IR Spectroscopy

1. ⦁ MS: Mass spectrometry
◦ light/electromagnetic radiation is not employed
◦ Molecular mass and formula
◦ Limited structural information
⦁ IR: Infrared spectroscopy
◦ Functional groups
⦁ UV/Vis: Ultraviolet/visible spectroscopy
◦ Conjugated double bonds

2. Functional
groups
Conjugated systems,
chromophores
Energy
Wavenumber

3. ⦁ ΔE = h ν = h c / λ
⦁ ν = c / λ
⦁ .
⦁ High frequency, large wavenumber and short
wavelength  high E

4. scissor rock twist wag
•Cause of the vibrations: change in dipole moment  absorption bands
•Thus, if no change in dipole moment  NO PEAK! (ex.: symmetrical
alkyne, H-CC-H)
•Higher energy IR absorptions: stretching  presence of a functional
group in a molecule
•Lower energy region: bending “fingerprint” region, whole molecule
information
•Intensity of absorption band  size of change in dipole moment
Energy
IR spectroscopy

5. Regions of the Infrared Spectrum
What affects frequency?
Remember: high frequency (λ) = large wave number ( ) = high E
1.Larger mass  lower frequency  lower E
2.Stronger bond  higher E needed
a.Higher bond polarity  higher intensity and higher frequency
b. Bond order (relative “stiffness” of the bond) higher order  higher E
C≡C > C=C > C-C , C=O > C-O, C ≡N > C=N > C-N

6. Compare effects of atomic masses and bond rigidity

8. Hetero atoms on the IR
• Observe difference between 1, 2 and 3 amines:
•N-H peaks can be broader because of the H bonding.
• 1 and 2 amines will exhibit N-H stretches in the 3300-3500cm-1 region.
• 1 amines will show 2 such peaks, while 2 amines will have only 1.
• 3 amines will have no such peaks.
•OH peaks are always broad because of intense H bonding, except in very
dilute solutions.

9. Additional effects:
3. Effects of e-delocalization:
Is the neighboring group/atom e-withdrawing or e-donating?
Double bonds with single bond character  lower frequency

11. Practice problems: indicate the functional groups in the following compounds.

12. Indicate the positions of functional groups in the following spectra.

15. ⦁ Nominal mass vs exact mass
⦁ Exact mass used to calculate fragment’s mass
⦁ m/z  molecular mass of the compound, or
of a fragment
⦁ Fragmentation  structural info

16.  [M+1] peak
Large [M+2] peak
Small [M+2] peak

17. Peak @ m/z-2 

18. α-cleavage
⦁ Weakest bond or most stable fragment
⦁ Cleavage: to obtain the most stable
fragment
⦁ e- can leave f
-
rom lone pair, while X
retains the e
⦁ Bond strength:
◦ C-Br < C-C  heterolytic cleavage
◦ C-Cl ~ C-C  heterolytic and homolytic
cleavage

19. ⦁ α-cleavage of C-C bond, homolytic
⦁ α-cleavage of C-C bond, heterolytic

20. More stable
Less stable

21. ⦁ McLafferty rearrangement