SPECTROSCOPIC METHODS OF ORGANIC MOLECULES Aspirin (acetylsalicylic acid) 0 20 40 60 80 100 120 500 1000 1500 2000 2500 3000 3500 4000 wavenumber/ cm -1 %T Wavenumber = 1/ wavelength Professor Ioannis P. Gerothanassis Department of Chemistry, University of Ioannina FEBRUARY 2018
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SPECTROSCOPIC METHODS OF ORGANIC MOLECULES. Basics in IR... · consequence of changing stretching vibrations. Ranges from 4000 to 1600 cm-1. 2. The Fingerprint region: Identifies
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SPECTROSCOPIC METHODS OF ORGANIC MOLECULES
Aspirin (acetylsalicylic acid)
0
20
40
60
80
100
120
5001000150020002500300035004000
wavenumber/ cm-1
%T
Wavenumber = 1/ wavelength
Professor Ioannis P. GerothanassisDepartment of Chemistry, University of Ioannina
What is Infrared? (Cont.)Humans, at normal body temperature, radiate most
strongly in the infrared, at a wavelength of about 10microns (A micron is the term commonly used inastronomy for a micrometer or one millionth of ameter). In the image to the left, the red areas are thewarmest, followed by yellow, green and blue(coolest).
The image to the right shows a cat in the infrared. The yellow-white areas are the warmest and the purple areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture. Notice the cold nose and the heat from the cat's eyes, mouth and ears. 4
Fourier Transform Infrared (FT-IR) Spectroscopy, Prof. V. KrishnakumarProfessor and Head, Department of Physics, Periyar University, Salem – 636 011, India
IR: Masses, Atoms and Springs
F = force, restoring back to equilibrium
position
k = characteristic stretching constant
Δr = displacement from the equilibrium
position
A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds).
Fourier Transform Infrared (FT-IR) Spectroscopy, Prof. V. KrishnakumarProfessor and Head, Department of Physics, Periyar University, Salem – 636 011, India
Let’s examine values as absolute numbers. Which is the vibrational frequency of the C – H bond?
Why ?
where mc and mH are the atomic weights of carbon and hydrogen, respectively, and 6.02x1023 is the Avogadro number.Taking into consideration that
K = 5 X 105 dynes cm-1
C – H = 9 x 1013 Hz or cycles s-1 8
Therefore, the C – H bond vibrates 9 x 1013 times s-1
To avoid such extreme numbers the term wavenumber is introduced
(cm-1)
where C is the velocity of light = 3 x 1010 cm s-1
Therefore, for the C – H bond
≈ 3000 cm-1
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The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency ν can be
transferred to the atoms involved in the vibration
The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon
• In the IR region of the electromagnetic spectrum, the absorption of radiation by a sample is due to changes in the vibrational energy states of a molecule.
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http://social.ocr.org.uk/files/ocr/An Introduction to Infrared and UV-Visible Spectroscopy.ppt
Symmetrical stretching:2 bonds increase or decrease in length simultaneously.
H
H
C
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Introduction and Principle of IR Spectrophotometry, Rajaram Kshetri B.pharm, IOM
Asymmetrical stretchingone bond length is increased and other is decreased.
H
H
C
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Introduction and Principle of IR Spectrophotometry, Rajaram Kshetri B.pharm, IOM
Bending vibrations• Vibration or oscillation not along the line of bond• These are also called as deformations• In this, bond angle is altered• Occurs at low energy: 1400-666 cm-1
• 2 types:a) In plane bending: scissoring, rockingb) Out plane bending: wagging, twisting
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Introduction and Principle of IR Spectrophotometry, Rajaram Kshetri B.pharm, IOM
In plane bendingi. Scissoring:• This is an in plane blending• 2 atoms approach each other• Bond angles are decrease
H
H
CC18
Introduction and Principle of IR Spectrophotometry, Rajaram Kshetri B.pharm, IOM
ii. Rocking:• Movement of atoms take place in the same direction.
H
H
CC
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Introduction and Principle of IR Spectrophotometry, Rajaram Kshetri B.pharm, IOM
Out plane bendingi. Wagging:• 2 atoms move to one side of the plane. They move up and down the
plane.ii. Twisting:• One atom moves above the plane and another atom moves below
the plane.
H
H
CC
H
H
CC
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Introduction and Principle of IR Spectrophotometry, Rajaram Kshetri B.pharm, IOM
INFRARED SPECTROSCOPY
• How much movement occurs in the vibration of a C-C bond?
0.1 Å
1.54 Åstretching vibration For a C-C bond with a bond length of 1.54 Å, the variation is about 0.1 Å.
bending vibration4o 0.1 Å
For C-C-C bond angle a change of 4o
is typical. This moves a carbon atom about 0.1 Å.
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http://social.ocr.org.uk/files/ocr/An Introduction to Infrared and UV-Visible Spectroscopy.ppt
Simple harmonic oscillator
0
3
12
54
ω
( )212 eqE K r r= −
12osc
kωπ µ
=
Energy
Oscillation
12 oscE hν ν ω = +
12 osc
E hhc
ννε ν ω = = +
Permitted energy levels ofthe harmonic oscillator
ν - Vibrational quantum number
If ν = 0, 12 osc
E hhc
ννε ω = =
Zero-point energy
Selection rule for the harmonic oscillator undergoing vibrational changes ∆ν = ± 1
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Fourier Transform Infrared (FT-IR) Spectroscopy, Prof. V. KrishnakumarProfessor and Head, Department of Physics, Periyar University, Salem – 636 011, India
• Bond – not a perfect Elastic nature
• Does not obey exactly simple harmonic motion
• Dissociates
• Not a ideal parabola
( ){ } 21 expeq eqE D a r r = − −
A – constant; Deq – Dissociation energy
Morse curve
Anharmonic Oscillator
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Fourier Transform Infrared (FT-IR) Spectroscopy, Prof. V. KrishnakumarProfessor and Head, Department of Physics, Periyar University, Salem – 636 011, India
IR Stretching Frequencies: What Do they Depend On?
Directly on the strength of the bonding between the two atoms (ν ~ k)
Inversely on the reduced mass of the two atoms (ν ~ 1/m)
Expect: ν will increase with increasing bond strength (bond order) and decreasing mass 24
http://social.ocr.org.uk/files/ocr/An Introduction to Infrared and UV-Visible Spectroscopy.ppt
The spectrophotometer give the spectra of certain wave by indicating the quality of wave (i.e. its wavenumber) and the quantity of wave that how much wave is absorbed by the molecule (Transmittance).
Parameters
Transmittance is the fraction of incident light electromagnetic radiation at a specified wavelength that passes through a sample.(Lambert-Beer Law) It is a ratio so it has no unit. 29
Infrared Spectroscopy, S. YOUSUF RAZA ZAIDI
INTERPRETATION OF INFRARED SPECTRA
The most useful regions are as follows:• 3200-3700 cm-1: various types of O-H and N-H stretching
vibrations.• 2700-3100 cm-1: different types of C-H stretching vibrations. • 1680-1750 cm-1:C=O stretches feature very strongly in IR
spectra and the type of carbonyl group can be determined from the exact position of the peak.
Too many bonds absorb in the region of 600-1600 cm-1 to allow confident assignment of individual bands. However, this region is useful as a fingerprint of a molecule, i.e. if the spectrum is almost identical to an authentic reference spectrum then the structure can be assigned with some confidence.
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http://social.ocr.org.uk/files/ocr/An Introduction to Infrared and UV-Visible Spectroscopy.ppt
Infrared SpectroscopyFor isopropyl alcohol, CH(CH3)2OH, the infrared absorption bandsidentify the various functional groups of the molecule.
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Fourier Transform Infrared (FT-IR) Spectroscopy, Prof. V. KrishnakumarProfessor and Head, Department of Physics, Periyar University, Salem – 636 011, India
The IR SpectrumThere are two type of IR Spectra from which we can obtained the information about the quality of molecule.
1. The Functional Group region: Identifies the functional group with the consequence of changing stretching vibrations. Ranges from 4000 to 1600 cm-1.
2. The Fingerprint region: Identifies the exact molecule with the consequence of changing bending vibrations. Ranges from 1600 to 625cm-1.
Focus your analysis on this region. This is where most stretching frequencies appear.
Fingerprint region: complex and difficult to interpret reliably.
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Infrared Spectroscopy, S. YOUSUF RAZA ZAIDI
IR SPECTRUM OF ALKANESAlkanes have no functional groups. Their IR spectrum displays only C-C and C-H bond vibrations. Of these the most useful are the C-H bands, which appear around 3000 cm-1. Since most organic molecules have such bonds, most organic molecules will display those bands in their spectrum.
IR SPECTRUM OF ALKENESBesides the presence of C-H bonds, alkenes also show sharp, medium bands corresponding to the C=C bond stretching vibration at about 1600-1700 cm-1. Some alkenes might also show a band for the =C-H bond stretch, appearing around 3080 cm-1 as shown below. However, this band could be obscured by the broader bands appearing around 3000 cm-1 (see next slide)
IR SPECTRUM OF ALKYNESThe most prominent band in alkynes corresponds to the carbon-carbon triple bond. It shows as a sharp, weak band at about 2100 cm-1. The reason it’s weak is because the triple bond is not very polar. In some cases, such as in highly symmetrical alkynes, it may not show at all due to the low polarity of the triple bond associated with those alkynes.
Terminal alkynes, that is to say those where the triple bond is at the end of a carbon chain, have C-H bonds involving the sp carbon (the carbon that forms part of the triple bond). Therefore they may also show a sharp, weak band at about 3300 cm-1 corresponding to the C-H stretch.
Internal alkynes, that is those where the triple bond is in the middle of a carbon chain, do not have C-H bonds to the sp carbon and therefore lack the aforementioned band.
IR SPECTRUM OF A NITRILEIn a manner very similar to alkynes, nitriles show a prominent band around 2250 cm-1 caused by the CN triple bond. This band has a sharp, pointed shape just like the alkyne C-C triple bond, but because the CN triple bond is more polar, this band is stronger than in alkynes.
IR SPECTRUM OF AN ALCOHOLThe most prominent band in alcohols is due to the O-H bond, and it appears as a strong, broad band covering the range of about 3000 - 3700 cm-1. The sheer size and broad shape of the band dominate the IR spectrum and make it hard to miss.
Carbonyl compounds are those that contain the C=O functional group. In aldehydes, this group is at the end of a carbon chain, whereas in ketones it’s in the middle of the chain. As a result, the carbon in the C=O bond of aldehydes is also bonded to another carbon and a hydrogen, whereas the same carbon in a ketone is bonded to two other carbons.
Aldehydes and ketones show a strong, prominent, stake-shaped band around 1710 - 1720 cm-1 (right in the middle of the spectrum). This band is due to the highly polar C=O bond. Because of its position, shape, and size, it is hard to miss.
Because aldehydes also contain a C-H bond to the sp2 carbon of the C=O bond, they also show a pair of medium strength bands positioned about 2700 and 2800 cm-1. These bands are missing in the spectrum of a ketone because the sp2
IR SPECTRUM OF A CARBOXYLIC ACIDA carboxylic acid functional group combines the features of alcohols and ketones because it has both the O-H bond and the C=O bond. Therefore carboxylic acids show a very strong and broad band covering a wide range between 2800 and 3500 cm-1 for the O-H stretch. At the same time they also show the stake-shaped band in the middle of the spectrum around 1710 cm-1 corresponding to the C=O stretch.
IR SPECTRA OF AMINESThe most characteristic band in amines is due to the N-H bond stretch, and it appears as a weak to medium, somewhat broad band (but not as broad as the O-H band of alcohols). This band is positioned at the left end of the spectrum, in the range of about 3200 - 3600 cm-1.
Primary amines have two N-H bonds, therefore they typically show two spikes that make this band resemble a molar tooth. Secondary amines have only one N-H bond, which makes them show only one spike, resembling a canine tooth. Finally, tertiary amines have no N-H bonds, and therefore this band is absent from the IR spectrum altogether. The spectrum below shows a secondary amine.
IR SPECTRUM OF AMIDESThe amide functional group combines the features of amines and ketones because it has both the N-H bond and the C=O bond. Therefore amides show a very strong, somewhat broad band at the left end of the spectrum, in the range between 3100 and 3500 cm-1 for the N-H stretch. At the same time they also show the stake-shaped band in the middle of the spectrum around 1710 cm-1 for the C=O stretch. As with amines, primary amides show two spikes, whereas secondary amides show only one spike.
Infrared SpectroscopyA molecule can be characterized (identified) by its molecularvibrations, based on the absorption and intensity of specific infraredwavelengths, however, IR depends upon the state of the matter (solid-liquid-solution-gas phase)