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7 CH 2 =CH-CH=CH 2 Absorption spectrum for 1,3-butadiene

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Normal Vibrational Modes 16

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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 19

Dipole Moments in IR 20

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 Infrared Absorption and Chemical Structure

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 Infrared Absorption and Chemical Structure

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 Infrared Absorption and Chemical Structure

IR Spectra of Alkanes C-H stretching: cm -1 C-H bending: fingerprint Functional-Group Infrared Absorptions

IR Spectra of Alkyl Halides Normally at the low-wavenumber end Commonly obscured by other peaks C-F stretch: cm -1 MS and NMR are more useful for identifying alkyl halides Functional-Group Infrared Absorptions

IR Spectra of Alkenes Functional-Group Infrared Absorptions

IR Spectra of Alkenes Functional-Group Infrared Absorptions

IR Spectra of Alkenes Functional-Group Infrared Absorptions

IR Spectra of Alkenes Functional-Group Infrared Absorptions

IR Spectra of Alcohols and Ethers O-H stretch (H-bonded): cm -1 O-H stretch (not H-bonded): 3600 cm -1 C-O stretch: cm -1 (ROH and ethers) Functional-Group Infrared Absorptions

Problems 1)The IR spectrum of phenylacetylene is shown below. Which absorption bands can you identify? 31

2)Which of the following compounds most likely corresponds to the IR spectrum below? 32

The Infrared Spectrometer Most modern IR spectrometers are Fourier- transform spectrometers Liquid samples can be analyzed undiluted (neat), as a mineral oil dispersion (mull), or as a solution (CHCl 3 or CH 2 Cl 2 as solvent) Solid samples can be analyzed as a fused KBr pellet Obtaining an Infrared Spectrum

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 34

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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 Introduction to Mass Spectrometry

Fragmentation Reactions Introduction to Mass Spectrometry

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 38

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 39