Infrared Spectroscopy (IR) Basics & Instruments
To Do’s • Read Chapter 11. Skip Raman. • Complete the end-of-chapter problems, 11-2 and 11-5 Answer Keys are available in CHB204H
Molecular Vibration • Atoms in a molecule are not fixed in space. They vibrate. • To a first approximation, it is like 2 balls, A and B, attached to each other by a spring. • If it was really a spring, any energy state would be possible. But, for a molecule, the vibrational energy is quantized. 1st excited vibrational energy state A
B
Molecular vibration
E
ΔEvib
ground vibrational energy state
Electromagnetic Radiation and IR absorption
When we put energy into the system, a transition from ground state the the excited state occurs when; ΔEvib = hν ν=
c λ
Wave Numbers • Typical IR absorptions occur at frequencies (ν) approximately 1013 s-1, an inconvenient large number. • A more convenient unit, ν (the wave number in cm-1), is used in IR. ν= Also, ν =
ν c 1 λ
c = speed of light (3 x 1010 cms-1) λ = wavelength
Classic IR Instrument Iref
Isample
%T =
Isample Iref
x 100
l : light intensity
Or, use absorbance (A) Iref A = - log ( ) Isample
A Typical IR Spectrum
%T
Wavenumber (cm-1), reciprocal centimeter
ΔEvib is related to bond strength, mass of atoms etc
Structual analysis
FT-IR Instrument
Michaelson Interferometer
• IR frequecy, ~1014 Hz, is too fast for the rapid changes in power to be directly measured as a function of time. Can not measure the FID signal directly. • Michaelson interferometer creates an interference pattern at a frequency that is a factor of 1010 times slower.
Michaelson Interferometer I0
d
When x = 0, I = 0.5 I0 x = λ/4, I = 0
d+x I 0.5 I0
I I(x) = 0.5 cos (2πx/λ)
x − λ/2 − λ/4
λ/4
λ/2
See YouTube video: http://www.youtube.com/watch?v=j-u3IEgcTiQ
IR Light with Different Frequencies I 0.5 I0
x − λ/2 − λ/4
λ/2
λ/4 I
0.5 I0
x − λ/2 − λ/4
I
λ/4
λ/2 0.5 I0
x λ/4 λ/2
− λ/2 − λ/4
0
0 x Signals are canceled out, except for x=0.
Interferogram and Fourier Transform
baseline
With sample
FT
Sample Preparation • Sample holder must be transparent to IR- salts • Liquids – Salt Plates (large NaCl crystals) – Neat, 1 drop – Samples dissolved in volatile solvents- 0.1-10%
• Solids – KBr pellets – Mulling (dispersions), i.e. nujol (paraffin oil) mull
• Quantitative analysis-sealed cell with NaCl/NaBr/ KBr windows
IR Samples (a) “Neat” samples
(b) KBr disks
Common IR Solvents
ATR-IR • Attenuated total reflectance • Penetrates several micrometers
ZnSe, germanium or diamond
Stretching Bands Hooke’s law in classical mechanics MA
A
B
MB
f spring constant reduced mass µ =
(1) f C-C
∴
MA x M B MA + M B
stronger bond larger f
2π ν=
C≡C
f
√
and ν =
µ
1
√
2πc
ν c
f µ
c : speed of light f : proportional to bond strength (2) µ
ν C=C
ν=
1
C-H C-C C-O C-N
ν small µ
large ν
large µ
small ν
Phenylpropiolaldehyde
-C≡C-
C=O
Cyclohexanol (dilute solution)
O-H C-O
Mass Effects on CH Bend Frequency for X(CH3)4
Bendings
CHCl3 and CDCl3
Cl H
C
Cl
Cl Cl
H
C
Cl Cl
C-H stretch C-H bending Both stretching and bending bands are affected by mass.
Typical IR Bands
Peak Intensity Oδ C δ
More efficient interaction
vibration causes a large change in dipole
• In order to interact with electromagnetic radiation, the bond has to change the dipole moment upon vibration
C-H C-O C=O
strong
C-C
weak
Pentynes
C≡C 2150 cm-1
Mechanical Coupling • Two identical, separate groups vibrate with same frequencies, independently from each other. • When part of the same molecule, they cannot vibrate independently. • Displacement of atoms in one group causes effect in the other group. • In-phase (symm.) and out-of-phase (unsymm.) combinations of the starting vibrations are observed.
CH2 Stretching Vibrations
Hexane
CH2 and CH3, symmetric and anti-symmetric
http://www.quimica3d.com/EN/IR/hexane.php
Hexane
CH3 bending
http://www.quimica3d.com/EN/IR/hexane.php
Isopropyl and tert-butyl Groups
http://www.quimica3d.com/EN/IR/23dimethylbutane.php
Carboxylic Acid Anhydrides
Overtone and Combination Bands
0 -> 1 : Fundamental bands 0 -> 2 etc : Overtone bands (ν0-2 ~ 2 x ν0-1) Combination bands are observed when two or more fundamental vibrations are excited simultaneously.
- Both overtone and combination bands are generally weak.
Aromatics
overtones
CH bendings
Fermi Resonance • Mechanical coupling between a fundamental band and an overtone band. • Some can be diagnostic.
Benzoyl Chloride
overtone
CH out-of-plane bending 865 cm-1
Secondary Amides
NH deformation ~ 1550 cm-1 overtone
Aldehydes
CH deformation ~ 1400 cm-1 overtone