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