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INSTRUMENTAL ANALYSISCHEM 4811

CHAPTER

DR. Neelam KumariAssOCIATE professor

DEPARTMENT of chemistryMeerut college meerut

U.P.

CHAPTER 4

INFRARED SPECTROSCOPY

(IR)

CHAPTER 4

INFRARED SPECTROSCOPY

(IR)

IR SPECTROSCOPY

- Used for qualitative identification of organic andinorganic compounds

- Used for checking the presence of functional groups in molecules

- Can also be used for quantitative measurements of compounds

- Each compound has its unique IR absorption pattern

- Wavenumber with units of cm-1 is commonly used

- Wavenumber = number of waves of radiation per centimeter

- Used for qualitative identification of organic andinorganic compounds

- Used for checking the presence of functional groups in molecules

- Can also be used for quantitative measurements of compounds

- Each compound has its unique IR absorption pattern

- Wavenumber with units of cm-1 is commonly used

- Wavenumber = number of waves of radiation per centimeter

IR SPECTROSCOPY

- The IR region has lower energy than visible radiation andhigher energy than microwave

The IR region is divided into

Near-IR (NIR): 750 nm – 2500 nm

Mid-IR: 2500 nm – 20000 nm

Far-IR: 20000 nm – 400000 nm

- The IR region has lower energy than visible radiation andhigher energy than microwave

The IR region is divided into

Near-IR (NIR): 750 nm – 2500 nm

Mid-IR: 2500 nm – 20000 nm

Far-IR: 20000 nm – 400000 nm

IR ABSORPTION BY MOLECULES

- Molecules with covalent bonds may absorb IR radiation

- Absorption is quantized- Molecule shows change in dipolemoment during vibrational transitions

- Molecules move to a higher energy state

- IR radiation is sufficient enough to cause rotation and vibration

- Radiation between 1 and 100 µm will cause excitation tohigher vibrational states

- Radiation higher than 100 µm will cause excitation tohigher rotational states

- Molecules with covalent bonds may absorb IR radiation

- Absorption is quantized- Molecule shows change in dipolemoment during vibrational transitions

- Molecules move to a higher energy state

- IR radiation is sufficient enough to cause rotation and vibration

- Radiation between 1 and 100 µm will cause excitation tohigher vibrational states

- Radiation higher than 100 µm will cause excitation tohigher rotational states


- Absorption spectrum is composed of broad vibrationalabsorption bands

- Molecules absorb radiation when a bond in the molecule vibratesat the same frequency as the incident radiant energy

- Molecules vibrate at higher amplitude after absorption

- A molecule must have a change in dipole moment duringvibration in order to absorb IR radiation

- Absorption spectrum is composed of broad vibrationalabsorption bands

- Molecules absorb radiation when a bond in the molecule vibratesat the same frequency as the incident radiant energy

- Molecules vibrate at higher amplitude after absorption

- A molecule must have a change in dipole moment duringvibration in order to absorb IR radiation


Absorption frequency depends on

- Masses of atoms in the bonds

- Geometry of the molecule

- Strength of bond

- Other contributing factors

Absorption frequency depends on

- Masses of atoms in the bonds

- Geometry of the molecule

- Strength of bond

- Other contributing factors

DIPOLE MOMENT (µ)

µ = Q x r

Q = charge and r = distance between charges

- Asymmetrical distribution of electrons in a bond renders thebond polar

- A result of electronegativity difference

- µ changes upon vibration due to changes in r

- Change in µ with time is necessary for a molecule to absorbIR radiation

µ = Q x r

Q = charge and r = distance between charges

- Asymmetrical distribution of electrons in a bond renders thebond polar

- A result of electronegativity difference

- µ changes upon vibration due to changes in r

- Change in µ with time is necessary for a molecule to absorbIR radiation

DIPOLE MOMENT (µ)

- The repetitive changes in µ makes it possible for polar moleculesto absorb IR radiation

- Symmetrical molecules do not absorb IR radiation since theydo not have dipole moment (O2, F2, H2, Cl2)

- Diatomic molecules with dipole moment are IR-active(HCl, HF, CO, HI)

- Molecules with more than two atoms may or may not beIR active depending on whether they have permanent

net dipole moment

- The repetitive changes in µ makes it possible for polar moleculesto absorb IR radiation

- Symmetrical molecules do not absorb IR radiation since theydo not have dipole moment (O2, F2, H2, Cl2)

- Diatomic molecules with dipole moment are IR-active(HCl, HF, CO, HI)

- Molecules with more than two atoms may or may not beIR active depending on whether they have permanent

net dipole moment

PRINCIPAL MODES OF VIBRATION

Stretching

- Change in bond length resulting from change ininteratomic distance (r)

Two stretching modes- Symmetrical and asymmetrical stretching

- Symmetrical stretching is IR-inactive (no change in µ)

Stretching

- Change in bond length resulting from change ininteratomic distance (r)

Two stretching modes- Symmetrical and asymmetrical stretching

- Symmetrical stretching is IR-inactive (no change in µ)


Bending

- Change in bond angle or change in the position of a group ofatoms with respect to the rest of the molecule

Bending Modes- Scissoring and Rocking

- In-plane bending modes (atoms remain in the same plane)

- Wagging and TwistingOut-of-plane (oop) bending modes (atoms move out of plane)

Bending

- Change in bond angle or change in the position of a group ofatoms with respect to the rest of the molecule

Bending Modes- Scissoring and Rocking

- In-plane bending modes (atoms remain in the same plane)

- Wagging and TwistingOut-of-plane (oop) bending modes (atoms move out of plane)


3N-6 possible normal modes of vibration

N = number of atoms in a molecule

Degrees of freedom = 3N

H2O for example- 3 atoms

- Degrees of freedom = 3 x 3 = 9- Normal modes of vibration = 9-6 = 3


N = number of atoms in a molecule

Degrees of freedom = 3N

H2O for example- 3 atoms

- Degrees of freedom = 3 x 3 = 9- Normal modes of vibration = 9-6 = 3


Linear Molecules

- Cannot rotate about the bond axis

- Only 2 degrees of freedom describe rotation


CO2 for example- 3 atoms

- Normal modes of vibration = 9-5 = 4

Linear Molecules

- Cannot rotate about the bond axis

- Only 2 degrees of freedom describe rotation


CO2 for example- 3 atoms

- Normal modes of vibration = 9-5 = 4

TRANSITIONS

Fundamental

- Excitation from the ground state Vo to the first excited state V1

- The most likely transition and have strong absorption bands

Fundamental

- Excitation from the ground state Vo to the first excited state V1

- The most likely transition and have strong absorption bands

Vo

V1

V2

V3

FUNDAMENTAL TRANSITIONS

Overtone

- Excitation from ground state to higher energy states V2, V3, ….

- Result in overtone bands that are weaker than fundamental

- Frequencies are integral multiples of fundamental absorption

- Fewer peaks are seen than predicted on spectra due to IR-inactivevibrations, degenerate vibrations, weak vibrations

- Additional peaks may be seen due to overtones

Overtone

- Excitation from ground state to higher energy states V2, V3, ….

- Result in overtone bands that are weaker than fundamental

- Frequencies are integral multiples of fundamental absorption

- Fewer peaks are seen than predicted on spectra due to IR-inactivevibrations, degenerate vibrations, weak vibrations

- Additional peaks may be seen due to overtones

COUPLING

- The interaction between vibrational modes

- Two vibrational frequencies may couple to produce a newvibrational frequency

Combinational Bandν1 + ν2 = ν3

Difference Bandν1 - ν2 = ν3

- The interaction between vibrational modes

- Two vibrational frequencies may couple to produce a newvibrational frequency

Combinational Bandν1 + ν2 = ν3

Difference Bandν1 - ν2 = ν3

VIBRATIONAL MOTION

- Consider a bond as a spring

μf

2π1ν~

c

c = speed of light (cm/s)f = force constant (dyn/cm; proportional to bond strength)

f for a double bond = 2f for a single bondf for a triple bond = 3f for a single bond

c = speed of light (cm/s)f = force constant (dyn/cm; proportional to bond strength)

f for a double bond = 2f for a single bondf for a triple bond = 3f for a single bond

- M1 and M2 are masses of vibrating atoms connecting the bond

21

21

MM

MMgramsinmassreducedμ

INSTRUMENTATION

Components

- Radiation source

- Sample holder

- Monochromator

- Detector

- Computer

Components

- Radiation source

- Sample holder

- Monochromator

- Detector

- Computer

RADIATION SOURCES

Intensity of radiation should- Be continuous over the λ range and cover a wide λ range

- Constant over long periods of time- Have normal operating temperatures between 1100 and 1500 K

- Have maximum intensity between 4000 and 400 cm-1

(mid-IR region)

- Modern sources include furnace ignitors, diesel engine heaters

- Sources are usually enclosed in an insulator to reduce noise

Intensity of radiation should- Be continuous over the λ range and cover a wide λ range

- Constant over long periods of time- Have normal operating temperatures between 1100 and 1500 K

- Have maximum intensity between 4000 and 400 cm-1

(mid-IR region)

- Modern sources include furnace ignitors, diesel engine heaters

- Sources are usually enclosed in an insulator to reduce noise

RADIATION SOURCES

Mid-IR Sources

Nernst Glowers- Heated ceramic rods

- Cylindrical bar- Made of zirconium oxide, cerium oxide, thorium oxide

- Heated electrically to 1500 K – 2000 K- Resistance decreases as temperature increases

- Can overheat and burn out

Mid-IR Sources

Nernst Glowers- Heated ceramic rods

- Cylindrical bar- Made of zirconium oxide, cerium oxide, thorium oxide

- Heated electrically to 1500 K – 2000 K- Resistance decreases as temperature increases

- Can overheat and burn out

RADIATION SOURCES

Mid-IR Sources

Globar- Silicon carbide bar

- Heated electrically to emit continuous IR radiation

- More sensitive than the Nernst glower

Mid-IR Sources

Globar- Silicon carbide bar

- Heated electrically to emit continuous IR radiation

- More sensitive than the Nernst glower

RADIATION SOURCES

Mid-IR Sources

Heated Wire Coils- Heated electrically to ~ 1100 oC

- Similar in shape to incandescent light bulb filament- Nichrome wire is commonly used

- Rhodium is also used- May easily fracture and burn out

Mid-IR Sources

Heated Wire Coils- Heated electrically to ~ 1100 oC

- Similar in shape to incandescent light bulb filament- Nichrome wire is commonly used

- Rhodium is also used- May easily fracture and burn out

RADIATION SOURCES

Near-IR Sources

Quartz halogen lamp- Contains tungsten wire filament

- Also contains iodine vapor sealed in a quartz bulb

- Evaporated tungsten is redeposited on the filamentincreasing stability

- Output range is between 25000 and 2000 cm-1

Near-IR Sources

Quartz halogen lamp- Contains tungsten wire filament

- Also contains iodine vapor sealed in a quartz bulb

- Evaporated tungsten is redeposited on the filamentincreasing stability

- Output range is between 25000 and 2000 cm-1

RADIATION SOURCES

Far-IR Sources

High pressure Hg discharge lamp- Made of quartz bulb containing elemental Hg

- Also contains inert gas and two electrodes

Far-IR Sources

High pressure Hg discharge lamp- Made of quartz bulb containing elemental Hg

- Also contains inert gas and two electrodes

RADIATION SOURCES

IR Laser Sources

- Excellent light source for measuring gases

Two typesGas-phase (tunable CO2) laser and solid-state laser

Laser- Light source that emits monochromatic radiation

IR Laser Sources

- Excellent light source for measuring gases

Two typesGas-phase (tunable CO2) laser and solid-state laser

Laser- Light source that emits monochromatic radiation

MONOCHROMATORS

- Salt prisms and metal gratings are used as dispersion devices

- Mirrors made of metal with polished front surface

- Spectrum is recorded by moving prism or grating suchthat different radiation frequencies pass through

the exit slit to the detector

- Spectrum obtained is %T verses wavenumber (or frequency)

- Salt prisms and metal gratings are used as dispersion devices

- Mirrors made of metal with polished front surface

- Spectrum is recorded by moving prism or grating suchthat different radiation frequencies pass through

the exit slit to the detector

- Spectrum obtained is %T verses wavenumber (or frequency)

FT SPECTROMETERS

- Based on Michelson interferometer

- Employs constructive and destructive interferences

- Destructive interference is a maximum when two beams are180o out of phase

- An FT is used to convert the time-domain spectrum obtained intoa frequency-domain spectrum

- The system is called FTIR

- Based on Michelson interferometer

- Employs constructive and destructive interferences

- Destructive interference is a maximum when two beams are180o out of phase

- An FT is used to convert the time-domain spectrum obtained intoa frequency-domain spectrum

- The system is called FTIR

FT SPECTROMETERS

- Has higher signal-to-noise ratio

- More accurate and precise than dispersive monochromators(Conne’s advantage)

- Much greater radiation intensity falls on the detector dueto the absence of slits

(throughput or Jacquinot’s advantage)

- Has higher signal-to-noise ratio

- More accurate and precise than dispersive monochromators(Conne’s advantage)

- Much greater radiation intensity falls on the detector dueto the absence of slits

(throughput or Jacquinot’s advantage)

FT SPECTROMETERS

Focusing Mirrors of Interferometer

- Made of metal and polished on the front surface

- Gold coated to prevent corrosion

- Focus source radiation onto the beam splitter

- Focus light emitting from sample onto detector

Focusing Mirrors of Interferometer

- Made of metal and polished on the front surface

- Gold coated to prevent corrosion

- Focus source radiation onto the beam splitter

- Focus light emitting from sample onto detector

FT SPECTROMETERS

Beam Splitter of Interferometer

- Made of Ge or Si coated onto a highly polishedIR-transparent substrate

- Ge coated on KBr is used for mid-IR

- Si coated on quartz is used for NIR

- Mylar film is used for far-IR

- Transmits and reflects 50% each of beam (ideally)

Beam Splitter of Interferometer

- Made of Ge or Si coated onto a highly polishedIR-transparent substrate

- Ge coated on KBr is used for mid-IR

- Si coated on quartz is used for NIR

- Mylar film is used for far-IR

- Transmits and reflects 50% each of beam (ideally)

DETECTORS

Two classes

Thermal detectors

and

photon-sensitive detectors

Two classes

Thermal detectors

and

photon-sensitive detectors

DETECTORS

Thermal Detectors

- Bolometers

- Thermocouples

- Thermistors

- Pyroelectric devices

Thermal Detectors

- Bolometers

- Thermocouples

- Thermistors

- Pyroelectric devices

DETECTORS

Thermal Detectors

Bolometers- Very sensitive electrical resistance thermometer

- Older ones were made of Pt wire(resistance change = 0.4% per oC)

- Modern ones are made of silicon placed in a wheatstone bridge

- Is a few micrometers in diameter

- Fast response time

Thermal Detectors

Bolometers- Very sensitive electrical resistance thermometer

- Older ones were made of Pt wire(resistance change = 0.4% per oC)

- Modern ones are made of silicon placed in a wheatstone bridge

- Is a few micrometers in diameter

- Fast response time

DETECTORS

Thermal Detectors

Thermocouples- Made up of two wires welded together at both ends

- Two wires are from different metals- One welded joint (hot junction) is exposed to IR radiation

- The other joint (cold junction) is kept at constant temperature- Hot junction becomes hotter than cold junction

- Potential difference is a function of IR radiation- Slow response time

- Cannot be used for FTIR

Thermal Detectors

Thermocouples- Made up of two wires welded together at both ends

- Two wires are from different metals- One welded joint (hot junction) is exposed to IR radiation

- The other joint (cold junction) is kept at constant temperature- Hot junction becomes hotter than cold junction

- Potential difference is a function of IR radiation- Slow response time

- Cannot be used for FTIR

DETECTORS

Thermal Detectors

Thermistors- Made of fused mixture of metal oxides

- Increasing temperature decreases electrical resistancewhich is a function of IR radiation

- Resistance change is about 5% per oC

- Slow response time

Thermal Detectors

Thermistors- Made of fused mixture of metal oxides

- Increasing temperature decreases electrical resistancewhich is a function of IR radiation

- Resistance change is about 5% per oC

- Slow response time

DETECTORS

Thermal Detectors

Pyroelectric Detectors- Made of dielectric materials (insulators), ferroelectric materials,

or semiconductors

- A thin crystal of the material is placed between two electrodes

- Change in temperature changes polarization of materialwhich is a function of IR radiation exposed

- The electrodes measure the change in polarization

Thermal Detectors

Pyroelectric Detectors- Made of dielectric materials (insulators), ferroelectric materials,

or semiconductors

- A thin crystal of the material is placed between two electrodes

- Change in temperature changes polarization of materialwhich is a function of IR radiation exposed

- The electrodes measure the change in polarization

DETECTORS

Photon Detectors

- Made of materials such as lead selenide (PbSe),indium gallium arsenide (InGaSa), indium antimonide (InSb)

- Materials are semiconductors

- S/N increases as temperature decreases (cooling is necessary)

- Very sensitive and very fast response time

- Good for FTIR and coupled techniques

Photon Detectors

- Made of materials such as lead selenide (PbSe),indium gallium arsenide (InGaSa), indium antimonide (InSb)

- Materials are semiconductors

- S/N increases as temperature decreases (cooling is necessary)

- Very sensitive and very fast response time

- Good for FTIR and coupled techniques

RESPONSE TIME

- The length of time that a detector takes to reach a steadysignal when radiation falls on it

- Response time for older dispersive IR spectrometerswere ~ 15 minutes

- Response time is very slow in thermal detectors due to slowchange in temperature near equilibrium

- The length of time that a detector takes to reach a steadysignal when radiation falls on it

- Response time for older dispersive IR spectrometerswere ~ 15 minutes

- Response time is very slow in thermal detectors due to slowchange in temperature near equilibrium

RESPONSE TIME

Semiconductors

- Are insulators when no radiation falls on them but conductorswhen radiation falls on them

- There is no temperature change involved

- Measured signal is due to change in resistance uponexposure to IR radiation ( is rapid)

- Response time is the time required to change the semiconductorfrom insulator to conductor (~ 1 ns)

Semiconductors

- Are insulators when no radiation falls on them but conductorswhen radiation falls on them

- There is no temperature change involved

- Measured signal is due to change in resistance uponexposure to IR radiation ( is rapid)

- Response time is the time required to change the semiconductorfrom insulator to conductor (~ 1 ns)

SATURATION

- Very high levels of light can saturate detector

- Linearity is very important especially inthe mid-IR region

- Very high levels of light can saturate detector

- Linearity is very important especially inthe mid-IR region

TRANSMISSION (ABSORPTION) TECHNIQUE

- Can be used for both qualitative and quantitative analysis

- Provides very high sensitivity at relatively low cost

- Sample cell material must be transparent to IR radiation(NaCl, KBr)

- Samples should not contain water since materials aresoluble in water

- Can be used for both qualitative and quantitative analysis

- Provides very high sensitivity at relatively low cost

- Sample cell material must be transparent to IR radiation(NaCl, KBr)

- Samples should not contain water since materials aresoluble in water


Solid Samples

Three sampling techniquesMulling, pelleting, thin film

Mulling- Samples are ground to power with a few drops of viscous

liquid like Nujol (size < 2 µm)

- 2-4 mg sample is made into a mull (thick slurry)

- Mull is pressed between salt plates to form a thin film

Solid Samples


Mulling- Samples are ground to power with a few drops of viscous

liquid like Nujol (size < 2 µm)

- 2-4 mg sample is made into a mull (thick slurry)

- Mull is pressed between salt plates to form a thin film


Solid Samples


Pelleting- 1 mg ground sample is mixed with 100 mg of dry KBr powder

- Mixture is compressed under very high pressure

- Small disk with very smooth surfaces forms (looks like glass)

Solid Samples


Pelleting- 1 mg ground sample is mixed with 100 mg of dry KBr powder

- Mixture is compressed under very high pressure

- Small disk with very smooth surfaces forms (looks like glass)


Solid Samples


Thin Film- Molten sample is deposited on the surface of KBr or NaCl plates

- Sample is allowed to dry to form a thin film on substrate

- Good for qualitative identification of polymers but notgood for quantitative analysis

Solid Samples


Thin Film- Molten sample is deposited on the surface of KBr or NaCl plates

- Sample is allowed to dry to form a thin film on substrate

- Good for qualitative identification of polymers but notgood for quantitative analysis


Liquid Samples

- Many liquid samples are analyzed as is

- Dilution with the appropriate solvent may be necessary(CCl4, CS2, CH3Cl)

- Solvent must be transparent in the region of interest

- Salt plates are hygroscopic and water soluble (avoid water)

- Special cells are used for water containing samples (BaF2, AgCl)

Liquid Samples

- Many liquid samples are analyzed as is

- Dilution with the appropriate solvent may be necessary(CCl4, CS2, CH3Cl)

- Solvent must be transparent in the region of interest

- Salt plates are hygroscopic and water soluble (avoid water)

- Special cells are used for water containing samples (BaF2, AgCl)


Gas Samples

- Gas cells have longer pathlengths to compensate for verylow concentrations of gas samples

- Generally about 10 cm(commercial ones are up to 120 cm)

- Temperature and pressure are controlled if quantitativeanalysis is required

Gas Samples

- Gas cells have longer pathlengths to compensate for verylow concentrations of gas samples

- Generally about 10 cm(commercial ones are up to 120 cm)

- Temperature and pressure are controlled if quantitativeanalysis is required


Background Absorption

- Absorption from material other than sample

Two sources- Solvent

- Air in the light path

- Background spectrum is subtracted from sample spectrum

- CO2 and H2O absorb IR radiation(sealed desiccants are used to reduce CO2 and H2O absorption)

Background Absorption

- Absorption from material other than sample

Two sources- Solvent

- Air in the light path

- Background spectrum is subtracted from sample spectrum

- CO2 and H2O absorb IR radiation(sealed desiccants are used to reduce CO2 and H2O absorption)

ATTENUATED TOTAL REFLECTION(ATR) TECHNIQUE

- Uses high refractive index optical element

- Called internal reflection element (IRE) or ATR crystal(germanium, ZnSe, silicon, diamond)

- Sample has lower refractive index than the ATR crystal

- Sample is placed on the ATR crystal

- When light travelling in the crystal hits the crystal-sampleinterface, total internal reflection occurs if the angle

of incidence is greater than the critical angle

- Uses high refractive index optical element

- Called internal reflection element (IRE) or ATR crystal(germanium, ZnSe, silicon, diamond)

- Sample has lower refractive index than the ATR crystal

- Sample is placed on the ATR crystal

- When light travelling in the crystal hits the crystal-sampleinterface, total internal reflection occurs if the angle

of incidence is greater than the critical angle

ATTENUATED TOTAL REFLECTION(ATR) TECHNIQUE

- The beam of light penetrates the sample a very short distance

- This is known as the evanescent wave

- Light beam is attenuated (reduced in intensity) if the evanescentwave is absorbed

- IR absorption spectrum is obtained from the interactionsbetween the evanescent wave and the sample

- The beam of light penetrates the sample a very short distance

- This is known as the evanescent wave

- Light beam is attenuated (reduced in intensity) if the evanescentwave is absorbed

- IR absorption spectrum is obtained from the interactionsbetween the evanescent wave and the sample

SPECULAR REFLECTANCE TECHNIQUE

- Occurs when angle of reflected light equals angle of incident light(45o angle of incidence is typically used)

- Occurs when light bounces off a smooth surface

- Nondestructive method for studying thin films

- Beam passes through thin film of sample where absorption occurs

- Beam is reflected from a polished surface below the sample

- Reflected beam passes through sample again

- Occurs when angle of reflected light equals angle of incident light(45o angle of incidence is typically used)

- Occurs when light bounces off a smooth surface

- Nondestructive method for studying thin films

- Beam passes through thin film of sample where absorption occurs

- Beam is reflected from a polished surface below the sample

- Reflected beam passes through sample again

DIFFUSE REFLECTANCE TECHNIQUE

- DR OR DRIFTS

- Used to obtain IR or NIR spectrum from a rough surface

- Beam penetrates sample to about 100 µm

- Reflected light is scattered at many angles

- Scattered radiation is collected with a large mirror

- Good for powdered samples (sample is mixed with KBr powder)

- DR OR DRIFTS

- Used to obtain IR or NIR spectrum from a rough surface

- Beam penetrates sample to about 100 µm

- Reflected light is scattered at many angles

- Scattered radiation is collected with a large mirror

- Good for powdered samples (sample is mixed with KBr powder)

IR EMISSION

- Certain samples are characterized using emission spectrum

- Sample molecules are heated to occupy excited vibrational states

- Radiation is emitted upon returning to the ground state

- Emitted IR radiation is directed into the spectrometer

- Radiation source is not needed (sample is the source)

- Method is nondestructive

- Emitted radiation is used to identify sample

- Certain samples are characterized using emission spectrum

- Sample molecules are heated to occupy excited vibrational states

- Radiation is emitted upon returning to the ground state

- Emitted IR radiation is directed into the spectrometer

- Radiation source is not needed (sample is the source)

- Method is nondestructive

- Emitted radiation is used to identify sample

FTIR MICROSCOPY

- For forensic analysis

- Analysis of paint chips from cars (hit-and-run accidents)

- Examination of fibers, drugs, explosives

- Characterization of pharmaceuticals, adhesives and gemstones

- For forensic analysis

- Analysis of paint chips from cars (hit-and-run accidents)

- Examination of fibers, drugs, explosives

- Characterization of pharmaceuticals, adhesives and gemstones

NEAR-IR (NIR) SPECTROSCOPY

- Region covers 750 nm – 2500 nm (13000 cm-1 – 4000 cm-1)

- Long wavelength end of IR region

- Bands occurring in this region are due to OH, NH, and CH bonds

- Bands are primarily overtone and combination bands

- Light source is tungsten-halogen lamp

- Detector is lead sulfide photodetector

- Quartz or fused silica sample cells with long pathlengths are used

- Region covers 750 nm – 2500 nm (13000 cm-1 – 4000 cm-1)

- Long wavelength end of IR region

- Bands occurring in this region are due to OH, NH, and CH bonds

- Bands are primarily overtone and combination bands

- Light source is tungsten-halogen lamp

- Detector is lead sulfide photodetector

- Quartz or fused silica sample cells with long pathlengths are used


Primary absorption bands seen in NIR

C−H Bands2100 – 2450 nm and 1600 – 1800 nm

N−H Bands1450 – 1550 nm and 2800 – 3000 nm

O−H Bands1390 – 1450 nm and 2700 – 2900 nm

Primary absorption bands seen in NIR

C−H Bands2100 – 2450 nm and 1600 – 1800 nm

N−H Bands1450 – 1550 nm and 2800 – 3000 nm

O−H Bands1390 – 1450 nm and 2700 – 2900 nm


- Used for quantitative analysis of solid and liquid samplescontaining OH, NH, CH bonds

- For quantitative characterization of polymers, food,proteins, agricultural products

- Pharmaceutical tablets can be analyzed nondestructively

- Forensic analysis of unknown wrapped powders believed to bedrugs are analyzed without destroying the wrappers

- Used for quantitative analysis of solid and liquid samplescontaining OH, NH, CH bonds

- For quantitative characterization of polymers, food,proteins, agricultural products

- Pharmaceutical tablets can be analyzed nondestructively

- Forensic analysis of unknown wrapped powders believed to bedrugs are analyzed without destroying the wrappers

RAMAN SPECTROSCOPY

- Light scattering technique for studying molecular vibrations

- Change in polarization is necessary for a vibration to beseen in Raman spectrum

- Implies change in distribution of electron cloud aroundvibrating atoms

- Polarization is easier for long bonds than for short bonds

- IR-inactive radiations are Raman-active

- Light scattering technique for studying molecular vibrations

- Change in polarization is necessary for a vibration to beseen in Raman spectrum

- Implies change in distribution of electron cloud aroundvibrating atoms

- Polarization is easier for long bonds than for short bonds

- IR-inactive radiations are Raman-active

RAMAN SPECTROSCOPY

Principles

- Part of the radiation is scattered by molecules whenthe radiation passes through sample

Three types of scattering occurs- Rayleigh scattering- Stokes scattering

- Anti-Stokes scattering

Principles

- Part of the radiation is scattered by molecules whenthe radiation passes through sample

Three types of scattering occurs- Rayleigh scattering- Stokes scattering

- Anti-Stokes scattering

RAMAN SPECTROSCOPY

Principles

Rayleigh scattering- Result of elastic collisions between photons and sample molecules

- Energy is same as incident radiation

Stokes scattering- Scattered photons have less energy than the incident radiation

- Results in spectral lines called Raman lines

Anti-Stokes scattering- Raman lines result from photons scattered with more energy

Principles

Rayleigh scattering- Result of elastic collisions between photons and sample molecules

- Energy is same as incident radiation

Stokes scattering- Scattered photons have less energy than the incident radiation

- Results in spectral lines called Raman lines

Anti-Stokes scattering- Raman lines result from photons scattered with more energy

RAMAN SPECTROSCOPY

Principles

- Stokes and anti-stokes are due to inelastic collisions

- The process is not quantized

- Raman lines are shifted in frequency from Rayleigh frequency

- Radiation measured is visible or NIR

Principles

- Stokes and anti-stokes are due to inelastic collisions

- The process is not quantized

- Raman lines are shifted in frequency from Rayleigh frequency

- Radiation measured is visible or NIR

RAMAN SPECTROSCOPY

Advantages over IR

- Aqueous solutions can be analyzed

- Fewer and much sharper lines so better for quantitative analysis

Techniques- Resonance Raman Spectroscopy

- Surface-Enhanced Raman Spectroscopy (SERS)- Raman Microscopy

Advantages over IR

- Aqueous solutions can be analyzed

- Fewer and much sharper lines so better for quantitative analysis

Techniques- Resonance Raman Spectroscopy

- Surface-Enhanced Raman Spectroscopy (SERS)- Raman Microscopy

APPLICATIONS OF IR SPECTROSCOPY

Quantitative

- Extent of absorption and Beer’s law can be used to determineconcentration of unknown analytes in sample

- Absorption band unique to the analyte molecule should beused for measurements

- Generally performed with samples in solutions

- Light scattering may occur with pellets whichdeviates from Beer’s law

Quantitative

- Extent of absorption and Beer’s law can be used to determineconcentration of unknown analytes in sample

- Absorption band unique to the analyte molecule should beused for measurements

- Generally performed with samples in solutions

- Light scattering may occur with pellets whichdeviates from Beer’s law


Quantitative

- Measure absorption intensities of standard solutions andunknown at exactly the same wavenumber

- All measurements must be made from the same baseline

- Plot a calibration curve

- Use the relationship obtained to determine the concentrationof unknown

- Not as accurate as using UV-VIS spectroscopy

Quantitative

- Measure absorption intensities of standard solutions andunknown at exactly the same wavenumber

- All measurements must be made from the same baseline

- Plot a calibration curve

- Use the relationship obtained to determine the concentrationof unknown

- Not as accurate as using UV-VIS spectroscopy


Quantitative

- Determination of impurities in raw materials (quality control)

- Analysis of contaminations from oil or grease

- Determination of reaction rates of slow reactions

Quantitative

- Determination of impurities in raw materials (quality control)

- Analysis of contaminations from oil or grease

- Determination of reaction rates of slow reactions


Qualitative

- Identification of unknown samples by matching the absorptionspectra with that of known compounds

- Identification of functional groups present in a sample(classification of unknowns)

Qualitative

- Identification of unknown samples by matching the absorptionspectra with that of known compounds

- Identification of functional groups present in a sample(classification of unknowns)

PREDICTING STRUCTURE OF UNKNOWN

- Identify the major functional groups from the strongabsorption peaks

- Identify the compound as aromatic or aliphatic

- Subtract the FW of all functional groups identified from the givenmolecular weight of the compound

- Look for C≡C and C=C stretching bands

- Look for other unique CH bands (e.g. aldehyde)

- Use the difference obtained to deduce the structure

- Identify the major functional groups from the strongabsorption peaks

- Identify the compound as aromatic or aliphatic

- Subtract the FW of all functional groups identified from the givenmolecular weight of the compound

- Look for C≡C and C=C stretching bands

- Look for other unique CH bands (e.g. aldehyde)

- Use the difference obtained to deduce the structure

INTERPRETATION OF IR SPECTRA

Functional Group Region- Strong absorptions due to stretching from hydroxyl, amine,

carbonyl, CHx

4000 – 1300 cm-1

Fingerprint Region- Result of interactions between vibrations

1300 – 910 cm-1

Functional Group Region- Strong absorptions due to stretching from hydroxyl, amine,

carbonyl, CHx

4000 – 1300 cm-1

Fingerprint Region- Result of interactions between vibrations

1300 – 910 cm-1


Hydrocarbons

- Absorption bands are due to the stretching or bending ofC−H and C−C bonds

- C−C stretching vibrations are distributed across thefingerprint region (not useful for identification)

- C−C bending vibrations occur below 500 cm-1

(not useful for identification)

- Observed bands are due to C−H stretching or bending

Hydrocarbons

- Absorption bands are due to the stretching or bending ofC−H and C−C bonds

- C−C stretching vibrations are distributed across thefingerprint region (not useful for identification)

- C−C bending vibrations occur below 500 cm-1

(not useful for identification)

- Observed bands are due to C−H stretching or bending


Cyclic Alkanes- No peak around 1375 cm-1 due to absence of methyl groups

- Two peaks at ~ 900 cm-1 and 860 cm-1 due to ring deformation

Alkenes- Contain many more peaks than alkanes

- Peaks of interest are due to stretching and bending of C−Hand C=C bonds

- C=C band will not appear if there is symmetrical substitutionabout the C=C bond

Cyclic Alkanes- No peak around 1375 cm-1 due to absence of methyl groups

- Two peaks at ~ 900 cm-1 and 860 cm-1 due to ring deformation

Alkenes- Contain many more peaks than alkanes

- Peaks of interest are due to stretching and bending of C−Hand C=C bonds

- C=C band will not appear if there is symmetrical substitutionabout the C=C bond


Alkynes- C≡C peak appears around 2100 – 2200 cm-1

- Terminal alkyne ≡C−H stretch occurs near 3300 cm-1

Aromatic Hydrocarbons- C→H absorption occurs above 3000 cm-1

- Aromatic C=C ring stretching absorption around1400 – 1600 cm-1 appears as doublet

- Aromatic C↓H oop band around 690 – 900 cm-1

- Overtones around 1660 – 2000 cm-1

Alkynes- C≡C peak appears around 2100 – 2200 cm-1

- Terminal alkyne ≡C−H stretch occurs near 3300 cm-1

Aromatic Hydrocarbons- C→H absorption occurs above 3000 cm-1

- Aromatic C=C ring stretching absorption around1400 – 1600 cm-1 appears as doublet

- Aromatic C↓H oop band around 690 – 900 cm-1

- Overtones around 1660 – 2000 cm-1


Alcohols

- OH band in neat aliphatic alcohols is a broad band centeredat ~ 3300 cm-1 due to hydrogen bonding (3100 – 3600 cm-1)

- OH band in dilute solutions of aliphatic alcohols is a sharppeak ~ 3600 cm-1

- C−C−O stretch ~ 1048 cm-1 for primary alcohols

- Decreasing frequency by 10 cm-1 in the order 1o>2o>3o

- Methyl bending vibrations at ~ 1200 – 1500 cm-1

Alcohols

- OH band in neat aliphatic alcohols is a broad band centeredat ~ 3300 cm-1 due to hydrogen bonding (3100 – 3600 cm-1)

- OH band in dilute solutions of aliphatic alcohols is a sharppeak ~ 3600 cm-1

- C−C−O stretch ~ 1048 cm-1 for primary alcohols

- Decreasing frequency by 10 cm-1 in the order 1o>2o>3o

- Methyl bending vibrations at ~ 1200 – 1500 cm-1


Phenol

- CO→H stretch is broad band

- C→H stretch ~ 3050 cm-1

- C−C→O band ~ 1225 cm-1

- C −O−H bend ~ 1350 cm-1

- Aromatic ring C stretching between 1450 – 1600 cm-1

- Monosubstituted bands ~ 745 – 895 cm-1 and 1650 – 2000 cm-1

Phenol

- CO→H stretch is broad band

- C→H stretch ~ 3050 cm-1

- C−C→O band ~ 1225 cm-1

- C −O−H bend ~ 1350 cm-1

- Aromatic ring C stretching between 1450 – 1600 cm-1

- Monosubstituted bands ~ 745 – 895 cm-1 and 1650 – 2000 cm-1


Aliphatic Acids

- Broad OH band around 2900 cm-1

- C−H stretching bands from CH3 and CH2 stick out at thebottom of the broad OH band

- C=O stretch ~ 1710 cm-1

- In-plane C −O−H bend ~ 1410 cm-1 andoop C −O−H bend ~ 930 cm-1

- C −C−O stretch dimer at ~ 1280 cm-1

Aliphatic Acids

- Broad OH band around 2900 cm-1

- C−H stretching bands from CH3 and CH2 stick out at thebottom of the broad OH band

- C=O stretch ~ 1710 cm-1

- In-plane C −O−H bend ~ 1410 cm-1 andoop C −O−H bend ~ 930 cm-1

- C −C−O stretch dimer at ~ 1280 cm-1


Carboxylic Acids, Esters, Ketones, Aldehydes

- Characterized by very strong carbonyl (C=O) stretchingband between 1650 cm-1 and 1850 cm-1

- Fermi resonance seen in aldehydes(doublet due to resonance with an overtone of the aldehydic

C−H bend at 1390 cm-1)

Carboxylic Acids, Esters, Ketones, Aldehydes

- Characterized by very strong carbonyl (C=O) stretchingband between 1650 cm-1 and 1850 cm-1

- Fermi resonance seen in aldehydes(doublet due to resonance with an overtone of the aldehydic

C−H bend at 1390 cm-1)


Nitrogen-Containing Compounds

- 1o amines (NH2) have scissoring mode and lowfrequency wagging mode

- 2o amines (NH) only have wagging mode (cannot scissor)

- 3o amines have no NH band and are characterized by C−Nstretching modes ~ 1000 – 1200 cm-1 and 700 – 900 cm-1

- 1o, 2o, 3o amides are similar to their amine counterpartsbut have additional C=O stretching band


- 1o amines (NH2) have scissoring mode and lowfrequency wagging mode

- 2o amines (NH) only have wagging mode (cannot scissor)

- 3o amines have no NH band and are characterized by C−Nstretching modes ~ 1000 – 1200 cm-1 and 700 – 900 cm-1

- 1o, 2o, 3o amides are similar to their amine counterpartsbut have additional C=O stretching band



- C=O stretching called amide I in 1o and 2o amides andamide II in 3o amides

- N−H stretch doublet ~ 3370 – 3291 cm-1 for 1o amines

- 1o N−H bend at ~ 1610 cm-1 and 800 cm-1

- Single N−H stretch ~ 3293 cm-1 for 2o but absent in 3o amine

- C−N stretch weak band ~ 1100 cm-1


- C=O stretching called amide I in 1o and 2o amides andamide II in 3o amides

- N−H stretch doublet ~ 3370 – 3291 cm-1 for 1o amines

- 1o N−H bend at ~ 1610 cm-1 and 800 cm-1

- Single N−H stretch ~ 3293 cm-1 for 2o but absent in 3o amine

- C−N stretch weak band ~ 1100 cm-1


Amino Acids [RCH(NH2)COOH]

- IR spectrum is related to salts of amines and salts of acids

- Broad CH bands that overlap with each other

- Broad band ~ 2100 cm-1

- NH band ~ 1500 cm-1

- Carboxylate ion stretch ~ 1600 cm-1

Amino Acids [RCH(NH2)COOH]

- IR spectrum is related to salts of amines and salts of acids

- Broad CH bands that overlap with each other

- Broad band ~ 2100 cm-1

- NH band ~ 1500 cm-1

- Carboxylate ion stretch ~ 1600 cm-1


Halogenated Compounds

- C→X strong absorption bands in the fingerprint andaromatic regions

- More halogens on the same C results in an increase in intensityand a shift to higher wavenumbers

- Absorption due to C−Cl and C−Br occurs below 800 cm-1

Halogenated Compounds

- C→X strong absorption bands in the fingerprint andaromatic regions

- More halogens on the same C results in an increase in intensityand a shift to higher wavenumbers

- Absorption due to C−Cl and C−Br occurs below 800 cm-1

LIMITATIONS OF IR SPECTROSCOPY

- Short pathlength

- Pathlength may vary from sample to sample

- Sample cells are soluble in water

- Short pathlength

- Pathlength may vary from sample to sample

- Sample cells are soluble in water

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