Lab 11

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 …