I. Spectroscopy

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I. Spectroscopy

• Study of the interaction of electromagnetic
  radiation (light)
• with molecules

A. Electromagnetic Radiation
- not just visible light!!
http//www.dnr.sc.gov/ael/personals/pjpb/lecture/s
pectrum.gif
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1. Wave Behavior of Light
a. Wavelength (?)

• often given in cm or nm

b. Frequency (?)

• of wave cycles that pass a point in a second
• s-1

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2. Particle Behavior
a. Photon massless packet of energy
b. A little bit of math!

• First c ??, where c is the speed of light
• (3.00 x 108 m s-1)
• ? h? , where h is Plancks Constant
• since we know that ? c/?
• then we also know that ? hc/?

• So??
• this means that the energy of the light is
  directly
• proportional to the frequency
• energy of light is inversely proportional to
  wavelength

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3. Absorption

• light of particular energy can be absorbed by a
  molecule
• resulting in the molecule adopting an excited
  state
• - this absorption can be measured!!

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B. Infrared (IR) Spectroscopy

• Upon absorption of light in the infrared region
  of the
• electromagnetic spectrum by organic molecules
• results in molecular motion

1. Types of Motion
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2. What we observe

• data is recorded using wavenumbers ( ) where
• 1/?, (cm-1)
• this means energy is directly proportional to
• wavenumber, i.e. higher wavenumber, higher
  energy
• can think of as weights on springs

a. Effect of Atom Type
C(sp3) H 2960 - 2850 cm-1 O
 H (alcohol) 3600 - 3200 cm-1
b. Effect of Bond Strength
CC 1680 - 1620 cm-1 CC 2260 - 2100 cm-1
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3. Sample Spectrum
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4. Table of IR Data
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5. Trends
a. Hydrogen Region 3330 - 2700 cm-1
O H(alc) 3650 - 3200 cm-1 O H(CA) 3300 -
2500 cm-1 N H(amine) 3500 - 3250 cm-1 N
H(amide) 3500 - 3400 cm-1
C(sp3) H 2960 - 2850 cm-1 C(sp2) H 3150 -
3050 cm-1 C(sp) H 3300 cm-1
b. Triple Bond Region 2300 - 2000 cm-1
CC(alkyne) 2260 - 2100 cm-1 CN(nitrile)
2260 - 2220 cm-1
c. Double Bond Region 1900 - 1500 cm-1
C O(ald) 1740 - 1690 cm-1 C O(ket) 1750 -
1680 cm-1 C O(est) 1750 - 1735 cm-1
C O(CA) 1760 - 1710 cm-1 C O(am) 1690 - 1630
cm-1 C C 1680 - 1620 cm-1
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5. Trends
d. Fingerprint Region
- Not much information, except!! C(sp2, aromatic,
bending) H Mono-substituted 770 - 730, 720 -
680 cm-1 Di-substituted (ortho) 770 - 735
cm-1 Di-substituted (meta) 810 - 750
cm-1 Di-substituted (para) 860 - 800 cm-1
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II. Sample Spectra
A. Spectrum 1

• strong versus medium versus weak Why?
• Dipole moment of molecule must change as
  vibration occurs
• Number of bonds absorbing

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B. Spectrum 2

• Sharp versus Broad Signals Why?
• multiple rotational energy levels in each
  vibrational energy level
• ??

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Multiple Rotational Energy Levels??
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C. Spectrum 3 What functional groups are
present?
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D. Spectrum 4 What functional groups are
present?
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E. Spectrum 5 What functional groups are
present?
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F. Spectrum 6 Propose a structure for
a compound with MF C7H12O
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G. Spectrum 7 Propose a structure for
a compound with MF C5H12O