Spectroscopy

1
Spectroscopy

• Molecules move
• Movement can be monitored with electromagnetic
  radiation, e.g. light

2
Energy of Light
Fig. 18.2 Bailey and Bailey

• Visible light only a small portion of
  electromagnetic spectrum
• Short wavelength, high energy

Fig. 18.1 Bailey and Bailey
3
Molecular Motions

• Stretch
• Twist
• Scissor
• Rock-n-roll
• Others

4
Relating Motion and Energy

• We will not be completely quantitative

5
Points to Make

• As you can see, stretching gets the most
  attention
• Short wavelength (high energy) portion called the
  functional group region (4000-1300 cm-1)
• 1300-909 cm-1 referred to as fingerprint region

6
One Angle

• In the following molecule, predict what molecular
  motions should be observed in an IR spectrum

7
One Angle

• Where the bands should appear (looked up not
  memorized)
• CH stretch
• CH3 2962 and 2872 cm-1
• CH2 2926 and 2853 cm-1
• CH bend
• CH3 1375 and 1450 cm-1
• CH2 1465 cm-1

8
The Other Angle

• Using Fig 18.4, identify the following molecule.
  The formula is C6H11

9
Ultraviolet (UV) Spectroscopy

• Very little useful information obtained
• One saving grace is that C-C bonds can be
  detected 212 nm
• Another saving grace is that conjugated compounds
  can be detected

10
Nuclear Magnetic Resonance (NMR) Spectroscopy

• Much more information can be obtained
• Like IR, molecules should be relatively simple
• Equivalent hydrogen atoms
• Nuclei flip in a magnetic field of the correct
  strength
• Position in ppm (0 for TMS internal standard)
• Integration used to determine number of Hs
• Shifting of peaks
• Splitting of peaks

11
NMR Spectroscopy Problems

• A compound has the formula C2H6O and yields 1
  peak by NMR spectroscopy. Propose a structure.

12
NMR Spectroscopy Problems

• The molecule shown below gives a singlet, a
  quartet, and a triplet in 323 ratio. At what
  ppm do these peaks show up as? (reference sheet)

13
NMR Spectroscopy Problems

• A compound has the formula C9H10O2 and yields the
  following NMR spectrum. Propose a structure.

14
NMR Spectroscopy Problems

• The molecule shown below gives a singlet, a
  septet and a doublet in a 316 ratio. At what
  ppm should these peaks show up as? (reference
  sheet)

15
NMR Spectroscopy Problems

• The molecule shown below gives a triplet and a
  quartet in a 32 ratio. At what ppm do these
  peaks show up as? (reference sheet)

16
NMR Spectroscopy Problems

• A compound has the formula ___ and yields the
  following NMR spectrum. Propose a structure.

C5H10O
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Mass Spectroscopy

• Analogy of smashing a vase with a hammer, and
  determining what the vase looked like by putting
  the pieces back together
• Difficulty is that all bonds can potentially
  break, but not all bonds break equally

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Important Mass Spectroscopy Terms

• Molecular ion- peak where molecule is not broken,
  just lost an e (ABC in this figure)
• Fragment ions- all other ions
• Base peak- most abundant peak in spectrum (a
  fragment ion)

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Mass Spectroscopy Problems

• What should the molecular ion of CH3Br exist as?
• Below is the mass spectrum of 2,2-dimethylpropane.
  Indicate the identity of the major peaks

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18.22a, 18.23f

• Write a structure consistent with the following
  data A straight-chained ketone with the formula
  C8H16O (M128) and major m/e ions at 29, 57, 71,
  and 99.

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End of Material for Exam I