CHMBD 430 Organic Spectral Analysis

1
Fall 2007

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

CHMBD 430 Organic Spectral Analysis
2

• 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
• (this is more for describing a bond that spans
  many energies)

3

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• We have learned
• That IR radiation can couple with the vibration
  of covalent bonds, where that particular
  vibration causes a change in dipole moment
• The IR spectrometer irradiates a sample with a
  continuum of IR radiation those photons that can
  couple with the vibrating bond elevate it to the
  next higher vibrational energy level (increase
  in A)
• When the bond relaxes back to the n0 state, a
  photon of the same n is emitted and detected by
  the spectrometer the spectrometer reports this
  information as a spectral band centered at the n
  of the coupling
• The position of the spectral band is dependent on
  bond strength and atomic size
• The intensity of the peak results from the
  efficiency of the coupling e.g. vibrations that
  have a large change in dipole moment create a
  larger electrical field with which a photon can
  couple more efficiently

4

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Remember, most interesting molecules are not
  diatomic, and mechanical or electronic factors in
  the rest of the structure may effect an IR band
• From a molecular point of view (discounting
  phase, temperature or other experimental effects)
  there are 10 factors that contribute to the
  position, intensity and appearance of IR bands
• Symmetry
• Mechanical Coupling
• Fermi Resonance
• Hydrogen Bonding
• Ring Strain
• Electronic Effects
• Constitutional Isomerism
• Stereoisomerism
• Conformational Isomerism
• Tautomerism (Dynamic Isomerism)

5

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Symmetry H2O
• For a particular vibration to be IR active there
  must be a change in dipole moment during the
  course of the particular vibration
• For example, the carbonyl vibration causes a
  large shift in dipole moment, and therefore an
  intense band on the IR spectrum
• For a symmetrical acetylene, it is clear that
  there is no permanent dipole at any point in the
  vibration of the C?C bond. No IR band appears on
  the spectrum

6

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Symmetry H2O
• Most organic molecules are fortunately
  asymmetric, and bands are observed for most
  molecular vibration
• The symmetry problem occurs most often in small,
  simple symmetric and pseudo-symmetric alkenes and
  alkynes
• Since symmetry elements cancel the presence of
  bonds where no dipole is generated, the spectra
  are greatly simplified

7

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Symmetry H2O
• Symmetry also effects the strength of a
  particular band
• The symmetry problem occurs most often in small,
  simple symmetric and pseudo-symmetric alkenes and
  alkynes
• Since symmetry elements cancel the presence of
  bonds where no dipole is generated, the spectra
  are greatly simplified

8

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Mechanical Coupling
• In a multi-atomic molecule, no vibration occurs
  without affecting the adjoining bonds
• This induces mixing and redistribution of energy
  states, yielding new energy levels, one being
  higher and one lower in frequency
• Coupling parts must be approximate in E for
  maximum interaction to occur (i.e. C-C and C-N
  are similar, C-C and H-N are not)
• No interaction is observed if coupling parts are
  separated by more than two bonds
• Coupling requires that the vibration be of the
  same symmetry

9

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Mechanical Coupling
• For example, the calculated and observed n for
  most CC bonds is around 1650 cm-1
• Butadiene (where the two CC systems are
  separated by a dissimilar C-C bond) the bands are
  observed at 1640 cm-1 (slight reduction due to
  resonance, which we will discuss later)
• In allene however, mechanical coupling of the two
  CC systems gives two IR bands at 1960 and 1070
  cm-1 due to mechanical coupling
• For purposes of this course, when we discuss the
  group frequencies, we will point out when this
  occurs

10

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Fermi Resonance
• A Fermi Resonance is a special case of mechanical
  coupling
• It is often called an accidental degeneracy
• In understanding this, for many IR bands, there
  are overtones of the fundamental (the ns you
  are taught) at twice the wavenumber
• In a good IR spectrum of a ketone (2-hexanone,
  here) you will see a CO stretch at 1715 cm-1 and
  a small peak at 3430 cm-1 for the overtone

overtone
fundamental
11

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Fermi Resonance
• Ordinarily, most overtones are so weak as not to
  be observed
• But, if the overtone of a particular vibration
  coincides with the band from another vibration,
  they can couple and cause a shift in group
  frequency and introduce extra bands
• If you first looked at the IR (working cold) of
  benzoyl chloride, you may deduce that there were
  two dissimilar CO bonds in the molecule

12

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Fermi Resonance
• In this spectrum, the out of plane bend of the
  aromatic C-H bonds occurs at 865 cm-1 the
  overtone of this band coincides with the
  fundamental of CO at 1730 cm-1
• The band is split by Fermi resonance (1760 and
  1720 cm-1)

13

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Fermi Resonance
• Again, we will cover instances of this in the
  discussion of group frequencies, but this occurs
  often in IR of organics
• Most observed
• Aldehydes the overtone of the C-H deformation
  mode at 1400 cm-1 is always in Fermi resonance
  with the stretch of the same band at 2800 cm-1

14

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Hydrogen Bonding
• One of the most common effects in chemistry, and
  can change the shape and position of IR bands
• Internal (intramolecular) H-bonding with carbonyl
  compounds can serve to lower the absorption
  frequency

1680 cm-1
1724 cm-1
15

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Hydrogen Bonding
• Inter-molecular H-bonding serves to broaden IR
  bands due to the continuum of bond strengths that
  result from autoprotolysis
• Compare the two IR spectra of 1-propanol the
  first is an IR of a neat liquid sample, the
  second is in the gas phase note the shift and
  broadening of the O-H stretching band

Gas phase
Neat liquid
16

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Hydrogen Bonding
• Some compound, in addition to intermolecular
  effects for the monomeric species can form dimers
  and oligomers which are also observed in neat
  liquid samples
• Carboxylic acids are the best illustrative
  example the broadened O-H stretching band will
  be observed for the monomer, dimer and oligomer

17

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Ring Strain
• Certain functional group frequencies can be
  shifted if one of the atoms hybridization is
  affected by the constraints of bond angle in ring
  systems
• Consider the CO band for the following
  cycloalkanones
• 1815 1775 1750
  1715 1705 cm-1
• We will discuss the specific cases for these
  shifts during our coverage of group frequencies

18

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Electronic Effects - Inductive
• The presence of a halogen on the a-carbon of a
  ketone (or electron w/d groups) raises the
  observed frequency for the p-bond
• Due to electron w/d the carbon becomes more
  electron deficient and the p-bond compensates by
  tightening

19

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Electronic Effects - Resonance
• One of the most often observed effects
• Contribution of one of the less good resonance
  forms of an unsaturated system causes some loss
  of p-bond strenght which is seen as a drop in
  observed frequency

20

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Electronic Effects - Resonance
• In extended conjugated systems, some resonance
  contributors are out-of-sync and do not
  resonate with a group
• Example

21

• IR Spectroscopy
• I. Introduction
• The IR Spectrum Factors that affect group
  frequencies
• Electronic Effects - Sterics
• Consider this example
• In this case the presence of the methyl group
  misaligns the conjugated system, and resonance
  cannot occur as efficiently
• The effects of induction, resonance and sterics
  are very case-specific and can yield a great deal
  of information about the electronic structure of
  a molecule