8/2/2019 IR SELF

1/51

Infrared Spectroscopy

8/2/2019 IR SELF

2/51

Spectroscopy is an instrumentally aided studiesof the interactions between matter (samplebeing analyzed) and energy (any portion of the

electromagnetic spectrum, EMS)

Chemists can use portions of the EMS to selectively

manipulate the energies contained within a molecule, to

uncover detailed evidence of its chemical structure and

bonding.

EMS refers to the seemingly diverse collection of

radiant energy, from cosmic rays to X-rays to visiblelight to microwaves, each of which can be considered as

a wave or particle traveling at the speed of light.

8/2/2019 IR SELF

3/51

=>

EMS and Molecular Effects

8/2/2019 IR SELF

4/51

Energy (E) E = hn = hc/l where h is Plancks constant, c is the speed of

light, nis frequency or the number of vibrations

per secondand lis the wavelength

Wavenumber (n) n = 1/ l given in cm-1

Period (P) P = 1/n the time between a vibration

= hcn

Energy, frequency, and wavenumber are directly proportional to each other.

8/2/2019 IR SELF

5/51

Method Abbrev. Energyused

Units

Ultraviolet-VisibleSpectroscopy

UV-Vis ultraviolet-visible

nm

InfraredSpectroscopy IR infrared mm orcm-1

Nuclear MagneticResonance

NMR radiofrequencies

Hz

Mass Spectroscopy MS electronvolts

amu

The four most common spectroscopic methods used in organic analysis are:

What actually happens to the sample during an analysis?

{How do the sample and energy interact?}

8/2/2019 IR SELF

6/51

What happens when a sample absorbsUV/Visenergy?

excitation of ground state electrons

(typically p and n electrons)

Eelectronic increases momentarily

What happens when a sample absorbsIRenergy?

stretching and bending of bonds

(typically covalent bonds)

Evibration increases momentarily

UV/Vis

p

p*

samplepp*

transition

IR

-O-H -O(3500 cm-1)

H

(200 nm)

Matter/Energy Interactions

8/2/2019 IR SELF

7/51

Infrared spectroscopy (IR) measures the bondvibration frequencies in a molecule and is used

to determine the functional group

Mass spectrometry (MS) fragments themolecule and measures the masses

Nuclear magnetic resonance (NMR)spectroscopy detects signals from hydrogenatoms and can be used to distinguish isomers

Ultraviolet (UV) spectroscopy uses electrontransitions to determine bonding patterns

8/2/2019 IR SELF

8/51

Just below red in the visible region

Wavelengths usually 2.5-25 mm

More common units are wavenumbers, or cm-1,

the reciprocal of the wavelength incentimeters (104/mm = 4000-400 cm-1)

Wavenumbers are proportional to frequency

and energy

The IR Region

The IR region is divided into three regions: the near, mid, and far IR. The mid IR region is

of greatest practical use to the organic chemist.

8/2/2019 IR SELF

9/51

Molecular Vibrations and IR Spectroscopy

8/2/2019 IR SELF

10/51

The infrared region of the spectrum encompasses radiation withwave numbers ranging from about 12,800 to 10 cm or wavelengthsfrom 0.78 to 1000m.Form standpoints of both application and instrumentation, the infrared

spectrum is conveniently divided into Near, Mid, and Far- infraredradiations.

Region Wavelength()Range, m

Wavenumber(), Range,cm

Frequency ()Range, Hz

Near 0.78 to 2.5 12,800 to 4000 3.8X 1014 to1.2X1014

Middle 2.5 to 50 4000 to 200 1.2X1014 to6.0x10

Far 50 to 1000 200 to 10 6.0X10 to3.0x10

Most Used 2.5 to 15 4000 to 670 1.2X1014

to2.0x10

8/2/2019 IR SELF

11/51

Molecules are made up of atoms linked by chemical bonds. Themovement of atoms and chemical bonds like spring and balls(vibration).The IR Spectroscopic Process

1. The quantum mechanical energy levels observed in IRspectroscopy are those of molecular vibration

2. We perceive this vibration as heat

3. When we say a covalent bondbetween two atoms is of acertain length, we are citing an average because the bondbehaves as if it were a vibrating spring connecting the twoatoms.

4. For a simple diatomic molecule, this model is easy tovisualize:

8/2/2019 IR SELF

12/51

What is a vibration in a molecule?

Any change in shape of the molecule- stretching of bonds,bending of bonds, or internal rotation around single bonds

Vibrations

There are two main vibrational modes :1. Stretching - change in bond length (higher

frequency)

Stretching vibration

8/2/2019 IR SELF

13/51

Stretching Types

Symmetric Asymmetric

Bending Types

In-plane (Scissoring)

2.Bending - change in bond angle (lower frequency)

Out-plane (Twisting)

H

H

CC

H

H

CC

RockWag

8/2/2019 IR SELF

14/51

Modes of vibrations

Stretching: change in bond distance.

Occurs at higher energy: 4000-1250 cm1.

-CH2-

H2O

Bending: change in bond angle.

Occurs at lower energy: 1400-666 cm1.

8/2/2019 IR SELF

15/51

More complex types of stretching and bending are possible

Can a vibration change the dipole moment of a molecule?

Infrared active vibrations (those that absorb IR

radiation) must result in a change of dipole moment

Asymmetrical stretching/bending and internal rotation

change the dipole moment of a molecule. Asymmetrical

stretching/bending are IR active.

Symmetrical stretching/bending does not. Not IR active

8/2/2019 IR SELF

16/51

Fundamental Vibrations (Absorption Frequencies)

A molecule has as many as degrees of freedom as the total

degree of freedom of its individual atoms.

Each atom has 3 degree of freedom (x,y,z)

A molecule of n atoms therefore has 3n degrees of freedom.

Non linear molecules (e.g. H2O)

Vibrational degrees of freedom or Fundamental Vibrations = 3n6

HO

H HO

H HO

H

Symmetrical

Stretching (s

OH)

3652 cm-1

Asymmetrical

Stretching (as

OH)

3756 cm-1

Scissoring

(s HOH)

1596 cm-1

8/2/2019 IR SELF

17/51

For linear molecule (e.g. CO2):

Vibrational degrees of freedom or Fundamental Vibrations = 3n5

O C O O C O

Symmetrical

Stretching (s CO2)

1340 cm-1

Asymmetrical

Stretching (as CO2)

2350 cm-1

O C O O C O

Scissoring (bending outof the plane of the paper)

(sCO2)666 cm-1

Scissoring (bending in

the plane of the paper)

(sCO2)666 cm-1

Th t h i i b d th i l f t th t h i l

8/2/2019 IR SELF

18/51

The technique is based on the simple fact that a chemical

substance shows marked absorption in the IR region of

Electromagnetic spectrum.

After absorption of IR radiation ,the molecule of achemical substance vibrates at many rates of vibration,giving rise to close packed bands called IR absorption

spectra.

Various bands will be present in the IR Spectra, which willCorrespond to characteristic functional groups and bondspresent in chemical substances.

f d

8/2/2019 IR SELF

19/51

Criteria for IR Radiation(a) Dipole MomentIR radiation is not energetic and thus confined largely tomolecular species that have small energy differences betweenvarious vibrational and rotational states.

In order to absorb IR radiation, the molecule must undergoA net change in dipole moment as a consequence of itsVibrational or rotational motion.

Only under these conditions can the alternating electrical

field of its radiation interact with the molecule and causechanges in the amplitude of one its motions.

For eg: the charge distribution around a molecule such as

hydrogen chlorideis not symmetric because chlorine hashi her electron densit than h dro en.

8/2/2019 IR SELF

20/51

As a hydrogen chloride molecule vibrates, a regularfluctuation in the dipole moment occurs, and a field isestablished that can interact with the electrical field

associated with the radiation.

No net change in the dipole occurs during vibration of

homonuclear species such as Oxygen, nitrogen or chlorine;such compounds cannot absorb IR radiation.

Similarly the rotation of asymmetric molecule around the

centre of mass results in a periodic fluctuation that caninteract with the radiation.

A l t b d ill t d t th

8/2/2019 IR SELF

21/51

As a covalent bond oscillates due to theoscillation of the dipole of the molecule a varyingelectromagnetic field is produced.

The greater the dipole moment change through thevibration, the more intense the EM field that isgenerated

8/2/2019 IR SELF

22/51

If the frequency of the radiation exactly matches with the

natural frequency of molecule, a net transfer of energy takesplace that results in change in amplitude of molecular

vibration.

(b) Correct wavelength

When a wave of infrared light encounters this

oscillating EM field generated by the oscillatingdipole of the same frequency, the two waves couple,and IR light is absorbed.

The coupled wave now vibrates with twice theamplitude

8/2/2019 IR SELF

23/51

IR beam from spectrometer

EM oscillating wave

from bond vibration

coupled wave

Vib ti l F

8/2/2019 IR SELF

24/51

Vibrational Frequency:The value of stretching vibrational frequency of a bond can be calculated fairly &accurately by the Hooks Law which may be represented as

=1

2cvvc =

Km1m2

m1+m2

K

=

1

2cWhere

is the reduced massm1m2

m1+m2& m1 & m2 are the masses of the atoms concerned in grams in a particular bond,

K=- Force constant of bond and relates to the strength of bond.For a single bond, it is approximately 5 x105 g sec-2 It become double and triple for double & triple bonds respectively.

C = velocity of radiation = 2.998 x 10 10 cm sec-1

Th th l f ib ti l f

8/2/2019 IR SELF

25/51

Thus the value of vibrational frequency orwave number depends upon:

i) Bond strengthand

ii) Reduced mass

If the bond strength increases or

reduced mass decreases,The value of the vibrational frequency

increases.

Let us calculate the approximate frequency of C-H stretching vibration from

8/2/2019 IR SELF

26/51

Let us calculate the approximate frequency of C-H stretching vibration fromthe following data:

K= 5 X 10 5 gm sec-2

Mass of carbon (m1)= 20x 10-24gm

Mass of hydrogen atom= 1.6 x 10-24gm

=

1

2cv

Km1m2

m1+m2

=7

2 x 22

5 x 105 gm sec-2

20x 10-24

gm x 1.6 x 10-24

gm

(20 + 1.6) 10-24 gm

= 9 x 1013 sec-1

The above value frequency can be converted into wave number as follows

8/2/2019 IR SELF

27/51

The above value frequency can be converted into wave number as follows

=vcv =

9.3 x 1013 sec-1

3.0x 105 m-1=

3.1 x 105 m-1

= 3100 cm-1C=C stretching is expected to absorb at higher frequency than the C-C stretching.

It is due to the higher bond strength (value of k) of the double bond compared tosingle bond.

Similarly, O-H strengthing absorbs at higher frequency compared to C-C bon.d.it can be described on the basis of reduced mass o-h compared with c-c , f-h.but this is not true.ACTUALLY, F-H absorbs at the higher frequency.This can be explained due to the higher elctronegativity of flourine compared to thatof oxygen.

8/2/2019 IR SELF

28/51

=

1

2pcn

K

m

m =m1 m2

m1 + m2

n = frequencyin cm-1

c = velocity of light

K = force constantin dynes/cm

m= atomic masses

SIMPLE HARMONIC

OSCILLATOR

C CC CC C > >

multiple bonds have higher Ks

m=reduced mass

( 3 x 1010 cm/sec )

THE EQUATION OF A

This equation describes the

vibrations of a bond.

where

8/2/2019 IR SELF

29/51

HOOKES LAW

x0 x1

Dx

K

-F = K(Dx)

m1 m2

K

Moleculeas aHookesLawdevice

Restoring force =

stretch

compressForce constant

8/2/2019 IR SELF

30/51

2150 1650 1200

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

increasing K

C-H > C-C > C-O > C-Cl > C-Br3000 1200 1100 750 650

increasing mincreasing m

8/2/2019 IR SELF

31/51

lpetit@clemson.edu VIBRATIONAL SPECTROSCOPY STUDIES OF GLASS STRUCTURE : IR spectroscopy 31

What about CO2?

Linear Molecule! O=C=O

Vibrational states: 3n-5 = 3*3-5= 4

8/2/2019 IR SELF

32/51

lpetit@clemson.edu VIBRATIONAL SPECTROSCOPY STUDIES OF GLASS STRUCTURE : IR spectroscopy 32

Symmetric and asymmetric stretch and bend:

IR and/or Raman active?

the symmetric stretch in carbon dioxide is not IR activebecause there is not change in the dipole moment. However,

the symmetric stretch is Raman active because thepolarizability of the molecule changes

The asymmetric stretch is IR active due to a change in

dipole moment.

The bending modes create a dipole perpendicular to themolecular axis thus is also infrared active.

What about CO2?

8/2/2019 IR SELF

33/51

lpetit@clemson.edu VIBRATIONAL SPECTROSCOPY STUDIES OF GLASS STRUCTURE : IR spectroscopy 33

Example: SiO2

Website: http://www.agu.org/reference/minphys/18_williams.pdf

8/2/2019 IR SELF

34/51

Number of possible vibrational modes

8/2/2019 IR SELF

35/51

BUT WHAT IS 3N, 5 & 6. HOWS IT COME?

Number of possible vibrational modes

CAN WE KNOW THE POSSIBLE VIBRATION?

YES, but how?

First let us know the basic formula

3N-5 for linear molecules

3N-6 nonlinear moleculesN: number of atoms in a molecules

Atoms are never fixed in the space but move

8/2/2019 IR SELF

36/51

Atoms are never fixed in the space but moveabout continuously

Each atom may be said to posses certain numberof degrees of freedom of movement.

The number of degrees of freedom is equal tothe sum of the coordinates necessary to locate allthe atoms of a molecule in space.

As per above cardinal rules, each

8/2/2019 IR SELF

37/51

As per above cardinal rules, eachatom has three degrees of freedomof movement corresponds to thethree Cartesian co-ordinates (X, Y &Z) necessary to describe its position

relative to other atoms in a molecule.

X

Y

Z

Cartesian

co-ordinates

8/2/2019 IR SELF

38/51

That means 3 coordinates are needed tolocate a point in space.

Each coordinate corresponds to one degreeof freedom for one of the atom.

So a molecule containing N-atom is said tohave

3N degree of freedom.

8/2/2019 IR SELF

39/51

An isolated atom which is considered as a point mass hasonly translational degrees of freedom.

It can not have vibrational and rotational degrees offreedom.

3n degrees of freedom=

Transitional+ Rotational + Vibrational

But when atom combine to form a molecule, no of degrees of freedom i.e thetotal number of freedom in a molecule will be equal to the 3n

where, n is the number of atoms in a molecule.So in conclusion a molecule which is finite dimensions willthus be made up of rotational, vibrational and transitionaldegrees of freedoms.

1) TRANSLATION MOTION

8/2/2019 IR SELF

40/51

Corresponds to the movement of the entire moleculethrough space whilethe position of the atoms relative toeach other remain fixed.

)

In this sense, the molecule may be considered as a singleparticle with the mass of the molecule located at itscenter of gravity and possesses three degree of translationfreedom.

ORDefinition of translation motion require 3 coordinates andthus this common motion requires 3 of the 3N degree offreedom.

O

HH

2) ROTATIONAL MOTION :

8/2/2019 IR SELF

41/51

But in the special case of linear molecule all theatom lie on a straight line and only 2 rotation canbe define (because rotation around the bond axis

is not possible)

)

Another 3 degree of freedom are needed todescribe the rotational motion of the molecule.

(i.e. poly atomic molecule has generally 3 degreeof rotation freedom).

O

HH

O

HH

O

HH

3)VIBRATIONAL MOTION :

8/2/2019 IR SELF

42/51

Now the rest i.e. substrate transition &rotation motion from 3N degree of freedom.

)

3N-6

For non linear molecule

3 of translation motion + 3 of rotational motion = 6

Suppose N number atom made a molecule

From previous discussion we have explained

Now, remaining motion would be =

For linear molecule

8/2/2019 IR SELF

43/51

For linear molecule

3 of translation motion + 2 of rotational motion = 5

Suppose N number atom made a molecule

From previous discussion we have explained

Now, remaining motion would be

3N-5=

Let us clarify the above theory with a Example using water molecule

8/2/2019 IR SELF

44/51

y y p g

O

HH

TRANSLATION

Movement of the whole molecule at varying speed(i.e. dependent on collision) and thus, with continually

differing amounts of KE.

O

HH

ROTATIONMovement around three principle axes ( Through Center of mass).

O

HH

O

HH

Internal movement as through the chemical bonds are springs that are compressed or extended during vibrationsalong the bond direction (stretching) or bent at an angle to the bond direction but in the same plane ( Scscissoring)

O

HH

O

HH

O

HH(and reverse) (and reverse) (and reverse)

i e & 3N-5(for linear molecule) 3 of

8/2/2019 IR SELF

45/51

i.e. & 3N 5(for linear molecule) 3 oftranslation motion + 2 of rotational motion=5

Fermi Resonance:

8/2/2019 IR SELF

46/51

Ferm Resonance

A Fermi resonance is the shifting of the energies and intensities of absorption bandsin an infrared or Raman spectrum. It is a consequence of quantum mechanicalmixing.[1] The phenomenon was explained by the Italian physicist Enrico Fermi.

http://en.wikipedia.org/wiki/Infrared_spectrumhttp://en.wikipedia.org/wiki/Raman_spectroscopyhttp://en.wikipedia.org/wiki/Fermi_resonancehttp://en.wikipedia.org/wiki/Enrico_Fermihttp://en.wikipedia.org/wiki/Enrico_Fermihttp://en.wikipedia.org/wiki/Fermi_resonancehttp://en.wikipedia.org/wiki/Raman_spectroscopyhttp://en.wikipedia.org/wiki/Infrared_spectrum

8/2/2019 IR SELF

47/51

8/2/2019 IR SELF

48/51

O

HH

8/2/2019 IR SELF

49/51

O

HH

O

HH

8/2/2019 IR SELF

50/51

2)ROTATIONAL MOTION :Another 3 degree of freedom are needed to describe the rotational motion of the molecule. (i.e. poly atomic molecule hasgenerally 3 degree of rotation freedom) But in the special case of linear molecule all the atom lie on a straight line and only2 rotation can be define (because rotation around the bond axis is not possible)

8/2/2019 IR SELF

51/51