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Dong-Sun Lee / cat-lab / SWU
Chapter 26 B
2010-Fall Version
Molecular AbsorptionSpectrometry
IR spectrometry
Infrared absorption spectrometry
1) The IR regions of spectrum
Designation Wavelength Frequency(Hz) Wave number (cm–1) Transition
Near IR 780~2500nm 1.2~3.8×1014 12,800 ~4,000 Molecular vibration
Overtone region
Mid IR 2.5~50m 6×1012 ~1.2×1014 4,000 ~ 200 Molecular vibration
(Fundamental region)
Conjugation region 2,500 ~ 2,000 Triple bond
2,000 ~1,540 Double bond
Group frequency 4,000~1,300 Functional group
Finger print region 1,300 ~ 650 Complete molecule
Far IR 50 ~ 1000 m 3×1011 ~6×1012 200 ~ 10 Molecular rotation
2) Origin of IR spectra
Atoms or atomic groups in a molecules are in continuous motion with respect to one another. IR spectra originate from the difference modes of vibration and rotation of a molecule, whereas the UV-visible absorption bands are primarily due to electronic transition.
In order to absorb IR radiation, a molecule must undergo a net change in dipole moment as a consequence of its vibrational or rotational motion. The dipole moment is determined by the magnitude of the charge difference and the distance between the two centers of charge. The change in bond length or angle due to vibrational or rotational motion must cause a net change in the dipole moment of the molecule.
No net change in dipole moment occurs during the vibration or rotation of homonuclear species such as O2, N2, or Cl2 ; consequently, such compounds cannot absorb in the IR. Vibrational modes which do not involve a change in dipole moment are said to be IR-inactive. With exception of a few compounds of this type, all molecular species exhibit IR-active.
Vibrations and characteristic frequencies of acetaldehyde.
-C-H bending
1460 cm–1
1365 cm–1
C-C stretching
1165 cm–1
C=O stretching
1730 cm–1
C-H stretching
of CH3
2960 cm–1
2870 cm–1
C-H stretching
of CHO
2720 cm–1
IR spectra of acetaldehyde.
Vibrational modes for methylene group(a) and breathing vibration for a ring compound (b).
3) Types of vibration
1) Stretching (or valency ) vibration :
Symmetric
Asymmetric
2) Bending ( or deformation ) vibration :
In-plane bending
Scissoring
Rocking
Out of plane
Wagging
Twisting
3) Breathing of ring compounds
Let us consider the vibration of a mass attatched to a spring that is hung from an immovable object. If the mass is displaced a distance y from its equilibrium position by application of a force along the axis of the spring, the restoring force is proportional to the displacement (Hooke’s law). F = –ky
Where F is the restoring force and k is the force constant, which depends upon the stiffness of the spring.
The potential energy E, is a maximum when the spring is stretched or compress to its maximum amplitude A, and decreases parabolically to zero at the rest or equilibrium position.
dE = –Fdy = kydy dE = k ydy
E = ½ k y2
The vibration frequency vm , of the oscillation is dependent upon the force constant and reduced mass .
v m = (1/2)(k / ) = (1/2) {k (m1m2) / (m1+m2)}
v = (1/2c)(k / ) = 5.3×10–12 (k / )
4) Mechanical model of stretching vibration
Reduced mass and force constants for various atom pairs.
5) Vibrational modes
Fundamental ( normal ) vibration modes
1) Non-linear molecule : 3n – 6 vibrational modes
3 possible rotational modes
2) Linear molecule : 3n – 5 vibrational modes
2 possible rotational modes
where is the number of atoms in the molecule, and 3n cartesian coordinates are called as degree of freedom .
Example
linear molecule :
CO2 : 3n – 5 = 3 ×3 – 5 = 4
non-linear molecule :
H2O : 3n – 6 = 3 ×3 – 6 = 3
HCHO : 3n – 6 = 3 ×4 – 6 = 6
Illustration of vibrational modes in H2O and CO2.
IR spectrum of H2O and CO2.
Single and double beam spectra of atmospheric water vapor and CO2.
Vibrational modes for formaldehyde.
IR spectra of formaldehyde.
Instrumentation of IR spectrometer
Dispersive IR spectrometer
Single beam is not very practical because of the absorption of IR radiation by
atmospheric H2O and CO2. Double beam Sample cell is usually placed in front of the monochromator to minimize the effects of IR emission and stray radiation from the cell compartment. Detecting method Optical null system Ratio recording system
Nondispersive IR spectrometer
Filter photometer Dielectric filter spectrometer Special purpose spectrometer
Fourier Transform IR spectrometer
Interferometer
Components of dispersive IR spectrometer
Region of electromagnetic spectrum
Near IR Mid IR Far IR
Wavenumber (cm–1) 12,500 4,000 200 10
Wavelength (m) 0.8 2.5 50 1,000
Source of radiation Tungsten filament Nernst glower, Globar, High-pressure
lamp or coil of nichrome wire mercury-arc lamp
Optical system One or two Two to four plane Double beam
quartz prisms or diffraction gratings grating for use
prism grating with either a foreprism to 700 m ;
double monochromator monochromator or interferometer
IR filters for use to 1000 m
Detector Photoconductive Thermopile, Golay
cells thermister, or pyroelectric
semiconductor
Optical null double beam IR spectrometer
Fourier transform IR spectroscopy
FT techniques are possible because the units of time and frequency are inversely related. A function in the time domain can be transformed into its equivalent function in the frequency domain. The mechanism by which the instrument generates the time domain signal depends on the form of spectroscopy. IR radiation can be analyzed by means of a scanning Michelson interferometer.
Fourier analysis is a procedure in which a curve is decomposed into a sum of sine and cosine terms, called a Fourier series.
y = a0 sin(0x)+b0 cos(0 x)+a1sin(1x)+ b1cos(1 x) + a2sin(2x)+ b2cos (2 x) + ……
= [ an sin(nx) + bn cos (n x)]
where = 2 /(x2 – x1)
A curve to be decomposed into a sum of sine and cosine terms by Fourier analysis.
Fourier series reconstruction of the curve in left Fig. Solid line is the original curve and dashed lines are made from a series of n=0 to n=2, 4 or 8 in the Fourier series equation :
y = [ an sin(nx) + bn cos (n x)]
The Nobel Prize in Physics 1907Albert Abraham Michelson, (December 19, 1852 - May 9, 1931), was born in Strzelno, Poland (then Strelno, Provinz Posen Kingdom of Prussia). He came to the United States with his parents when he was two years old.
Michelson was an American physicist known for his work on the measurement of the speed of light. In 1907 he received a Nobel prize for physics.
http://nobelprize.org/physics/laureates/1907/michelson-bio.html
Interferometry
The heart of a Fourier transform infrared specrtophotometer is the interferometer.
Radiation from the source at the left strikes a beamsplitter, which transmits some light and reflects some light. For the sake of this discussion, consider a beam of monochromatic radiation. (In fact, the Fourier transform spectrophotometer uses a continuum source of infrared radiation, not a monochromatic source.)
For simplicity, suppose that the beamsplitter reflects half of the light and transmits half. When light strikes the beamsplitter at point O, some is reflected to a stationary mirror at a distance OS and some is transmitted to a movable mirror at a distance OM. The rays is transmitted and half is reflected.
One recombined ray travels in the direction of the detector, and another heads back to the source.
Schematic diagram of Michelson interferometer. Detector response as a function of retardation (= 2[OM – OS] ) is shown for the case of monochromatic incident radiation of wavelength .
Michelson Interferometer
The Michelson interferometer produces interference fringes by splitting a beam of monochromatic light so that one beam strikes a fixed mirror and the other a movable mirror. When the reflected beams are brought back together, an interference pattern results.
Precise distance measurements can be made with the Michelson interferometer by moving the mirror and counting the interference fringes which move by a reference point. The distance d associated with m fringes is
d = m/2http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/michel.html
In general, the paths OM and OS are not equal, so the two waves reaching the detector are not in phase. If the two waves are in phase, they interfere constructively to give a wave with twice the amplitude. If the waves are one-half wavelength (180°) out of phase, they interfere destructively and cancel. For any intermediate-phase difference, there is partial cancellation.
The difference in pathlength followed by the two waves in the interferometer is 2(OM-OS). This difference is called the retardation , .
Constructive interference occurs whenever is an integral multiple of the wavelength () of the light.
A minimum appears when is a half-integral multiple of .
If mirror M moves away from the beamsplitter at a constance speed, light reaching the detector goes through a sequence of maxima and minima as the interference alternates between constructive and destructive phases.
A graph of output light intensity versus retardation, , is called an interferogram.
If the light from the source is monochromatic, the interferogram is a simple cosine wave:
I() = B()cos(2π/ ) = B()cos(2π )
where I() is the intensity of light reaching the detector and and is the wavenumber (=1/ ) of the light.
Clearly, I is a function of the retardation, .
B() is a constant that accounts for the intensity of the light source, efficiency by beamsplitter (which never gives exactly 50% reflection and 50% transmission), and response of the detector.
All these factors depend on . In the case of monochromatic light, there is only one value of .
Interferograms produced by different spectra
Figure a) shows the interferogram produced by monochromatic radiation of wavenumber o=2 ㎝ -1. The wavelength (repeat distance) of the interforogram can be seen in the figure to be =0.5 ㎝ , which is equal to 1/ o = 1/(2 ㎝ -1).
Figure b) shows the interferogram that results from a source with two monochromatic waves (o = 2 and o = 8 ㎝ -1) with relative intensities 1:1. There is a short wave oscillation ( = 1/8 ㎝ ) superimposed on a long wave oscillation ( = 1/2 ㎝ ). The interferogram is a sum of two terms:
I() = B1cos(2π 1 ) + B2cos(2π 2 )
where B1 = 1, 1 = 2 ㎝ -1, B2 = 1, and 2 = 8 ㎝ -1.
Fourier analysis decomposes a curve into its component wavelengths. Fourier analysis of the interferogram in Figure a) gives the (trivial) result that the interferogram is made from a single wavelength function, with = 1/2 ㎝ . Fourier analysis of the interferogram in Figure b) gives the slightly more interesting result that the interferogram is composed of two wavelengths ( = 1/2 ㎝ and = 1/8 ㎝ ) with relative contributions 1:1. We say that the spectrum is
the Fourier transform of the interferogram.
The interferogram in Figure c) is a less trivial case in which the spectrum consists of an absorption band centered at o = 4 ㎝ -1.
The interferogram is the sum of contributions from all source wavelengths.
The Fourier transform of the interferogram in Figure c) is indeed the third spectrum in Figure c). That is, decomposition of the interferogram into its component wavelength gives back the band centered around o = 4 ㎝ -1. Fourier analysis of the interferogram gives back the intensities of its component wavelengths.
The interferogram in Figure d) is obtained from the two absorption bands in the spectrum at the left. The Fourier transform of this interferogram gives back the spectrum to its left.
Michelson interferometer http://www.3dimagery.com/michelsn.html
Interference pattern created by Michelson interferometer
Diagram of a Michelson interferometer.A two dimensional representation of the interference of two monochromatic wavefronts of the same frequency.
Formation of interferograms at the output of the Michelson interferometer.
(a) Spectrum of a continuum light source.
(b) Inteferogram of the light source in (a) produced at the output of the Michelson interferometer.
http://www.infrared-analysis.com/info1.htm
Layout of Fourier transform infrared spectrometer.
Often, benchtop instruments purge the FT-IR spectrometer with an inert gas or dry, CO2-free air to reduce the background absorption from water vapor and CO2.
He-Ne
Most FT-IR spectrometers are of the single beam type.
To obtain the spectrum of sample, the background spectrum is first obtained by FT of the interferogram from background (solvent, ambient water, and carbon dioxide). This is normally a measurement with no sample in the beam.
Next, the sample spectrum is obtained.
Finally, the ratio of the single beam sample spectrum to that of the background spectrum is calculated, and absorbance or transmittance versus wavelength or wavenumber is plotted
(a) Interferogram obtained from a typical FTIR spectrometer for methylene chloride. (b) IR spectrum of methylene chloride produced by the Fourier transformation of the data in (a).
Sample Preparation In general the amount of sample necessary to obtain a good IR spectrum is the order of 1 to 5 mg (sample/KBr = 1~5mg/100mg). Since almost all substances absorb IR radiation at some wavelengths, cell window materials, cell pathlengths, and solvents must be carefully chosen for the wavelength region and sample of interest.
Solid substances
Solid state forces such as intermolecular hydrogen bonding render such spectra somewhat unreliable for diagnostic purposes.
1) Sample must be finely ground so that the particle size is smaller than the wavelength(1m) of IR
radiation. Otherwise pronounced scattering of the incident light occurs.
2) These small particles must now be suspended in a medium of similar refractive index.
A) Mulls
Mulls are normally prepard by grinding a few mg of the powdered sample with an agate(alumina) mortar and pestle. A few drops of the mineral oil (Nujol; medicinal paraffin: refined mixture of saturated hydrocarbons) are then added. Grinding is continued in the presence of the oil until a smooth paste is obtained. A small amount of the resulting paste is then spread between two polished NaCl plates and placed in the spectrometer.
Nujol shows absorption in the region near 2950 cm–1 for (CH), at 1450 cm–1 for asy (methylene and methyl group CH) and 1380 cm–1 for sym (methyl group CH).
If Nujol absorption is severe in a region of interest, chlorinated(hexachlorobutadiene) or fluorinated(Fluorolube) oils can be used.
B) KBr pellet
1 mg of sample is mixed with 100 mg of dry KBr (spectroscopic grade) powder in a mortar, the mixture is then compressed under ~60MPa(60atm: 5000~10,000 Kg at 5 mmHg) in a die to form a transparent pellet(=disc) pellet. And the pellet is mounted in a suitable holder and then can be placed directly into the spectrometer. Properly made pellets are quite clear and the KBr is transparent in the IR region out of ~25 cm–1.
Many substances tend to react with KBr under pressure or even while mixing. Thus, with unknown samples it is usually wise to obtain a spectrum of the material in a mull as well for comparison purposes. In addition, KBr is quite hygroscopic and the spectra obtained are difficult to reproduce.
While mulls and pellets are satisfactory for qualitative analysis, neither technique is well suited for quantitative analysis.
Infrared transmitting materials
Pure liquid(neat) substances
A drop of the pure liquid is placed between two NaCl plates which are then clamped together in a demountable cell. Spectra of pure liquids often show strong intermolecular hydrogen bonding and association effects.
Solution samples
The first problem when using solution samples for IR spectrometry is to find a suitable solvent. Choice of solvent depends on the region of the spectrum of most interest. By using “window areas”, that is, transparent areas of the solvent, the whole spectrum may be covered. For instance, the most common use of carbon tetrachloride is from 4000 to 1300 cm–1 and for carbon disulfide, 1300 to 660 cm –1. NaCl cells are employed, the most useful thickness being 0.1 mm and 0.5 mm.
M2000 FT-IR spectrometers
http://www.midac.com/m_series.htm
How to Operate MIDAC Spectrometer The program that we are using to operate spectrometer is called LAB CALC To start Lab Calc from Windows
1. Open File Manager
2. Find an lc (lc stands for LAB CALC) directory
3. On the right side of the File Manager window find a file named lc.exe and press Enter
4. When MIDAC FT-IR screen appears press any key
Alternative way to start Lab Calc from Windows
1. In Program Manager find a START UP icon
2. In Start Up window find MS-DOS FT-IR icon and click on it
3. When MIDAC FT-IR screen appears press any key
Before running any samples you have to set up parameters
1. When Lab Calc screen appears press F2 key (F2 = Menu)
2. After pressing this key next screen appears and you will see the following menu at the bottom of the screen
http://patsy.hunter.cuny.edu/GStud/pevsner/midac.htm
Environment Collect Arithmetic I-Peak File Draw Plot Text Quit
3 Environment will be highlighted and you will also see a submenu directory
Template+ parms
Mode Display
Limits
Axes
FileSave
Windows
Status
Collar
- Choose Template and press Enter
- Another (pink) submenu appears: choose Master Method press Enter
- Then you will see a yellow submenu choose STD-IR and press enter
4. Press F2 key again -Choose Mode Display/Paged and press Enter
5. Press F2 key -Choose Directory
A pink "Enter Default Directory Window" will appear
Type a directory in which you want to store you data. For example, if I want to store the data in my file I would type c:\alex . Spectra of my samples will automatically be stored in this directory. There is a directory called U761 where your spectra can be stored. Each group should also create their own subdirectory in U761 and stored their files in there. For example, suppose I was assigned to the first group. I would create a subdirectory called one in the directory U761 . Therefore when it comes to choosing a default directory I would type C:\U761\one.
6. Press F2 key
- Choose Filesave/Autosave and press Enter You done with Environment, press to highlight Collect and press Enter.
On the bottom of the screen you will see the following menu
Name Memo Type Gain Resolution Scans Align Begin
Remember before you run any samples you have to take a spectrum of background. Background is also called reference.
1. Highlight Name and press Enter
- Type the name of your reference 4. Highlight Memo
-Type background or reference
5. Highlight Type
-Choose Reference. The sample that you will run know will be taken as the reference. You have to take spectrum of the reference only once. Computer will automatically store reference spectrum in its memory. Every time you run your sample, computer will use the last background spectrum that you took as the reference.
6. Highlight Gain and type 0 7. Highlight Resolution, choose 2 cm
8. Highlight Scans and type 10
9. Now you ready to take run a spectrum. Highlight Begin and press Enter
When Spectrometer finished scanning, a screen with the spectrum will appear. In the lower right corner of the window you will see the question
Return to Collect ? Yes NO
If you want to play with the spectrum choose NO, if don’t choose Yes
You have the spectrum of a background. Now you ready to take the spectrum of your analyte.
1. Do through the same steps as you did for reference except one thing
2. When you get to the Type choose Absorbance
3. Gain, Resolution and number of Scans will be the same as before
To Quit Lab Calc
-Press F2
-Highlight Quit/Yes
Processing of spectra is done on another computer, therefore you data files have to be
copied on the floppy disk. To do that
-Open File Manager and find your directory
-On the right side of the screen you will see the files that are stored in your directory. All of them have spc extension. Highlight the files you want to copy.
- From File menu choose Copy , type b:\ and press Enter
How to approach the analysis of an IR spectrum1. Is a carbonyl group present ? C=O 1820~1660 cm–1 (strong absorption)
2. If C=O is present, check the following types. (If absent, go to 3)
Acids is OH also present ? OH 3400~2400 cm–1 (broad absorption)
Amides is NH also present ? NH 3500 cm–1 (medium absorption)
Esters is C-O also present ? C-O 1300~1000 cm–1 (strong absorption)
Anhydrides have two C=O absorptions near 1810 and 1760 cm–1.
Aldehydes is aldehyde CH present ? Two weak absorptions near 2850 and 1760 cm–1 .
Ketones The above 5 choices have been eliminated.
3. If C=O is absent
Alcohols / Phenols check for OH OH 3600~3300 cm–1 (broad absorption) C-O 1300~1000 cm–1 .
Amines check for NH NH 3500 cm–1 . (medium absorption)
Ethers Check for C-O (and absence of OH) 1300~1000 cm–1 .
4. Double bons and / or aromatic rings
C=C 1650 cm–1 (weak absorption) aromatic ring 1650~1450 cm–1
aromatic and vinyl CH 3000 cm–1
5. Triple bonds CN 2250cm–1 (sharp absorption) C C 2150cm–1 (sharp absorption)
acetylenic CH 3300 cm–1
6. Nitro group two strong absorptions at 1600~1500 cm–1 and 1390~1300 cm–1
7. Hydrocarbons none of the above are found, CH 3000 cm–1 (major absorption)
IR spectrum of n-butanal (n-butyraldehyde).
Frequency of various group vibrations in the group frequency region and in the fingerprint region.
