ir spectroscopy

Infrared Spectroscopy

Syllabus:

Infrared absorption spectroscopy: instrumentation, FTIR, advantages ofFTIR, applications of IR, qualitative and quantitative analysis, advantagesand limitations of quantitative IR methods.


• IR Spectroscopy is the interaction between matter and IR radiation.

• All molecular species except “homonuclear diatomics” (e.g., O2, H2, N2, etc.) are detectable

• IR light absorption due to changes in rotational and vibrational energy in molecule

• Absorbed energy causes molecular motions which create a net change in the dipole moment.

Infrared radiationRegion cm-1 μmNear IR 12000 – 4000 0.8 – 2.5 Mid IR 4000 – 400 2.5 – 25 Far IR 400 – 10 25 – 1000

Vibrations of MoleculesSymmetrical

stretchingAntisymmetrical

stretching Scissoring

Rocking Wagging Twisting

Theory of Infrared Absorption Spectroscopy

• IR photons have low energy. The only transitions that have comparable energy differences are molecular vibrations and rotations.

Theory of Infrared Absorption Spectroscopy

• In order for IR absorbance to occur two conditions must be met:

1. There must be a change in the dipole moment of the molecule as a result of a molecular vibration (or rotation). The change (or oscillation) in the dipole moment allows interaction with the alternating electrical component of the IR radiation wave. Symmetric molecules (or bonds) do not absorb IR radiation since there is no dipole moment.

2. If the frequency of the radiation matches the natural frequency of the vibration (or rotation), the IR photon is absorbed and the amplitude of the vibration increases.

DE = hn• There are three types of molecular transitions that occur in IR

a) Rotational transitions • When an asymmetric molecule rotates about its center of mass, the

dipole moment seems to fluctuate.

• DE for these transitions correspond to n < 100 cm-1

• Quite low energy, show up as sharp lines that subdivide vibrational peaks in gas phase spectra.

b) Vibrational-rotational transitions • complex transitions that arise from changes in the molecular dipole

moment due to the combination of a bond vibration and molecular rotation.

c) Vibrational transitions • The most important transitions observed in qualitative mid-IR

spectroscopy. • n = 13,000 – 675 cm-1 (0.78 – 15 mM)

IR Spectroscopy

I. IntroductionC. The IR Spectroscopic Process

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

2. We perceive this vibration as heat

3. When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms

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

IR Spectroscopy

I. IntroductionC. The IR Spectroscopic Process

5. There are two types of bond vibration:• Stretch – Vibration or oscillation along the line of the bond

• Bend – Vibration or oscillation not along the line of the bond

H

H

CH

HC

scissor

asymmetric

H

HCC

H

HCC

H

HCC

H

HCC

symmetric

rock twist wagin plane out of plane

C.The IR Spectroscopic Process6.As a covalent bond oscillates – due to the oscillation of the dipole of

the molecule – a varying electromagnetic field is produced

7.The greater the dipole moment change through the vibration, the more intense the EM field that is generated


C. The IR Spectroscopic Process8. When a wave of infrared light encounters this oscillating EM

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

9. The coupled wave now vibrates with twice the amplitude


IR beam from spectrometer

EM oscillating wavefrom bond vibration

“coupled” wave

D. The IR Spectrum1. Each stretching and bending vibration occurs with a characteristic frequency as

the atoms and charges involved are different for different bonds

The y-axis on an IR spectrum is in units of % transmittance

In regions where the EM field of an osc. bond interacts with IR light of the same n – transmittance is low (light is absorbed)

In regions where no osc. bond is interacting with IR light, transmittance nears 100%


IR Spectroscopy

D. The IR Spectrum2. The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number

of waves per centimeter in units of cm-1 (Remember E = hn or E = hc/l)

IR Spectroscopy

D. The IR Spectrum3. This unit is used rather than wavelength (microns) because wavenumbers

are directly proportional to the energy of transition being observed – chemists like this, physicists hate it

High frequencies and high wavenumbers equate higher energyis quicker to understand than

Short wavelengths equate higher energy

4. This unit is used rather than frequency as the numbers are more “real” than the exponential units of frequency

5. IR spectra are observed for the mid-infrared: 600-4000 cm-1

6. The peaks are Gaussian distributions of the average energy of a transition

IR Spectroscopy

D. The IR Spectrum7. In general:

Lighter atoms will allow the oscillation to be faster – higher energyThis is especially true of bonds to hydrogen – C-H, N-H and O-H

Stronger bonds will have higher energy oscillationsTriple bonds > double bonds > single bonds in energy

Energy/n of oscillation

The Vibrational Modes of Water

Mechanical Model of Stretching Vibrations1. Simple harmonic oscillator.

• Hooke’s Law (restoring force of a spring is proportional to the displacement)

F = -ky

Where: F = Forcek = Force Constant(stiffness of spring)y = Displacement

• Natural oscillation frequency of a mechanical oscillator depends on: a) mass of the object b) force constant of the spring (bond)

• The oscillation frequency is independent of the amount of energy imparted to the spring.

• Frequency of absorption of radiation can be predicted with a modified Hooke’s Law.

Where: n = wavenumber of the abs. peak (cm-1)c = speed of light (3 x 1010 cm/s) k = force constantm = reduced mass of the atoms

21

21

mn k

c

yx

yx

MMMM

m Where: Mx = mass of atom x in kgMy = mass of atom y in kg

• Force constants are expressed in N/m (N = kg•m/s2)

- Range from 3 x 102 to 8 x 102 N/m for single bonds - 500 N/m is a good average force constant for single bonds when

predicting k. - k = n(500 N/m) for multiple bonds where n is the bond order

IR Sources and DetectorsSources - inert solids that heat electrically to 1500 – 2200 K.• Emit blackbody radiation produced by atomic and molecular oscillations

excited in the solid by thermal energy. • The inert solid “glows” when heated.

• Common sources:

1. Nernst glower - constructed of a rod of a rare earth oxide (lanthanide) with platinum leads.

2. Globar - Silicon carbide rod with water cooled contacts to prevent arcing.

3. Incandescent wire - tightly wound wire heated electrically. Longer life but lower intensity.

Detectors – measure minute changes in temperature.

1. Thermal transducer • Constructed of a bimetal junction, which has a temperature dependant

potential (V). (similar to a thermocouple)• Have a slow response time, so they are not well suited to FT-IR.

2. Pyroelectric transducer • Constructed of crystalline wafers of triglycine sulfate (TGS) that have a

strong temperature dependent polarization. • Have a fast response time and are well suited for FT-IR.

3. Photoconducting transducer • Constructed of a semiconducting material (lead sulfide,

mercury/cadmium telluride, or indium antimonide) deposited on a glass surface and sealed in an evacuated envelope to protect the semiconducting material from the environment.

• Absorption of radiation promotes nonconducting valence electrons to a conducting state, thus decreasing the resistance (W) of the semiconductor.

• Fast response time, but require cooling by liquid N2.

FTIR Background

• FTIR is a modern spectroscopic method which operates in the IR region (molecular vibrations and rotations)

• The “FT” in FTIR gives the wavelength selection method (Fourier Transformation)

• Prior to FTIR, grating and prism spectrometers were used

What is FTIR• Fourier-transform infrared spectroscopy is a vibrational

spectroscopic technique, meaning it takes advantage of asymmetric molecular stretching, vibration, and rotation of chemical bonds as they are exposed to designated wavelengths of light.

• Fourier transform is to transform the signal from the time domain to its representation in the frequency domain

FTIR seminar

Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light source

(ceramic)

Detector

(DTGS)

Beam splitter

FT Optical System Diagram

The Interferometer

Simplest interferometer design Beamsplitter for dividing the incoming IR beam into

two parts Two plane mirrors for reflecting the two beams back

to the beamsplitter where they interfere either constructively or destructively depending on the position of the moving mirror

Position of moving mirror is expressed as Optical Path Difference (OPD)

An Interferometer

Albert Abraham Michelson(1852-1931)

OPD = Distance travelled by red beamminus distance travelled by yellow beam

OPD = 0 atthe white line

Interference signal Interference signal

EM waves with same amplitude and frequency, out of phase

EM waves with same amplitude and frequency, in phase (OPD = 0)

A A

A A

• When EM-waves interact, interference is observed

• Depending on the relative phase of the waves, interference is either destructive or constructive

destructive interference constructive interference

Michelson interferometer producing Interference

• Collect data in the time domain and convert to the frequency domain by Fourier Transform.

Multiplexing (FT) Spectrometers

• Detectors are not fast enough to respond to power variations at high frequency (1012 to 1015 Hz) so the signal is modulated by a Michelson interferometer to a lower frequency that is directly proportional to the high frequency.

1. Michelson Interferometer

B. Multiplexing (FT) Spectrometers

• The source beam is split into two beams.

• One beam goes to a stationary mirror and the other goes to a moveable mirror.

• Movement of the mirror at a constant rate and recombination of the two beams results in a signal that is modulated by constructive and destructive interference (Interferogram).


• The frequency of the radiation (n) is directly related to the frequency of the interferogram (f).

nnc

f m2

n = frequency of radiationf = frequency of inteferogramnm = velocity of the mirror c = speed of light (3.00 x 1010

cm/s)

• FT-IR spectrometers use a polychromatic source and collect the entire spectrum simultaneously and decode the spectrum by Fourier Transform.

Fourier Transform

Time domain: I vs. δ Frequency domain: I vs. v

FT

δ

I

v

I

Fourier transform defines a relationship between a signal in time domain and its representation in frequency domain.

Being a transform, no information is created or lost in the process, so the original signal can be recovered from the Fourier transform and vice versa.

Fourier transformation is the mathematical relation between the interferogram and the spectrum (in general, between time domain signal and frequency signal)

Mathematics

• Optical path difference is • Intensity of the detector has maxima at

and minima at

)(I

)(I 2,1,0, nnl

l )2/1( n

Mathematics (contd.)

• The resulting interferogram is described as an infinitely long cosine wave • where =intensity as F(v) • For non-monochromatic source treat each frequency as if it resulted in a

separate cosine train.

)2cos()(lnB )(nB


• An infrared source is typically approximated as a black body radiator and the summation can be replaced by an integral.

• At = 0, signal always has a strong maxima called Centreburst• Outwards from the centreburst the cosine waves cancel and

reinforce and the amplitude of the interferogram dies off.

nnn dBI )2cos()()(0


• Spectroscopists are interested in the spectrum in the frequency domain i.e intensity versus wavenumber

• If the mathematical form of the interferogram is known,spectrum in the frequency domain can be calculated by Fourier Transformation

nn dIB )2cos()()(

For mathematical validity this integration must be carried out over all possible values of delta i.e + to –

Measurement TechniquesThough some emission spectroscopy is done most of the

work is in the absorption spectra area using FTIR.• 1. If there is no sample present the spectrum is that of

a black body radiator modified by any transmission characteristics of the interferometer components.

• 2. If the interferometer chamber is not evacuated or purged with dry gas some absorption from the atmospheric Co2 and H20 is observed. This is called the background spectrum.

• 3. When the sample is introduced the spectrum now is a superposition of the absorption bands of the sample on an uneven background.

• 4. To obtain %T with wavenumber we ratio the single beam sample spectrum with the background spectrum.

Measurement Techniques(contd.)

FT IR Detectors:The two most popular detectors for a FTIR spectrometer are:• deuterated triglycine sulfate (DTGS): Is a pyroelectric detector that delivers rapid responses because it

measures the changes in temperature rather than the value of temperature. It operates at room temperature,

• mercury cadmium telluride (MCT). It must be maintained at liquid nitrogen temperature (77 °K)

to be effective. In general, the MCT detector is faster and more sensitive than

DTGS detector. Thermal Detectors are not used in FT IR: • The response times of thermal detectors (for example,

thermocouple and thermistor) used in dispersive IR instruments are too slow for the rapid scan times (1 sec or less) of the interferometer.

2. FT-IR instrument


• Mirror length of travel ranges from 1 to 20 cm.

• Use multiple scans and signal averaging to improve S/N.

• Scan rates from 0.1 to 10 cm/s

• Detectors are usually pyroelectric or photoconducting.

• Cost $10,000 - $20,000

• Have virtually replaced dispersive instruments.

Performance Characteristics

• Range: 7800 to 350 cm-1 (less expensive) 25,000 to 10 cm-1 (Near to far IR, expensive)

• Resolution: 8 cm-1 to 0.01 cm-1

• Qualitative: Very good, functional groups are identifiable

• Quantitative: Dispersive – poor FTIR - fair

FT-IR Advantages

• Fellgett's (multiplex) Advantage (High S/N ratio comparing with dispersive instruments)

• All frequencies are measured at the same time

• Hard to do samples having low transmission and weak spectra can be done with FTIR.

• High resolution, reproducibility and highly accurate frequency determination

• Technique allows high speed sampling with the aid of laser light interference fringes

Better sensitivity.In the interferometer, the radiation power transmitted on to the detector

is very high which results in high sensitivity.

No Stray light Fourier Transform allows only interference signals to contribute to

spectrum. Background light effects greatly lowers.

Precision AdvantageInternal laser control the scanner – built in calibration

Disadvantages of FTIR compared to Normal IR:

1) single-beam, requires collecting blank2) can’t use thermal detectors – too slow3) CO2 and H2O sensitive4)Too sensitive that it would detect the smallest contaminant

APPLICATIONS• Identification of inorganic compounds and organic

compounds• Measurement of toxic gas in fuels• Identification of components of an unknown mixture • Analysis of solids, liquids, and gases • In remote sensing• Can also be used on satellites to probe the space

02 January 2006 Introduction to FTIR

Advantages of FTIR spectroscopy

• Speed (Felgett advantage): All the frequencies are recorded simultaneously; a complete spectrum is measured in less than a second.

• Sensitivity (Jacquinot or Throughput advantage): In the interferometer, the radiation power transmitted on to the detector is very high which results in high sensitivity.

• Internally Calibrated (Connes advantage): FTIR spectrometers employ a HeNe laser as an internal wavelength calibration standard, no need to be calibrated by the user.

• Multicomponent capability: Since the whole infrared spectrum is measured continuously, all infrared active components can be identified and their concentrations determined.

02 January 2006 Introduction to FTIR

REFERENCES•Introduction to Spectroscopy ,Donald L. Pavia • Instrumental Analysis; Skoog, Holler and Crouch•Stuart; IR Spectroscopy-Fundamentals and Applications.• Instrumental methods-Willard, Merrit and Dean•Infrared Spectroscopy in Conservation Science,Michele R Derrick,Dusan Stulik,James M. Landry •http://resources.yesicanscience.ca/trek/scisat/final/grade9/spectrometer2.html•http://www.health.clinuvel.com/en/uv-light-a-skin•http://mmrc.caltech.edu/FTIR/FTIRintro.pdf

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