Infrared absorptions from organic molecules
Infrared spectra are most commonly plotted showing the transmittance of light through the sample as a function of the wave number.
Transmittance spectra have the baseline at the top. Absorptions are shown as downward peaks.
Infrared spectra are usually complicated, particularly in the fingerprint region from 400-1500 cm–1; however absorptions characteristic of the bonds shown in the diagram below are used to determine the functional group present.
In the spectrum above see
C-H stretching frequencies at 2900 cm–1
O-H stretching frequency at 3450 cm–1
C-O stretching frequency at 1050 cm–1
These relative frequencies at which the various stretching absorptions occur are consistent with a model of the bond as a spring.
Hooke's Law says that the frequency of vibration of a spring depends directly on the stiffness of the spring (force constant k - bond strength) and inversely on the masses of the bonded atoms.
The diagram shows that the stretching frequency for bonds
Transmittance spectra have the baseline at the top. Absorptions are shown as downward peaks.
Infrared spectra are usually complicated, particularly in the fingerprint region from 400-1500 cm–1; however absorptions characteristic of the bonds shown in the diagram below are used to determine the functional group present.
In the spectrum above see
C-H stretching frequencies at 2900 cm–1
O-H stretching frequency at 3450 cm–1
C-O stretching frequency at 1050 cm–1
These relative frequencies at which the various stretching absorptions occur are consistent with a model of the bond as a spring.
| ν2 = | k |
| m |
The diagram shows that the stretching frequency for bonds
between heavier atoms is lower (C-O and C-Cl lower than O-H, C–H and N–H)
that are stronger is higher (C≡C higher than C=C).
that are stronger is higher (C≡C higher than C=C).