1. 1 Increasing frequency

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

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16. Normal Vibrational Modes 16

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19. 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

20. Dipole Moments in IR 20

21. 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 21 12.3 Infrared Absorption and Chemical Structure

22. 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 22 12.3 Infrared Absorption and Chemical Structure

23. 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 23 12.3 Infrared Absorption and Chemical Structure

24. IR Spectra of Alkanes C-H stretching: 2850-2960 cm -1 C-H bending: fingerprint 24 12.4 Functional-Group Infrared Absorptions

25. 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 25 12.4 Functional-Group Infrared Absorptions

26. IR Spectra of Alkenes 26 12.4 Functional-Group Infrared Absorptions

27. IR Spectra of Alkenes 27 12.4 Functional-Group Infrared Absorptions

28. IR Spectra of Alkenes 28 12.4 Functional-Group Infrared Absorptions

29. IR Spectra of Alkenes 29 12.4 Functional-Group Infrared Absorptions

30. 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) 30 12.4 Functional-Group Infrared Absorptions

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

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

33. 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 33 12.5 Obtaining an Infrared Spectrum

34. 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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36. 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 36 12.6 Introduction to Mass Spectrometry

37. Fragmentation Reactions 37 12.6 Introduction to Mass Spectrometry

38. 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

39. 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