Infrared Spectroscopy

Theory of Infrared Absorption Spectroscopy

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

Theory of Infrared Absorption Spectroscopy

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

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

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

Theory of Infrared Absorption Spectroscopy

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

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

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

E = hE = h•• There are three types of molecular transitions that occur in IR There are three types of molecular transitions that occur in IR

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

dipole moment seems to fluctuate.dipole moment seems to fluctuate.

•• E for these transitions correspond to E for these transitions correspond to < 100 cm< 100 cm-1 -1

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

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

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

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

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

Vibrational Modes1.1. StretchingStretching - - the rhythmic movement along a bond axis the rhythmic movement along a bond axis

wit a subsequent increase and decrease in bond length.wit a subsequent increase and decrease in bond length.

2.2. BendingBending - - a change in bond angle or movement of a a change in bond angle or movement of a group of atoms with respect to the rest of the molecule.group of atoms with respect to the rest of the molecule.

The Vibrational Modes of Water

Mechanical Model of Stretching Vibrations1.1. Simple harmonic oscillator.Simple harmonic oscillator.

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

F = -F = -kyky

Where: Where: FF = Force = Forcek k = = Force ConstantForce Constant(stiffness of spring)(stiffness of spring)yy = Displacement = Displacement

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

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

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

Where: Where: = wavenumber of the abs. peak (cm = wavenumber of the abs. peak (cm-1-1))cc = speed of light (3 x 10 = speed of light (3 x 1010 10 cm/s) cm/s) k k = = force constantforce constant = reduced mass of the atoms = reduced mass of the atoms

21

21

k

c

yx

yx

MMMM

Where: Where: MMxx = mass of atom x in kg = mass of atom x in kg

MMyy = mass of atom y in kg = mass of atom y in kg

•• Force constants are expressed in N/m (N = kg•m/sForce constants are expressed in N/m (N = kg•m/s22))

-- Range from 3 x 10Range from 3 x 1022 to 8 x 10 to 8 x 1022 N/m for single bonds N/m for single bonds -- 500 N/m is a good average force constant for single bonds when 500 N/m is a good average force constant for single bonds when

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

Example 1:Example 1: Calculate the force constant of the carbonyl bond in the Calculate the force constant of the carbonyl bond in the following spectrum.following spectrum.

Example 2:Example 2: Predict the wavenumber of a peak arising from a nitrile Predict the wavenumber of a peak arising from a nitrile stretch.stretch.

Anharmonic oscillatorsAnharmonic oscillators

•• In reality, bonds act as anharmonic oscillators because as atoms get In reality, bonds act as anharmonic oscillators because as atoms get close, they repel one another, and at some point a stretched bond close, they repel one another, and at some point a stretched bond will break.will break.

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

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

•• Common sources: Common sources:

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

2.2. GlobarGlobar - - Silicon carbide rod with water cooled Silicon carbide rod with water cooled contacts to prevent arcing. contacts to prevent arcing.

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

DetectorsDetectors – measure minute changes in temperature. – measure minute changes in temperature.

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

potential (V). (similar to a thermocouple)potential (V). (similar to a thermocouple)

•• Have a slow response time, so they are not well suited to FT-IR.Have a slow response time, so they are not well suited to FT-IR.

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

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

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

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

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

•• Fast response time, but require cooling by liquid NFast response time, but require cooling by liquid N22. .

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

Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers

•• Detectors are not fast enough to respond to power variations at high Detectors are not fast enough to respond to power variations at high frequency (10frequency (101212 to 10 to 101515 Hz) so the signal is modulated by a Hz) so the signal is modulated by a Michelson Michelson interferometerinterferometer to a lower frequency that is directly proportional to the high to a lower frequency that is directly proportional to the high frequency. frequency.

1.1. Michelson InterferometerMichelson Interferometer

B.B. Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers

•• The source beam is split into two The source beam is split into two beams.beams.

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

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

Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers

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

c

f m2

= frequency of radiation= frequency of radiationf f = frequency of inteferogram= frequency of inteferogrammm = velocity of the mirror = velocity of the mirror

cc = speed of light (3.00 x 10 = speed of light (3.00 x 101010 cm/s) cm/s)

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

2.2. FT-IR instrumentFT-IR instrument

Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers

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

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

•• Scan rates from 0.1 to 10 cm/s Scan rates from 0.1 to 10 cm/s

•• Detectors are usually pyroelectric Detectors are usually pyroelectric or photoconducting. or photoconducting.

•• Cost $10,000 - $20,000Cost $10,000 - $20,000

•• Have virtually replaced Have virtually replaced dispersive instruments. dispersive instruments.

Performance CharacteristicsPerformance Characteristics

•• Range:Range: 7800 to 350 cm7800 to 350 cm-1-1 (less expensive) (less expensive) 25,000 to 10 cm25,000 to 10 cm-1-1 (Near to far IR, expensive) (Near to far IR, expensive)

•• Resolution: Resolution: 8 cm8 cm-1-1 to 0.01 cm to 0.01 cm-1-1

•• Qualitative: Qualitative: Very good, functional groups are Very good, functional groups are identifiable identifiable

•• Quantitative: Quantitative: Dispersive – poor Dispersive – poor FTIR - fair FTIR - fair