13.6 |
Analysis by IR Spectroscopy |
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IR spectroscopy has grown into an extremely versatile analytical tool. Most organic and inorganic groups (such as CH3, -C=C, M-C≡0) have characteristic frequencies and these frequencies provides finger prints, using which the groups in newly synthesized molecules can be identified. Although we can not “see” molecules, through various spectral methods, we can identify atoms, groups, bond lengths, relative locations (cis/trans, endo/exo) and so on. The IR frequencies of a few common groups are given in Table 13.2. |
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Table 13.2 Characteristic frequencies (in cm-1) of some molecular groups. |
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Group Approximate frequency Group Approximate frequency
C-I 550 -C ≡ C - 2200
C-Cl 725 -C ≡ N 2250
C=S 1100 S – H 2580
C-O- 1000 – 1200 -CH2 2930 (asym stretch)
2860 (sym stretch)
C-N 1000 – 1200 1470 (deformation)
C-C 1000 – 1200 = CH2 3030
C=N- 1600 Aromatic C-H 3060
C = C 1650 ≡C – H 3300
C = O 1600 – 1750 -N-H2 3400
O-H 3600
H-bonds 3200 – 3570
In special conformations, the group frequencies can deviate from the values in the table significantly.
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