solvent is unavailable, it is common practice to obtain spectra on the pure (neat) liquid. A drop of the neat liquid is squeezed between two rock-salt plated to give a layer that has a thickness of 0.01 mm or less. The two plates, held together are then mounted in the beam path. Such a technique does not give reproducible transmittance data, but the resulting spectra are usually satisfactory for qualitative investigations. 10

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13 Sample Handling -cont. Solids: Most organic compounds exhibit numerous absorption peaks throughout the mid-infrared region, and finding a solvent that does not have overlapping peaks is often impossible. As a consequence, spectra are often obtained on dispersions of the solid in a liquid or solid matrix. Pelleting: One of the most popular techniques for handling solid samples has been KBr pelleting. A milligram or less of the finely ground sample is intimately mixed with about 100 mg of dried potassium bromide powder. The mixture is then pressed in a die at 10,000 to 15,000 pounds per square inch to yield a transparent disk. The disk is then held in the instrument beam for spectroscopic examination. Mulls: Infrared spectra of solids that are not soluble in an infraredtransparent solvent or are not conveniently pelleted in KBr are often obtained by dispersing the analyte in a mineral oil or fluorinated hydrocarbon mull. Mulls are formed by grinding 2 to 5 mg of the finely powdered sample in the presence of one or two drops of a heavy hydrocarbon oil (Nujol). If hydrocarbon bands interfere, Fluorolube, a halogenated polymer, can be used. The resulting mull is then examined as a film between flat salt plates. 13

14 Theory IR is one of the first technique inorganic chemists used (since 1940) Molecular Vibration Newton s law of motion is used classically to calculate force constant F r r e F ky F The basic picture : atoms (mass) are connected with bonding electrons. r e is the equilibrium distance and F: force to restore equilibrium where y is displacement from equilibrium Natural frequency m 1 2 k Where k i is the force constant and i is reduce mass of a particular motion 14

15 Theory Displacement of atoms during vibration lead to distortion of electrical charge distribution of the molecule which can be resolve in dipole, quadrupole, octopole. In various directions => Molecular vibration lead to oscillation of electric charge governed by vibration frequencies of the system Oscillating molecular dipole can interact directly with oscillating electric vector of electromagnetic radiation of the same frequency E h hcv hc wavelength frequency Wave number h= Planck s constant ( x10-27 erg.sec) Vibrations are in the range to Hz => 30-3,000 cm -1 15

16 Calculating stretching frequencies 1 2 c k : Frequency in cm-1 c : Velocity of light => 3 * cm/s k : Force constant => dynes /cm : masses of atoms in grams m 1 m 2 m 1 + m 2 M 1 M 2 M 1 + M 2 (6.02 * ) 5.3x10 12 k C C K = 5* 10 5 dynes/cm C=C K = 10* 10 5 dynes/cm C C K = 15* 10 5 dynes/cm 16

17 IR spectrum may be broken into three regions 17

18 Vibrations Stretching frequency Bending frequency Modes of vibration Stretching H C H H Bending H H C Scissoring 1450 cm -1 O H H H Wagging 1350 cm -1 H Symmetrical 2853 cm -1 H Asymmetrical 2926 cm -1 H H Rocking 720 cm -1 H H Twisting 1250 cm -1 18

19 Vibrations General trends: Stretching frequencies are higher than bending frequencies (it is easier to bend a bond than stretching or compresing them) Bond involving Hydrogen are higher in freq. than with heavier atoms Triple bond have higher freq than double bond which has higher freq than single bond 19

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23 Symmetrical and asymmetrical stretch Symmetrical Stretch Asymmetrical Stretch H Methyl C H 2872 cm -1 H H C H H 2962 cm -1 Anhydride O O 1760 cm -1 O O 1800 cm -1 O O Amino N H H H 3300 cm -1 N 3400 cm -1 H Nitro N O O O 1350 cm -1 N 1550 cm -1 O 23

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26 Aromatic substitution: Out of plane bending 26

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38 IR Spectroscopy of Polymers 1. Polymer identificatication ; By comparasion of the positions of aabsorptions in the IR spectrum of a polymer sample with the characteristic absorption regions, leads to identification of the bonds and functional groups present in the polymer. -For conformation the spectrum must be compared in detail with that of an authentic sample since two spectra should be identical if the identification is correct. That is why the IR spectra of polymer can be considered as a fingerprint for this purpose. Ex: Samples of flexible PVC, usually show strong C=O streching absorptions due to the ester groups in phthalate ester plasticizers. 2. Branched polymers CH 3 end group absorption in IR (1378 cm -1) in the IR spectrum of PE can be used to Measure the degree of branching 38

39 Average Compositions of Copolymers Average compositions of copolymers can be determined if the different repeat units give caharacteristic absorptions which are not coincident. Ex: poly(styrene-co-(n-butylacrylate)) Out of plane bending (C-H) benzene rings in styrene repeat unit C=O streching absorption of ester groups in n-butyl acrylate repeat units Spectra are recorded as absorbance, A, against wavenumber or (wavelength) where; I o A log I I O and I are the intensities of the radiation incident to and after transmission through the sample respectively. 39

40 Beer-lambert equation A cl Molar absorptivity of absorption Path length of the radiatio through the sample The difference betwwen the maximum and the background absorbance at the position ( obtained by linking the baseline before and after the absorption) c c A A As long as the ratio of mlar absorptivities has been determined previously using samples of known composition 40

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42 In situ FTIR spectra of copolymerization of ethylene and vinyl acetate in scco2 (experimental conditions: T = 50 C; P = 27.6 MPa; ethylene mole fraction in feed = 0.902). Published in: William Z. Xu; Paul A. Charpentier; Ind. Eng. Chem. Res. 2009, 48, DOI: /ie801275g Copyright 2009 American Chemical Society 42

43 FT-IR spectra of (a) pristine MMT-k10, (b) TEA-modified MMT-k10, and (c) THFextracted PMMA/clay composites (from sample PMMA3). Published in: Liqiang Cui; Naresh H. Tarte; Seong Ihl Woo; Macromolecules 2008, 41, DOI: /ma Copyright 2008 American Chemical Society 2a) the band at 3630 cm 1 is typical for smectites due to internal hydroxyl groups of clay minerals. The stretching vibration of water is observed at 3430 cm 1. The band at 1635 cm 1 is assigned to water deformation band. Si O stretching and Al O stretching are observed at 1040 and 570 cm 1, respectively. 2b), the C H stretching bands are found at 2855 and 2950 cm 1, confirming the presence of ethyl group in the layers of the clay. Additionally, the weak OH stretching band (3630 cm 1 ) and the weak band (1635 cm 1 ) derived from water molecule, 2c) The IR spectrum of THF-extracted PMMA/clay composites shows a distinct absorption at 1730 cm 1 (CO stretching) and cm 1 (C O stretching), which are 43 the characteristic bands of PMMA

44 glycidoxypropyltrimethoxylsilane (GPS) thioxanthone photoinitiator (HPTX) PMMA FT-IR spectra of (a) SiO2 (raw SPs), (b) (c) (d) h. Published in: Fengting Chen; Xuesong Jiang; Rui Liu; Jie Yin; ACS Appl. Mater. Interfaces 2010, 2, DOI: /am900758j Copyright 2010 American Chemical Society 44