Infrared Spectroscopy

SpectroscopyThe energy of the interaction of

electromagnetic radiaiton (light) with moleculesSeveral different techniques:

Infrared spectroscopy Proton nuclear magnetic resonance Carbon-13 nuclear magnetic resonance

Infrared SpectroscopyProvides a simple and rapid instrumental

technique that can give evidence for the presence of various functional groups. Depends on the interaction of molecules and

atoms with electromagnetic radiationInfrared radiation causes atoms and groups of

atoms of organic compounds to vibrate with increases amplitude about the covalent bonds that connect them.

Infrared Spectrometer

Fourier Transform Infrared SpectrometerSplit radiation beam from the IR source so that it reflects simultaneously from a moving mirror and a fixed mirror, leading to interferencePass through the sample to the dectector and recorded as interferogram

http://www.wooster.edu/chemistry/analytical/ftir/default.html

Electromagnetic Radiation Radiation energy – has

wavelike properties Frequency (ν, Greek nu) – the

number of peaks (maxima) that pass by a fixed point per unit time (s-1 or Hz)

Wavelength (λ, Greek lambda) – the length from one wave maximum to the next

Amplitude – the height measured from the middle point between peak and trough (maximum and minimum)

Intensity of radiant energy is proportional to amplitude

Planck Equation Planck – energy radiated by a heated

object is quantized Radiant energy emitted in discrete units or

quanta The smallest quantity of energy that can be emitted in

the form of electromagnetic radiation

E = hν or h = 6.626 x 10-34 J•s (Planck’s constant) unit of E is J/proton

high energy radiation – higher ν, shorter λ low energy radiation – lower ν, higher λ

Vibration of moleculesTwo atoms are joined by a covalent bond that

can undergo a stretchingTwo factors contributed to stretching

Mass of bonded atomsRelative stiffness of the bond

http://upload.wikimedia.org/wikipedia/commons/6/60/Scissoring.gif

Vibration of molecules

Vibrations of atomsStretch: the distance between bonded atoms

oscillatesBend: bond angles oscillate

H

CH H

CH

Stretch bend

Characteristic Infrared Absorptions of Groups

Group Bond Frequency (cm-1)

Alkyl C-H 2853 - 2962

Alcoohol O-H 3590 - 3650

Amine N-H 3300 - 3500

Bond Frequency range (cm-1)

2100 -2260

2220 - 2260

1620 – 1680

1630 - 1780

C C

C N

C C

C O

A complete characteristics are given in table 2.7 page 79