Chapter 16

1
Chapter 16 Infrared Spectroscopy
2
Introduction

• Useful range from about 2.5 mm to 50 mm.
• Infrared used to determine the major functional
  groups present.
• Quantitative measurements possible but subject to
  large amount of error.
• Atoms or groups of atoms in molecules are in
  continuous motion with different modes of
  vibration relative to each other.
• Absorption of radiation changes amplitude of
  vibration but not frequency.

3
Vibrational Modes

• Increased amplitude produces a change in dipole
  moment.
• ? qr. where ? dipole moment, q change
  displacement, and r displacement from
  equilibrium.
• Only vibrations that cause this change in
  electric dipole moment will be associated with an
  absorption of infrared radiation.
• E.g. Symmetric and antisymmetric modes of
  vibration are possible with CO2
• symmetric mode of vibration has no net change in
  its dipole moment, while antisymmetic mode has
  net change in dipole moment. The antisymmetric
  mode would be infrared active and the symmetric
  mode would not.

4
VIBRATIONAL MODELS

• Mechanical model spring connects one or two
  moving bodies. Restoring force, F, pulling on
  atoms to return to initial positions.
• Force related to force constant stiffness of
  spring (bond).
• F ?ky. Negative sign means restoring force.
  (Hooks law)
• dE ?Fdy or upon integrating between equilibrium
  position and y gives E ½ky2.
• Potential energy curve parabolic maximum when
  spring is stretched or compressed and minimum at
  equilibrium position.

5
Vibrational frequency

• F ma where a acceleration and m mass of the
  substance moving.
• Acceleration is written as
• Solution y A cos 2pumt where
• .um vibrational frequency and
• A maximum amplitude of the motion.
• Leads to
• Two moving particles frequency uses reduced mass
  .

6
Quantum Mechanical Vibrations

• Harmonic oscillator is used to obtain wave
  equation for potential energy of oscillator.
• where v vibrational quantum ( integer).
• Quantum Mechanical Model of Atomic Movement only
  certain energies are allowed.
• near equilibrium position, molecular vibrations
  similar to mechanical model..
• Frequency of the vibration from the mechanical
  model as a reasonable estimate of true
  vibrational frequency .
• But quantum mechanical energy is .
• Quantum mechanical selection rule which states
  that Dv 1 (due to conservation of momentum of
  the combined photon and molecular system).
• and
  where
• k force constant (N/m) and m reduced mass
  (kg).

7
GROUP FREQUENCIES

• Estimation of frequencies of vibration for
  various groups possible when force constant
  known.
• E.g.1 force constant of CO bond is 1.23x103 N/m,
  determine vibrational frequency of this CO
  group.
• mO
• reduced mass of
• Substituting
• Units

8
Sample Problem

• E.g.2 C-H stretch of alkane occurs at ? 2900
  cm?1 determine frequency of deuterated analog
  using mechanical equation
• ratio of two equations for two forms of compound
• or
• reduced mass for each of two bonds will be mC
  1.99x10?26 kg,
• mD 3.32x10?27 kg, mH 1.66x10?27 kg and
• .mC-H 1.53x10?27 kg and mC-D 2.84x10?27 kg
• Substituting
• Deuterating convenient way to confirm presence
  of particular type of bond, since frequency shift
  is relatively large and predictable.

9
INSTRUMENTATION

• Instrumentation is same as for other absorption
  instruments except that the sources, detector and
  the optical material are designed specifically
  for this spectral region.
• Sources We have mentioned these detectors in
  our discussion of general features of the
  absorption spectrophotometer.
• Nernst Glower cylinder composed of rare-earth
  oxides which is heated to some temperature before
  current can be passed directly through it.
• Since it has a negative temperature coefficient,
  it is necessary to regulate the current (T kept
  at about 1800K)..
• Globar SiC rod (T kept at about 1600K) kept at
  desired temperature by passing current through
  it positive temperature coefficient..
• Both of these sources suffer from having low
  intensities ( 10?7 - 10?9 W) and has led
  authors to claim that source is energy limited.

10
Detectors

• Low flux of photons and low energy of infrared
  radiation make it more difficult to detect.
• photon detector- based upon photoconductive
  effect that occurs in certain semiconductor
  materials. Absorption of light causes resistance
  to decrease. Voltage drop across load resistor
  measured HgCdTe must be cooled to 77K and PbS2
  can be operated at room temperature.
• Photon detectors (also called quantum detectors)
  have rapid response and thus are used with FTIR .
  
• Material deposited on surface of a nonconducting
  material and is sealed in an evacuated tube to
  protect the semiconductor from reaction with the
  atmosphere.

11
Thermal Detectors

• Absorption of IR radiation produces a heating
  effect which alters a physical property of the
  detector. Thermal detectors are usable over a
  wide wavelength range.
• Thermocouple or therompile.Instrumental Methods
  of Analysis, Willard, Merritt, Dean Settle, p.
  193.thermocouples-piece of blackened foil placed
  over 2 wires made of dissimilar metals.
• detector is placed inside vacuum to improve its
  response (minimize heat loss due to conduction).
  
• Several thermocouples in series thermopile has
  higher sensitivity since voltages of individual
  thermocouples are additive.
• Temperature differences of about 10?6 K can be
  determined this way.
• Thermocouple is low impedance circuit so that
  high impedance preamplifier is necessary to avoid
  signal modification by the amplifier circuitry.
  The response time of these detectors is on order
  of 100 ms. 2s
• Null Point Wheatsone Bridg
• thermistor or bolometer- change in resistance is
  measured and related to amount of photons hitting
  detector.
• Resistance measured with Wheatstone bridge
• Radiation falls on R1R2 adjusted until circuit
  balanced (no voltage drop between C and D. Then
  R1/R2 R3/R4
• Knowledge of all other resistances makes it
  possible to determine R1

12
INSTRUMENTS

• Dispersive instruments have been the traditional
  instrument design for IRs high resolution is
  possible, FT-IR is essentially the same in modern
  instruments. The FT-IR has a much higher
  sensitivity which is very important due to the
  low signal levels in the IR region. This extra
  sensitivity makes it possible to use the FT-IR
  for quantitative work.
• Sample Handling Solids, liquids and gases can be
  analyzed with IR.
• Gases are generally are often constructed with a
  cylindrical glass body and are usually about 10
  cm in length.
• The pressure can be from a few mm Hg up to
  several atm. depending upon the absorption
  characteristics of sample.
• Liquids and solutions
• Only a short pathlength (1mm) is required for
  liquids.
• Some solvents absorb in IR region interfering
  with signal from sample.
• Pathlength of thin IR sample holder can be
  determined by observation of the interference
  patterns associated with constructive
  interference from reflection of light from the
  two internal surfaces of cell .
• E.g. Determine the pathlength from the fringes
  below

13
FT-IR

• Michaelson Interferometer.
• Laser-fringe reference system provides
  sampling-interval. Signal averaging can only be
  accomplished if the positioins of the mirror are
  precisely know. This is achieved by using a
  helium-neon laser as a reference. Radiation at
  exactly 632.8 nm traverses the same optical path
  as the IR beam.
• A separate detector measures the interferogram
  produced, giving a sinusoidal signal with maxima
  separated by the laser frequency at 15,803 cm-1.
• This signal is used to trigger the sampling of
  the IR signal very reproducibly
• IR Source
• Detector

14
Quantitative Analysis

• IR has traditionally been used for qualitative
  analysis.
• Difficult to use quantitatively due to chemical
  or instrumental effects.
• Large sloping background often interferes with
  normal spectrum.
• The base line method corrects involves selection
  of absorption band of the substance under
  analysis which is sufficiently separated from
  other matrix peaks and corrected as shown below.
• Mixtures can be determined by same methods
  described earlier. Need to set up the correct
  number of simultaneous equations.
• Convenient for measuring concentrations of gases