Instant Notes: Analytical Chemistry

spectrum, and therefore aqueous solutions may readily be studied. Overtones

and combination vibrations occur less often. Vibrations of symmetrical struc-

tures, such as R-C∫C-R, which are weak in the IR, appear as strong bands in

the R.

E10 – Infrared and Raman spectrometry: principles and instrumentation 237

Table 1. H–X group frequencies

X Wavenumber (cm-^1 ) Comment

H-C (aliphatic) 2960–2900 Strong

H-C (aromatic) 3050 - 3000 Strong

H-C (alkyne) 3300

H-O 3600 Free OH

H-O 3500 - 2500 H-bonded

H-N 3500–3300 Broad

H-S 2600 - 2500 Weak

Table 2. Multiple bonds

Bond Wavenumber (cm-^1 ) Comment

Single C-C Approx. 1200 Variable

Aromatic C:::C 1600, 1500

Double C=C 1650

Triple C∫C 2200 Weak

Single -C-N 1100 Variable

Double -C=N- 1670

Triple -C∫N 2250 Strong

S t r e t c h i n g B e n d i n g

Hydrocarbons

and

C–H groups

Oxygen

compounds

Heteroatom

compounds

4000 3000 2000 1500

Wavenumber (cm–1)

1000 500

m–s

m

–C–H(sat) m C–C*

=C–H

s =C–H v C=C

v C–O

v C=C

v Ar–H

v O–H

w H–CO s C=O

m N–H v C–N*

m–s C=N s N=O s C–CI

m–s C–H

m–s Ar–H

m–s N–H

m =C–H

s O–H

w

Chain rock

v Ar ring

Free H-bonded

Fig. 3. Chart of characteristic infrared group frequencies. *Not so useful for characterization.