Infrared Spectroscopy
Chapter 12
Energy
Table 12.1, p.472
Final Exam Schedule, Thursday, May 22, 10:30 AM
Fang, MD10A
Kunjappu, MD10B
Kunjappu, MD10C
Metlitsky, MD10D
1127N
Zamadar
2143N
320A
Infrared spectroscopy causes
molecules to vibrate
A non-linear molecule having n atoms may have many different vibrations. Each
atom can move in three directions: 3n. Need to subtract 3 for translational motion
and 3 for rotations
# vibrations = 3 n – 6
(n = number of atoms in non-linear molecule)
Infrared radiation does not cause all possible vibrations to vibrate.
For a vibration to be caused by infrared radiation (infrared active) requires that the
vibration causes a change in the dipole moment of the molecule. (Does the
moving of the atoms in the vibration causes the dipole to change. Yes: should
appear in spectrum. No: should not appear.)
Consider
C=C bond
stretch…
H
H
F
H
H
H
F
H
ethylene
1,1 difluoro ethylene
What about 1,2 difluoro ethylene?
Different bonds have different resistances to stretching, different frequencies of vibration
Table 12.4, p.478
Typical Infra-red spectrum.
wavelength
Frequency, measured in “reciprocal centimeters”, the number
of waves in 1 cm distance.
Energy.
Figure 12.2, p.475
C-H
C=O
“fingerprint region”, complex
vibrations of the entire molecule.
Vibrations characteristic of
individual groups.
Figure 12.2, p.475
BDE of C-H
414
464
556
472
Table 12.5, p.480
BDE and CC stretch
376
727
966
Table 12.5, p.480
Alkane bands
Figure 12.4, p.480
Recognition of Groups: Alkenes (cyclohexene).
Compare these two
C-H stretches
Sometimes weak
if symmetric
Recognition of Groups: Alkynes (oct-1-yne)
This is a terminal alkyne and we expect to see
1. Alkyne C-H
2. Alkyne triple bond stretch (asymmetric)
Recognition of Groups: Arenes. (methylbenzene, toluene)
Out-of-plane bend;
strong
Recognition of Groups: Alcohols
The O-H stretch depends on whether there is hydrogen bonding present
Compare –O-H vs -O-H….O Hydrogen bonding makes it easier to move the H
with H bonding as it is being pulled in both directions; lower frequency
Recognition of Groups: Alcohols
Recognition of Groups: Ethers
No O-H bond stretch present but have C-O in
same area as for alcohol.
C-O stretch in assymetric ethers
sp3
CH3
O
sp2
Recognition of Groups: Amines
Easiest to recognize is N-H bond stretch: 3300 – 3500 cm-1. Same area as
alcohols. Note tertiary amines, NR3, do not have hydrogen bonding.
Hydrogen bonding can shift to lower frequency
Esters
One C=O stretch and two C-O stretches.
Recognition of Groups: Carbonyl
C=O stretch can be recognized reliably in area of 1630 – 1820 cm-1
•Aldehydes will also have C(O)-H stretch
•Esters will also have C-O stretch
•carboxylic acid will have O-H stretch
•Amide will frequently have N-H stretch
•Ketones have nothing extra
What to check for in an IR
spectrum
C-H vibrations about 3000 cm-1 can detect vinyl and terminal alkyne
hydrogens.
O-H vibrations about 3500 cm-1
C=O vibrations about 1630 – 1820 cm-1
C-O vibrations about 1000-1250 cm-1