Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
Infrared spectroscopy
Theory: Different chemical bonds have different vibration frequency When the molecule exposed to IR radiation, the atoms vibrate around the bond in a spring shape Bond is the space between 2 nuclei
Unit of IR is wave number (ϋ) = 1/λ (in cm) = cm-1 E α 1/λ, so E α ϋ, that means as ϋ increase the E. also increase (and λ decrease)
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Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
The main parts of an IR spectrometer are as follows 1) IR radiation source 2) monochromators 3) sample cells 4) detectors 1. Infrared sources: require a source of IR radiant energy. The rod is electrically heated to 1500 K to produce IR radiations. A) Nernst glower or Nernst filament: they are made up of sintered mixtures of oxides of Zr, Th, Ce, etc. -Used in near IR region. -Intensity of radiation produced is more intense. -Used for detection of carbohydrates & proteins. B) Globar source: made up of silicon carbide. They are self starters. used in middle IR region. used to detect simple functional groups. C) incandescent lamp: it is made up of nichrome wire. used in near IR instruments. Has a low spectral emissivity. It is heated up to 1100 K. Used to detect complex organic molecules D) mercury arc: high pressures are used. used in far IR region. intensity of radiation is greater. used to detect inorganic complexes. E) tungsten lamp: they heated up to 3500 K used in mid IR region. intensity of radiation is mild. used to detection of organic functional grps
2. MONOCHROMATORS They convert polychromatic light into mono chromatic light. They must be constructed of materials which transmit the IR. They are of 3 types. a) metal halide prisms b) NaCl prisms c) gratings A) metal halide prisms: prisms which are made up of KBr are used in the wavelength region from 12-25µm. LiF prisms are used in the wavelength region from 0.2-6µm. CeBr prisms used in wavelength region from 15-38µm. B) NaCl prisms: Used in the whole wave length region from 4000-650cm-1. they have to be protected above 20 °C because of hygroscopic nature. c) gratings: They offer better resolution at low frequency than prisms.
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Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
3. Sample cells: sample cells Sample cells made up of alkali halides like NaCl or KBr are used. Aqueous solvents cannot be used as they dissolve alkali halides. Only organic solvents like chloroform is used. IR spectroscopy has been used for the characterization of solid, liquid, gas samples. 4. Detectors They convert the radiation into electrical signal. There are mainly 5 types of detectors used in IR. Thermo couple detectors: made up of bismuth& antimony coated by metal oxides. If wires of 2 dissimilar metals joined head to tail, then a difference in temperature between head & tail causes a current to flow in the wire. This current proportional to the intensity of radiation. also called as thermopile detectors. material should be thermally active. used in dispersive instruments. Response time is 30 seconds. They give responses for all frequencies. Thermister detectors: Thermister detectors These are made up of sintered oxides of Mn, Co, Ni. The material should be thermally sensitive. The response time is 4 seconds . pyro electric detectors These are made up of TGS, LiTaO3. They are used in FTIR instruments. It involves multiple scanning. Principle involved is electric polarization. Golay detectors: Golay detectors The material used is CO2. The material should be inert in nature. used in non dispersive IR instruments. Response time is 0.01 sec. Principle involved is expansion of gases. Photo conductive detectors: Photo conductive detectors Photoelectric detectors such as the mercury cadmium telluride detector comprise a film of semiconducting material deposited on a glass surface, sealed in an evacuated envelope. Absorption of IR promotes nonconducting valence electrons to a higher, conducting state. These detectors have better response characteristics than pyroelectric detectors and are used in FT-IR instruments - particularly in GC - FT-IR.
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Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
Sample Preparation and Handling Infrared spectroscopy permits the analyst to select from a wide variety of sample preparation techniques. Polystyrene film is commercially used for calibration of wave numbers. (3026, 3002, 2924, 1602, 1495 & 906).
Gaseous samples: Gas phase spectra can be taken at room temperature. What is required is a sample with a vapor pressure of several millimeters and a pathlength of about a decimeter (10 cm). The cells consist of a glass or metal body, two IR-transparent end windows (NaCl or KBr), and valves for filling gas from external sources. KBr is transparent from the wavelength 4000250 cm-1 and sodium chloride from 4000-600 cm-1. Liquid Samples: For liquid samples two polished disks of NaCl or KBr are used. A thin film is prepared by depositing a drop of the liquid between the two plates and mounting them in the beam of the spectrometer. Silver chloride plates can be used for samples that dissolve the salt plates. Solid samples: Neat Spectra (thin film) A thin layer of a solution is used on an infrared cell and the solvent is allowed to evaporate. Solvents such as CS2, CH2Cl2 and CCl4, which are transparent in the region of interest are usually used. Nujol mull (mineral oil) A suspension of a solid in a liquid is prepared. The commercial Nujol sample often used for this purpose is mineral oil. The major disadvantage of using a Nujol mull is that the information in the C-H stretching region is lost because of the absorptions of the mulling agent. To eliminate this problem, it may be necessary to run a second spectrum in a different mulling agent that does not contain any C-H bonds. Typical mulling agents that are used for this purpose are perfluoro- or perchlorohydrocarbons. Examples include Florolube, hexachlorobutadiene, perfluorokerosene or perfluorohydrocarbon oil. 4
Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
KBr pellets A KBr pellet is a dilute suspension of a solid sample in solid KBr. It is usually obtained by first grinding the sample in anhydrous KBr at a ratio of approximately 1 part sample to 100 parts KBr. The ground up sample mixture is then placed in a hydraulic press and subjected to pressures of 15000 psi for about 20 seconds resulting in a KBr pellet that is reasonably transparent both to visible light and infrared radiation. The only limitation of KBr is that it is hydroscopic.
Please refere to the following link: http://cnx.org/contents/
[email protected]:41/Physical_Methods_in_Chemistry_ Instrumentation Dispersive infrared spectrophotometer An infrared spectrophotometer comprises of the following: • A source generates light across the spectrum of interest. • A monochromater to produce a single spectral line from a broadband (multi-wavelength) source. • A slit selects the collection of wavelengths that shine through the sample at any given time. • A beam splitter that separates the incident beam in two; half goes to the sample, and half to a reference. • The sample absorbs light according to its chemical properties. • A detector collects the radiation that passes through the sample, and in double-beam operation, compares its energy to that going through the reference. • The detector puts out an electrical signal, which is normally sent directly to an analog recorder. A link between the monochromator and the recorder allows you to record energy as a function of frequency or wavelength. Dispersive IR spectrometers use a diffraction grating in a monochromator to disperse the different wavelengths of light. Diffraction grating consists usually of thousands of narrow, closely spaced parallel slits (or grooves). Because of interference the intensity of the light getting pass through the slits depends upon the direction of the light propagation. There are selected directions at which the light waves from the different slits interfere in phase and in these directions the maximums of the light intensity are observed. These selected directions depend upon wavelength, and so the light beams with different wavelength will propagate in different directions to produce a single spectral line from a broadband (multi-wavelength) source.
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Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
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Practical Organic Spectroscopy CHEM 333 Islamic University in Madinah
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