15-1 infrared spectroscopy unit 15. 15-2 electromagnetic radiation electromagnetic radiation: ...
TRANSCRIPT
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15-1
Infrared Spectroscopy
Unit 15
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15-2
Electromagnetic Radiation
Electromagnetic radiation: light and other forms of radiant energy
Wavelength (): the distance between consecutive peaks on a wave
Frequency (): the number of full cycles of a wave that pass a given point in a second
Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”)
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15-3
Electromagnetic Radiation
Common units used to express wavelength
Angstrom (Å) 1 Å = 10-10 m
Relationto MeterUnit
1 mm = 10-3 m
1 nm = 10-9 m1 m = 10-6 m
Nanometer (nm)Micrometer (m)
Millimeter (mm)Meter (m) ----
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15-4
Molecular Spectroscopy
Molecular spectroscopy: the study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure• we study three types of molecular spectroscopy
Vibrational energy levels
Nuclear spin states
Electronic energy levels
Infrared
Radio fequency
Ultraviolet-visible
Absorption ofElectromagneticRadiation Results in Transition Between
Region of theElectromagneticSpectrum
Nuclear magneticresonance
Infrared
Ultraviolet-visible
Type of Spectroscopy
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15-5
Infrared Spectroscopy
The vibrational IR extends from 2.5 x 10-6 m (2.5 m) to 2.5 x 10-5 m (25 m)• the frequency of IR radiation is commonly expressed
in wavenumbers• wavenumber : the number of waves per centimeter,
with units cm-1 (read reciprocal centimeters)• expressed in wavenumbers, the vibrational IR extends
from 4000 cm-1 to 400 cm -1
()-
n = = 400 cm-1= 4000 cm-1 = n 10-2 m•cm -1
2.5 x 10-6 m
10-2 m•cm -1
2.5 x 10-5 m
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15-6
Infrared Spectroscopy
IR spectrum of 3-methyl-2-butanone
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15-7
Molecular Vibrations
• atoms joined by covalent bonds undergo continual vibrations relative to each other
• the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed
• the energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm-1
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15-8
Molecular Vibrations
For a molecule to absorb IR radiation• the bond undergoing vibration must be polar and• its vibration must cause a periodic change in the bond
dipole moment Covalent bonds which do not meet these criteria
are said to be IR inactive • the C-C double and triple bonds of symmetrically
substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds
H3C
C C
CH3
H3C CH3
H3C-C C-CH3
2,3-Dimethyl-2-butene 2-Butyne
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15-9
Molecular Vibrations
For a nonlinear molecule containing n atoms, there are 3n - 6 allowed fundamental vibrations
For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex
The simplest vibrational motions are bending and stretching
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15-10
Molecular vibrations
Fundamental stretching and bending vibrations for a methylene group
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15-11
Molecular Vibrations
Consider two covalently bonded atoms as two vibrating masses connected by a spring• the total energy is proportional to the frequency of
vibration• the frequency of a stretching vibration is given by an
equation derived from Hooke’s law for a vibrating spring
K = a force constant, which is a measure of the bonds’ strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm
m = reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in grams
K = 4.12
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15-12
Molecular Vibrations
From this equation, we see that the position of a stretching vibration• is proportional to the strength of the vibrating bond• is inversely proportional the masses of the atoms
connected by the bond The intensity of absorption depends primarily on the
polarity of the vibrating bond
K = 4.12
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15-13
Correlation Tables
Characteristic IR absorptions for the types of bonds and functional groups we deal with most often
C-HN-HO-H
C=OC=C
C-O 1000-1250 strong
weak to medium1600-16801630-1820 strong
2700-3300 weak to medium
weak to strong3200-3650medium3100-3550
IntensityStretchingFrequency (cm-1)Bond
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Hydrocarbons
C-H
CH3
C=CC-H
C-HC C
C-C
C-HC=CC-H
Intensity
2850 - 3000 Medium
Stretching 3300 Medium to strongStretching 2100-2250 Weak
Weak to medium1600 - 1680StretchingWeak to medium3000 - 3100Stretching
Alkyne
Alkene
AlkaneVibration
Stretching
Hydro-carbon
Frequency(cm-1)
Bending 1450-1475 MediumBending 1375 and 1450 Weak to medium(Not useful for interpretation - too many bands
AreneStretching 3030 Weak to medium
Medium1450-1600StretchingBending 690-900 Strong
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15-15
Alkanes
IR spectrum of decane
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15-16
Alkenes
IR spectrum of cyclohexene
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15-17
Alkynes
IR spectrum of 1-octyne
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15-18
Aromatics
IR spectrum of toluene
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15-19
Alcohols
• IR spectrum of 1-hexanol
O-H (free)
O-H (H bonded)
C-O
Bond IntensityFrequency, cm-1
Medium1000 - 1250
Medium, broad3200 - 3500
3600-3650 Weak
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15-20
Ethers
IR spectrum of dibutyl ether
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15-21
Ethers
IR spectrum of anisole
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15-22
Amines
IR spectrum of 1-butanamine
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15-23
IR of Molecules with C=O Groups
C=O
C=O
C-H
O HRCOHO
RCHO
RCR'O
C=O
Strong1700-1725StretchingCarboxylic acids
Aldehydes
VibrationCarbonyl GroupFrequency
(cm-1) Intensity
Stretching 1630-1820 StrongStretching 2720 Weak
Stretching 2500-3300 Strong (broad)
KetonesStretching 1630-1820 Strong
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15-24
IR of Molecules with C=O Groups
C=O
sp3 C O
sp2 C O
C=O
C=O
N H
CNRC N
RCOCRO O
RCOR'O
RCNH2
O
C O Stretching 900-1300 Strong
Strong1735-1800StretchingCarboxylic esters
Stretching 1000-1100 Strong
Stretching 1200-1250 Strong
Acid anhydrides
Stretching 1740-1760 and1800-1850
Strong
Strong1630-1680StretchingAmides
Stretching 3200, 3400 Medium(1° amides have two N-H stretches)(2° amides have one N-H stretch)
NitrilesStretching 2200-2250 Medium
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15-25
Aldehydes and Ketones
IR spectrum of menthone
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15-26
Carbonyl groups
The position of C=O stretching vibration is sensitive to its molecular environment• as ring size decreases and angle strain increases,
absorption shifts to a higher frequency
• conjugation shifts the C=O absorption to lower frequency
1850 cm-11780 cm-11745 cm-11715 cm-1
O O O O
1690 cm-1 1700 cm-11717 cm-1
O OH
O
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15-27
Carboxylic acids
IR spectrum of pentanoic acid
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15-28
Esters
IR of ethyl butanoate