Transcript Rovibrational Spectroscopy

Rotational and Vibrational
Spectroscopy
Lecture Date: January 30th, 2008
Vibrational and Rotational Spectroscopy
 Core techniques:
– Infrared (IR) spectroscopy
– Raman spectroscopy
– Microwave spectroscopy
The Electromagnetic Spectrum
 The basic!
 Microwave
 Infrared (IR)
The History of Infrared and Raman Spectroscopy
 Infrared (IR) Spectroscopy:
– First real IR spectra measured by Abney and Festing in 1880’s
– Technique made into a routine analytical method between 19031940 (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
 Raman Spectroscopy:
– In 1928, C. V. Raman discovers that small changes occur the
frequency of a small portion of the light scattered by molecules.
The changes reflect the vibrational properties of the molecule
– In the 1970’s, lasers made Raman much more practical. NearIR lasers (1990’s) allowed for avoidance of fluorescence in
many samples.
W. Abney, E. R. Festing, Phil. Trans. Roy. Soc. London, 1882, 172, 887-918.
Infrared Spectral Regions
 IR regions are traditionally sub-divided as follows:
Region
Wavelength
Wavenumber
(), 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)
Frequency
(), Hz
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
Rotational and Vibrational Spectroscopy: Theory
 Overview:
– Separation of vibrational and rotational contributions to energy is
commonplace and is acceptable
– Separation of electronic and rovibrational interactions
 Basic theoretical approaches:
– Harmonic oscillator for vibration
– Rigid rotor for rotation
 Terminology:
– Reduced mass (a.k.a. effective mass):
m1m2

m1  m2
See E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, “Molecular Vibrations”, Dover, 1955.
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:
B0 = 10.4398 cm-1
D0 = 0.0005319 cm-1
r0 = 1.2887 Å
1 k
2c u
 is the reduced mass
h is Planck’s constant
0 = 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)
Vibrational Spectroscopy: Theory
 Harmonic oscillator – based on the classical “spring”
1
m 
2
k
u
E  v  12 h m
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
Note – all E are
potential energies (V)!
 Since v must be a whole number (see Ex. 16-1, pg. 386):
h
E  h m 
2
k

and
  5.3 10
(wavenumbers)
12
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
 Potential energy of a harmonic oscillator:
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)
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 and IR Absorption
 Number of modes:
– Linear: 3n – 5 modes
– Non-linear: 3n – 6 modes
 Types of vibrations:
– Stretching
– Bending
Symmetric
No change in dipole
IR-inactive
Asymmetric
Change in dipole
IR-active
 Examples:
– CO2 has 3 x 3 – 5 = 4 normal
modes
Scissoring
Change in dipole
IR-active
 IR-active modes require dipole changes during rotations
and vibrations!
Vibrational Modes: Examples
 IR-activity requires
dipole changes
during vibrations!
 For example, this
Inactive
Active
Active
Active
is Problem 16-3
from Skoog:
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.
Rayleigh and Raman Scattering
 Only objects whose dimension is ~1-1.5  will scatter EM
radiation.
 Rayleigh scattering:
– occurs when incident EM radiation induces an oscillating dipole in
a molecule, which is re-radiated at the same frequency
 Raman scattering:
– occurs when monochromatic light is scattered by a molecule, and
the scattered light has been weakly modulated by the
characteristic frequencies of the molecule
 Raman spectroscopy measures the difference between
the wavelengths of the incident radiation and the
scattered radiation.
The Raman Effect
 The incident radiation excites “virtual states” (distorted
or polarized states) that persist for the short timescale of
the scattering process.
 Polarization changes
are necessary to form
the virtual state and
hence the Raman
effect
 This figure depicts
“normal” (spontaneous)
Raman effects
Virtual state
Virtual state
hv1
hv1
hv1 – hv2
Anti-Stokes line
hv1 – hv2
Stokes line
Excited state
(vibrational)
Scattering timescale ~10-14 sec
(fluorescence ~10-8 sec)
H. A. Strobel and W. R. Heineman, Chemical Instrumentation: A Systematic Approach, 3rd Ed. Wiley: 1989.
Ground state
(vibrational)
More on Raman Processes
 The Raman process:
inelastic scattering of a photon
when it is incident on the electrons in a molecule
– When inelastically-scattered, the photon loses some of its energy
to the molecule (Stokes process). It can then be experimentally
detected as a lower-energy scattered photon
– The photon can also gain energy from the molecule (anti-Stokes
process)
 Raman selection rules are based on the polarizability of
the molecule
 Polarizability: the “deformability” of a bond or a molecule
in response to an applied electric field. Closely related to
the concept of “hardness” in acid/base chemistry.
P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd Ed. Oxford: 1997.
More on Raman Processes
 Consider the time variation of the dipole moment induced
by incident radiation (an EM field):
Induced dipole moment
 (t )   (t ) (t )
EM field
polarizability
 If the incident radiation has frequency  and the
polarizability of the molecule changes between min and
max at a frequency int as a result of this rotation/vibration:
 (t )    12  cos intt  0 cos t
mean polarizability
 = max - min
 Expanding this product yields:
 (t )  0 cos t  14 0 cos(  int )t  cos(  int )t 
Rayleigh line
Anti-Stokes line
P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd Ed. Oxford: 1997.
Stokes line
The Raman Spectrum of CCl4
Rayleigh line
(elastic scattering)
0 = 20492 cm-1
0 = 488.0 nm
Stokes lines
(inelastic scattering)
Observed in
“typical”
Raman
experiments
Anti-Stokes lines
(inelastic scattering)
459
314
218
-218
-314
-459
400
200
0
-200
-400
Raman shift cm-1
0 = (s - 0)
Figure is redrawn from D. P. Strommen and K. Nakamoto, Amer. Lab., 1981, 43 (10), 72.
Raman-Active Vibrational Modes
 Modes that are more polarizable are more Raman-active
 Examples:
– N2 (dinitrogen) symmetric stretch
 cause no change in dipole (IR-inactive)
 cause a change in the polarizability of the bond – as the bond gets
longer it is more easily deformed (Raman-active)
– CO2 asymmetric stretch
 cause a change in dipole (IR-active)
 Polarizability change of one C=O bond lengthening is cancelled by
the shortening of the other – no net polarizability (Raman-inactive)
 Some modes may be both IR and Raman-active, others
may be one or the other!
The Raman Depolarization Ratio
 Raman spectra are excited by linearly polarized radiation


(laser).
The scattered radiation is polarized differently depending
on the active vibration.
Using a polarizer to capture the two components leads to
the depolarization ratio p:
I
p
I
 The depolarization ratio p can be useful in interpreting the
actual vibration responsible for a Raman signal.
Instrumentation for Vibrational Spectroscopy
 Absorption vs. Emission for IR 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 space applications.
 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)
Why Build Instruments for Fourier Transform Work?




Advantages:
– 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
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, 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

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
2
0.5
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
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:
Fourier Transform - IR Spectrum
IR Source
Beamsplitter
Michelson
Interferometer
Detector
Sample
Interferogram
Fixed Mirror
Moving Mirror
IR Sampling Methods: Absorbance Methods
 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.
 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
 Cells: For liquids or dissolved samples. Includes internal reflectance
cells (CIRCLE cells)
 Photoacoustic (discussed later)
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
IR Sources
 Nernst glower:
a rod or cylinder made from several grams
of rare earth oxides, heated to 1200-2200K by an electric
current.
 Globar:
similar to the Nernst glower but made from silicon
carbide, electrically heated. Better performance at lower
frequencies.
Incandescent Wires: nichrome or rhodium, low intensity

 Mercury Arc:
high-pressure mercury vapor tube, electric
arc forms a plasma. Used for far-IR
Tungsten filament: used for near-IR

 CO2 Lasers (line source):
high-intensity, tunable, used for
quantitation of specific analytes.
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)
Raman Spectrometers
 The basic design dispersive Raman scattering system:
Sample
Wavelength
Selector
Detector
(photoelectric transducer)
(90° angle)
Radiation
source
 Special considerations:
– Sources: lasers are generally the only source strong enough to
scatter lots of light and lead to detectable Raman scattering
– Avoiding fluorescence: He-Cd (441.6 nm), Ar ion (488.0 nm,
514.5 nm), He-Ne (632.8), Diode (782 or 830), Nd/YAG (1064)
Modern Raman Spectrometers
 FT-Raman spectrometers – also make use of Michelson
interferometers
– Use IR (1 m) lasers, almost no problem with fluorescence for
organic molecules
– Have many of the same advantages of FT-IR over dispersive
– But, there is much debate about the role of “shot noise” and
whether signal averaging is really effective
 CCD-Raman spectrometers – dispersive spectrometers
that use a CCD detector (like the ICP-OES system
described in the Optical Electronic lecture)
– Raman is detected at optical frequencies!
– Generally more sensitive, used for microscopy
– Usually more susceptible to fluorescence, also more complex
 Detectors - GaAs photomultiplier tubes, diode arrays, in
addition to the above.
More on Raman
 Raman can be used to study aqueous-phase samples
– IR is normally obscured by H2O modes, these happen to be less
intense in Raman
– However, the water can absorb the scattered Raman light and
will damp the spectrum, and lower its sensitivity
 Raman has several problems:
– Susceptible to fluorescence, choice of laser important
– When used to analyze samples at temperatures greater than
250C, suffers from black-body radiation interference (so does
IR)
– When applied to darkly-colored samples (e.g. black), the Raman
laser will heat the sample, can cause decomposition and/or
more black-body radiation
Applications of Raman Spectroscopy
 Biochemistry:


water is not strongly detected in Raman
experiments, so aqueous systems can be studied.
Sensitive to e.g. protein conformation.
Inorganic chemistry: also often aqueous systems.
Raman also can study lower wavenumbers without
interferences.
Other unique examples:
– Resonance Raman spectroscopy: strong enhancement (102 –
106 times) of Raman lines by using an excitation frequency close
to an electronic transition (Can detect umol or nmol of analytes).
– Surface-enhanced Raman (SERS): an enhancement obtained
for samples adsorbed on colloidal metal particles.
– Coherent anti-Stokes Raman (CARS): a non-linear technique
using two lasers to observe third-order Raman scattering – used
for studies of gaseous systems like flames since it avoids both
fluorescence and luminescence issues.
Applications of Raman Spectroscopy
 Raman in catalysis research (see C&E News, Oct. 13,
2006, pg. 59):
– Useful for the study of zeolite interiors
– Fluorescence can be a problem, but one approach is to use UV
light (257 nm) which avoids it just like switching to the IR (but at
the risk of decomposition) – See work from the Stair group at
Northwestern
– For uses of SERS: Catal. Commun 3 547 (2002).
 Raman microscopy: offers sub-micrometer lateral
resolution combined with depth-profiling (when combined
with confocal microscopy)
Comparison of IR and Raman Spectroscopy
 Advantages of Raman over IR:
– Avoids many interferences from solvents, cells and sample
preparation methods
– Better selectivity, peaks tend to be narrow
– Depolarization studies possible, enhanced effects in some cases
– Can detect IR-inactive vibrational modes
 Advantages of IR over Raman:
– Raman can suffer from laser-induced fluorescence and
degradation
– Raman lines are weaker, the Rayleigh line is also present
– Raman instruments are generally more costly
– Spectra are spread over many um in the IR but are compressed
into several nm (20-50 nm) in the Raman
 Final conclusion – they are complementary techniques!
Interpretation of IR and Raman 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 and Raman Spectra of an Organic Compound
O
OH
The IR and Raman spectra of
flufenamic acid (an analgesic/antiinflammatory drug):
CF3
FT-IR Flufenamic acid A ldri ch as recd
0.30
0.25
Ab s
0.20
0.15
0.10
0.05
FT-Raman Flufenamic acid Aldrich as recd
60
50
In t
40
30
20
10
0
3500
3000
2500
2000
Raman shi ft (cm-1)
1500
1000
500
IR and Raman Spectra of an Organic Compound
O
OH
The IR and Raman spectra of
flufenamic acid (an analgesic/antiinflammatory drug):
CF3
FT-IR Flufenamic acid A ldri ch as recd
0.30
0.25
Note – materials
usually limit IR
in this region
Ab s
0.20
0.15
0.10
0.05
FT-Raman Flufenamic acid Aldrich as recd
60
50
In t
40
30
20
10
0
1600
1400
1200
1000
Wavenumbers (cm-1)
800
600
400
200
IR and Raman Spectra of an Organic Compound
The IR and Raman spectra of tranilast:
Tranilast Form I FV101031-171A 1 FTIR
0.6
0.5
Ab s
0.4
0.3
0.2
0.1
Tranilast Form I FV101031-171A 1 FT-Raman
500
O3
C4
O
400
C5
C3
C6
C2
In t
N1
C15
300
C18
C14
H3C
C16
C11
C10
C8
C9
C1
N
H
C12
C13
C7
O
200
O
O5
O2
C17
H3C
100
3500
O
OH
O1
O4
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
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.
Applications of Far IR Spectroscopy
 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.
 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 Near IR Spectroscopy
 Near IR – heavily used in process chemistry
 Amenable to quantitative analysis usually in conjunction with
chemometrics (calibration requires many standards to be run)
 While not a 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
 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)
 1st derivative (and 2nd derivative) allows for easier identification of
bands
Photoacoustic Spectroscopy
 First discovered in 1880 by A. G. Bell
 The IR “version” of photoacoustic sampling is generally
applied to two types of system (UV-Vis spectrometry can
also be performed):
IR Radiation

Gas:
All gas (or all-liquid)
systems:
IR Radiation

The solid-gas system:
IR-Transparent Gas
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

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.
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).
Hybrid/Hyphenated Techniques: Interfaces
 Interfaces between vibrational spectrometers and other
analytical instruments
 GC-FTIR: gaseous column effluent passed through light
pipes
 Similar Technique: TGA-IR, for identification of evolved
gases from thermal decomposition
Figure from Skoog et al.
Homework Problems
Chapter 16:
16-7
Chapter 18:
18-2
Further Reading
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.