Rovibrational Spectroscopy

8/11/2019 Rovibrational Spectroscopy

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Lectu re Date: January 30th, 2008

Rotational and VibrationalSpectroscopy

Vibrational and Rotational Spectroscopy

Core techniques:

Infrared (IR) spectroscopy

Raman spectroscopy

Microwave spectroscopy


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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 1880s

Technique made into a routine analytical method between 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

1980s, led to a great increase in sensitiv ity 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 1970s, lasers made Raman much more practical. Near-

IR lasers (1990s) allowed for avoidance of fluorescence in

many samples.

W. Abney, E. R. Festing, Phil. Trans. Roy. Soc. London, 1882, 172, 887-918.


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Infrared Spectral Regions

IR regions are traditionally sub-divided as follows:

Region Wavelength

(), m

Wavenumber

(), cm-1

Frequency

(), Hz

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 to6.0 x 1012

Far 50 to 1000 200 to 10 6.0 x 1012 to

3.0 x 10

11

After Table 16-1 o f 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 andwavelength scales:

Image from www.asu.edu

100001 cm

Convertingwavelength () to

wavenumbers:


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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):

See E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, Molecular Vibrations, Dover, 1955.

21

21

mm

mm

Rotational Spectroscopy: Theory

Rotational energy levels can bedescribed as follows:

R. Woods and G. Henderson, FTIR Rotational Spectroscopy, J. Chem. Educ., 64, 921-924 (1987)

DJBJJ3)1()1()(

crhB2

0

28/

23 /4c

BD

Where:c is the speed of lightk is the Hookes law force constantr0 is the vibrati onally-averaged bond length

The rotational constant:

The centrifugal distortion coefficient:

u

k

cc

2

1

Example for HCl:B0 = 10.4398 cm-1

D0 = 0.0005319 cm-1

r0 = 1.2887

is the reduced massh is Plancks constant

0 = 2990.946 cm-1 (from IR)k = 5.12436 x 105 dyne/cm-1

ForJ = 0, 1, 2, 3


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Vibrational Spectroscopy: Theory

Harmonic oscillator based on the classical spring

mhvE 2

1

m is the natural frequency of the oscillator (a.k.a. the fundamental vibrational wavenumber)kis the Hookes law force constant (now for the chemical bond)

u

km

2

1

v is the vibrational quantum numberh is Plancks constant

Since v must be a whole number (see Ex. 16-1, pg. 386):

The potential energy function is:2

21 )()(

eHO rrkrE

Note allEarepotential energies (V)!

or 2221 )()2()(

emHO rrcrE

khhE m

2

k12103.5 and(wavenumbers)

ris the distance (bond distance)re is the equilibrium distance

Vibrational Spectroscopy: Theory

Potential energy of a harmonic oscillator:

Figure from Skoog et al.


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Anharmonic Correct ions

Anharmonic motion: when the restoring force is notproportional 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

Effects: at higher quantum numbers, E gets smaller, andthe ( = +/-1) selection rule can be broken

Double ( = +/-2), triple ( = +/-3), and higher order transitions

can occur, leading to overtone bands at higher frequencies (NIR)

2)()1()( e

rra

eMorse ehcDrE De is the dissociation energy

e

m

hcD

ca

2

)2( 2

Vibrational Coupling

Vibrations in a molecule may couple changing eachothers 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 samesymmetry (i.e. between two carbonyl vibrations)


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Vibrational Modes and IR Absorption

Number of modes: Linear: 3n 5 modes

Non-linear: 3n 6 modes

Types of vibrations: Stretching

Bending

Examples: CO2 has 3 x 3 5 = 4 normal

modes

SymmetricNo change in dipole

IR-inactive

AsymmetricChange in dipole

IR-active

ScissoringChange in dipole

IR-active

IR-active modes require dipole changes during rotationsand vibrations!

Vibrational Modes: Examples

IR-activity requiresdipole changes

during vibrations!

For example, thisis Problem 16-3

from Skoog:

InactiveActiveActive

Active

Inactive

Inactive

Active


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IR Spectra: Formaldehyde

Certain types of vibrations have distinct IR frequencies hence thechemical usefulness of the spectra

The gas-phase IR spectrum of formaldehyde:

Formaldehyde spectrum f rom: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir2Results generated usi ng B3LYP//6-31G(d) in Gaussian 03W.

Tables and simulation results can help assign the vibrations!

(wavenumbers, cm-1)

Rayleigh and Raman Scattering

Only objects whose dimension is ~1-1.5 will scatter EMradiation.

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 betweenthe wavelengths of the incident radiation and the

scattered radiation.


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The Raman Effect

Polarization changesare necessary to form

the virtual state and

hence the Raman

effect

This figure depictsnormal (spontaneous)

Raman effects

H. A. Strobel and W. R. Heineman, Chemical Instrumentation: A Systematic Approach, 3rd Ed. Wiley: 1989.

hv1

Scattering timescale ~10-14 sec

(fluorescence ~10-8 sec)

Virtual state

Virtual state

hv1

Ground state(vibrational)

The incident radiation excites virtual states (distorted

or polarized states) that persist for the short timescale ofthe scattering process.

Excited state(vibrational)

hv1 hv2Stokes line

hv1 hv2Anti-St okes line

More on Raman Processes

The Raman process: inelastic scattering of a photonwhen 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 ofthe molecule

Polarizability: the deformability of a bond or a moleculein 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, 3rdEd. Oxford: 1997.


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More on Raman Processes

Consider the time variation of the dipole moment inducedby incident radiation (an EM field):

)()()( ttt

P. W. Atkins and R. S. Friedman,Molecular Quantum Mechanics, 3rdEd. Oxford: 1997.

EM fieldInduced dipole moment

Expanding this product yields:

tttt )cos()cos(cos)( intint041

0

Rayleigh line Anti-Stokes line Stokes line

polarizability

If the incident radiation has frequency and thepolarizability of the molecule changes between min and

max at a frequency int as a result of this rotation/vibration:

ttt coscos)( 0int21

mean polarizability = max -min

The Raman Spectrum of CCl4

Figure is redrawn from D. P. Strommen and K. Nakamoto,Amer. Lab., 1981, 43 (10), 72.

Observed intypicalRaman

experiments

0 = 20492 cm-1

0 = 488.0 nm

Anti-Stokes lines

(inelastic scattering)

-218

Raman shift cm-1

0 = (s -0)

-200

Stokes lines

(inelastic scattering)

-400400 200

218314

-314

-459

459

0

Rayleigh line

(elastic scattering)


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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, othersmay be one or the other!

The Raman Depolarization Ratio

Raman spectra are excited by linearly polarized radiation(laser).

The scattered radiation is polarized differently dependingon the active vibration.

Using a polarizer to capture the two components leads tothe depolarization ratio p:

I

Ip

The depolarization ratio p can be useful in interpreting theactual vibration responsible for a Raman signal.


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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).

Fourier-transform methods: based on interferometry, a method of

interfering and modulating IR radiation to encode it as a functionof its f requency.

Radiation

SourceSample

Wavelength

Selector

Detector

(transducer)

Radiation

SourceInterferometer 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 atonce, 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 signalsin 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 interferometerswith tiny mirror movements


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Inteferometers for FT-IR and FT-Raman

The Michelson

interferometer, theproduct of a famous

physics experiment:

Producesinterference

patterns frommonochromatic

and white light

Figures from Wikipedia.org

Inteferometers

For monochromaticradiation, the

interferogram looks like

a cosine curve

For polychromaticradiation, each

frequency is encoded

with a much slower

amplitude modulation

The relationshipbetween frequencies:

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

cvf M2

Where: is the frequency of the radiationc is the speed of light in cm/svm is the mirror velocity in cm/s


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The Basics of the Fourier Transform

The conversion from time- to frequency domain:

50 100 150 200 250

-1

-0.5

0.5

1

50 100 150 200 250

0.5

1

1.5

2

FT

50 100 150 200 250

-1.5

-1

-0.5

0.5

1

1.5

2

50 100 150 200 250

0.5

1

1.5

2

2.5

1

0

/21 N

k

Nikn

kn ed

Nf

b

a

dtftKg )(),()(1 )texp(),( itK Continuous:

Discrete:

FT

FTIR Spectrometer Design

Michelson

Interferometer

IR Source

Sample

Moving MirrorFixed Mirror

Beamsplitter

Detector

Interferogram

Fourier Transform - IR Spectrum

It is possible to build a detector that detects multiplefrequencies for some EM radiation (ex. ICP-OES with CCD,UV-Vis DAD)

FTIR spectrometers are designed around the Michelsoninterferometer, which modulates each IR individualfrequency with an additional unique frequency:


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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 matrixis 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 reflectancecells (CIRCLE cells)

Photoacoustic (discussed later)

IR Sampling Methods: Reflectance Methods

Specular reflection: directreflection 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)

Diffuse reflection (DRIFTS): atechnique that collects IR

radiation scattered off of fine

particles and powders. Used

for both surface and bulk

studies.

Figures f rom http://www.nuance.northwestern.edu/KeckII/ftir7.asp

ATR

DRIFTS


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IR Sources

Nernst glower: a rod or cylinder made from several gramsof rare earth oxides, heated to 1200-2200K by an electric

current.

Globar: similar to the Nernst glower but made from siliconcarbide, electrically heated. Better performance at lower

frequencies.

Incandescent Wires: nichrome or rhodium, low intensity

Mercury Arc: high-pressure mercury vapor tube, electricarc forms a plasma. Used for far-IR

Tungsten filament: used for near-IR

CO2 Lasers (line source): high-intensity, tunable, used forquantitation 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 ~47C

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)


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Raman Spectrometers

The basic design dispersive Raman scattering system:

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)

SampleWavelength

Selector

Detector

(photoelectric transducer)

Radiation

source

(90 angle)

Modern Raman Spectrometers

FT-Raman spectrometers also make use of Michelsoninterferometers

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 spectrometersthat 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, inaddition to the above.


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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

250C, 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

Appl icat ions of Raman Spectroscopy

Biochemistry: water is not strongly detected in Ramanexperiments, 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 obtainedfor 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.


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Interpretation of IR and Raman Spectra

General Features:

Stretching frequencies are greater (higher wavenumbers) thancorresponding 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 bandsand 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)

alkynes (CC-H)

Broad

Variable

Sharp

3000 alkane (C-C-H)

alkene (C=C-H)

2100-2300 alkyne (CC-H)

nitrile (CN-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, es ters,

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.


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IR and Raman Spectra of an Organic Compound

The IR and Raman spectra of

flufenamic acid (an analgesic/anti-

inflammatory drug):

CF3

O OH

FT-IR Flufenamic acid Aldrich as recd

0.05

0.10

0.15

0.20

0.25

0.30

Abs

FT-Raman Flufenamic acid Aldrich as recd

0

10

20

30

40

50

60

Int

500100015002000250030003500

Raman shift (cm-1)


The IR and Raman spectra offlufenamic acid (an analgesic/anti-

inflammatory drug):

CF3

O OH

FT-IR Flufenamic acid Aldrich as recd

0.05

0.10

0.15

0.20

0.25

0.30

Abs

FT-Raman Flufenamic acid Aldrich as recd

0

10

20

30

40

50

60

Int

2004006008001000120014001600

Wavenumbers (cm-1)

Note materialsusually limit IRin this region


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The IR and Raman spectra of tranilast:

Tranilast Form I FV101031-171A1 FTIR

0.1

0.2

0.3

0.4

0.5

0.6

Abs

Tranilast Form I FV101031-171A1 FT-Raman

100

200

300

400

500

Int

500100015002000250030003500

Wavenumbers (cm-1)

O

O

NH

O

OHO

C1

C6C2

C3

C4

C5

C7

N1

C8

C9

C10

C11

C12

C13

C14

C15

C16

C17

C18

H3C

H3C

O4

O5

O3

O2 O1

IR Frequencies and Hydrogen Bonding Effects

IR frequencies are sensitive tohydrogen-bonding strength and

geometry (plots of relationships

between crystallographic distances

and vibrational frequencies):

G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford, 1997.


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Appl ications of Far IR Spectroscopy

Far IR is used to study low frequency vibrations, like those between

metals and ligands (for both inorganic and organometal lic 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 semiconductorproperties.

The Far IR region also overlaps rotational bands, but these arenormally not detectable in condensed-phase work

Terahertz Spectroscopy

A relatively new technique, addresses an unused portionof 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 instrumentdesign, accesses a region of crystalline phonon bands

P. F. Taday and D. A. Newnham, Spectroscopy Europe, , www.spectroscopyeurope.comG. Winnewisser, Vibrational Spectroscopy 8 (1995) 241-253


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Appl ications of Near IR Spectroscopy

Near IR heavily used in process chemistry

Amenable to quantitative analysis usually in conjunction withchemometrics (calibration requires many standards to be run)

While not a qualitative technique, it can serve as a fast and usefulquantitative 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.


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Near IR Spectrum of Acetone

NIR taken in transmission mode (via a reflective gold plate) on aFoss NIRsystems spectrometer

Useful for quick solvent identification

Near IR Spectrum of Water (1st Derivative)

1st derivative (and 2nd derivative) allows for easier identification ofbands


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Photoacoustic Spectroscopy

First discovered in 1880 by A. G. Bell

The IR version of photoacoustic sampling is generallyapplied to two types of system (UV-Vis spectrometry canalso be performed):

All gas (or all-liquid)systems:

The solid-gas system:

Solid

IR-Transparent Gas

Gas:

IR Radiation

IR Radiation

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 intensity-modulated light hits a solid surface (or a confined gas or

liquid).

Gas

Solid

Modulated IR Radiation

x

PA Cell

Thermal Wave (attenuates rapidly)

J. F. McClelland.Anal. Chem. 55(1), 89A-105A (1983)M. W. Urban. J. Coatings Technology. 59, 29 (1987).

Microphone

P(x)

P0

IR is absorbed by a vibrational transition,

followed by non-radiative relaxation

P R P ex

R

P

surface

( )(

1 0

0

+ )

surface reflectivity

incident IR beam power

- absorption coefficient

- thermal diffusion length

1

(Psurface)


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The Thermal Diffusion Length

Urban, M. W. J. Coatings Technology. 1987, 59, 29Quintanilla, L., Rodriguez-Cabello, J. C., Jawhari, T. and P astor, J. M.. Polymer. 1993, 34, 3787.

The thermal diffusion length is:

PET

PVF2

0.15 cm/sec IR 1.2 cm/sec IR


= / 2

The thermal diffusivity a is:

The variable , the modulation frequency of the IRradiation, is directly proportional to interferometer mirror

velocity, and is defined as:

(cm/sec)eterinterferomMichelsonofocityMirror vel

rs)(wavenumbeFrequencyIR

4

M

M

ak

C

k

C

thermal conductivity

density

specific heat

2a

The Thermal Diffusion Length

Urban, M. W. and Koenig, J. L. Appl. Spec. 1986, 40, 994.Quintanilla, L., Rodriguez-Cabello, J. C., Jawhari, T. and P astor, J. M.. Polymer. 1993, 34, 3787.

The mirror velocity is therefore inversely related to thethermal 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) Thermal Diffusion Length (microns)

0.15 8.9

0.30 6.3

0.60 4.5

0.90 3.61.20 3.1

The thermal diffusivity was taken to be 1.3 * 10-3 cm2/sec, and the absorption coefficient of the carbonyl band wasassumed to be 2000 cm-1.


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A Typical Photoacoustic FTIR Spectrum

A PA-FTIR Spectrum of a silicone sealant:

The spectrum shows peaks where the IR radiation is beingabsorbed due to vibrational energy level transitions.

Paroli, R. M., Delgado, A. H., and Cole, K. C. Canadian J. Appl. Spectr. 1994, 39, 7.

IR Modulation

frequency is high

IR Modulation

frequency is low

Differences between a PA-FTIR spectrum and a regular IRspectrum: IR modulation frequency effects (weak CH3 and CH2 bands)

Saturation of strong bands in the spectrum

Photoacoustic Saturation

Strong bands in PA-FTIR spectra oftenshow saturation.

Saturation occurs when the vibrationaltransition is being pumped to its

excited state faster than it can release

energy.

A high absorption coefficient coincideswith faster saturation.

A Saturated Band

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.


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Depth-Profiling Studies with PA-FTIR

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.

Thermal diffusion lengthallows for IR depthprofiling with PA-FTIR

Example: a layer ofpoly(vinylidine fluoride

(PVF2) on poly(ethylene

terephthalate) (PET)

PET

PVF2

PVF2 top layer is 6 micrometers thick.

The carbonyl band, due to the PET, is m arked with a red dot ().

Data acquired with a Digilab FTS-20E with a home-built PA cell.

0.15 cm/sec IR 1.2 cm/sec IR


= / 2

Appl ications of FT Microwave Spectroscopy

Under development for: real-time, sensitive monitoring ofgases 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 Fabry-

Perot interferometers (PNFTMW)

Diagram from http://physics.nist.gov/Divisions/Div844/facilities/ftmw/ftmw.htmlFor more information, s ee E. Arunan, S. Dev. And P. K. Mandal, Applied Spectroscopy Reviews, 39, 131-181 (2004).

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


30/313

Hybrid/Hyphenated Techniques: Interfaces

Interfaces between vibrational spectrometers and other

analytical instruments

GC-FTIR: gaseous column effluent passed through lightpipes

Similar Technique: TGA-IR, for identification of evolvedgases from thermal decomposition

Figure from Skoog et al.

Homework ProblemsChapter 16:

16-7

Chapter 18:

18-2


31/31

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.

Documents

Rovibrational Spectroscopy