http://lawrencekok.blogspot.com

Prepared by Lawrence Kok

Tutorial on Infrared Spectroscopy and Dual Beam Infrared Spectrometer.

Velocity of light (c ) = frequency (f) x wavelength (λ)• c = f λ• All electromagnetic waves travel at the speed of light (3.00 x 108ms-1)• Radiation with high ↑ frequency – short ↓ wavelength• Electromagnetic radiation has a particle nature and each photon carry a quantum of energy given by

h = plank constant = 6.626 x 10-34 Js c = speed of light = 3.00 x 108ms-1

f = frequency λ = wavelength

hc

E Short ↓ λ, Higher ↑ frequency, Higher Energy ↑

E = hf

Electromagnetic Radiation

Picture from :http://www.azimuthproject.org/azimuth/show/Blog+-+a+quantum+of+warmth

Electromagnetic Radiation and Spectroscopy

Radiowaves

Nuclear spin

Nuclear Magnetic Resonance Spectroscopy

• Organic structure determination

• MRI and body scanning

Infra Red

Molecular vibration

Infra Red Spectroscopy

UV or visible

Transition of outer most valence electrons

• Organic structure determination

• Functional gp determination• Measuring bond strength• Measuring level of alcohol in

breath

Electromagnetic Radiation

UV Spectroscopy

Atomic Absorption Spectroscopy

• Quantification of metal ions• Detection of metal in various

samples

Electromagnetic Radiation Interact with Matter (Atoms, Molecules) = Spectroscopy

Dipole change

Vibrational motion -result in a change dipole moment – IR active

Polar molecule will absorb IR• H-CI, as bond stretches, distance

between atoms increases, results in change in dipole moment

Condition for molecular vibration to absorb a photon /IR• Vibration causing oscillation in developing a change

in dipole bet opposite charged centres• Vibration of bond in HCI will cause the dipole in

bond to oscillate• Causes a change in dipole momentOscillation of bonds - Lead to oscillation of dipole - lead to change in

dipole moment

Molecular Vibration

Change in dipole moment

IR frequency = Natural frequency for bond – Resonance will happen.• HCI bond has natural vibrational frequency • IR frequency match the vibrational frequency in HCI, then IR is

absorb and molecule excited to vibrational state

• IR absorb by bond will result in greater vibration in amplitude

IR frequency = Natural frequency for bond – Resonance will happen

IR frequency is applied

IR absorption due to molecular Vibration

IR radiation absorbed by molecule is converted into vibrational energy within the molecule.  IR frequency match the natural vibrational frequency of molecule , then IR is absorbed and resonance happen (excited vibrational state).Absorption on Y-axis and wavenumber on the X-axis.  Absorption plotted two ways.  Transmittance, %(T) and Absorbance (A)Transmittance of 100% means IR Absorbance of 0% .Transmittance of 0% means IR Absorbance of 100%

Transmittance or Absorbance of IR Spectra

IR radiation absorbed Bonds undergo Resonance

IR radiation absorbed by molecule is converted into vibrational energy within the molecule.  IR frequency match the natural vibrational frequency of molecule , then IR is absorbed and resonance happen (excited vibrational state).Absorption on Y-axis and wavenumber on the X-axis.  Absorption plotted two ways.  Transmittance, %(T) and Absorbance (A)Transmittance of 100% means IR Absorbance of 0% .Transmittance of 0% means IR Absorbance of 100%

Abso

rba

nce

Wavenumber, cm-1

Transmittance or Absorbance of IR Spectra

Click here to view

Absorbance is High ↑

Absorbance is Low

IR radiation absorbed Bonds undergo Resonance

IR radiation absorbed by molecule is converted into vibrational energy within the molecule.  IR frequency match the natural vibrational frequency of molecule , then IR is absorbed and resonance happen (excited vibrational state).Absorption on Y-axis and wavenumber on the X-axis.  Absorption plotted two ways.  Transmittance, %(T) and Absorbance (A)Transmittance of 100% means IR Absorbance of 0% .Transmittance of 0% means IR Absorbance of 100%

Tra

nsm

itta

nce

/%A

bso

rba

nce

Wavenumber, cm-1

Transmittance or Absorbance of IR Spectra

Click here to view

Wavenumber, cm-1

Transmittance is Low ↓

Absorbance is High ↑

Transmittance is 100%

Absorbance is Low

IR radiation absorbed Bonds undergo Resonance

Infra Red Spectroscopy• Fingerprinting region (400- 1500)cm-1 – due to vibration of entire molecule, not independent functional gp• IR active – it must have an electric dipole that changes as it vibrates• IR does not have enough energy to excite electrons, only causes molecular vibration• Wavenumber = Reciprocal of wavelength (1/λ) , Unit = cm-1

• Wavenumber = 1/ Wavelength = number of waves cycles in one centimeter• Wavenumber α frequency• Wavenumber used in IR spectroscopy• Higher Wavenumber ↑ = Lower wavelength ↓= Higher ↑ frequency = = Greater Energy ↑

Infrared Spectroscopy

Infra Red Spectroscopy• Fingerprinting region (400- 1500)cm-1 – due to vibration of entire molecule, not independent functional gp• IR active – it must have an electric dipole that changes as it vibrates• IR does not have enough energy to excite electrons, only causes molecular vibration• Wavenumber = Reciprocal of wavelength (1/λ) , Unit = cm-1

• Wavenumber = 1/ Wavelength = number of waves cycles in one centimeter• Wavenumber α frequency• Wavenumber used in IR spectroscopy• Higher Wavenumber ↑ = Lower wavelength ↓= Higher ↑ frequency = = Greater Energy ↑

Infrared wavelength from (2500 – 25000)nm → Convert to wavenumber (400– 4000)cm-1

Wavenumber between 400cm-1 – 4000cm-1

Wavelength, λ = 2500nm (convert to cm) → λ = 0.00025cm → Wavenumber = 1/λ = 1/0.00025 = 4000 cm-1

Wavelength, λ = 25000nm (convert to cm) → λ = 0.0025cm → Wavenumber = 1/λ = 1/0.0025 = 400 cm-1

Wavenumber bet 400cm-1 – 4000cm-1

IR absorption by different functional gps

Fingerprinting region

Infrared Spectroscopy

Infra Red Spectroscopy• Fingerprinting region (400- 1500)cm-1 – due to vibration of entire molecule, not independent functional gp• IR active – it must have an electric dipole that changes as it vibrates• IR does not have enough energy to excite electrons, only causes molecular vibration• Wavenumber = Reciprocal of wavelength (1/λ) , Unit = cm-1

• Wavenumber = 1/ Wavelength = number of waves cycles in one centimeter• Wavenumber α frequency• Wavenumber used in IR spectroscopy• Higher Wavenumber ↑ = Lower wavelength ↓= Higher ↑ frequency = = Greater Energy ↑

Infrared wavelength from (2500 – 25000)nm → Convert to wavenumber (400– 4000)cm-1

Wavenumber between 400cm-1 – 4000cm-1

Wavelength, λ = 2500nm (convert to cm) → λ = 0.00025cm → Wavenumber = 1/λ = 1/0.00025 = 4000 cm-1

Wavelength, λ = 25000nm (convert to cm) → λ = 0.0025cm → Wavenumber = 1/λ = 1/0.0025 = 400 cm-1

Wavelength, λ low ↓ → Wavenumber, 1/λ is High ↑ Speed of light, c = fλ, f = c/λ → c x 1/λ Wavenumber, 1/λ is High ↑ → f = c x 1/λ → f is High ↑ (cause 1/λ is high) → Energy = hf High ↑

Wavenumber bet 400cm-1 – 4000cm-1

IR absorption by different functional gps

Fingerprinting region

Infrared Spectroscopy

Operating Principle of a Double Beam Infrared Spectrometer

Double beam splitter • Directs half radiation through sample and other half

through reference• Allow radiation passing through sample and compare it

with reference• Two beams recombined at detector.• Signal from sample/reference are compared to determine

if sample absorb radiation emitted from source

Reference • Solvent used to dissolve sample• Reference use to eliminate instrument

fluctuation, absorption due to impurities in solvent and all interferences. • Absorption of IR is only due to solute using the

reference

Monochromator• Allow radiation of a particular wavelength to

pass through

Fourier transformation • Allow several wavelength through the sample at the same time and analyse the results • Using mathematical techniques to determine the amplitude/intensity of each single frequency• Fourier transformation- Intensity of IR radiation at each frequency determined separately

Recorder/Output• Scanning wavenumber from 400cm-1 to 4000cm-1

• Spectrum of Abs/Trans against frequency/wavenumber is produced

Light Source• Provide IR

radiation

Strength of bondSingle, Double, Triple Bonds

IR absorption frequency depends

Stronger bond• Higher energy needed for

resonance • Higher

frequency/wavenumber absorbed

Weaker bond• Lower energy needed for

resonance • Lower

frequency/wavenumber absorbed

IR absorption frequency

Strong bond

Weak bond

Strength of bondSingle, Double, Triple Bonds

Mass of atoms

Lighter/Lower Mass atom • Higher energy frequency for

vibrationIR absorption

frequency depends

Heavier/Higher Mass atom• Lower energy/frequency for

vibration

Stronger bond• Higher energy needed for

resonance • Higher

frequency/wavenumber absorbed

Weaker bond• Lower energy needed for

resonance • Lower

frequency/wavenumber absorbed

IR absorption frequency

Strong bond

Weak bond

C- H = 2840cm-1

C- CI = 600cm-1

Strength of bondSingle, Double, Triple Bonds

Mass of atoms

Lighter/Lower Mass atom • Higher energy frequency for

vibration

Stretching Vibration Vs Bending Vibration

IR absorption frequency depends

Heavier/Higher Mass atom• Lower energy/frequency for

vibration

Bending Vibration• Less energy needed for

resonance • Lower

frequency/wavenumber needed

Stretching Vibration• More energy needed to

stretch bond• Higher

frequency/wavenumber needed

Stronger bond• Higher energy needed for

resonance • Higher

frequency/wavenumber absorbed

Weaker bond• Lower energy needed for

resonance • Lower

frequency/wavenumber absorbed

IR absorption frequency

Strong bond

Weak bond

C- H = 2840cm-1

C- CI = 600cm-1

C- H stretch = 2840cm-1

C- H bend = 1400cm-1

IR Spectroscopy on Alcohol Level Determination

Major absorption for alcohol • 3230cm-1 – (O-H) stretch• 2950cm-1 – (C-H) stretch• 1400cm-1 – (C-H) bend• 1200cm-1 – (C-O) stretch

Sample cell

Reference cell

Breath sample inBreath sample outInfrared

Intoximeter – Measure the alcohol content in breath

IR Spectroscopy on Alcohol Level Determination

Major absorption for alcohol • 3230cm-1 – (O-H) stretch• 2950cm-1 – (C-H) stretch• 1400cm-1 – (C-H) bend• 1200cm-1 – (C-O) stretch

• Alcohol concentration cannot be measured from O-H vibration (3230cm-1) due to water vapour present in atmosphere and breath• C-H vibration ( 2950cm-1 ) is used • Propanone, ketone bodies which is produced by diabetic patients also absorb at 2950cm-1

• A second sensor to detect propanone was used. (To eliminate the absorption due to propanone)

Sample cell

Reference cell

Breath sample inBreath sample outInfrared

Intoximeter – Measure the alcohol content in breath

Different bonds absorb IR radiation at frequency/wavenumber . Use IR to identify the bonds and the functional group in a molecule.Common functional groups• (COOH), (O-H), (C=O), (C=C), (C-O), (C-H)

IR spectra of organic compounds with different functional groups

IR Absorption by different types of bonds/functional gps

IR spectrum with different functional gps and fingerprint region

Fingerprinting region • Range from (1500-

400cm -1)• Specific to each

molecule

IR spectra of organic compounds with different functional groups

Propanal (CH3CH2CHO)• (2840-3000) → C-H stretch• (2720) → C-H stretch of CHO• (1680-1740) → C=O stretch

Hex-1-ene CH2=CH(CH2)3CH3

• (2840-3000) → C-H stretch• (1610-1680) → C-C stretch • (1200- 1400) → C-H bend

Hex-1-yne CH2≡CH(CH2)3CH3

• (3350) → C≡ C stretch• (2840-3000) → C-H stretch • (1200- 1400) → C-H bend

IR spectra of organic compounds with different functional groups

Chloromethane CH3CI• (2840-3000) → C-H stretch• (1200-1400) → C-H bend • (700-800) → C-CI stretch

Halogenoalkane Aldehyde

Alkene Alkyne

IR spectra of organic compounds with different functional groups

Methanol (CH3OH)• (3230-3550) → O-H stretch Broad Absorption due to H bonding between molecules• (2840-3000) → C-H stretch• (1000-1300) → C-O stretch

Ethanol(CH3CH2OH)• (3230-3550) → O-H stretch Broad Absorption due to H bonding between molecules• (2840-3000) → C-H stretch• (1000-1300) → C-O stretch• (1200- 1400) → C-H bend

Phenol (C6H5OH)• (3230-3550) → O-H stretch Broad Absorption due to H bonding between molecules• (2840-3000) → C-H stretch• (1400-1500) → C=C aromatic stretch• (1000-1300) → C-O stretch

Benzoic acid (C6H5COOH)• (3230-3550) → O-H stretch Broad Absorption due to H bonding between molecules• (2840-3000) → C-H stretch• (1400-1500) → C=C aromatic stretch• (1000-1300) → C-O stretch• (1680-1740) → C=O stretch

Click here to animated Spectra Click here to search IR spectra

IR Spectra search for different Organic Molecules

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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