CHMBD 449 Organic Spectral Analysis

1
Fall 2005

• Chapter 2 IR Spectroscopy
• Spectroscopic Process
• IR Absorption Process
• Uses of IR
• Covalent bonds
• Vibrational Modes
• Absorption Trends

CHMBD 449 Organic Spectral Analysis
2

• IR Spectroscopy
• Introduction
• Spectroscopy is the study of the interaction of
  matter with the electromagnetic spectrum
• Electromagnetic radiation displays the properties
  of both particles and waves
• The particle component is called a photon
• The energy (E) component of a photon is
  proportional to the frequency . Where h is
  Plancks constant and n is the frequency in Hertz
  (cycles per second)
• 
  E hn
• The term photon is implied to mean a small,
  massless particle that contains a small
  wave-packet of EM radiation/light we will use
  this terminology in the course

3

• IR Spectroscopy
• Introduction
• Because the speed of light, c, is constant, the
  frequency, n, (number of cycles of the wave per
  second) can complete in the same time, must be
  inversely proportional to how long the
  oscillation is, or wavelength
• Amplitude, A, describes the wave height, or
  strength of the oscillation
• Because the atomic particles in matter also
  exhibit wave and particle properties (though
  opposite in how much) EM radiation can interact
  with matter in two ways
• Collision particle-to-particle energy is lost
  as heat and movement
• Coupling the wave property of the radiation
  matches the wave property of the particle and
  couple to the next higher quantum mechanical
  energy level

c
___
hc
___
n
? E hn
l
l
c 3 x 1010 cm/s
4

• IR Spectroscopy
• Introduction
• The entire electromagnetic spectrum is used by
  chemists

Frequency, n in Hz
1015
1013
1010
105
1017
1019
Wavelength, l
10 nm
1000 nm
0.01 cm
100 m
0.01 nm
.0001 nm
Energy (kcal/mol)
300-30
300-30
10-4
gt 300
10-6
UV
X-rays
IR
g-rays
Radio
Microwave
Visible
5

• IR Spectroscopy
• Introduction
• Every spectroscopic method works using the same
  principle
• Each method uses a method of irradiation,
  absorption-excitation, re-emission-relaxation,
  and detection.

excited state
Absorption Molecule takes on the quantum energy
of a photon that matches the energy of a
transition and becomes excited
Detection Photons that are reemitted and
detected by the spectrometer correspond to
quantum mechanical energy levels of the molecule
hn
Relaxation
Excitation
Energy
rest state
rest state
Irradiation Molecule is bombarded with photons
of various frequencies over the range desired
hn
hn
hn
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• IR Spectroscopy
• Introduction
• The IR Spectroscopic Process
• The quantum mechanical energy levels observed in
  IR spectroscopy are those of molecular vibration
• We perceive this vibration as heat
• When we say a covalent bond between two atoms is
  of a certain length, we are citing an average
  because the bond behaves as if it were a
  vibrating spring connecting the two atoms
• For a simple diatomic molecule, this model is
  easy to visualize

7

• IR Spectroscopy
• Introduction
• The IR Spectroscopic Process
• There are two types of bond vibration
• Stretch Vibration or oscillation along the line
  of the bond
• Bend Vibration or oscillation not along the
  line of the bond

asymmetric
symmetric
scissor
rock
twist
wag
in plane
out of plane
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• IR Spectroscopy
• Introduction
• The IR Spectroscopic Process
• Each stretching and bending vibration occurs with
  a characteristic frequency
• Typically, this frequency is on the order of 1.2
  x 1014 Hz
• (120 trillion oscillations per sec. for the
  H2 vibration at 4100 cm-1)
• The corresponding wavelengths are on the order of
  2500-15,000 nm or 2.5 15 microns (mm)
• When a molecule is bombarded with electromagnetic
  radiation (photons) that matches the frequency of
  one of these vibrations (IR radiation), it is
  absorbed and the bonds begin to stretch and bend
  more strongly (emission and absorption)
• When this photon is absorbed the amplitude of the
  vibration is increased NOT the frequency
• The molecule will slowly decay to its resting
  state by emission of a photon of this particular
  frequency, which is detected by the spectrometer
  (detection)

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IR Spectroscopy

• IR Spectroscopy
• Introduction
• The IR Spectroscopic Process
• The result of the spectroscopic process is a
  spectrum of the various stretches and bends of
  the covalent bonds in an organic molecule

10

• IR Spectroscopy
• Introduction
• The IR Spectrum
• The x-axis of the IR spectrum is in units of
  wavenumbers, n, which is the number of waves per
  centimeter in units of cm-1 (Remember E hn or
  E hc/l)
• This unit is used rather than wavelength
  (microns) because wavenumbers are directly
  proportional to the energy of transition being
  observed chemists like this, physicists
  hate it
• High frequencies and high wavenumbers equate
  higher energy
• is quicker to understand than
• Short wavelengths equate higher energy
• This unit is used rather than frequency as the
  numbers are more real than the exponential
  units of frequency
• IR spectra are observed for what is called the
  mid-infrared 400-4000 cm-1
• The peaks are Gaussian distributions of the
  average energy of a transition

11

• IR Spectroscopy
• Introduction
• The IR Spectrum bond differences
• So how does the IR detect different bonds?
• The potential energy stretching or bending
  vibrations of covalent bonds follow the model of
  the classic harmonic oscillator (Hookes Law)

Remember E ½ ky2 where y is spring
displacement k is spring constant
Potential Energy (E)
Interatomic Distance (y)
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• IR Spectroscopy
• Introduction
• The IR Spectrum bond differences
• Aside Physically here are the movements we are
  discussing
• Stretching vibration a typical C-C bond with a
  bond length of 154 pm, the displacement is
  averages 10 pm
• Bending vibration For C-C-C bond angle a change
  of 4 is typical, which corresponds to an average
  displacement of 10 pm.

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• IR Spectroscopy
• Introduction
• The IR Spectrum bond differences
• The energy levels for these vibrations are
  quantized as we are considering quantum
  mechanical particles
• Only discrete vibrational energy levels exist

rotational transitions (in microwave region)
Potential Energy (E)
Vibrational transitions, n
Interatomic Distance (r)
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• IR Spectroscopy
• Introduction
• The IR Spectrum bond differences
• However, the application of the classical
  vibrational model fails apart for two reasons
• As two nuclei approach one another through bond
  vibration, potential energy increases to
  infinity, as two positive centers begin to repel
  one another
• At higher vibrational energy levels, the
  amplitude of displacement becomes so great, that
  the overlapping orbitals of the two atoms
  involved in the bond, no longer interact and the
  bond dissociates
• We say that the model is really one of an
  aharmonic oscillator, for which the simple
  harmonic oscillator model works well for low
  energy levels

15

• IR Spectroscopy
• Introduction
• The IR Spectrum bond differences
• Here is the derivation of Hookes Law we will
  apply for IR theory
• Vibrational frequency given by
• ½
• n 1 K
• 2pc m
• where
• n frequency
• c speed of light
• K force constant bond strength
• m reduced mass m1m2/(m1m2)

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• IR Spectroscopy
• Introduction
• The IR spectrum bond differences
• Vibrational frequency given by
• ½
• n 1 K
• 2pc m
• What does this mean for the different covalent
  bonds in an organic molecule?
• Lets consider reduced mass, m, first
• The C-H and C-C single bonds differ by only 16
  kcal/mole
• 99 kcal mol-1 vs. 83 kcal mol-1 (similar
  K)
• Due to the reduced mass term, these two bonds of
  similar strength show up in very different
  regions of the IR spectrum
• C-C 1200 cm-1 m (12 x 12)/(12 12)
  6 (.408)
• C-H 3000 cm-1 m (1 x 12)/(1 12)
  0.92 (.95)

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• IR Spectroscopy
• Introduction
• The IR spectrum bond differences
• Vibrational frequency given by
• ½
• n 1 K
• 2pc m
• When greater masses are added, the trend is
  similar (Ks here are different)
• C-I 500 cm-1
• C-Br 600 cm-1
• C-Cl 750 cm-1
• C-O 1100 cm-1
• C-C 1200 cm-1
• C-H 3000 cm-1
• A smaller atom therefore gives rise to a higher
  wavenumber
• (and ? n and E)

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• IR Spectroscopy
• Introduction
• The IR spectrum bond differences
• Lets consider reduced bond strength (force
  constant, K)
• A CC bond is stronger than a CC bond is
  stronger than a C-C bond
• Therefore higher wavenumbers result from stronger
  bonds K
• wavenumber, cm-1
• From IR spectroscopy we find CC 2100
• CC
  1650
• 
  CC 1200
• Which is in good accord with the heats of
  formation (Hf) for each bond
• kcal mol-1
• CC 200
• 
  CC 146
  CC 83

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• IR Spectroscopy
• I. Introduction
• The IR Spectrum Peak Intensities
• The y-axis of the IR spectrum is in units of
  transmittance, T, which is the ratio of the
  amount of IR radiation transmitted by the sample
  (I) to the intensity of the incident beam (I0)
  Transmittance is T x 100
• T I / I0
• T (I / I0) X 100
• IR is different than other spectroscopic methods
  which plot the y-axis as units of absorbance (A).
  A log(1/T)
• As opposed to chromatography or other
  spectroscopic methods, the area of a IR band (or
  peak) is not directly proportional vs.
  concentration of other functionalities, it can be
  used vs. itself if standardized!!!

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• IR Spectroscopy
• I. Introduction
• The IR Spectrum
• The intensity of an IR band is affected by two
  primary factors
• Whether the vibration is one of stretching or
  bending
• Electronegativity difference of the atoms
  involved in the bond
• For both effects, the greater the change in
  dipole moment in a given vibration or bend, the
  larger the peak.
• The greater the difference in electronegativity
  between the atoms involved in bonding, the larger
  the dipole moment
• Typically, stretching will change dipole moment
  more than bending
• It is important to make note of peak intensities
  to show the effect of these factors
• Strong (s) peak is tall, transmittance is low
• Medium (m) peak is mid-height
• Weak (w) peak is short, transmittance is high
• Broad (br) if the Gaussian distribution is
  abnormally broad

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• II. Infrared Group Analysis
• A. General
• The primary use of the IR spectrometer is to
  detect functional groups
• Because the IR looks at the interaction of the EM
  spectrum with actual bonds, it provides a unique
  qualitative probe into the functionality of a
  molecule, as functional groups are merely
  different configurations of different types of
  bonds
• Since most types of bonds in covalent molecules
  have roughly the same energy, i.e., CC and CO
  bonds, C-H and N-H bonds they show up in similar
  regions of the IR spectrum
• Remember all organic functional groups are made
  of multiple bonds and therefore show up as
  multiple IR bands (peaks)
• There are 4 principle regions

4000 cm-1
2700 cm-1
2000 cm-1
1600 cm-1
400 cm-1
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We will pick up next time with peak intensities,
width of bands and some simple symmetry rules, as
well as instrument design Monday we should
finally get to functional groups where we will
apply in depth the general topics we have
discussed in the introductory material No
Problem set for today! But take this time to
review some organic - bond strengths both
inter and intra-molecular - bond distances for
more organic-y bonds - hybridization models -
Periodic table and properties you should know
the position and ENs of H, B, C, N, O, F, Si,
P, S, Cl, Br and I