Infrared Spectroscopy

Spectroscopy The energy of the interaction of electromagnetic radiaiton (light) with molecules Several different techniques: Infrared spectroscopy Proton nuclear magnetic resonance Carbon-13 nuclear magnetic resonance

Infrared Spectroscopy Provides 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 radiation Infrared 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 Spectrometer Split radiation beam from the IR source so that it reflects simultaneously from a moving mirror and a fixed mirror, leading to interference Pass through the sample to the dectector and recorded as interferogram y/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 = x Js (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 molecules Two atoms are joined by a covalent bond that can undergo a stretching Two factors contributed to stretching Mass of bonded atoms Relative stiffness of the bond issoring.gif issoring.gif

Vibration of molecules

Vibrations of atoms Stretch: the distance between bonded atoms oscillates Bend: bond angles oscillate

Characteristic Infrared Absorptions of Groups GroupBondFrequency (cm -1 ) AlkylC-H AlcooholO-H AmineN-H BondFrequency range (cm- 1 ) – A complete characteristics are given in table 2.7 page 79

Examples

Alkenes substituted