Increasing frequency
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3
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Absorption spectrum for 1,3-butadiene
CH2=CH-CH=CH2
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Normal Vibrational Modes
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IR Absorption Intensity
• Overall peak intensity is related to the concentration of the
sample
• Relative peak intensity is additive: A large number of similar
groups (e.g., alkyl) will increase the intensity of a given peak
• Relative peak intensity is also due to the dipole moment
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Dipole Moments in IR
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Dipole Moments in IR
• Recall: Dipole moment is related to the charge
separation and distance between two atoms
• As the bond stretches, the dipole increases
• As the bond compresses, the dipole decreases
• With a match in frequency the bond dipole
gains energy as the light wave loses energy
12.3 Infrared Absorption and Chemical Structure
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Dipole Moments in IR
• The electric field of a light wave cannot
interact with a bond that has no dipole
• Bonds with no dipole will not absorb in IR
• Conversely, groups with large dipoles (e.g.,
C=O, O-H) provide intense absorptions
12.3 Infrared Absorption and Chemical Structure
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Dipole Moments in IR
• Molecular vibrations that occur but do not
give rise to IR absorption are said to be
infrared-inactive
• Any vibration that does give rise to an
absorption is said to be infrared-active
12.3 Infrared Absorption and Chemical Structure
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IR Spectra of Alkanes
• C-H stretching: 2850-2960 cm-1
• C-H bending: fingerprint
12.4 Functional-Group Infrared Absorptions
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IR Spectra of Alkyl Halides
•
•
•
•
Normally at the low-wavenumber end
Commonly obscured by other peaks
C-F stretch: 1000-1100 cm-1
MS and NMR are more useful for identifying
alkyl halides
12.4 Functional-Group Infrared Absorptions
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IR Spectra of Alkenes
12.4 Functional-Group Infrared Absorptions
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IR Spectra of Alkenes
12.4 Functional-Group Infrared Absorptions
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IR Spectra of Alkenes
12.4 Functional-Group Infrared Absorptions
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IR Spectra of Alkenes
12.4 Functional-Group Infrared Absorptions
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IR Spectra of Alcohols and Ethers
• O-H stretch (H-bonded): 3200-3400 cm-1
• O-H stretch (not H-bonded): 3600 cm-1
• C-O stretch: 1050-1200 cm-1 (ROH and ethers)
12.4 Functional-Group Infrared Absorptions
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Problems
1) The IR spectrum of phenylacetylene is shown
below. Which absorption bands can you
identify?
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2) Which of the following compounds most
likely corresponds to the IR spectrum below?
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The Infrared Spectrometer
• Most modern IR spectrometers are Fouriertransform spectrometers
• Liquid samples can be analyzed undiluted
(neat), as a mineral oil dispersion (mull), or as
a solution (CHCl3 or CH2Cl2 as solvent)
• Solid samples can be analyzed as a fused KBr
pellet
12.5 Obtaining an Infrared Spectrum
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Mass Spectrometry
• Spectroscopic technique used for:
– Determination of molecular mass
– Determination of partial or whole molecular structure
– Confirmation of suspected molecular structure
• The instrument used is a mass spectrometer
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Electron-Impact Mass Spectra
• The sample is vaporized in a vacuum and
subjected to an electron beam of high energy
• The energy of the beam is typically ~70 eV
(6700 kJ/mol)
• This easily exceeds that of chemical bonds
• A radical-cation is produced
12.6 Introduction to Mass Spectrometry
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Fragmentation Reactions
12.6 Introduction to Mass Spectrometry
37
• Each of the fragments are separated according to
their mass-to-charge ratio (m/z)
• Only ions appear in the mass spectrum
– neutral molecules and radicals do not appear
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The Mass Spectrum of Methane
• Molecular ion (M): The ion derived from electron ejection
only (no fragmentation)
• Base peak: The ion of greatest relative abundance in the
spectrum
• M and base peak are commonly different
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