Vibrational spectroscopy
6.3.1 Fundamentals of vibrational spectroscopy
Definition Vibrational spectroscopy:is concerned with the d t ti f t iti detection of transitions between energy levels in molecules that result from stretching and bending vibrations of the interatomic bonds.
asymmetrical
Vibrational spectroscopy
Kinds of vibrational spectroscopy ¾Infra-red spectroscopy
(more sensitive to polarized group)6.3.1 Fundamentals of vibrational spectroscopy
symmetrical
¾Raman spectroscopy (more
sensitive to non-polarized)Both methods are concerned with vibrations in molecules , they differ in the manner in which interaction with the exciting radiation occurs .
Linear PE: (a) IR, (b)
Raman
Fig. 6-14 Dipole moment of HCl
Vibrational spectroscopy
Vibrating of Disulfide carbon
Symmetrical stretching
Infrared inactive 6.3.2 Infrared spectroscopy
Asymmetrical stretching
Bending
Infrared active
Infrared inactive Fig. 6-15 Vibration of Disulfide carbon
m1
low
High/cm-1
High/cm-1
low
Vibrational spectroscopy Methylbenzene(甲苯)
2005.2 S. Gu
v =0 represents the ground state v =l the excited vibrational state
6.3.3 Raman spectroscopy
(1)
(2)
(3)
Vibrational spectroscopy ¾The essential prerequisite for Raman scattering is a change in the polarizability of the bond when vibrations occur.
Polarizability may be thought of as a measure of 6.3.3 Raman spectroscopy
¾Polarizability may be thought of as a measure of the
Fig. 6-16 Motion state of linear molecules Degrees of freedom (H2O) : 3×3−6 = 3
Vibraitonal modes (methylene group)
：2926cm-1（s）as
ν
Asymmetrical
s
ν: 2853 cm Symmetrical
δ：1468 cm-1(m) δr：720 cm-1(CH
1306～1303cm-1（w）γ
t ：1250cm
scissoring rocking wagging
Hexane
Four peaks
p
Spectral interpretation always starts at the high end, because there are the best group frequencies and they are the easiest to interpret. No peaks appear above 3000 cm-1, the cut-off for unsaturated C-H. the four peaks below 3000 cm-1 are saturated C-H stretching modes.
Hexane
The peak at 2962 cm-1 is
assigned to the antisymmetric
assigned to the antisymmetric
stretch of the CH3group. This
vibration is always found in
the range 2962±10 cm-1. there
are actually two degenerate
antisymmetric stretching
modes (only one shown).Hexane
At 2926cm-1, the CH2
antisymmetric stretch
absorbs.Normal range:
2926±10 cm-1.
Hexane
At 2872cm-1, the CH3
symmetric stretch
absorbs.Normal range:
2872±10 cm-1.Hexane
At2853-1,the CH
At 2853cm, the CH2
symmetric stretch
absorbs.Normal range:
2853±10 cm-1.
Vibrational spectroscopy Hexane
1470cm-1This is the C-H bending
region, expanded to show the
nearly overlapping peaks for
the CH3and CH2bends.
Vibrational spectroscopy
Hexane
rocking When four or more CH2groups are
in a chain, a vibration at 720±10
cm-1corresponds to concerted
rocking of all of the CH2’s.
Vibrational spectroscopy
Hexanol
3334 cm-1–OH stretch. Normal range: 3350±150 cm-1.
This is a very characteristic group frequency. All of the
peaks due to the OH group are broad due to hydrogen
bonding.
Vibrational spectroscopy Hexanol 1430 cm -1–OH bend . Normal range: 1400±100 cm -1. This broad peak is buried under the CH bending modes.
Vibrational spectroscopy
Hexanol
660 cm -1–OH wag. While not a group frequency, this is another band due to the OH.
Vibrational spectroscopy Aromatic ring expansion (Methylbenzene )
At 1601 cm -1, the
symmetric ring strethch absorbs. Normal range: 1590±10 cm -1. This ib ti h di l
Only not
symmetrically substituted.
vibration has a dipole change (and absords in IR) only when not
symmetrically substituted. The intensity of this band also varies with the
substituent. Compare to p-xylene from the overlay menu.
Vibrational spectroscopy
Aromatic ring expansion (Methylbenzene )
At 1500cm -1, a different ring stretch absorbs. Range: 1500±10cm -1. Variable intensity
Vibrational spectroscopy 6.3.6 Comparing of IR and Raman Spectroscopy
asymmetrical
symmetrical
Fig. 6-17 Linear PE: (a) IR, (b) Raman。