Lecture 23 Overview

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Lecture 23 - Overview

• 4. Infrared Spectroscopy
• 4.1 The Infrared Absorption Process
• 4.2 Uses of the Infrared Spectrum
• 4.3 The Modes of Stretching and Bending
• 4.4 Bond Properties and Absorption Trends

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References and Additional Resources

• Crews, P., Rodriguez, J., Jaspars, M. Organic
  Structural Analysis. Sections 8.1, 8.3 and 8.4.
• Dr. Neil Glagovich,Central Connecticut State
  University, Department of Chemistry
  http//www.chemistry.ccsu.edu/glagovich/teaching/3
  16/index.html

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4.1 The Infrared Absorption Process
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4.1 The Infrared Absorption Process
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4.1 The Infrared Absorption Process
Lots of places just use the symbol for
wavelength, but use units of wavenumber!
Chemists normally use wavenumbers because they
are directly proportional to energy. That is,
higher wavenumbers correspond to a higher energy.
Microns is used frequently in older literature.
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4.1 The Infrared Absorption Process
What does an IR spectrum look like?
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4.1 The Infrared Absorption Process
What does an IR spectrum look like?
At 100 transmittance all light passes through
at 0 transmittance, all light is absorbed and
none passes through a sample.
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4.1 The Infrared Absorption Process

• Molecules are excited to higher energy states
  when they absorb IR radiation.
• The absorption of IR radiation is quantized and
  corresponds to transitions between vibrational
  energy levels.
• The absorption of IR radiation corresponds to
  energy changes between 8 40 kJ/mol.
• Frequencies of IR radiation that match the
  natural vibration frequencies of a molecule are
  absorbed.
• Very important Not all bonds absorb IR energy
  (even if there is a frequency match).

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4.1 The Infrared Absorption Process

• Very important Not all bonds absorb IR energy
  (even if there is a frequency match).
• ONLY BONDS THAT CAN EXHIBIT A PERMANENT CHANGE IN
  THEIR DIPOLE MOMENT ARE CAPABLE OF ABSORBING IR
  RADIATION.
• Example The stretching and bending vibrational
  modes of water.

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4.2 Uses of the Infrared Spectrum

• No two different molecules will have the same IR
  spectra
• All types of bonds have a different frequency of
  vibration
• The same type of bonds in two different molecules
  will have different environments.
• If two IR spectra are peak-for-peak identical,
  they are likely the same.
• Structural information can also be obtained from
  IR spectra.

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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Stetching a rhythmical movement along the bond
  axis such that the interatomic distance is
  increasing or decreasing.
• May be symmetric or asymmetric

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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Stetching a rhythmical movement along the bond
  axis such that the interatomic distance is
  increasing or decreasing.
• May be symmetric or asymmetric
• Example -CH2

Symmetric Stretch
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Stetching a rhythmical movement along the bond
  axis such that the interatomic distance is
  increasing or decreasing.
• May be symmetric or asymmetric
• Example -CH2

Asymmetric Stretch
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Stetching a rhythmical movement along the bond
  axis such that the interatomic distance is
  increasing or decreasing.
• May be symmetric or asymmetric
• Example -CH2

Symmetric Stretch
Asymmetric Stretch
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Bending a change in bond angle between bonds
  with a common atom or the movement of a group of
  atoms with respect to the remainder of the
  molecule without movement of the atoms in the
  group with respect to one another.
• May be scissoring, rocking, wagging or twisting.

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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Bending a change in bond angle between bonds
  with a common atom or the movement of a group of
  atoms with respect to the remainder of the
  molecule without movement of the atoms in the
  group with respect to one another.
• May be scissoring, rocking, wagging or twisting.
• Example -CH2

Scissoring
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Bending a change in bond angle between bonds
  with a common atom or the movement of a group of
  atoms with respect to the remainder of the
  molecule without movement of the atoms in the
  group with respect to one another.
• May be scissoring, rocking, wagging or twisting.
• Example -CH2

Rocking
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Bending a change in bond angle between bonds
  with a common atom or the movement of a group of
  atoms with respect to the remainder of the
  molecule without movement of the atoms in the
  group with respect to one another.
• May be scissoring, rocking, wagging or twisting.
• Example -CH2

Wagging
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Bending a change in bond angle between bonds
  with a common atom or the movement of a group of
  atoms with respect to the remainder of the
  molecule without movement of the atoms in the
  group with respect to one another.
• May be scissoring, rocking, wagging or twisting.
• Example -CH2

Twisting
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.
• Bending a change in bond angle between bonds
  with a common atom or the movement of a group of
  atoms with respect to the remainder of the
  molecule without movement of the atoms in the
  group with respect to one another.
• May be scissoring, rocking, wagging or twisting.
• Example -CH2

Rocking
Scissoring
Wagging
Twisting
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4.3 The Modes of Stretching and Bending

• The two simplest vibrational modes are stretching
  and bending.

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4.3 The Modes of Stretching and Bending

• In a heteronuclear molecule, the different
  electron densities and electronegativities of the
  atoms leads to the molecule having a dipole
  moment. When the molecule vibrates, the dipole
  moment changes.
• Heteronuclear diatomics such as CO, HF and NO
  give rise to an IR spectrum
• In homonuclear diatomics, the atoms do not have
  charges and no dipole is present.
• Stretching the bond does not create a dipole
  moment. It is therefore not possible to transfer
  the energy.
• Therefore, homonuclear diatomics such as N2, O2
  and H2 do not give rise to an IR spectrum

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4.3 The Modes of Stretching and Bending

• The selection rule does not require the molecule
  to have a dipole moment, only that the dipole
  moment changes during the vibration.
• Although each of the CO bonds in CO2 is polar,
  the molecule has no dipole moment as the dipole
  moments for the individual bonds cancel.

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4.3 The Modes of Stretching and Bending

• The selection rule does not require the molecule
  to have a dipole moment, only that the dipole
  moment changes during the vibration.
• Although each of the CO bonds in CO2 is polar,
  the molecule has no dipole moment as the dipole
  moments for the individual bonds cancel.

The symmetrical stretch of CO2 is inactive in the
IR because this vibration produces no change in
the dipole moment of the molecule.
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4.3 The Modes of Stretching and Bending

• The selection rule does not require the molecule
  to have a dipole moment, only that the dipole
  moment changes during the vibration.
• Although each of the CO bonds in CO2 is polar,
  the molecule has no dipole moment as the dipole
  moments for the individual bonds cancel.

The asymmetrical stretch of CO2 gives a strong
band in the IR at 2350 cm1. You may notice this
band in samples which you run on the instruments
in other labs, since CO2 is present in the
atmosphere.
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4.3 The Modes of Stretching and Bending

• The selection rule does not require the molecule
  to have a dipole moment, only that the dipole
  moment changes during the vibration.
• Although each of the CO bonds in CO2 is polar,
  the molecule has no dipole moment as the dipole
  moments for the individual bonds cancel.

The two scissoring or bending vibrations are
equivalent and therefore, have the same
frequency and are said to be degenerate,
appearing in an IR spectrum at 666 cm1.
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4.3 The Modes of Stretching and Bending

• A molecule consisting of n atoms has a total of
  3n degrees of freedom, corresponding to the
  Cartesian coordinates of each atom in the
  molecule.
• In a nonlinear molecule, 3 of these degrees are
  rotational and 3 are translational and the
  remaining correspond to fundamental vibrations.
• In a linear molecule, 2 degrees are rotational
  and 3 are translational.

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4.3 The Modes of Stretching and Bending

• For a molecule consisting of n atoms, the net
  number of fundamental vibrations is
• 3n-6 (degrees of freedom) for a non-linear
  molecule
• 3n-5 (degrees of freedom) for a linear molecule
• Not all are necessarily IR observable!

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4.4 Bond Properties and Absorption Trends

• The stretching frequency of a bond can be
  approximated by Hookes Law. In this
  approximation, two atoms and the connecting bond
  are treated as a simple harmonic oscillator
  composed of 2 masses (atoms) joined by a spring

m2
m1

• This system has a reduced mass expressed by

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4.4 Bond Properties and Absorption Trends

• The natural frequency of vibration of a bond is
  given by

• where K is the force constant of the spring.
• Stronger bonds have larger force constants and
  vibrate at higher frequencies than weaker bonds.
• Bonds between atoms of higher masses vibrate at
  lower frequencies than bonds between lighter
  atoms.

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4.4 Bond Properties and Absorption Trends

• Some general trends
• Triple bonds are stronger than double, which are
  stronger than single bonds between the same two
  atom.
• CC (2150 cm-1) CC (1650 cm-1) C-C (1200 cm-1)
• As the atom bonded to carbon increases in mass,
  the vibration frequency decreases (weaker bond).
• C-O (1100 cm-1) gt C-Cl (750cm-1) gt C-Br (600cm-1)
  gt C-I (500cm-1)
• Bonds are stronger in the order spgtsp2gtsp3
• C-H (3300 cm-1) C-H (3100 cm-1) -C-H (2900
  cm-1)
• Resonance gives bonds more single bond character.

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4.4 Bond Properties and Absorption Trends

• If the mass of our system are expressed in amu
  and our force constant has units of dynes/cm, the
  following expression can be derived

• Which simplifies to

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4.4 Bond Properties and Absorption Trends

• Problem Calculate the stretching frequency for a
  C-C bond (1650 cm-1 by experiment), a C-H bond
  (3000 cm-1 by experiment), and a C-D bond (2206
  cm-1 by experiment). Assume they all have the
  same K value of 10x105 dynes/cm