Infrared spectroscopy
Probing molecular structure based on bond vibrations
Unlike UV-visible spectroscopy, which examines transitions between electronic states (promotion of electrons from one orbital to another), we are interested in absorption of light to induce bond vibration.
This is also quantized hν = ΔEexcited-ground
IR terminology As with UV-vis we are interest in:
1) What is the energy of a transition (E=hυ)?
•instead of l, usually express as wavenumber ( )
2) How likely is the transition to occur (ε)– this is measured by how intense the peak is, usually expressed as %tranmittance (low %t=intense)
υ ~
υ ~ = 1/λ = υ/c
What Hooke’s Law tells us…
1. The larger the force constant (i.e. the stronger the bond), the higher the
frequency of vibration and the higher the frequency of light absorbed to cause this
vibration.
All else being equal:
νsingle bonds < νdouble bonds < νtriple bonds
2. The smaller the masses of the balls (atoms), the higher the frequency:
νC-H > νC-D >> νC-O
HH
H
HH
H
DD
D
DD
D
C-H 3036 cm-1
C-C 1479 cm-1
C-H rocking 1000-1300 cm-1
C-H 2381 cm-1
C-C 1330 cm-1
C-H oop
Overtones and Combination bands
HH
H
HH
H
combination bands
vibrating strings
fundamental υ
2 • υ
3 • υ
observe bands that are integer values of fundamental υ (overtones)
Combination bands from two different vibrations coupled together = υ1+ υ2
Overtone and combination bands lower intensity than fundamental frequencies. Occur at slightly lower than integral frequencies
Dipole moments
O
Only vibrations that give rise to changes in dipole will be “IR active” (allowed transitions).
+q -q d
Dipole moment (µ) = q • d
+q -q d
no C=C band
C=O band (~1700 cm-1)
C=C band CH3
CH3
large Δµ
Vibrational modes
νas
A B
Linear triatomic
IR-active only if A≠B
Bent triatomic
xx
degenerate
νs
IR-active: 2350 cm-1 in CO2
νs stretch
νas stretch
Scissoring or bending
(C20H42)
Alkanes
CH stretch CH3
CH3
weak CC (pseudosymmetric as chains get longer)
CH2 rock
CH2 rock
~720 cm-1
branching increases CC bands
no CH2 rock?
(C20H42)
Alkanes CH2 rocking
CH stretch CH3
CH3
weak CC (pseudosymmetric as chains get longer)
CH2 rock
CH2 rock
~720 cm-1
CH2 rock: concerted motion increases in intensity as linear chain length increases without branching
H
H
H
HH
H H
H