Post on 31-Mar-2015
Lab 9
STRUCTURAL DETERMINATION(IR)
Electromagnetic Spectrum:
Frequency (v): the number of complete wave cycles in a second.
Given in hertz(Hz)
Wavelength(λ): distance between any two peaks or troughs of the wave.C: speed of light (3x10 m/sec)
Energy of the waves: It is proportional to its frequency and inversely proportional to its wavelength.
Electromagnetic Spectrum:
8
E____ = hc
λ
Electromagnetic Spectrum:
energy
Infrared Region (IR)
IR photons do not have enough energy to cause electronic transition, but they
can cause groups of atoms to vibrate with respect to the bonds that connect
them.
Each trough is caused because energy is being absorbed from that particular
frequency of infra-red radiation to excite bonds in the molecule to a higher state
of vibration.
Molecular vibration:the covalent bond between two atoms acts like a spring.
1-Stretching
The bond will stretched or compressed.
Symmetrical stretching Asymmetrical stretching
Infrared Region (IR)
2-Bending:
• In- plane bending (scissoring and rocking )
• Out of plane bending (wagging and twisting)
scissoring rocking wagging twisting
Infrared Region (IR)
Infrared Spectra:4000-2500 cm-1 single bonds N-H, C-H, O-H
N-H 3300-3500 broad with spikes
O-H 3000-3600 broad
C-H 2800-3300
2500-2000 cm-1 triple bonds C≡C ,C ≡ N
C≡C 2100-2200 moderate or weak
2000-1500 cm-1 double bond C=C ,C=O
C=O ≈1750
C=C 1640-1680
Below 1500 cm-1 “fingerprint” region
Fingerprint region: (from 1500-500 cm¯¹)
Contains very complicated series of absorptions, mainly due to all manner of bending
vibrations.
Infrared Spectra:
• C-H stretching (2800 – 3300) cm -1
• Alkenes : C=C (1640 – 1680) cm -1
• Alkynes : C≡C (2100 – 2200) cm -1
Infrared Spectra:
Infrared Spectra:
O-H alcohol:
3000 - 3600 cm-¹ broad peak
Infrared Spectra:
Carbonyl compounds:
C=O ≈ 1700 cm-1
Ketones, aldehydes, carboxylic acids
Infrared Spectra:
Amines:N-H: 3300-3500 cm-¹ broad with spikes.
N-H2: 3300-3500 cm-¹ fork like.
Infrared Spectra:
Examples …