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Molecular absorption

spectroscopy

FTIR

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IR spectral regions

REGION WAVELENGTHS

(), m WAVENUMBERS( ), cm-1 FREQUENCY (), HzNear 0.78 to 2.5 12800 to 4000 3.8 x 1014 to 1.2 x 1014

Middle 2.5 to 50 400 to 200 1.2 x 1014

to 6.0 x 1012

Far 50 to 1000 200 to 10 6.0 x 1014 to 2.0 x 1011

Most

used

2.5 to 15 4000 to 670 1.3 x 1014 to 2.0 x 1013

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IR absorption spectrum of a

polystyrene film

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IR spectra

Frequency is seldom used as the abscissa

Inconvenient size of unit

1.3 x 1014

to 2.0 x 1013

Hz or s-1

This axis is often referred to frequency

The terminology is not correct

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IR spectra

Plot of %T vs. cm-1

Linear wavenumber scale is preferred

Direct proportionality betweenwavenumber and both energy and

frequency

hcchhE

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Molecules

Are composed of atoms held toghether bychemical bonds

The atoms in a molecule are always

moving or vibrating The intensities of vibrations increase when

IR radiation is absorbed

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Each chemical bond requires a precise amount

of energy to make it vibrate Each frequency of IR radiation provides a preciseamount

Radiation is absorbed by a molecule only if the

frequency of the radiation provides energy in theprecise amount required by one of the bonds in

the molecule

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Molecules can be large or small

The atoms they contain may be the same

or different

The bonds between them may vary from

weak to small

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IR active

To absorb IR radiation

The molecule absorbs IR radiation that

corresponds to the energy difference for

the vibrational transition

Energy is quantized

A molecule must undergo a net change

in dipole moment as it vibrates or

rotates

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Dipole moment

The charge distribution around a molecule

is not symmetric because one of the atom

has a higher electron density

Example HCl

Is determined by the magnitude of the

charge difference and the distance

between the two centers of charge

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DIPOLE MOMENTS

Only bonds which have significant dipole moments willabsorb infrared radiation.

Bonds which do not absorb infrared include:

Symmetrically substituted alkenes and alkynes

C C RR

R

R R

R

Many types of C-C Bonds

Symmetric diatomic molecules

H-H Cl-Cl

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C

OThe carbonyl group is oneof the strongest absorbers

O H C OAlso O-H and C-O bonds

d+d-

STRONG ABSORBERS

+ +

- -

C

O

C

O

d+d-

oscillating dipoles couple and

energy is transferred

infrared beam

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Rotational transitions

Small energy is required to cause a change in

rotational level

100 cm-1 or > 100m

Rotational levels are quantized

Absorption of gases in the FIR region is

characterized by discrete, well defined lines

In liquids and solids; intramolecular collisionsand interactions cause broadening of the lines

into a continuum

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Vibrational-rotational

transitions Vibrational energy levels are quantized

E between quantum states correspond to MIR

region for most molecules

Spectrum of a gas

A series of closely spaced lines

There are several rotational energy levels for each

vibrational level

Spectra ofsolids and liquids

Broad vibrational bands

Rotation is restricted in solids and liquids

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Depending on the number of ways the

bond can move (bend, stretch etc.), each

type of bond may absorb IR radiation at

one or more specific frequencies.

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Types of molecular vibrations

Stretching

A continuous change in the interatomic distance

along the axis of the bands between the two atoms

Symmetrical

Asymmetrical

Bending

A change in the angle between two bonds

4 types Scissoring

Rocking

Wagging

Twisting

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Stretching vibrations

Linear molecule

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Stretching vibrations

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Bending vibrations

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Hookes Law

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=1

2pcK

larger K,

higher frequency

larger atom masses,

lower frequency

constants

2150 1650 1200

C=C > C=C > C-C=

C-H > C-C > C-O > C-Cl > C-Br

3000 1200 1100 750 650

increasing K

increasing

1m

21

21

mm

mm

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Stretching vibrations of CO2

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Stretching vibrations of CO2

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Bond order affects bond strength, so bond order

affects the position of absorption bands

The approximate wavenumber of an absorption can be

calculated from Hookes law

is the wavenumber of the stretching vibration

f is the force constant

m1 and m2 are the masses of the atoms

2

1

21

21 )(2

1

mmmmf

cp

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The exact position of the absorption band

depends on

electron delocalization

the electronic effect of neighboring substituents

hydrogen bonding

CH3CCH2CH2CH3

O O O-

C O C O

at 1720 cm1 at 1680 cm1

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Putting an atom other than carbon next to the carbonyl group

causes the position of the carbonyl absorption band to shift

The predominant effect of the nitrogen of an amide is electron

donation by resonance

The predominant effect of the oxygen of an ester is inductive

electron withdrawal

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The position of a CO absorption varies because

CH3CH2 OH

CH3CH2 O CH2CH3

C

O

H3C OHC

O-

H3C OH

C

O

H3C O CH3C

O-

H3C O CH3

~1050 cm1

~1050 cm1

~1250 cm1

~1250 cm1 and 1050 cm1

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FTIR spectrometer

The main optical components are

The IR source

The interferometer The beamsplitter

The laser

The IR detector

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IR source

The IR source produces IR radiation beam

that travels through the spectrometer

passing through the sample and to the

detector

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Interferometer

Beamsplitter

Two mirrors

fixed

movable

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Beamsplitter

Separates the IR beam into two beams of

nearly equal energy

One beam is reflected from the beam splitter

to a fixed mirror

The other beam is transmitted to a moving

mirror and back to the beam splitter where

the beams recombine

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Laser

Used as an internal calibrator

control the moving mirrors position signals the capture of data

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IR transducer

After being absorbed at specific

frequencies by the sample,

the remainder of the IR beam is focused onto

the detector

The detector produces signal to the

amount of IR radiation striking it and sends

the signal to the computer for processing

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FTIR spectrometer

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FTIR spectrometer

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Emits IR radiation across the region of

interest

IR beam is deflected off a mirror

Directs the IR beam into the interferogram

Where the spectral encoding takes place

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Interferometer

IR source

Beamsplitter

Moving

mirror

Fixed

mirror

Detector

Sample

He-Ne Laser

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The computer reads the interferogram and

uses Fourier transformation to decode the

intensity information for each frequency

and presents a spectrum

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The spectrometer measures the intensity

of a speciallyencoded IR beam after it has

passed through the sample

The resulting signal, interferogram

Contains infromation about all frequencies

present in the beam

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Laser

Produces a single frequency of red light

that follows the same path as the IR beam

Calibrates the instrment internally

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FOURIER TRANSFORMATION

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Applications of IR spectrometry

Major application (MIR region)

Structural investigations of molecular

compounds particularly organic

compounds

M j li ti f IR

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Major applications of IR

spectrometry

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NIR and FIR regions

NIR region (4000 to 14,000 cm-1)

Quantitative determination of low MW

hydrocarbons, H2O, CO2, S

FIR region (15 to 100 m)

Structural determination of inorganic and

metal-organic species

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What produces the IR spectrum

Materials that contain C (organic

compounds)

Many compounds; natural and synthetic

inorganic materials

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Sample handling

No solvent is transparent throughout the

region of interest

Sample handling

Most difficult

Time-consuming

part of IR spectrometric analysis

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Gases

Sample expands into an evacuated

cylindrical cells with suitable windows (Fig

16.3)

Variable lengths

A few cm to 10 m or longer

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Measuring gases

Placed in a sealed cell

Specified T and P can be maintained

Gases are often present at low

concentrations

Mirrors are used to deflect the IR beam back

and forth before the beam is allowed to exit

Increased the interaction between gas sample andIR beam

To acquire more spectral information

This gas cell provides a fixed

pathlength that is 2 m long

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Solutions

Water and alcohols are not suitable

solvents

SOLVENTS FOR IR

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SOLVENTS FOR IR

SPECTROMETRYNo solvent is transparent throughout the region of interest

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Liquids

Pure (neat) liquid

Very thin film, a short pathlength

A drop of neat liquid is squeezed between

two salt plates

Held together by capillary action

Placed in the beam of IR radiationNon reproducible transmittance data

Satisfactory for qualitative analysis

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Measuring liquids

Liquid cells Holds the liquid between two crystal made

from materials that completely transmit IR

radiation

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Cells

Liquid cells

(compared to UV/Vis spectroscopy)

Narrow pathlengths

0.01 to 1 mm

Relatively high sample concentrations

0.1% to 10%

Short pathlengths

Low molar absorbtivity

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Liquid cells

Designed for easy assembly/disassembly

Use Teflon spacers

Allow variations in path lengths

Fixed path-length cells

Use a syringe to fill or empty

Demountable IR cell for liquid

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Demountable IR cell for liquid

sample

C IR i d t i l

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Common IR window materials

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Selection of window materials

Cost

Range of transparency

Solubility in solvent

Reactivity with sample or solvent

Most common NaCl and KBr

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Solids

Mulls

Pellets

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Solids

Solid in a liquid or solid matrix

Samples are ground to fine powder

To avoid effects of scattered radiation

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Pelleting

Most common technique; KBr pelleting

Halide salts have a property ofcold flow

Translucent property when pressure is

applied to finely powdered salts

~1 mg (or less) sample is mixed with ~

100 mg dried KBr

Mix and grind in a mortar and pestle

Apply 10,000 to 15,000 psi in a die

Produce a transparent disk or pellet

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Pelleting

Best done in vacuum or store the pellet in

a dessicator before measurement

Bands at 3450 and 1640 cm-1 due to

absorbed moisture

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Mulls

Solids

Not soluble in an IR transparent solvent

KBr pelleting is not convenient

Grind ~ 2 5 mg fine powder in a drop of

mulling agents

Heavy hydrocarbon (Nujol)

Fluorolube (halogenated polymer)

Examine as thin film between two salt

discs

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Qualitative analysis

FTIR, NMR and MS

Indentify

Organic

Inorganic species

Biological

Limitations to the use of

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Limitations to the use of

correlation chartsTo determine the identity or structure Not possible using the correlation charts

alone (only as a guide).

Overlapping of group frequencies Instrumental limitations

Spectral variations as a function of physical

states Neat liquid

Solution

Pellet

Mull

Table of group frequencies for

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Table of group frequencies for

organic functional groups

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Group frequency region of MIR

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Group frequency region of MIR

Active or inactive vibrations in

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Active or inactive vibrations in

IR spectrum?

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Documents

ftir july2010