IR Spectroscopy
Spectroscopy: Branch of science in which light or other electromagnetic radiation
is resolved into its component wavelengths to produce spectra, which are graphs of
intensity vs. wavelength or frequency of radiation.
Current usage broadens this definition to include some methods that don’t involve
electromagnetic radiation, such as mass spectroscopy (also called mass spectrometry), or
electron spectroscopy.
This is a useful approach to study matter, since matter will interact with electromagnetic
radiation to absorb or emit certain frequencies of radiation which are characteristic of the
sample of matter.
As we saw when we studied the H atom, radiation is absorbed or emitted only in the
frequency of light matches the energy difference of 2 quantum levels of the sample of
matter.
ν
hE =∆
IR Spectroscopy
Tool for examining vibrations in molecules. IR spectra are usually taken in the
range m
µ
λ
0.155.2~ −, which corresponds to 6504000
−
cm-1.
(e.g. ,
cmmm 46 105.2105.25.2 −− ⋅=⋅=
µ
1
44000
105.2
1−
−=
⋅
cm
cm )
The whole IR region is considered to be around
( ) to ()
cm
5
108~ −
⋅
λ
1
500,12 −
cm cm
1
10−1
10 −
cm
λ
λ
1
190 - 300 nm 52,600 – 30,000 cm-1 ultraviolet electronic
transitions
330 - 800 nm 30,000 – 12,500 cm-1 visible Electronic
transitions often in
conjugated
molecules
800 - 2500 nm 12,500 – 4,000 cm-1 “near IR” overtone vibrations
2.5 - 15µm 4,000 – 650 cm-1 “mid IR” fundamental
vibrations
1.5·10-3 - 10-1 cm 650 – 10 cm-1 “far IR” “floppy” vibrations
10-1 – 10 cm 10 – 0.1 cm-1 microwave Rotations
>10 cm < 0.1 cm-1 microwave NMR
