FTIR Interpretation

INFRARED SPECTROSCOPY:
FUNDAMENTALS AND
APPLICATIONS
Barbara H. Stuart
University of Technology, Sydney, Australia

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Infrared Spectroscopy: Fundamentals and Applications

hydrocarbons appear in the 3000–2800 cm−1 range and the C–H stretching bands of methyl groups and methylene groups are readily differentiated. For methyl groups, asymmetric C–H stretching occurs at 2870 cm−1 , while symmetric C–H stretching occurs at 2960 cm−1 . By comparison, methylene groups show asymmetric stretching at 2930 cm−1 and symmetric stretching at 2850 cm−1 . C–H bending gives rise to bands in the region below 1500 cm−1 . Methyl groups produce two bending bands, i.e. a symmetrical band at 1380 cm−1 and an asymmetrical band at 1475 cm−1 . Methylene groups give rise to four bending vibrations: scissoring (1465 cm−1 ), rocking (720 cm−1 ), wagging (1305 cm−1 ) and twisting (1300 cm−1 ). The intensity of the methylene CH2 rocking band is useful as four or more CH2 groups are required in a chain to produce a distinct band near 720 cm−1 . Shorter chains show a more variable band, for instance, the CH2 rocking band for C4 H10 is near 734 cm−1 . Although these are the main characteristic bands associated with aliphatic hydrocarbons, there are a number of bands that appear in the spectra of such compounds as there is a wide range of structures possible. The main infrared bands for alkanes are summarized in
Table 4.1.

Table 4.1 Characteristic infrared bands of aliphatic hydrocarbons
Wavenumber (cm−1 )

Assignment

2960
2930
2870
2850
1470
1465
1380
1305
1300
720

Alkanes
Methyl symmetric C–H stretching
Methylene asymmetric C–H stretching
Methyl asymmetric C–H stretching
Methylene symmetric C–H stretching
Methyl asymmetrical C–H bending
Methylene scissoring
Methyl symmetrical C–H bending
Methylene wagging
Methylene twisting
Methylene rocking

3100–3000
1680–1600
1400
1000–600

Alkenes
=C–H stretching
C=C stretching
=C–H in-plane bending
=C–H out-of-plane bending

3300–3250
2260–2100
700–600

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