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Infra-red Spectroscopy: Part-1

Infra-red (IR) spectroscopy is an essential tool in the analysis of molecules of forensic significance. IR can help in unambiguous identification of molecules.
Regions of IR-Spectrum

1. Near IR: From 12,820—4000 cm-1
2. Mid-IR: from 4000 cm-I to 400 cm-1
3. Far-IR:  from 400—10 cm-1

For most forensic purposes, the mid-IR region is commonly used for identification purposes.
The Mid-IR region can be further divided into 3 main regions:
(a) from 4000 to 1400 cm-1
(b) from 1400 to 900 cm-1
(c) from 900 to 400 cm-1
Some of the common vibrations associated with the groups in organic molecules are given below.

Spectral Range, (cm-1)

Group

Vibration type
3700--3200 -OH O-H stretching 
3400--3330 -NH2 Antisymmetric stretching
3300--3250 -NH2 symmetric stretching
3065-3030 C-Haromatic C-H stretching
3020-2950 C-H3 Antisymmetric stretching
2960-2910 C-H2 Antisymmetric stretching
2970-2860 C-H3 Symmetric stretching
2860-2840 C-H2 Symmetric stretching
2590-2560 SH S-H stretching
2600-2350 B-H B-H stretching
2450-2275 P-H P-H stretching

to be cont.….

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