Download - Molecular Vibrations and IR Spectroscopy
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Molecular Vibrationsand
IR Spectroscopy
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VibrationsWhat is a vibration in a molecule?
Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bondshttp://academic.pgcc.edu/~ssinex/VibMol/form.html
Can a vibration change the dipole moment of a molecule?
Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active. Symmetrical stretching/bending does not. Not IR active
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Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of 2500-15,000 nm or 2.5 – 15 m or mcm)
What wavelength of electromagnetic radiation is involved in causing vibrations in molecules?
For a vibration at 4111 cm-1 (the stretch in H2), how many vibrations occur in a second?120 x 1012 vibrations/sec or a vibration
every 8 x 10-15 seconds!WOW !!!!!
120 trillion vibration per second!!!!
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How does the mass influence the vibration?
H2 I2
MM =2 g/moleMM =254 g/mole
The greater the mass - the lower the wavenumber
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How much movement occurs in thevibration of a C-C bond?
For a C-C bond with a bond length of 154 pm, the variation is about 10
pm.
For C-C-C bond angle a change of 4o is typical.This moves a carbon atom about 10 pm.
4o 10 pm
10 pm
154 pm
stretching vibration
bending vibration
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Vibrational Distortions for CCl2FCClF2
http://www2.ess.ucla.edu/~schauble/MoleculeHTML/CCl2FCClF2_html/
CCl2FCClF2_page.html
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A little physics of electromagnetic radiation
• Energy (E) E = h = hc/ – where h is Planck’s constant, c is the speed of
light, is frequency or the number of vibrations per secondand is the wavelength
• Wavenumber (’) ’ = 1/ – given in cm-1
• Period (P) P = 1/– the time between a vibration
= hc’
Energy, frequency, and wavenumber are directly proportional to each other.
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What type of vibrations would occur in pentane?
Let’s examine the IR spectrum of pentane.
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IR spectrum
Increasing wavenumber (energy, frequency)
Increasing absorption of IR radiation
Increasing wavelength
C-Hstretching
C-Cbending
C-Hbending
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IR Spectra of chloroform and deuterochloroform
Mode of vibration CHCl3Calculated
*
CHCl3Measured
CDCl3Measured
C-H stretching 3002 3020 2256C-H bending 1120 1219 912
C-Cl stretching 701 773 737C-Cl bending 418 671 652* Spartan ’02 AM1 minimization
Shift of peak due to replacement of H with D (2x mass)In
crea
sing
abso
rban
ce
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Some results• Calculated values using computational
software give lower wave numbers
• Increasing mass of substituted atoms shifts wave numbers to lower values
(Excel spreadsheet)
• Stretching energies > bending energies > internal rotation energies (occur at higher wavelengths)
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Does the stretching energy have any relationship to the
strength of the bond?Wavenumber vs. Bond Energy
010002000300040005000
200 300 400 500 600Bond energy (kJ / mole)
Wav
enum
ber (
cm-1)
W = 6.3286BE + 401.38
r2 = 0.7979
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Let’s examine the carbonyl group on three compounds
formaldehyde phosgene acetone
How does the C=O stretching energy compare for these three molecules?
2053 cm-1 1951 cm-1 2063 cm-1
The carbonyl group has a range of 1700-3000 cm-1.
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Functional group analysis in organic compounds
• Unlike atomic spectroscopy where sharp energy transitions occur due to well quantized electron transitions, molecular spectroscopy tends to show bands.
• Molecular vibrations are influenced by the surrounding groups!
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4000 3000 2000 1000
WAVENUMBER (cm-1)
Basic Functional Groups
C-H
O-H
CC C=C
alkenesaromatic
C=O C-O
C-H
O-Hbend
ing
stre
tchi
ng
C-C
400
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http://wwwchem.uwimona.edu.jm/spectra/iranim/index.html
Animations Showing Vibrations and Accompanying
Spectrahttp://www.chem.umass.edu/~nermmw/Spectra/irspectra/index.htm
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Use of IR spectra• Identification of functional groups on a
molecule – this is a very important tool in organic chemistry
• Spectral matching can be done by computer software and library spectra
• Since absorbance follows Beer’s Law, can do quantitative analysis