Transcript Rovibrational Spectroscopy

Infrared and Microwave
Spectroscopy
Lecture Date: February 6th , 2013
The History of Infrared Spectroscopy
 Infrared (IR) Spectroscopy:
– Herschel first recognized the existence of
IR and its relation to the heating of water
– First real IR spectra measured by Abney
and Festing in 1880’s
– IR spectroscopy became a routine
analytical method as spectra were
measured and instruments developed from
1903-1940 (especially by Coblentz at the
US NBS)
– IR spectroscopy through most of the 20th
century is done with dispersive (grating)
instruments, i.e. monochromators
– Fourier Transform (FT) IR instruments
become common in the 1980’s, led to a
great increase in sensitivity and resolution
W. Abney, E. R. Festing, Phil. Trans. Roy. Soc. London, 1882, 172, 887-918.
J. F. W. Herschel
W. Coblentz
Infrared Spectral Regions
 IR regions are traditionally sub-divided as follows:
Region
Wavelength
Wavenumber
Frequency (), Hz
(), m
(), cm-1
Near
0.78 to 2.5
12800 to 4000
3.8 x 1014 to 1.2 x 1014
Mid
2.5 to 50
4000 to 200
1.2 x 1014 to 6.0 x 1012
Far
50 to 1000
200 to 10
6.0 x 1012 to 3.0 x 1011
After Table 16-1 of Skoog, et al. (Chapter 16)
What is a Wavenumber?
 Wavenumbers (denoted cm-1) are a measure of frequency
– For an easy way to remember, think “waves per centimeter”
 Relationship of wavenumbers to the usual frequency and
wavelength scales:
 Converting
wavelength () to
wavenumbers:
 cm 
1
10000

Image from www.asu.edu
Vibrational Spectroscopy: Theory
 In IR spectroscopy, IR photons is absorbed and converted
by a molecule into energy of molecular vibration
m1
m2
r
 A simple harmonic oscillator is a mechanical system
consisting of a point mass connected to a massless
spring. The mass is under the action of a restoring force
proportional to the displacement of the particle from its
equilibrium position and the force constant k of the spring
(under the classical Hooke’s law)
Vibrational Spectroscopy: Harmonic Oscillator
 The quantum version of the classical oscillator (spring):
1
m 
2
E  v  12 h m
k
u
m is the natural frequency of the oscillator (a.k.a. the fundamental vibrational wavenumber)
k is the Hooke’s law force constant (now for the chemical bond)
v is the vibrational quantum number
h is Planck’s constant

 v must be a whole number, so:
h
E  h m 
2
k

and
  5.3 10
(wavenumbers)
12
m1m2
m1  m2
k

 The potential energy function is:
EHO (r )  12 k (r  re ) 2 or EHO (r )  12  (2c m ) 2 (r  re ) 2
r is the distance (bond distance)
re is the equilibrium distance
Vibrational Spectroscopy: Theory
 The potential energy of vibrations fits the parabolic

function fairly well only near the equilibrium internuclear
distance.
The anharmonic oscillator model is a more accurate
description for the overall motion
Figure from Skoog et al.
Anharmonic Corrections
 Anharmonic motion: when the restoring force is not
proportional to the displacement.
– More accurately given by the Morse potential function than by the
harmonic oscillator equation.
– Primarily caused by Coulombic (electrostatic) repulsion as atoms
approach
EMorse(r )  hcDe (1  e  a ( r re ) ) 2
a
De is the dissociation energy
 (2c m ) 2
2hcDe
 Effects: at higher quantum numbers, E gets smaller, and
the ( = +/-1) selection rule can be broken
– Double ( = +/-2), triple ( = +/-3), and higher order transitions
can occur, leading to overtone bands at higher frequencies (NIR)
Rotational Spectroscopy: Theory
 Vibrational spectra of condensed phases appear as bands

rather than lines.
When vibrational spectra of gaseous molecules are
observed under high-resolution conditions, each band can
be found to contain a large number of closely spaced
components resulting from rotational energy levels.
m1
m2
r0
Rotational Spectroscopy: Theory
 Rotational energy levels can be
described as follows:
 ( J )  ( J  1) B  ( J  1)3 D
For J = 0, 1, 2, 3…
The rotational constant:
B  h / 8 2 r02c
The centrifugal distortion coefficient:
D  4 B 3 /  c2
c 
Where:
c is the speed of light
k is the Hooke’s law force constant
r0 is the vibrationally-averaged bond length
Example for HCl:
B = 10.4398 cm-1
D = 0.0005319 cm-1
r0 = 1.2887 Å
1 k
2c u
 is the reduced mass
h is Planck’s constant
c = 2990.946 cm-1 (from IR)
k = 5.12436 x 105 dyne/cm-1
R. Woods and G. Henderson, “FTIR Rotational Spectroscopy”, J. Chem. Educ., 64, 921-924 (1987)
Rovibrational Spectroscopy: Theory
 A vibrational absorption transition from  to +1 gives
rise to three sets of rotational lines called branches:
 Lower-frequency P branch: =1, J=-1
 Higher-frequency R branch: =1, J=+1
 Q branch: branch: =1, J=0
The selection rules allow
transitions with  = +1
and J = ±1 (the
transition with J = 0 is
normally not allowed
except those with an odd
number of electrons (e.g.
NO)).
P
R
Molecular Vibrations: Total Modes
 How many vibrational modes are possible in a molecule?


A molecule has as many degrees of freedom as the total
degree of freedom of its individual atoms. Each atom has
three degrees of freedom (corresponding to the Cartesian
coordinates), thus in an N-atom molecule there will be 3N
degrees of freedom.
Translation: the movement of the entire molecule while the
positions of the atoms relative to each other remain fixed.
There are 3 degrees of translational freedom.
Rotational transitions: interatomic distances remain
constant but the entire molecule rotates with respect to
three mutually perpendicular axes. There are 3 degrees of
rotational freedom in a nonlinear molecule and 2 degrees
in a linear molecule.
Vibrational Modes and IR Absorption
 For a molecule with N atoms the
number of vibrational modes is:
– Linear: 3N – 5 modes
– Non-linear: 3N – 6 modes
 Types of vibrations:
Symmetric
No change in dipole
IR-inactive
Asymmetric
Change in dipole
IR-active
– Stretching
– Bending
 Examples:
– CO2 has 3 x 3 – 5 = 4 normal modes
Scissoring
Change in dipole
IR-active
 IR-active modes require molecular dipole moment
changes during rotations and vibrations.
Vibrational Coupling
 Vibrations in a molecule may couple – changing each
other’s frequency.
– In stretching vibrations, the strongest coupling occurs between
vibrational groups sharing an atom
– In bending vibrations, the strongest coupling occurs between
groups sharing a common bond
– Coupling between stretching and bending modes can occur when
the stretching bond is part of the bending atom sequence.
– Interactions are strongest when the vibrations have similar
frequencies (energies)
– Strong coupling can only occur between vibrations with the same
symmetry (i.e. between two carbonyl vibrations)
Vibrational Modes: Examples
 IR-activity requires
dipole changes
during vibrations!
 For example, this
is Problem 16-3
from Skoog (6th
edition):
Inactive
Active
Active
Active
Inactive
Active
Inactive
IR Spectra: Formaldehyde
 Certain types of vibrations have distinct IR frequencies – hence the

chemical usefulness of the spectra
The gas-phase IR spectrum of formaldehyde:
(wavenumbers, cm-1)
 Tables and simulation results can help assign the vibrations!
Formaldehyde spectrum from: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir2
Results generated using B3LYP//6-31G(d) in Gaussian 03W.
IR Frequencies and Hydrogen Bonding Effects
 IR frequencies are sensitive to
hydrogen-bonding strength and
geometry (plots of relationships
between crystallographic distances
and vibrational frequencies):
G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford, 1997.
Instrumentation for Vibrational Spectroscopy
 Two IR Absorption methods:
– Dispersive methods: Scanning of wavelengths using a grating
(common examples are double-beam, like a spectrometer
discussed in the optical electronic spectroscopy lecture).
Radiation
Source
Sample
Wavelength
Selector
Detector
(transducer)
– Fourier-transform methods: based on interferometry, a method of
interfering and modulating IR radiation to encode it as a function
of its frequency.
Radiation
Source
Interferometer
Sample
Detector
(transducer)
IR Emission Spectroscopy
 Emission is seldom used for chemical analysis
 The sample must be heated to a temperature
much greater than its surroundings (destroying
molecules)
 IR emission is widely used in astronomy and in
remote sensing applications on heated materials.
Fourier Transform IR Spectroscopy: Rationale

Advantages of FT methods:
– The Jacqinot (throughput) advantage: FT instruments have
few slits, or other sources of beam attenuation
– Resolution/wavelength accuracy (Connes advantage):
achieved by a colinear laser of known frequency
– Fellgett (multiplex) advantage: all frequencies detected at
once, signal averaging occurs and thermal noise grows more
slowly than signal (good with IR detectors)
 These advantages are critical for IR spectroscopy
 The need for FT instruments is rooted in the detector

– There are no transducers that can acquire time-varying signals
in the 1012 to 1015 Hz range – they are not fast enough!
Why are FT instruments not used in UV-Vis?
– The multiplex disadvantage (shot noise) adversely affects
signal averaging – it is better to multiplex with array detectors
(such as the CCD in ICP-OES)
– In some cases, there are technical challenges to building
interferometers with tiny mirror movements
Inteferometers for FT-IR and FT-Raman

The Michelson
interferometer, the
product of a famous
physics experiment:
Figures from Wikipedia.org

Produces
interference
patterns from
monochromatic
and white light
Inteferometers
 For monochromatic


radiation, the
interferogram looks like
a cosine curve
For polychromatic
radiation, each
frequency is encoded
with a much slower
amplitude modulation
The relationship
between frequencies:
f 
2vM

c
Where:
 is the frequency of the radiation
c is the speed of light in cm/s
vm is the mirror velocity in cm/s
 Example: mirror rate = 0.3 cm/s modulates 1000 cm-1 light at 600 Hz
 Example: mirror rate = 0.2 cm/s modulates 700 nm light at 5700 Hz
FTIR Spectrometer Design


It is possible to build a detector that detects multiple
frequencies for some EM radiation (ex. ICP-OES with CCD,
UV-Vis DAD)
FTIR spectrometers are designed around the Michelson
interferometer, which modulates each IR individual
frequency with an additional unique frequency well suited to
the time response of IR detectors:
Fourier Transform - IR Spectrum
IR Source
Beamsplitter
Michelson
Interferometer
Detector
Sample
Interferogram
Fixed Mirror
Moving Mirror

The Basics of the Fourier Transform
The conversion from time- to frequency domain:
b
Continuous: g ( )   K 1 (, t ) f (t )d K (, t )  exp(iωt )
a
Discrete:
1
fn 
N
N 1
d e
k 0
 2ikn / N
k
1
Time domain
0.5
2
Frequency domain
1.5
50
100
150
200
250
FT
-0.5
1
0.5
-1
50
100
150
200
50
100
150
200
250
2.5
2
1.5
2
1
1.5
0.5
50
100
150
200
250
FT
1
-0.5
0.5
-1
-1.5
250
IR Sampling Methods: Absorbance Methods
 KBr/CsI pellet: a dilute (~1%) amount of sample in the halide matrix



is pressed at >10000 psi to form a transparent disk.
– Disadvantages: dilution required, can cause changes in sample
Mulls: Solid dispersion of sample in a heavy oil (Nujol)
– Disadvantages: big interferences
Salt plates (NaCl): for liquids (a drop) and small amounts of solids.
Sample is held between two plates or is squeezed onto a single
plate.
Cells: For liquids or dissolved samples. Includes internal reflectance
cells (CIRCLE cells)
Common IR Solvents
The horizontal lines indicate regions where solvent transmits at least
25% of the incident radiation in a 1 mm cell.
IR Sampling Methods: Reflectance Methods
 Specular reflection: direct
ATR
reflection off of a flat surface.
– Grazing angles
 Attenuated total reflection
(ATR): Beam passed through
an IR-transparent material with
a high refractive index, causing
internal reflections.
 Depth is ~2 um (several
wavelengths)
DRIFTS
 Diffuse reflection (DRIFTS): a
technique that collects IR
radiation scattered off of fine
particles and powders. Used
for both surface and bulk
studies.
Figures from http://www.nuance.northwestern.edu/KeckII/ftir7.asp
Hybrid/Hyphenated Techniques: Interfaces
 Interfaces between vibrational spectrometers and other

analytical instruments are possible
Example: In GC-FTIR, gaseous GC column effluent is
passed through “light pipes”
 Another Example: TGA-IR, for identification of evolved
gases from thermal decomposition
Figure from Skoog et al.
IR Sources
Nernst Glower
A rod or cylinder made from
several grams of rare earth
oxides, heated to 1200-2200K
by an electric current.
1-50 µm
(mid- to far-IR)
Globar
Similar to the Nernst glower but
made from silicon carbide SiC,
electrically heated. Better
performance at lower
frequencies.heated
1-50 µm
(mid- to far-IR)
Tungsten (W)
filament lamp
Heated to 1100 K
0.78-2.5 µm
(Near-IR)
Hg arc lamp
High-pressure mercury vapor
tube, electric arc forms a
plasma.
50 - 300 µm
(far-IR)
CO2 laser
High-intensity, tunable radiation
used for quantitation of specific
analytes
9-11 µm
IR Detectors
 Thermal transducers
– Response depends upon heating effects of IR radiation
(temperature change is measured)
 Slow response times, typically used for dispersive instruments or
special applications
 Pyroelectric transducers
– Pyroelectric: insulators (dielectrics) which retain a strong electric
polarization after removal of an electric field, while they stay
below their Curie temperature.
– DTGS (deuterated triglycine sulfate): Curie point ~47°C
 Fast response time, useful for interferometry (FTIR)
 Photoconducting transducers
– Photoconductor: absorption of radiation decreases electrical
resistance. Cooled to LN2 temperatures (77K) to reduce thermal
noise.
– Mid-IR: Mercury cadmium telluride (MCT)
– Near-IR: Lead sulfide (NIR)
Interpretation of IR Spectra
 General Features:
– Stretching frequencies are greater (higher wavenumbers) than
corresponding bending frequencies
 It is easier to bend a bond than to stretch it
– Bonds to hydrogen have higher stretching frequencies than those
to heavier atoms.
 Hydrogen is a much lighter element
– Triple bonds have higher stretching frequencies than double
bonds, which have higher frequencies than single bonds

Strong IR bands often correspond to weak Raman bands
and vice-versa
Interpretation of IR and Raman Spectra
Characteristic Vibrational Frequencies for Common Functional Groups
Frequency (cm-1)
Functional Group
Comments
3200-3500
alcohols (O-H)
amine, amide (N-H)
Broad
Variable
Sharp
alkynes (CC-H)
3000
alkane (C-C-H)
alkene (C=C-H)
2100-2300
alkyne (CC-H)
nitrile (CN-H)
1690-1760
carbonyl (C=O)
ketones, aldehydes,
acids
1660
alkene (C=C)
imine (C=N)
amide (C=O)
Conjugation lowers
amide frequency
1500-1570
1300-1370
nitro (NO2)
1050-1300
alcohols, ethers, esters,
acids (C-O)
See also Table 17-2 of Skoog, et al.
More detailed lists are widely available. See R. M. Silverstein and F. X. Webster, “Spectrometric Identification of Organic Compounds”, 6th Ed., Wiley, 1998.
IR Absorption and Chemical Structure
IR and Raman Spectra of an Organic Compound
The diamond ATR IR spectrum of tranilast (polymorphic Form I):
0.65
T ra milast F o rm I
0.60
O3
C4
O
0.55
C5
C3
C6
C2
N1
0.50
C15
C18
0.45
C14
H3C
C16
C11
C10
C8
C9
Absorbance
N
H
C12
C13
C7
O
O
O5
0.40
C1
O2
C17
H3C
O
OH
O1
O4
0.35
0.30
0.25
0.20
0.15
0.10
0.05
4000
3500
3000
2500
Wavenumbers (cm-1)
2000
1500
1000
IR and Raman Spectra of an Organic Compound
O
The diamond ATR IR spectrum of
flufenamic acid (an analgesic/antiinflammatory drug):
0.32
OH
CF3
F lufe nam ic A c id A ldr ic h a s r ecd
0.30
0.28
0.26
0.24
0.22
Absorbance
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
4000
3500
3000
2500
Wavenumbers (cm-1)
2000
1500
1000
Far IR Spectroscopy in Condensed Phases
 Far IR is used to study low frequency vibrations, like those between
metals and ligands (for both inorganic and organometallic chemistry).
– Example: Metal halides have stretching and bending vibrations in the
650-100 cm-1range.
– Organic solids show “lattice vibrations” in this region
 Can be used to study crystal lattice energies and semiconductor
properties in solids via phonon modes.
 The far IR region also overlaps rotational bands, but these are
normally not detectable in condensed-phase work
Terahertz Spectroscopy
 A relatively new technique, addresses an unused portion
of the EM spectrum (the “terahertz gap”):
– 50 GHz (0.05 THz) to 3 THz (1.2 cm-1 to 100 cm-1)
 Made possible with recent innovations in instrument
design, accesses a region of crystalline phonon bands
P. F. Taday and D. A. Newnham, Spectroscopy Europe, , www.spectroscopyeurope.com
G. Winnewisser, Vibrational Spectroscopy 8 (1995) 241-253
Applications of FT Microwave Spectroscopy
 Under development for:
real-time, sensitive monitoring of
gases evolved in process chemistry, plant and vehicle
emissions, etc…
– Current techniques have limits (GC, IR, MS, IMS)
– Normally use pulsed-nozzle sources and high-precision FabryPerot interferometers (PNFTMW)
Compound
Detection Limit
(nanomol/mol)
Acrolein
0.5
Carbonyl sulfide
1
Sulfur dioxide
4
Propionaldehyde
100
Methyl-t-butyl ether
65
Vinyl chloride
0.45
Ethyl chloride
2
Vinyl bromide
1
Toluene
130
Vinyl cyanide
0.28
Acetaldehyde
1
Diagram from http://physics.nist.gov/Divisions/Div844/facilities/ftmw/ftmw.html
For more information, see E. Arunan, S. Dev. And P. K. Mandal, Applied Spectroscopy Reviews, 39, 131-181 (2004).
New Methods in FT Microwave Spectroscopy
 A new method using a “chirp” pulse (which excites a wide
range of frequencies) has been developed by the Pate
group at U. Virginia Charlottesville
– The CP-FTMW (chirped pulse FT microwave) method enhances
sensitivity by 100 to 10000 times and allows for studies of
molecular shape changes (occurring on picosecond timescales)
Diagram from B. H. Pate et al., Science, 2008, 320, 924
See C&E News: http://pubs.acs.org/cen/news/86/i20/8620notw1.html
Applications of Near IR Spectroscopy
 Near IR (NIR) is heavily used in process chemistry and materials
identification
 Amenable to quantitative analysis usually in conjunction with
chemometrics (calibration requires many standards to be run)
 While not a popular qualitative technique, it can serve as a fast and
useful quantitative technique especially using diffuse reflectance
 Accuracy and precision in the ~2% range
 Examples:
– On-line reaction monitoring (food, agriculture, pharmaceuticals)
– Moisture and solvent measurement and monitoring
 Water overtone observed at 1940 nm
– Solid blending and solid-state issues
Near IR Spectroscopy
Figure from www.asdi.com. For more information see:
1. Ellis, J.W. (1928) “Molecular Absorption Spectra of Liquids Below 3 m”, Trans. Faraday Soc. 1928, 25, pp. 888-898.
2. Goddu, R.F and Delker, D.A. (1960) “Spectra-structure correlations for the Near-Infrared region.” Anal. Chem., vol. 32 no. 1, pp. 140-141.
3. Goddu, R.F. (1960) “Near-Infrared Spectrophotometry,” Advan. Anal. Chem. Instr. Vol. 1, pp. 347-424.
4. Kaye, W. (1954) “Near-infrared Spectroscopy; I. Spectral identification and analytical applications,” Spectrochimica Acta, vol. 6, pp. 257-287.
5. Weyer, L. and Lo, S.-C. (2002) Spectra-Structure Correlations in the Near-infrared, In Handbook of Vibrational Spectroscopy, Vol. 3, Wiley, U.K., pp. 1817-1837.
6. Workman, J. (2000) Handbook of Organic Compounds: NIR, IR, Raman, and UV-Vis Spectra Featuring Polymers and Surfactants, Vol. 1, Academic Press, pp. 77-197.
Near IR Spectrum of Acetone
(nm)
 NIR taken in transmission mode (via a reflective gold plate) on a

Foss NIRsystems spectrometer
Useful for quick solvent identification
Near IR Spectrum of Water (1st Derivative)
(nm)
 1st derivative (and 2nd derivative) allows for easier identification of
bands
Photoacoustic Spectroscopy
 First discovered in 1880 by A. G. Bell
 When radiation is absorbed, the energy is converted to heat,

causing expansion and contraction of the sample at the modulation
frequency which is transferred to the surrounding air. Can be
detected with a microphone.
The IR “version” of photoacoustic sampling is generally applied to
two types of system
IR Radiation

Gas:
All gas (or all-liquid)
systems:
IR Radiation
IR-Transparent Gas

The solid-gas system:
Solid
A. G. Bell, Am. J. Sci. 20 (1880) 305.
A. G. Bell, Philos. Mag. 11 (1881), 510.
The Photoacoustic Effect for Solid-Gas Systems
 The photoacoustic effect is produced when intensitymodulated light hits a solid surface (or a confined gas or
liquid).
Psurface  (1  R) P 0  e
Modulated IR Radiation
 ( +
1

) x
R  surface reflectivity
P 0  incident IR beam power
 - absorption coefficient
PA Cell
Gas
(Psurface)
 - thermal diffusion length
Microphone
P0
x
Solid
P(x)
Thermal Wave (attenuates rapidly)
IR is absorbed by a vibrational transition,
followed by non-radiative relaxation
J. F. McClelland. Anal. Chem. 55(1), 89A-105A (1983)
M. W. Urban. J. Coatings Technology. 59, 29 (1987).
The Thermal Diffusion Length –Solids

The thermal diffusion length  is:

2a

 The thermal diffusivity a is:
k
a
C
k  thermal conductivity
0.15 cm/sec IR
1.2 cm/sec IR
PVF2
PET
 - thermal diffusion length
=/2
  density

C  specific heat
The variable , the modulation frequency of the IR
radiation, is directly proportional to interferometer mirror
velocity, and is defined as:
  4  M
  IR Frequency (wavenumbe rs)
 M  Mirror vel ocity of Michelson interferom eter (cm/sec)
Urban, M. W. J. Coatings Technology. 1987, 59, 29
Quintanilla, L., Rodriguez-Cabello, J. C., Jawhari, T. and Pastor, J. M.. Polymer. 1993, 34, 3787.
The Thermal Diffusion Length


The mirror velocity is therefore inversely related to the
thermal diffusion length, and therefore can be used to
control the maximum sampling depth.
Typical thermal diffusion lengths for the carbonyl band
(~1750 cm-1) of poly(ethylene terephthalate):
Mirror Speed (cm/sec)
0.15
0.30
0.60
0.90
1.20
Thermal Diffusion Length (microns)
8.9
6.3
4.5
3.6
3.1
The thermal diffusivity was taken to be 1.3 * 10-3 cm2/sec, and the absorption coefficient of the carbonyl band was
assumed to be 2000 cm-1.
Urban, M. W. and Koenig, J. L. Appl. Spec. 1986, 40, 994.
Quintanilla, L., Rodriguez-Cabello, J. C., Jawhari, T. and Pastor, J. M.. Polymer. 1993, 34, 3787.
A Typical Photoacoustic FTIR Spectrum
A PA-FTIR Spectrum of a silicone sealant:
IR Modulation
frequency is high
IR Modulation
frequency is low
 The spectrum shows peaks where the IR radiation is being
absorbed due to vibrational energy level transitions.
 Differences between a PA-FTIR spectrum and a regular IR
spectrum:
– IR modulation frequency effects (weak CH3 and CH2 bands)
– Saturation of strong bands in the spectrum
Paroli, R. M., Delgado, A. H., and Cole, K. C. Canadian J. Appl. Spectr. 1994, 39, 7.
Photoacoustic Saturation
 Strong bands in PA-FTIR spectra often
A Saturated Band
show saturation.
 Saturation occurs when the vibrational
transition is being pumped to its
excited state faster than it can release
energy.
 A high absorption coefficient coincides
with faster saturation.
Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy. Wiley: New York, 1980.
Paroli, R. M., Delgado, A. H., and Cole, K. C. Canadian J. Appl. Spectr. 1994, 39, 7.
Depth-Profiling Studies with PA-FTIR


Thermal diffusion length
allows for IR depth
profiling with PA-FTIR
Example: a layer of
poly(vinylidine fluoride
(PVF2) on poly(ethylene
terephthalate) (PET)
0.15 cm/sec IR
1.2 cm/sec IR
PVF2
PET
PVF2 top layer is 6 micrometers thick.
 - thermal diffusion length
=/2
The carbonyl band, due to the PET, is marked with a red dot ().
Data acquired with a Digilab FTS-20E with a home-built PA cell.
Urban, M. W. and Koenig, J. L. Appl. Spec. 1986, 40, 994.
Crocombe, R. A. and Compton, S. V. Bio-Rad FTS/IR Application Note 82. Bio-Rad Digilab Division, Cambridge, MA, 1991.
IR Microscopy
 Most FTIR microscopes image using array detectors
 IR spectra from a region are acquired at once, better S/N
– However, this is at the expense of resolution (limited to ca. 10 um),
in contrast with scanning techniques. Resolution in FTIR imaging is
of course limited by the diffration limit, which is even worse for IR
wavelengths.
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).
IR Microscopy: Image Analysis
 Extraction of data from FTIR micrographs is done by
color-coding peaks based on their IR frequency (a)


Suitable IR frequencies can be
chosen via a scatter plot (c) of
every point in the image vs. two
(or more) frequencies, followed
by location of the center-of-gravity
and possible statistical analysis
False color images can then be
constructed (b)
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).
IR Microscopy: Polymer Chemistry Applications
 FTIR microscopy can analyze compositional differences in

material science, chemical and biochemical applications
Example – the study of time-dependent processes like
dissolution of a polymer by a solvent
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).
IR Microscopy: Polymer Chemistry Applications
 A complex, solvent-dependent dissolution, diffusion and
molecular motion process is observed for polymers (e.g.
polymethylstyrene) above their entanglement mwt:
Figure from J. L. Koenig, S. Q. Wang, and R. Bhargava, Anal. Chem., 73, 361A-369A (2001).
Vibrational Circular Dichroism
 Analogous to electronic

(UV-visible) circular
dichroism (ECD), VCD
measures the differential
absorption of right and lefthanded circularly polarized
IR radiation
Much less sensitive than
ECD, but much higher
information content (many
more bands show effects
linked to chirality)
VCD of -pinene
R-enantiomer
S-enantiomer
Further Reading
Optional:
L. J. Bellamy, Advances in Infrared Group Frequencies, Methuen and
Co., 1968.
R. M. Silverstein and F. X. Webster, Spectrometric Identification of
Organic Compounds, 6th Ed., Wiley, 1998.
P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd.
Ed., Oxford, 1997.