Infrared Spectroscopy (IR)
Vibrational spectroscopy is an energy sensitive method and is used to
characterize compounds in terms of the strengths and number of the bonds
present. One can get/detect:
ƒ the presence of known compounds (finger print)
ƒ the components of an unknown compound (functional groups)
ƒ and thus a likely structure of a compound
ƒ changes in the concentration of a species during reaction
ƒ the properties of bonds (bond strength, force constants)
Since a bond in a molecule behaves like a spring, the harmonic/anharmonic
oscillator model is used to describe the 3N-6 or 3N-5 different ways a nonlinear or linear molecule, respectively, consisting of N atoms can vibrate.
These vibrational modes (normal modes) give rise to absorption bands of
characteristic energies/frequencies/wave numbers, intensities, and widths
(change of dipole moment required), which are detected and analyzed.
Vibrational energy levels in harmonic approximation
Zero point vibration/energy!
Please note that vibrations normally are more or less anharmonic
Vibrational levels in harmonic/anharmonic approximation
Δn = ±1
Potential curve of an harmonic oscillator
(En: Vibrational levels, E0: Zero point energy)
EVIB = hνosc(n + ½) - h2ν2/(4ED)· (n + ½)2 (Δn = ±1, ±2, ...)
In case of an anharmonic vibration, the distances of
neighbouring levels become smaller with increasing n (the
arrows symbolize transition probabilities and intensities.
Potential curve of an anharmonic oscillator
(E0: Zero point energy, ED: Dissociation energy)
Normal modes of vibration
3N – 6 modes (3N – 5 if linear)
ν~ ~
The three normal modes of H2O
and their wavenumbers
f /m
The four normal modes of
(linear) CO2
Typical wavenumbers of stretching/bending vibrations
“molecule“
stretching
C-H
2800 - 3000
N-N
3300 - 3500
H2 O
3600 - 3000
C=O
1700
C=C
1600
SO32-
970 (νs)
930 (νas)
bending
1600
620 (γ)
470 (δ)
IR - Spectrometer
grating, double beam
Sample
Fourier Transform (FT)
Sample
Examples
BaSO3
Cs2CrCl5·4H2O
ν(PH)
758
Sr(HPO2OH)2
RT
KMn(SeO2OH)3
3214
3080
2881
686
1226
2439 2433
1168
779
Transm ission
694
γ(OH)
1235
TT
3191
3032
2899
Sr(SeO2OH)2
RT
2440 2434
1170
δ(OH)
2300
2400
1225 1155
3176 3019
2303
2432
TT
3500
3175
2963
3000
2783
1241
685
1164
νB(OH)
2500
1300
1150
1000
850
700
ν/cm-1
400