Infrared Spectroscopy Chapter 12. Table 12.1, p.472 Energy.
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Transcript of Infrared Spectroscopy Chapter 12. Table 12.1, p.472 Energy.
Infrared Spectroscopy
Chapter 12
Table 12.1, p.472
Energy
Final Exam Schedule, Thursday, May 22, 10:30 AM
Fang, MD10A
Kunjappu, MD10B
Kunjappu, MD10C
320A
Metlitsky, MD10D 1127N
Zamadar 2143N
Infrared spectroscopy causes molecules to vibrate
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.)
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)
H
H
H
H
F
F
H
H
Consider C=C bond stretch…
ethylene 1,1 difluoro ethylene
What about 1,2 difluoro ethylene?
Table 12.4, p.478
Different bonds have different resistances to stretching, different frequencies of vibration
Figure 12.2, p.475
Frequency, measured in “reciprocal centimeters”, the number of waves in 1 cm distance.
Energy.
wavelength
Typical Infra-red spectrum.
Figure 12.2, p.475
C=OC-H“fingerprint region”, complex vibrations of the entire molecule.
Vibrations characteristic of individual groups.
Table 12.5, p.480
BDE of C-H
414
464
556
472
Table 12.5, p.480
BDE and CC stretch
727
966
376
Figure 12.4, p.480
Alkane bands
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 see1. Alkyne C-H2. 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
O
CH3sp3
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