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الموضوع: Infrared Spectroscopy

  1. #1
    عضو ذهبي الصورة الرمزية اسراء
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    Aug 2011
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    افتراضي Infrared Spectroscopy

    Infrared Spectroscopy

    1. Introduction
    As noted in a previous chapter, the light our eyes see is but a small part of a broad spectrum of electromagnetic radiation. On the immediate high energy side of the visible spectrum lies the ultraviolet, and on the low energy side is the infrared. The portion of the infrared region most useful for analysis of organic compounds is not immediately adjacent to the visible spectrum, but is that having a wavelength range from 2,500 to 16,000 nm, with a corresponding frequency range from 1.9*1013 to 1.2*1014 Hz. Photon energies associated with this part of the infrared (from 1 to 15 kcal/mole) are not large enough to excite electrons, but may induce vibrational excitation of covalently bonded atoms and groups. The covalent bonds in molecules are not rigid sticks or rods, such as found in molecular model kits, but are more like stiff springs that can be stretched and bent. The mobile nature of organic molecules was noted in the chapter concerning conformational isomers. We must now recognize that, in addition to the facile rotation of groups about single bonds, molecules experience a wide variety of vibrational motions, characteristic of their component atoms. Consequently, virtually all organic compounds will absorb infrared radiation that corresponds in energy to these vibrations. Infrared spectrometers, similar in principle to the UV-Visible spectrometer described elsewhere, permit chemists to obtain absorption spectra of compounds that are a unique reflection of their molecular structure. An example of such a spectrum is that of the flavoring agent vanillin, shown below.The complexity of this spectrum is typical of most infrared spectra, and illustrates their use in identifying substances. The gap in the spectrum between 700 & 800 cm-1 is due to solvent (CCl4) absorption. Further analysis (below) will show that this spectrum also indicates the presence of an aldehyde function, a phenolic hydroxyl and a substituted benzene ring. The inverted display of absorption, compared with UV-Visible spectra, is characteristic. Thus a sample that did not absorb at all would record a horizontal line at 100% transmittance (top of the chart).
    Frequency - Wavelength Converter
    Frequency in cm-1

    Wavelength in μ

    The frequency scale at the bottom of the chart is given in units of reciprocal centimeters (cm-1) rather than Hz, because the numbers are more manageable. The reciprocal centimeter is the number of wave cycles in one centimeter; whereas, frequency in cycles per second or Hz is equal to the number of wave cycles in 3*1010 cm (the distance covered by light in one second). Wavelength units are in micrometers, microns (μ), instead of nanometers for the same reason. Most infrared spectra are displayed on a linear frequency scale, as shown here, but in some older texts a linear wavelength scale is used. A calculator for interconverting these frequency and wavelength values is provided on the right. Simply enter the value to be converted in the appropriate box, press "Calculate" and the equivalent number will appear in the empty box.
    Infrared spectra may be obtained from samples in all phases (liquid, solid and gaseous). Liquids are usually examined as a thin film sandwiched between two polished salt plates (note that glass absorbs infrared radiation, whereas NaCl is transparent). If solvents are used to dissolve solids, care must be taken to avoid obscuring important spectral regions by solvent absorption. Perchlorinated solvents such as carbon tetrachloride, chloroform and tetrachloroethene are commonly used. Alternatively, solids may either be incorporated in a thin KBr disk, prepared under high pressure, or mixed with a little non-volatile liquid and ground to a paste (or mull) that is smeared between salt plates.
    2. Vibrational Spectroscopy
    A molecule composed of n-atoms has 3n degrees of freedom, six of which are translations and rotations of the molecule itself. This leaves 3n-6 degrees of vibrational freedom (3n-5 if the molecule is linear). Vibrational modes are often given descriptive names, such as stretching, bending, scissoring, rocking and twisting. The four-atom molecule of formaldehyde, the gas phase spectrum of which is shown below, provides an example of these terms. If a ball & stick model of formaldehyde is not displayed to the right of the spectrum, press the view ball&stick model button on the right. We expect six fundamental vibrations (12 minus 6), and these have been assigned to the spectrum absorptions. To see the formaldehyde molecule display a vibration, click one of the buttons under the spectrum, or click on one of the absorption peaks in the spectrum.
    Gas Phase Infrared Spectrum of Formaldehyde, H2C=O

    View CH2 Asymmetric Stretch
    View CH2 Symmetric Stretch
    View C=O Stretch
    View CH2 Scissoring
    View CH2 Rocking
    View CH2 Wagging
    Ball&Stick Model
    Spacefill Model
    Stick Model
    Motion Off


    The exact frequency at which a given vibration occurs is determined by the strengths of the bonds involved and the mass of the component atoms. For a more detailed discussion of these factors Click Here. In practice, infrared spectra do not normally display separate absorption signals for each of the 3n-6 fundamental vibrational modes of a molecule. The number of observed absorptions may be increased by additive and subtractive interactions leading to combination tones and overtones of the fundamental vibrations, in much the same way that sound vibrations from a musical instrument interact. Furthermore, the number of observed absorptions may be decreased by molecular symmetry, spectrometer limitations, and spectroscopic selection rules. One selection rule that influences the intensity of infrared absorptions, is that a change in dipole moment should occur for a vibration to absorb infrared energy. Absorption bands associated with C=O bond stretching are usually very strong because a large change in the dipole takes place in that mode.
    Some General Trends:i) Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.)
    ii) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms.
    iii) Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds.
    (Except for bonds to hydrogen). The general regions of the infrared spectrum in which various kinds of vibrational bands are observed are outlined in the following chart. Note that the blue colored sections above the dashed line refer to stretching vibrations, and the green colored band below the line encompasses bending vibrations. The complexity of infrared spectra in the 1450 to 600 cm-1 region makes it difficult to assign all the absorption bands, and because of the unique patterns found there, it is often called the fingerprint region. Absorption bands in the 4000 to 1450 cm-1 region are usually due to stretching vibrations of diatomic units, and this is sometimes called the group frequency region.
    3. Group Frequencies
    Detailed information about the infrared absorptions observed for various bonded atoms and groups is usually presented in tabular form. The following table provides a collection of such data for the most common functional groups. Following the color scheme of the chart, stretching absorptions are listed in the blue-shaded section and bending absorptions in the green shaded part. More detailed descriptions for certain groups (e.g. alkenes, arenes, alcohols, amines & carbonyl compounds) may be viewed by clicking on the functional class name. Since most organic compounds have C-H bonds, a useful rule is that absorption in the 2850 to 3000 cm-1 is due to sp3 C-H stretching; whereas, absorption above 3000 cm-1 is from sp2 C-H stretching or sp C-H stretching if it is near 3300 cm-1.
    Stretching Vibrations
    Bending Vibrations
    Functional Class
    Range (cm-1)
    Intensity
    Assignment
    Range (cm-1)
    Intensity
    Assignment
    Alkanes
    2850-3000 str CH3, CH2 & CH
    2 or 3 bands
    1350-1470
    1370-1390
    720-725
    med
    med
    wk
    CH2 & CH3 deformation
    CH3 deformation
    CH2 rocking
    Alkenes
    3020-3100
    1630-1680

    1900-2000
    med
    var

    str
    =C-H & =CH2 (usually sharp)
    C=C (symmetry reduces intensity)

    C=C asymmetric stretch
    880-995
    780-850
    675-730
    str
    med
    med
    =C-H & =CH2
    (out-of-plane bending)
    cis-RCH=CHR
    Alkynes
    3300
    2100-2250
    str
    var
    C-H (usually sharp)
    C≡C (symmetry reduces intensity)
    600-700 str C-H deformation
    Arenes
    3030
    1600 & 1500
    var
    med-wk
    C-H (may be several bands)
    C=C (in ring) (2 bands)
    (3 if conjugated)
    690-900 str-med C-H bending &
    ring puckering
    Alcohols & Phenols
    3580-3650
    3200-3550
    970-1250
    var
    str
    str
    O-H (free), usually sharp
    O-H (H-bonded), usually broad
    C-O
    1330-1430
    650-770
    med
    var-wk
    O-H bending (in-plane)
    O-H bend (out-of-plane)
    Amines
    3400-3500 (dil. soln.)
    3300-3400 (dil. soln.)
    1000-1250
    wk
    wk
    med
    N-H (1°-amines), 2 bands
    N-H (2°-amines)
    C-N
    1550-1650
    660-900
    med-str
    var
    NH2 scissoring (1°-amines)
    NH2 & N-H wagging
    (shifts on H-bonding)
    Aldehydes & Ketones
    2690-2840(2 bands)
    1720-1740
    1710-1720
    1690167517451780
    med
    str
    str

    str
    str
    str
    str
    C-H (aldehyde C-H)
    C=O (saturated aldehyde)
    C=O (saturated ketone)

    aryl ketone
    α, β-unsaturation
    cyclopentanone
    cyclobutanone
    1350-1360
    1400-1450
    1100
    str
    str
    med
    α-CH3 bending
    α-CH2 bending
    C-C-C bending
    Carboxylic Acids & Derivatives
    2500-3300 (acids) overlap C-H
    1705-1720 (acids)
    1210-1320 (acids)
    1785-1815 ( acyl halides)1750 & 1820 (anhydrides) 1040-11001735-1750 (esters) 1000-1300 1630-1695(amides)
    str
    str
    med-str

    str
    str
    str
    str
    str
    str
    O-H (very broad)
    C=O (H-bonded)
    O-C (sometimes 2-peaks)

    C=O
    C=O (2-bands)
    O-C
    C=O
    O-C (2-bands)
    C=O (amide I band)
    1395-1440







    1590-1650
    1500-1560
    med







    med
    med
    C-O-H bending







    N-H (1°-amide) II band
    N-H (2°-amide) II band
    Nitriles

    Isocyanates,Isothiocyanates,
    Diimides, Azides & Ketenes
    2240-2260

    2100-2270
    med

    med
    C≡N (sharp)

    -N=C=O, -N=C=S
    -N=C=N-, -N3, C=C=O

    To illustrate the usefulness of infrared absorption spectra, examples for five C4H8O isomers are presented below their corresponding structural formulas. The five spectra may be examined in turn by clicking the "Toggle Spectra" button. Try to associate each spectrum (A - E) with one of the isomers in the row above it. When you have made assignments check your answers by clicking on the structure or name of each isomer.



    4. Other Functional Groups
    Infrared absorption data for some functional groups not listed in the preceding table are given below. Most of the absorptions cited are associated with stretching vibrations. Standard abbreviations (str = strong, wk = weak, brd = broad & shp = sharp) are used to describe the absorption bands.
    Functional Class
    Characteristic Absorptions
    Sulfur Functions
    S-H thiols 2550-2600 cm-1 (wk & shp)
    S-OR esters 700-900 (str)
    S-S disulfide 500-540 (wk)
    C=S thiocarbonyl 1050-1200 (str)
    S=O sulfoxide
    sulfone sulfonic acidsulfonyl chloride sulfate
    1030-1060 (str)
    1325± 25 (as) & 1140± 20 (s) (both str)
    1345 (str)
    1365± 5 (as) & 1180± 10 (s) (both str)
    1350-1450 (str)
    Phosphorous Functions
    P-H phosphine 2280-2440 cm-1 (med & shp)
    950-1250 (wk) P-H bending
    (O=)PO-H phosphonic acid 2550-2700 (med)
    P-OR esters 900-1050 (str)
    P=O phosphine oxide
    phosphonatephosphate phosphoramide
    1100-1200 (str)
    1230-1260 (str)
    1100-1200 (str)
    1200-1275 (str)
    Silicon Functions
    Si-H silane 2100-2360 cm-1 (str)
    Si-OR 1000-1110 (str & brd)
    Si-CH3 1250± 10 (str & shp)
    Oxidized Nitrogen Functions
    =NOH oxime
    O-H (stretch)C=NN-O
    3550-3600 cm-1 (str)
    1665± 15
    945± 15
    N-O amine oxide
    aliphaticaromatic
    960± 20
    1250± 50
    N=O nitroso
    nitro
    1550± 50 (str)
    1530± 20 (as) & 1350± 30 (s)

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  2. #2
    مشرفة الاقسام الاكاديمية الصورة الرمزية تمارا احمد
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    Jul 2011
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    افتراضي رد: Infrared Spectroscopy

    Thanks a lot

    عضو في نادي ماركا الأكاديمي


  3. #3
    مراقب عام الصورة الرمزية Eiman
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    افتراضي رد: Infrared Spectroscopy

    شكرا اسراء
    من جد وجد ....... ومن سار على الدرب وصل

  4. #4
    الصورة الرمزية khaledabuarab
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    Feb 2012
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    jordan
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    افتراضي رد: Infrared Spectroscopy

    hi esraa how are you thanks for you about the best subject in there i study chemistry as analytical chemistry in yarmouk university in master dgree i hopp there will bee anather subj like that and gggooo onn

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  5. #5
    عضو ذهبي الصورة الرمزية اسراء
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    افتراضي رد: Infrared Spectroscopy

    شكرا لك اخي خالد
    ربنا يوفقك وعقبال الدكتوراه

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  6. #6
    عضو ذهبي الصورة الرمزية نداء محمد
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    افتراضي رد: Infrared Spectroscopy

    Thank you

    عضو في نادي ماركا الأكاديمي


  7. #7
    عضو ذهبي الصورة الرمزية Noor
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    افتراضي رد: Infrared Spectroscopy

    شكرا اسراء
    ربنا يبارك فيكي

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