Infrared Spectroscopy 03

8/13/2019 Infrared Spectroscopy 03

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Infrared (IR) Spectroscopy

IR deals with the interaction of infrared radiation with matter.The IR spectrum of a compound can provide importantinformation about its chemical nature and molecular structure.

Most commonly, the spectrum is obtained by measuring theabsorptionof IR radiation, although infrared emission andreflection are also used.

Widely applied in the analysis of organic materials, also usefulfor polyatomic inorganic molecules and for organometalliccompounds.


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Overview

1. Electromagnetic radiation

2. Vibrations

3. Principle of IR experiment

4. IR spectrum

5. Types of vibration

6. CGF/Fingerprint regions

7. IR activity of vibrations

8. Interpretation of IR spectra

9. Instrumentation

10. Sample preparation


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Electromagnetic Radiation

The propagation of electromagneticradiation in a vacuum is constant for allregions of the spectrum (= velocity oflight):

c =

1 = 10 10m 1 nm = 10 9m 1 m = 10 6m

Another unit commonly used is the wavenumber, which is linear with energy:

Work by Einstein, Planck and Bohr indicated that electromagnetic radiation can be

regarded as a stream of particles or quanta, for which the energy is given by the

Bohr equation:


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The Electromagnetic Spectrum


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LIMIT OF RED LIGHT: 800 nm, 0.8 m, 12500 cm-1

NEAR INFRARED: 0.8 -2.5 m, 12500 - 4000 cm-1

MID INFRARED: 2.5 - 50 m, 4000 - 200 cm-1

FAR INFRARED:50 - 1000 m, 200 - 10 cm-1

Divisions arise because of different optical materials and

instrumentation.

Infrared region


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Molecular spectra

There are three basic types of optical

spectra that we can observe formolecules:1.Electronic or vibronic spectra (UV-visible-near IR)(transitions between a specific vibrational and rotational level

of one electronic state and a vibrational and rotational level ofanother electronic state)

2.Vibrational or vibrational-rotational

spectra (IR region)(transitions from the rotational levels of one vibrational levelto the rotational levels of another vibrational level in the sameelectronic state)

3.Rotational spectra (microwave region)(transitions between rotational levels of the same vibrationallevel of the same electronic state)


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Infrared radiation in the range from 10,000 100 cm 1isabsorbed and converted by an organic molecule into energy ofmolecular vibration

> this absorption is quantized:

Vibrational spectra (I): Harmonic oscillator model

A simple harmonic oscillator is a mechanical system consisting ofa point mass connected to a massless spring. The mass is underaction of a restoring force proportional to the displacement of

particle from its equilibrium position and the force constantf(alsokin followings) of the spring.


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The vibrational frequency is increasing with:

increasing force constant f = increasing bond strength

decreasing atomic mass

Example:f cc>f c=c>f c-c

The vibrational energy V(r) can be calculated using the (classical) model of the

harmonic oscillator:

Using this potential energy function in the Schrdinger equation, the vibrationalfrequency can be calculated:


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Vibrational spectra (II): Anharmonic oscillator model

The actual potential energyof vibrations fits the

parabolic function fairlywell only near theequilibrium internuclear

distance. The Morsepotential function moreclosely resembles the

potential energy ofvibrations in a molecule for

all internuclear distances-anharmonic oscillatormodel.

Fig. 12-1


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The energy difference between

the transition from n to n+1corresponds to the energy of theabsorbed light quantum

The difference between two

adjacent energy levels gets

smaller with increasing n untildissociation of the moleculeoccurs (Dissociation energy ED)

EVI B ( En+1En) =h osc

Note:

Weaker transitions called overtonesare sometimes observed. These correspond to=2 or 3, and their frequencies are less than two or three times the fundamentalfrequency (=1) because of anharmonicity.

Typical energy spacings for vibrational levels are on the order of 10-20J. from theBolzmann distribution, it can be shown that at room temperature typically 1% or less ofthe molecules are in excited states in the absence of external radiation. Thus most

absorption transitions observed at room temperature are from the =0 to the =1 level.


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The vibrational spectra appear as bands rather than lines. When vibrational

spectra of gaseous diatomic molecules are observed under high-resolutionconditions, each band can be found to contain a large number of closelyspaced componentsband spectra. The structure observed is due to thata single vibrational energy change is accompanied by a number ofrotational energy changes. The form of such a vibration-rotation spectrumcan be predicted from the energy levels of a vibrating-rotating molecule.

> vibrational-rotational bands

Vibrational spectra (III): Rotation-vibration transitions

A vibrational absorption transition from to +1 gives rise to three sets

of lines called branches:

Lower-frequency P branch: =1, J=-1;

Higher-frequency R branch: =1, J=+1;

Q branch: branch: =1, J=0.


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Spectrum of the Rotating Oscillator

The selection rules allow only 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


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The IR absorption spectrum can be obtained with gas-phase orwith condensed-phase molecules. For gas-phase moleculesvibration-rotation spectra are observed, while in condensed

phases, the rotational structure is lost.

For most routine analytical applications of infraredspectrometry, spectra are obtained with condensed-phasesamples. Hence, the discuss here centers around the vibrationaltransitions observed with molecules present as pure liquid, as

solutions, or in the solid state.


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Molecular vibrations How many vibrations are possible (=fundamental vibrations)?

A molecule has as many degrees of freedom as the total degree offreedom of its individual atoms. Each atom has three degrees of freedom(corresponding to the Cartesian coordinates), thus in an N-atommolecule there will be 3N degree of freedom.

In molecules, movements of the atoms are constrained by interactionsthrough chemical bonds.

Translation- the movement of the entire molecule while the positionsof the atoms relative to each other remain fixed: 3 degrees of

translational freedom.Rotational transitionsinteratomic distances remain constant but theentire molecule rotates with respect to three mutually perpendicularaxes: 3 rotational freedom (nonlinear), 2 rotational freedom (linear).


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Fundamental Vibrations

Vibrationsrelative positions of the atoms change while the averageposition and orientation of the molecule remain fixed.


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There are two different types of vibrational modes:

Vibrations can either involve a change in bond length

(stretching) or bond angle (bending)

Vibration Types


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Principle of IR experiments

E-vector in electromagnetic radiationhas frequency Molecular vibrations involving change in dipole moment set upfluctuating electric field

Vibrational energies: fundamental (= one quantum)

Energy transferred to molecule by resonance when vibrationfrequency is the same as that of the electromagnetic radiation

IR SAMPLE SAMPLE*

(MOLECULE, GS) (VIB.)


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Vibrations which do not change the dipole moment areInfrared Inactive

(homonuclear diatomics).

Selection Rules

The energy associated with a quantum of light may be transferred to themolecule if work can be performed on the molecule in the form ofdisplacement of charge.

Selection rule:

A molecule wil l absorb infrared radiation i f the change in vibrational

states is associated with a change in the dipole moment () of themolecule.

= qrq: electrical charge, r: directed distance of that charge from somedefined origin of coordinates from the molecule.

Dipole moment is greater when electronegativity difference between theatoms in a bond is greater. Some electronegativity values are:H 2.2; C 2.55; N 3.04; O 3.44; F 3.98; P 2.19; S 2.58; Cl 3.16


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The theoretical number of fundamental vibrations (absorption

frequencies) will seldom be observed

> overtones (multiples of a given frequency), combination (sum oftwo other vibrations) or difference (the difference of two othervibrations) tones increase the number of bands

> the following effects will reduce the number of theoreticalbands: frequencies which fall outside the measured spectral region (400-

4000 cm 1) bands which are too weak bands are too close and coalesce occurrence of a degenerate band from several absorptions of the

same frequency lack of change in molecular dipole

Why not 3N-6/3N-5 bands in IR spectrum?


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Infrared Spectrum

of Carbon Dioxide


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Vibrational Modes for a CH2Group


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Absorption Regions


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Fingerprint region

In the region from 1300 to 400 cm-1, vibrational frequencies are

affected by the entire molecule, as the broader ranges for groupabsorptions in the figure belowfingerprint region.

Absorption in this fingerprint region is characteristic of the moleculeas a whole. This region finds widespread use for identification purpose

by comparison with library spectra.


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When two bond oscillators share a common atom, they seldom behave

as individual oscillators (unless the individual oscillation frequencies are

widely different).

The frequency of the asymmetric stretching vibration in CO2is at a

shorter wavelength (higher frequency) than for a carbonyl group in

aliphatic ketones (around 1715 cm1).

> there must be strong mechanical coupling or interaction!

Example: CO stretching band in

Methanol: 1034 cm 1

Ethanol: 1053 cm 1

not an isolated stretching vibration, but rather a coupled symmetric

stretching invloving CCO stretching

Coupled Interactions


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The vibrations must be of the same symmetry

The interaction is greatest, when the coupled groups absorb

(individually) near the same frequency--- the same energies of isolated

vibrations.

Strong coupling between stretching vibrations requires a common atombetween the two groups

Coupling between bending and stretching vibrations can occur if the

stretching bond forms one side of the changing angle.

A common bond is required for coupling of bending vibrations. Coupling is negligible when groups are separated by one or more carbon

atoms and the vibrations are mutually perpendicular.

Requirements for Coupled Interactions


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Hydrogen bonding alters the force constant of both groups:

the XH stretching bands move to lower frequency

the stretching frequency of the acceptor group (B) is also reduced,

but to a lesser degree

The XH bending vibration usually shifts to a shorter wavelength

Effect of Hydrogen Bonding


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y axis is %T or A

x axis is wavenumber (or wavelength)

Iosample I

T = I/Io %T = 100 I/Io

T transmission / transmittance

A = -log T

A absorbance(no units)

(Note A (but not T) concentration)

IR spectrum


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Absorbance spectrum of polystyrene

> generally used for quantitative

work

Ordinate Scaling

Transmittance spectrum of polystyrene

> traditionally used for spectral

interpretation


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Dispersive instruments: with a monochromator to be used inthe mid-IR region for spectral scanning and quantitativeanalysis.

Fourier transform IR (FTIR) systems: widely applied andquite popular in the far-IR and mid-IR spectrometry.

Nondispersive instruments: use filters for wavelengthselection or an infrared-absorbing gas in the detection systemfor the analysis of gas at specific wavelength.


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Dispersive IR spectrophotometers

Simplified diagram of a double beam infrared spectrometer

Modern dispersive IR spectrophotometers are invariably double-beam

instruments, but many allow single-beam operationvia a front-panelswitch.


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Double-beam operation compensates for atmospheric absorption, for thewavelength dependence of the source spectra radiance, the opticalefficiency of the mirrors and grating, and the detector instability, whichare serious in the IR region.single-beam instruments not practical.

Double-beam operation allows a stable 100% T baseline in the spectra.


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Dispersive spectrophotometers Designs

Null type instrument


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Components of dispersive spectrophotometers

Nernst Glower heated rare earth oxide rod(~1500 K)

1-50 m

(mid- to far-IR)

Globar heated SiC rod (~1500 K) 1-50 m(mid- to far-IR)

W filament lamp 1100 K 0.78-2.5 m

(Near-IR)

Hg arc lamp plasma 50 - 300 m(far-IR)

CO2 laser stimulated emission lines 9-11 m

1. IR source


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3. Optical system


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Reflection gratings( made from various plastics): the groovespacing is greater (e.g. 120 grooves mm-1). To reduce the effect ofoverlapping orders and stray radiation, filters or a preceding prism

are usually employed. Two or more gratings are often used withseveral filters to scan a wide region.

Mirrorsbut not lenses are used to focus and collimate the IRradiation. Generally made from Pyrex or another material with lowcoefficient of thermal expansion. Front surfaces coated with avacuum-deposited thin metal film of Al, Ag, or Au.


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Windowsare used for sample cells and to permit various compartmentto be isolated from the environment.

transparent to IR over the wavelength regioninert to the various chemicals analyzedcapable of being shaped, ground, and polished to the desiredoptical quality

Fourier Transform Infrared Spectrometer (FTIR)


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The Fourier transform method provides an alternatives to the useof monochromators based on dispersion.

In conversional dispersive spectroscopy, frequencies are separatedand only a small portion is detected at any particular instant, whilethe remainder is discarded. The immediate result is afrequency-domain spectrum.

Fourier transform infrared spectroscopy generates time-domainspectraas the immediately available data, in which the intensity isobtained as a function of time.

Direct observation of a time-domain spectrum is not immediatelyuseful because it is not possible to deduce, by inspection,frequency-domain spectra from the corresponding time-domainwaveform (Fourier transformis thus introduced).

Fourier Transform Infrared Spectrometer (FTIR)

Si l b FTIR S t t


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In one arm of the interferometer, the IR source radiation travels through the beamsplitter to the fixed mirror back to the beam splitter through the sample and to thedetector. In the other arm, the IR source radiation travels to the beam splitter to themovable mirror, back through the beam splitter to the sample and to the detector. Thedifference in pathlengths of the two beams is the retardation . An He-NE laser is usedas a monochromatic reference source. The laser beam is sent through the interferometer

in the opposite direction to that of the IR beam.

Single-beam FTIR Spectrometer


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Double-beam FTIR

Spectrometer

Interferometer


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Interferometer

Michelson interferometer

If moving mirror moves 1/4 (1/2 round-trip) waves are out of phase at beam-splitting mirror - no signal

If moving mirror moves 1/2 (1 round-trip) waves are in phase at beam-splittingmirrorsignal

...


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Interferograms


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Difference in pathlength called retardation Plot vs. signal - cosine wave with frequency proportional to lightfrequency butsignal varies at much lower frequency

One full cycle when mirror moves distance /2 (round-trip = )

Frequency of signal:

Substituting =c/

If mirror velocity is 1.5 cm/s

Bolometer, pyroelectric, photoconducting IR detectors can "see changes

on 10-4s time scale!

MMMM VVf 2

2/==

c

Vf MM

2=

VMMvelocity of moving mirror

10

10 10/103

/3

== scm

scm

f


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Computer needed to turn complex interferograms into spectra.


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Advantages of FTIR


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very high resolution (< 0.1 cm 1)Two closely spaced lines only separated if one complete "beat" is recorded. As

lines get closer together, must increase.(cm1) = 1/Mirror motion is 1/2 Resolution governed by distance movable mirror travels

very high sensitivity (nanogram quantity)can be coupled with GC analysis (> measure IR spectra in gas-phase)

High S/N ratios - high throughputFew optics, no slits mean high intensity of light

Rapid (


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Usually to improve resolution

decrease slit width but less lightmakes spectrum "noisier" - signalto noise ratio (S/N)

n # scans

S/N improves with more scans

(noise is random, signal is not!)

nN

S

SS

S

nN

S

i==

2)(

To improve S/N ratio


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For routine instrument calibration, run the spectrum ofpolystyrene film (or indene) at resolution 2 cm-1. Band

positions are available in the literature.

Higher resolution calibrations may be made from gas-

phase spectra (e.g. HCl gas).

Spectrum calibration

S l ti t h i


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Sample preparation techniques

The preparation of samples for infrared spectrometry is often the mostchallenging task in obtaining an IR spectrum. Since almost all substances absorbIR radiation at some wave length, and solvents must be carefully chosen for the

wavelength region and the sample of interest.

Infrared spectra may be obtained for gases, liquids or

solids (neat or in solution)


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Regions of transparency for common infrared solvents.

The horizontal lines indicate regions where solvent transmits at least25% of the incident radiation in a 1-mm cell.


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Solid samples

Spectra of solids are obtained as alkali halide discs (KBr), mulls

(e.g. Nujol, a highly refined mixture of saturated hydrocarbons) and

films (solvent or melt casting)

Alkali halide discs:

1. A milligram or less of the fine ground sample mixed with about

100 mg of dry KBr powder in a mortar or ball mill.

2. The mixture compressed in a die to form transparent disc.

Mulls

1. Grinding a few milligrams of the powdered sample with a mortar

or with pulverizing equipment. A few drops of the mineral oiladded (grinding continued to form a smooth paste).

2. The IR of the paste can be obtained as the liquid sample.


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1.Fundamental chemistry

Determination of molecular structure/geometry.

e.g. Determination of bond lengths, bond angles of

gaseous molecules

2. Qualitative analysissimple, fast, nondestructive

Monitoring trace gases: NDIR.Rapid, simultaneous

analysis of GC, moisture, N in soil. Analysis of fragments

left at the scene of a crime

Quantitative determination of hydrocarbons on filters, in

air, or in water

Main uses of IR spectroscopy:


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Near-infrared and Far-infrared absorption

The techniques and applications of near-infrared (NIR) and

far-infrared (FIR) spectrometry are quite different from those

discussed above for conventional, mid-IR spectrometry.

Near-infrared: 0.8 -2.5 m, 12500 - 4000 cm

-1

Mid-infrared: 2.5 - 50 m, 4000 - 200 cm-1

Far-infrared:50 - 1000 m, 200 - 10 cm-1


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Near-infrared spectrometry

NIR shows some similarities to UV-visible spectrophotometry and

some to mid-IR spectrometry. Indeed the spectrophotometers

used in this region are often combined UV-visible-NIR ones.

The majority of the absorption bands observed are due to

overtones (or combination) of fundamental bands that occur in

the region 3 to 6 m, usually hydrogen-stretching vibrations.

NIR is most widely used for quantitative organic functional-group

analysis. The NIR region has also been used for qualitative

analyses and studies of hydrogen bonding, solute-solvent

interactions, organometallic compounds, and inorganiccompounds.


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Far-infrared spectrometry

Almost all FIR studies are now carried out with FTIRspectrometers.

The far-IR region can provide unique information.

i) The fundamental vibrations of many organometallic and

inorganic molecules fall in this region due to the heavy atomsand weak bonds in these molecules.

ii) Lattice vibrations of crystalline materials occur in this region,

iii) Electron valence/conduction band transition in

semiconductors often correspond to far-IR wavelengths.


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Infrared Spectrum of CCl


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Infrared Spectrum of CCl4


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Infrared Spectroscopy 03

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