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, MD10D1127N Zamadar2143N

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) 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=O C-H “fingerprint region”, complex vibrations of the entire molecule. Vibrations characteristic of individual groups.

Table 12.5, p.480 BDE of C-H

Table 12.5, p.480 BDE and CC stretch

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 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

Recognition of Groups: Amines Easiest to recognize is N-H bond stretch: 3300 – 3500 cm -1. Same area as alcohols. Note tertiary amines, NR 3, 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 cm -1