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وبلاگ تخصصی شیمی - Infrared Spectroscopy
 
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سلامی چو بوی خوش هیدروکربنهای آروماتیک برشما که قلبهایتان همچون فلزات قلیایی الکترون های وجودتان را سخاوتمندانه به ما بخشیدند و مستحکم ترین پیوندقلبی را ایجاد کردند ، ما از با شما بودن چیزی فراتر از استوکیومتری زندگی ومولاریته شادیها آموختیم امیدواریم محلول زندگی تان همواره شفاف و معادلات زندگی تان پیوسته موازنه شده و پیوند خانوادگی تان یونی ترین پیوندها و بختتان همواره به سفیدی سدیم کلرید و محلول زندگی تان از عشق و محبت فراسیرشده باشد با بیشترین درصد خلوص دوستتان داریم وبا بالاترین غلظت به شما خوشامد میگوئیم .با مادرتماس باشید.
مدیر وبلاگ : علی ناطقی
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سه شنبه 23 فروردین 1390 :: نویسنده : علی ناطقی

 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).

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.

            Typical Infrared Absorption Frequencies

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

1690
1675
1745
1780
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)
    1040-1100
1735-1750 (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 acid
sulfonyl 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
phosphonate
phosphate
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=N
N-O

3550-3600 cm-1 (str)
1665± 15
945± 15
N-O amine oxide
aliphatic
aromatic

960± 20
1250± 50
N=O nitroso
nitro
1550± 50 (str)
1530± 20 (as) & 1350± 30 (s)


Test your ability to use information from infrared and mass spectrometry to identify an unknown compound. Clicking the button opens a display in which four different problems of this kind may be selected. Answers are provided once an effort to solve the problem has been made.





نوع مطلب : شیمی تجزیه، نانو شیمی، 
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جمعه 13 خرداد 1390 11:55 ق.ظ
لطفا كتابها و پاور پوینت های شیمی و شیمی تجزیه دستگاهی مثل hnfra red, raman , ..... را برام میل كنید . با تشكر از سایت خوبتون
علی ناطقیسلام دوست عزیز اسپکتروسکوپی رامان را در وبلاگ قرار دادیم قابل دانلود.اما IR,TEM.SEM,XRD NMRبه زودی در وبلاگ قرار خواهد گرفت.دوستانی که عجله دارند می توانند برای ما میل بزنند.
دوشنبه 29 فروردین 1390 03:29 ب.ظ
Hi great article and thanks again Jalbtan about this subject let
 
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