1. Infrared Spectroscopy
Energy Absorption and Vibration
IR electromagnetic radiation is just less energetic than visible light
This energy is sufficient to cause excitation of vibrational energy levels
Wavelength (l) = x 10-6 m
n = wavenumbers. Larger n = higher energy
Excitation depends on atomic mass and how tightly they are bound
Hooke’s Law for 2 masses connected by a spring
C—H Bond: Reduced Mass = (12+1)/(12x1) = 13/12 = 1.08
C—C Bond: Reduced Mass = (12+12)/(12x12) = 24/144 = 0.167
k = constant
f = force constant = bond strength
m-term = reduced mass

2. Many possible absorptions per molecule exist: stretching, bending,…
Vibrational modes leading to IR absorptions:

4. Using IR in Organic Chemistry
Functional Groups have characteristic IR absorptions
Fingerprint Region ( cm-1) is unique for every molecule and lets us match an unknown with a known spectrum

5. Regions of the Infrared Spectrum
cm-1 N-H, C-H, O-H (stretching)
N-H, O-H
3000 C-H
cm-1 CºC and C º N (stretching)
cm-1 double bonds (stretching)
C=O
C=C cm-1
Below 1500 cm-1 “fingerprint” region

6. 12.8 Infrared Spectra of Some Common Functional Groups
Alkanes, Alkenes, Alkynes
C-H, C-C, C=C, C º C have characteristic peaks
absence helps rule out C=C or C º C

10. IR of Alkenes
Alkene C—H absorbs at higher energy than alkanes because the force constant is stronger than alkanes (sp2 hybridization)
Substitution pattern of alkenes give characteristic absorptions
Terminal alkenes give 910, 990 cm-1
Geminal disubstituted gives 890 cm-1
trans disubstituted gives 970 cm-1

12. IR: Aromatic Compounds
Weak C–H stretch at 3030 cm1
Weak absorptions cm1 range
Medium-intensity absorptions 1450 to 1600 cm1

14. IR: Alcohols and Amines
O–H 3400 to 3650 cm1
Usually broad and intense
N–H 3300 to 3500 cm1
Sharper and less intense than an O–H

17. IR: Carbonyl Compounds
Strong, sharp C=O peak 1670 to 1780 cm1
Exact absorption characteristic of type of carbonyl compound
1730 cm1 in saturated aldehydes
1705 cm1 in aldehydes next to double bond or aromatic ring

18. 1715 cm1 in six-membered ring and acyclic ketones
C=O in Ketones
1715 cm1 in six-membered ring and acyclic ketones
1750 cm1 in 5-membered ring ketones
1690 cm1 in ketones next to a double bond or an aromatic ring
1735 cm1 in saturated esters
1715 cm1 in esters next to aromatic ring or a double bond
C=O in Esters