1. CHEM 430
IR SPECTROSCOPY
FALL 2011
Long
Lecture
Play
Dr. Justik

2. CHEM 430 – NMR Spectroscopy
Infrared and Raman Spectroscopy
11-1
Introduction
The method provides a rapid and simple method for observing the functional group species present in an organic molecule
The spectrum is a plot of the percentage of IR radiation that passes through the sample (% transmission) versus some function of the wavelength of the radiation related to covalent bonding
CHEM 430 – NMR Spectroscopy

3. CHEM 430 – NMR Spectroscopy
Infrared and Raman Spectroscopy
11-1
Introduction
Instrumentation.
Modern IR spectrometers are based on the Michelson interferometer
Fourier transform infrared ( FT– IR) spectrometers: The absorption spectrum is obtained by means of Fourier transformation of an interferogram.
Dispersive infrared spectrometers: Earlier instruments based on monochromators that disperse the radiation from an IR source into its component wavelengths - spectrum is obtained by measuring the amount of radiation absorbed by a sample as the wavelength is varied.
Raman spectroscopy provides information complementary to that obtained from IR spectroscopy.
CHEM 430 – NMR Spectroscopy

4. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration
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:
CHEM 430 – NMR Spectroscopy

5. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
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 H C H C
asymmetric
symmetric H C H C H C H C
scissor
rock
twist
wag
in plane
out of plane 5

6. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
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 ,000 nm or 2.5 – 15 microns (mm)
When a molecule is bombarded with electromagnetic radiation (photons) that match 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
CHEM 430 – NMR Spectroscopy

7. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
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
CHEM 430 – NMR Spectroscopy 7

8. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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 = ħn or E = ħc/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
CHEM 430 – NMR Spectroscopy 8

9. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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: cm-1
The peaks are Gaussian distributions of the average energy of a transition
CHEM 430 – NMR Spectroscopy 9

10. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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 (Hooke’s Law)
Remember: E = ½ ky2
where: y is spring displacement
k is spring constant
Potential Energy (E)
Interatomic Distance (y)
CHEM 430 – NMR Spectroscopy 10

11. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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.
10 pm
154 pm 4o
10 pm
CHEM 430 – NMR Spectroscopy 11

12. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
The energy levels for these vibrations are quantized as we are considering quantum mechanical particles
Only discrete vibrational energy levels exist:
Note there is no energy level below n = 0, at any temperature above absolute zero there is always the first vibrational energy level
rotational transitions – (in microwave region)
Potential Energy (E)
Vibrational transitions, n
Interatomic Distance (r)
CHEM 430 – NMR Spectroscopy 12

13. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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
CHEM 430 – NMR Spectroscopy 13

14. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
Here is the derivation of Hooke’s Law we will apply for IR theory:
Vibrational frequency given by:
n : frequency
K: force constant – bond strength
m: reduced mass = m1m2/(m1+m2)
Reduced mass is used, as each atom in the covalent bond oscillates about the center of the two masses
CHEM 430 – NMR Spectroscopy 14

15. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
What does this mean for the different covalent bonds in a molecule?
Let’s 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 cm-1 m = (12 x 12)/( ) = 6 (0.41)
C─H cm-1 m = (1 x 12)/(1 + 12) = 0.92 (0.95)
A smaller atom therefore gives rise to a higher wavenumber (and  n and E)
CHEM 430 – NMR Spectroscopy 15

16. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
What does this mean for the different covalent bonds in a molecule?
When greater masses are added, the trend is similar (K’s here are different)
C─I 500 cm-1
C─Br cm-1
C─Cl cm-1
C─O cm-1
C─C cm-1
C─H cm-1
A smaller atom therefore gives rise to a higher wavenumber (and  n and E)
and a larger atom gives rise to lower wavenumbers (and  n and E)
CHEM 430 – NMR Spectroscopy 16

17. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
What does this mean for the different covalent bonds in a molecule?
Let’s consider bond strength, K:
A C≡C bond is stronger than a C=C bond is stronger than a C-C bond
wavenumber, cm-1 DHf
From IR spectroscopy we find: C≡C ~
C=C ~
C—C ~
Note the good correlation with the heats of formation for each bond!
Stronger bonds give higher wavenumbers (and  higher n and E)
CHEM 430 – NMR Spectroscopy 17

18. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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!!!
CHEM 430 – NMR Spectroscopy 18

19. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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
CHEM 430 – NMR Spectroscopy 19

20. Vibrations of Molecules
Infrared and Raman Spectroscopy
11-2
Vibrations of Molecules
The IR Spectroscopic Process.
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)
Exact transmittance values are rarely recorded
CHEM 430 – NMR Spectroscopy 20

21. 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., C=C and C=O 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:
Bonds to H
O-H single bond
N-H single bond
C-H single bond
Triple bonds
C≡C
C≡N
Double bonds
C=O
C=N
C=C
Single Bonds
C-C
C-N
C-O
Fingerprint
Region
4000 cm-1
2700 cm-1
2000 cm-1
1600 cm-1
400 cm-1

22. 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 EN’s of H, B, C, N, O, F, Si, P, S,
Cl, Br and I

23. 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)
Exact transmittance values are rarely recorded

24. 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

25. 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)

26. 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

27. 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

28. 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

29. 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

30. IR Spectroscopy
I. Introduction
The IR Spectrum – Factors that affect group frequencies
Mechanical Coupling
For example, the calculated and observed n for most C=C bonds is around 1650 cm-1
Butadiene (where the two C=C 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 C=C 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

31. The IR Spectrum – Factors that affect group frequencies
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 n’s you are taught) at twice the wavenumber
In a good IR spectrum of a ketone (2-hexanone, here) you will see a C=O stretch at 1715 cm-1 and a small peak at 3430 cm-1 for the overtone
overtone
fundamental

32. 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 C=O bonds in the molecule

33. 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 C=O at 1730 cm-1
The band is “split” by Fermi resonance (1760 and 1720 cm-1)

34. 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
The N-H stretching mode of –(C=O)-NH- in polyamides (peptides for the biologists and biochemists) appears as two bands at 3300 and 3205 cm-1 as this is in Fermi resonance with the N-H deformation at 1550 cm-1

35. The IR Spectrum – Factors that affect group frequencies
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

36. The IR Spectrum – Factors that affect group frequencies
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

37. 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

38. 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 C=O band for the following cycloalkanones:
cm-1
We will discuss the specific cases for these shifts during our coverage of group frequencies

39. 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

40. 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

41. 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:

42. 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

43. Group Frequencies and Analysis Introduction
IR Spectroscopy
Group Frequencies and Analysis
Introduction
When approaching any IR spectrum be sure to use the larger-to-smaller region approach- do not immediately focus on any one single peak (even –OH or C=O)
From the Hooke’s Law derivation we are using we find that the IR can be conveniently be divided into four major regions:
Bonds to H
Triple bonds
Double bonds
Single Bonds
O-H
N-H
C-H
C≡C
C≡N
C=O
C=N
C=C
C-C
C-N
C-O
C-X
“Fingerprint
Region”
4000 cm-1
2700 cm-1
2000 cm-1
1600 cm-1
400 cm-1

44. IR Spectroscopy
Group Frequencies and Analysis
Introduction
If supporting information is available – molecular formula, chemical inferences – (i.e. this was the product of an oxidation reaction), assume this information is correct and the analysis of the IR should support it (later in your careers you can doubt information given to you)
If a molecular formula is available, do an HDI!
Many texts list various methods for approaching an IR spectrum; use the method that works best for you and stick to it.
The most common mistakes in spectral analysis are those of “jumping the gun” to a conclusion (usually based on some small, insignificant peak) or taking a random haphazard approach to the spectrum (gee, here is an IR, oh, let’s start looking for phosphorus this time)
Be methodical, develop a scheme and stick to it!

45. Group Frequencies and Analysis
IR Spectroscopy
Group Frequencies and Analysis
Before we begin – Each functional group will be described as follows:
Group
General – What is most recognizable? What makes it different from similar groups?
Group Frequencies (cm-1):
Bond
observed
n in cm-1
type of vibration
Exceptions and things to watch
Scale on bottom summarizes band positions and strengths
Strong - Medium - Weak -

46. Group Frequencies and Analysis The Hydrocarbons Alkanes
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkanes
General – due to the small electronegativity difference between C and H, hydrocarbon bands are of medium intensity at best and give simple spectra
Group Frequencies (cm-1):
C-H
Stretch
Strained ring systems may have higher n
-CH2-
~1465
Methylene bend (scissor)
-CH3
~1375
Methyl bend (sym)
-(CH2)4-
~720
Rocking motion 4 or more –CH2- (long chain band)
C-C
Not interpretively useful, small weak peaks

47. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkanes – Dodecane – C12H26

48. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkanes – Cyclopentane – C5H10

49. Group Frequencies and Analysis The Hydrocarbons Alkanes
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkanes
Additional – If the region is free of interference, the presence of certain alkyl groups can be discerned:
Methylene Methyl H C H C H C
Scissor
1465
Bendasymm
1450
Bendsymm
1375
usually overlap
gem-dimethyl
1380
1370
t -butyl
1390
1370

50. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkanes
Additional – Example: Compare 2,2-dimethylpentane vs. 2-methylhexane:
vs.

51. Group Frequencies and Analysis The Hydrocarbons Alkenes
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes
General – slightly more complex than alkanes; asymmetric C=C is observed as well as the sp2-C-H stretch. Still, bands are weak to medium in intensity
Group Frequencies (cm-1):
=C-H
Stretch
- Diagnostic for unsaturation- may be aromatic as well
Out-of-plane (oop) bend
- Can be used to determine degree of substitution
C=C
- Can be reduced by resonance
- Symmetrical C=C do not absorb
- trans- weaker than cis-

52. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes – 1-octene – C8H16
Note – you still have alkane present!

53. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes – trans-4-octene – C8H16
Note – absence of C=C band, shouldering of C-H band

54. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes – cis-2-pentene – C5H10
Note – shouldering of C-H band

55. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes – cyclopentene – C5H8
Note – increased complexity due to ring vibrations

56. Group Frequencies and Analysis The Hydrocarbons Alkenes
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes
Substitution – The out of plane =C-H bend produces strong bands but interference can come from aromatic rings (similar oop) and C-Cl bonds (~700)
monosubstituted
cis-1,2
trans-1,2
1,1-disubstitued
trisubstituted
tetrasubstituted
1000
900
800
700
none, with weak C=C

57. overtone usually observed
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes
Substitution – The monosubstitued band is very reliable; and the variance induced by electronic effects is observed
monosubstituted (R-)
monosubstitued
w/lone pair group
(ex. –Cl, -F, -OR)
w/conj. group
(ex. C=O, CN)
The shifts are similar for 1,1-disubstitued systems
1000
900
800
700
overtone usually observed

58. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes
Rings – Incorporation of a double bond endocyclic or exocyclic to a ring may shift the observed band
Endocyclic: Ring strain shifts the C=C band to lower n (ex. cyclopropene)
The adjacent C-C bond couples with the C=C system – if the resulting component vector is along the line of the C=C bond an increase in n occurs – this reaches a minima at 90o for cyclobutene (no net component along C=C bond) and rises again with cyclopropene
nC=C 1650
1646
1611
1566
1656

59. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes
Rings –
Endocyclic: If C=C at a ring fusion, absorption is reduced as if one further carbon was removed from the ring:
The presence of additional alkyl groups on the ring dramatically raises nC=C
nC=C 1611
nC=C 1656
1788
1883
nC=C 1566
1641
1675

60. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkenes
Rings –
Exocyclic: these C=C bonds give an increase in absorption n with decreasing ring size:
As the angle between the two C-C bonds is reduced – more p character is required (sp = 180°, sp2 = 120°, sp3 = 109.5°, “sp>3” = <109°
The p character of the double bond is reduced, but the stronger s bond is strengthened to a greater degree
Think of the allene example (“2-membered ring”) as an extreme example
nC=C

61. Group Frequencies and Analysis The Hydrocarbons Alkynes
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkynes
General – can be symmetric, psuedo-symmetric or internal – greatly reducing the number of observed bands
Group Frequencies (cm-1):
C-H
~3300
Stretch
- Diagnostic for terminal alkyne
CC
~2150
- Can be reduced by resonance
Symmetrical and psuedo-sym. CC do not absorb
Bend
(Text does not list)
Possible not to observe any bands for the CC system

62. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkynes – 1-hexyne – C6H10
Nice terminal, asymmetric, well behaved alkyne

63. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkynes – 3-hexyne – C6H10
A not-so-nice, internal, symmetrical alkyne

64. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Alkynes – 1-hexyne – C6H10
Nice terminal, asymmetric, well behaved alkyne

65. Group Frequencies and Analysis The Hydrocarbons
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings
General – not true alkenes; most of the small bands associated with them are not of diagnostic value; electronic effects of a single group on the ring can change the observed bands drastically
Group Frequencies (cm-1):
-C-H
Stretch
Also for alkenes
C-H
Out of plane (oop) bend
Can be used to determine substitution pattern
Overtone and combination bands
If observed, similar too oop
=C-H
Ring stretch – observed as two doublets (1600, 1580, 1500 & 1450)
Greatly dependent on substituents

66. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings – toluene – C7H8
Typical mono-substituted (EDG) ring

67. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings – o-xylene – C8H10
Typical ortho-substituted (EDG) ring

68. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings – m-xylene – C8H10
Typical meta-substituted (EDG) ring

69. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings – p-xylene – C8H10
Typical para-substituted (EDG) ring

70. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings – a-methylstyrene – C9H10
Conjugated mono-substituted ring

71. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings
Substitution – The aromatic out of plane =C-H bend produces strong bands but interference can come from alkenes (similar oop) and C-Cl (~700)
Consider this region to only be reliable for alkyl-, alkoxy-, halo-, amino-, and acetyl substituted rings
Interpretation is often unreliable for nitro-, carboxylic- and sulfonic groups
The overtone of these bands is the dominant source of the combination and overtone bands observed at

72. Group Frequencies and Analysis The Hydrocarbons
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings
Substitution –
900
800
700
600
mono
ortho
meta
para
1,2,4
1,2,3
1,3,5

73. Group Frequencies and Analysis The Hydrocarbons
IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Mononuclear aromatic rings
Substitution – The aromatic combination and overtone bands are a set of weak absorptions that occur from This is often obscured by C=O
mono
The general shape of the pattern is used for determining substitution pattern; typically only a neat liquid sample gives an intense enough set of bands for analysis
ortho
meta
para
1,2,3
1,3,5
1,2,4

74. IR Spectroscopy
Group Frequencies and Analysis
The Hydrocarbons
Polynuclear and Hetero- aromatic rings
General – All bands for these aromatic systems are similar to the mononuclear systems; shifts should be assumed, and analysis would be case-by-case

75. Group Frequencies and Analysis
IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Alcohols
General – the best recognized group on carefully selected spectra, but H-bonding effects can drastically change the position, intensity and shape of the O-H band
Group Frequencies (cm-1):
O-H (free)
Stretch
Seen in dilute solution or gas phase spectra
O-H
(H-bond)
The “classic” H-bonded band, seen in addition to the free band in solution
C-O-H
Bend
Often obscured by -CH3 bend
C-O
Can be used to determine 1o, 2o, 3o or phenolic structure

76. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Alcohols – 1-octanol
Neat liquid sample gives classic spectrum

77. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Alcohols – 1-octanol
Same sample in dilute CCl4 solution (solvent bands deleted for clarity)

78. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Phenols – p-cresol
Presence of aromatic bands, sharper -OH

79. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Alcohols –
Substitution – Using the position of the C-O stretching band, it is possible to suggest a 1o, 2o, 3o or phenolic structure to the alcohol; but these should be considered as base values, that may be changed by the effects of conjugation or an adjacent ring system
base value
phenol 1220
tertiary 1150
secondary 1100
primary 1050
nC-O 1070
nC-O 1017
nC-O 1070
nC-O 1060
nC-O 1030

80. Group Frequencies and Analysis
IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Ethers
General – like alkynes, the simplicity of the spectra may allow them to pass unnoticed – deduce from molecular formula if one should be present
Group Frequencies (cm-1):
C-O
Stretch (asymm.)
Absence of C=O and O-H will confirm it is not ester or alcohol
Simple alkyl ethers usually one band at 1120, aryl alkyl ethers give two bands – 1250 & 1040

81. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Ethers – diispropyl ether
Spectrum dominated by all other functionality

82. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Ethers –
Additional Types
Aryl and vinyl ethers – The effect of conjugation gives the C-O bond a small amount of double bond character, raising the observed n
Furthermore, strongly asymmetric systems (aryl alkyl and vinyl alkyl ethers) may show an additional weak C-O band for the symmetric stretch at 1040 and 850 respectively

83. IR Spectroscopy
Group Frequencies and Analysis
sp3 Oxygen – Alcohols, phenols and ethers
Ethers –
Additional Types
Epoxides – Most important bands are the ring deformation bands at nasym and nsym
Weaker “breathing mode” band is present at
Acetals and Ketals – Give four or five unresolved bands in the region

84. Group Frequencies and Analysis sp3 Nitrogen – Amines
IR Spectroscopy
Group Frequencies and Analysis
sp3 Nitrogen – Amines
Amines – Once presence is determined, the substitution at nitrogen is easy to determine; only the 3° amine may present a problem
Group Frequencies (cm-1):
N-H
(-NH2)
(2 bands)
Stretch (sym. and asym.)
Bend
(-NHR)
(1 band)
1500
Stretch
For alkyl amines, very weak – for aromatic 2° amines, stronger
~800
Oop bend
N-N
Remember 3° amines have no N-H bands

85. IR Spectroscopy
Group Frequencies and Analysis
sp3 Nitrogen – Amines
1° Amine – tert-butylamine
Two band –NH2 peak appears as small “w”

86. IR Spectroscopy
Group Frequencies and Analysis
sp3 Nitrogen – Amines
2° Amine – dibutylamine
Note weakness of –NH- band (can be mistaken as C=O overtone, if carbonyl is present)

87. IR Spectroscopy
Group Frequencies and Analysis
sp3 Nitrogen – Amines
3° Amine – tributylamine
Difficult to discern from alkane – molecular formula for confirmation almost requisite

88. Group Frequencies and Analysis sp3 Nitrogen – Amines Ammonium Salts
IR Spectroscopy
Group Frequencies and Analysis
sp3 Nitrogen – Amines
Ammonium Salts
Almost certainly never encountered in neat samples, but an important component of amino acids and many pharmaceuticals
Group Frequencies
N-H
Stretch
1° salts are at the higher n end of this band, 3° salts at the lower end
Additional band sometimes obs. at 2100
Bend
1° as two bands (sym. And asymm.), 2° at the upper end of this range, 3° absorbs weakly

89. IR Spectroscopy
Group Frequencies and Analysis
sp3 Nitrogen – Amines
Ammonium Salts – anilinium hydrochloride
Spectrum is of a KBr disc sample:

90. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
General – Along with alcohols, the most ubiquitous group on the IR spectrum. Although it is easy to determine if the C=O is present, deducing the exact functionality and factors that influence the position of the band provide the challenge
Base C=O Frequencies (cm-1):
C=O
1810
Stretch (sym.)
Anhydride band 1
1800
Acid Chloride
1760
Anhydride band 2
1735
Ester
1725
Aldehyde
1715
Ketone
1710
Carboxylic Acid
1690
Amide

91. C N O F S Cl IR Spectroscopy Group Frequencies and Analysis Carbonyls
General – The carbonyl C=O frequency is very sensitive to the effects we went over previously – a quick recap
Electronic Effects: Inductive vs. Resonance:
On first inspection, the ester, amide and acid halide/anhydride all possess lone pairs of electrons that can resonate with the C=O (which should lower n) C N O F S Cl

92. C N O F S Cl IR Spectroscopy Group Frequencies and Analysis Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
In the case of an oxygen or chlorine being adjacent to the carbonyl, each of these atoms resist the positive charge in the contributing resonance structure, and the inductive effect becomes a stronger factor C N O F S Cl
This inductive effect draws in s electrons from the C=O, which strengthens the p bond – these carbonyls appear at higher n

93. C N O F S Cl IR Spectroscopy Group Frequencies and Analysis Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
In the case of nitrogen, it is less electronegative than oxygen and has a greater acceptance of the positive charge in the contributing resonance structure, so the carbonyl is lowered in n C N O F S Cl
The inductive effect of nitrogen compared to an sp2 carbon is negligible by comparison

94. C N O F S Cl IR Spectroscopy Group Frequencies and Analysis Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
Likewise in aldehydes and ketones there is the inductive donation of electrons to the s bond of the carbonyl which slightly weakens and reduces the n of the p bond (and explains the small difference between aldehydes and ketones) C N O F S Cl
The inductive effect of nitrogen compared to an sp2 carbon is negligible by comparison

95. C N O F S Cl IR Spectroscopy Group Frequencies and Analysis Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
In addition, we discussed this effect in regards to a-halogenated carbonyls as one of the effects that can change group n C N O F S Cl
The inductive effect of chlorine will draw s electrons through a-carbon, weakening the C=O s and strengthening the p

96. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
General – Electronic Effects - Resonance:
Not only is the C=O n lowered by the effects of conjugation, the peak may also be broadened or split by the contribution of the two electronic conformers
The s-cis absorbs at higher n than the s-trans. Why?

97. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
General – Ring Strain Effects:
C=O groups that can be incorporated into a ring are sensitive to this effect. As ring size decreases more p-character must be used to make the single bonds take on the smaller angle (re: sp>3 = <109°). The p component of the C=O is weakened, but the s-bond strengthened, raising the overall n
Cyclic ketones, esters (lactones), amides (lactams) and anhydrides exhibit this behavior
To clear up confusion – there are two ways to strengthen the C=O
Remove s bond character – p bond becomes more stronger (better overlap) – this is a result inductive w/d
Remove p bond character – s bond becomes stronger – ring constraint

98. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
General – H-bonding effects:
C=O groups are reduced in n if some of the electron density is tapped off to form H-bonds:
This effect can be inter- or intra-molecular:

99. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon
Group Frequencies (cm-1):
C=O
1715
Stretch (sym.)
n Base, sensitive to change
conj. w/C=C
nC=C reduced to
conj. w/Ph
nring
Decreased ring size raises n
Bend

100. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Ketones – 2-hexanone
Typical aliphatic ketone

101. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Ketones – 4-methylacetophenone
Typical aromatic ketone,

102. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon
Group Frequencies (cm-1):
C=O
1715
Stretch (sym.)
n Base, sensitive to change
conj. w/C=C
nC=C reduced to
conj. w/Ph
nring
Decreased ring size raises n
Bend

103. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Aldehydes – Presence of the unique carbonyl C-H bond differentiates this group from ketones
Group Frequencies (cm-1):
C=O
1725
Stretch (sym.)
n Base, sensitive to change
conj. w/C=C
nC=C reduced to 1640
conj. w/Ph
nring
2820, 2720
Stretch
Fermi doublet; Higher n band often obscured by sp3 C-H

104. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Aldehydes – isovaleraldehyde
Typical aliphatic aldehyde – note appearance of Fermi doublet and C=O overtone

105. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Aldehydes – anisaldehyde
Typical aromatic aldehyde, note how C=O obscures the combination and overtone region – oop region would be used to determine substitution

106. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Carboxylic Acids – Various H-bonding effects lead to messy spectra, especially in the upper frequency ranges – be aware of the effects of monomeric, dimeric and oligomeric species on the spectrum
Group Frequencies (cm-1):
C=O
1710
Stretch (sym.)
n Base, sensitive to change; conjugation gives reduced n
C-O
Stretch
O-H
Overlaps C-H region in most cases; multiple “sub-peaks” can be seen for the dimeric and oligomeric species – simplified in non-polar solution or gas phase spectra

107. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Carboxylic Acids – propionic acid
Aliphatic carboxylic acid – neat sample
vs. CCl4 solution (right)

108. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Carboxylic Acids – o-toluic acid
Aromatic carboxylic acid, larger non-polar “end” of the molecule cuts down on the hydrogen bonding seen with the smaller, previous propionic acid

109. Group Frequencies and Analysis Carbonyls Carboxylic Acids - Salts
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Carboxylic Acids - Salts
Salts are expressed as possesing one single and one double bond – the true picture is one that is isoelectronic with the nitro group, with two bonds to oxygen with a bond order of 1.5
Group Frequencies:
1600
1400
Stretch (asymm.)
Stretch (sym.)

110. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Carboxylic Acids - Salts – ammonium benzoate
Here is an example of ammonium and carboxylate moieties:

111. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Carboxylic Acids – Amino Acids – L-alanine
Amino Acids combine the features of carboxylate and ammonium salts:

112. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Esters – Ester oxygen has an electron withdrawing effect that tends to draw in electrons within the C=O system, strengthening it compared to other carbonyls
Group Frequencies (cm-1):
C=O
1735
Stretch (sym.)
n Base, sensitive to change
conj. C=C
nC=C reduced to
w/Ph
nring
conj. of sp3 O
nC=O increases with smaller ring
C-O
Stretch, 2 bands

113. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Esters – methyl butyrate
Simple aliphatic ester

114. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Esters – methyl m-bromobenzoate
Conjugation on the carbonyl end:

115. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Esters – phenyl acetate
Conjugation on the sp3 oxygen end:

116. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Amides – Amide nitrogen acts as a conjugating group with C=O, reducing double bond character; amide nitrogen appears similar to amime, including the effects of substitution
Group Frequencies (cm-1):
C=O
1685
Stretch (sym.)
n Base, sensitive to change
Can be as low as 1630 w/conj.
N-H
~3300
Stretch
Similar to amines, but typically more intense
nC=O increases with smaller ring
Bend
~800
oop bend

117. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Amides – pivalamide
Primary aliphatic amide

118. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Amides – 2-pyrrolidone
Cyclic secondary amide - lactam

119. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Anhydrides – With acid halides, typically the highest n C=O; appears as two bands for the symmetric and asymmetric stretching modes
Group Frequencies (cm-1):
C=O
Stretch (asym.)
n Base, sensitive to change
conj. C=C
Stretch (sym.)
Two bands of variable relative intensity
nC=O increases with smaller ring
C-O
Stretch, multiple bands

120. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Anhydrides – iso-butyric anhydride
Typical anhydride

121. Group Frequencies and Analysis Carbonyls
IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Acid Halides – Acid bromides and iodides are not often encountered; acid chlorides are the most prevalent (and useful)
Group Frequencies (cm-1):
C=O
Stretch (sym.)
n Base, sensitive to change
conj. w/Ph
add. band
Fermi resonance with combination and overtone region of aromatic ring
C-Cl
Stretch
If below 600, not observed using NaCl windows
C-Br
Typically too low to obs.
C-I

122. IR Spectroscopy
Group Frequencies and Analysis
Carbonyls
Acid Halides – propionyl chloride
Overtones of low n peaks can confuse some spectra

123. Group Frequencies and Analysis sp2 and sp Nitrogen compounds
IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Nitriles – The “other” triple bond group observed in IR, due to the higher dipole change during the stretching vibration, this band is more intense than CCs.
Group Frequencies (cm-1):
CN
2250
Stretch (sym.)
n sensitive to change from conjugation; usually stronger than CC

124. IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds Carbonyls
Nitriles – benzonitrile

125. Group Frequencies and Analysis sp2 and sp Nitrogen compounds
IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Imines and Oximes – Often referred to as “derivatives” of carbonyl compounds, these groups are not often encountered in routine IR obs.
Group Frequencies (cm-1):
C=N (imine and oxime)
Stretch (sym.)
n sensitive to change from conjugation; usually stronger than CC
O-H
(oxime)
Stretch
H-bond effects
N-O

126. IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Imines and Oximes – acetone oxime

127. Group Frequencies and Analysis sp2 and sp Nitrogen compounds
IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Isocyanates and Isothiocyanates – Reactive groups, not often observed in routine qualitative IR
Group Frequencies (cm-1):
N=C=O
~2270
Stretch (sym.)
broad n band – coupled vibration
N=C=S
~2125
Stretch (1 or 2 bands)

128. IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Isocyanates and Isothiocyanates– tert-butyl isocyanate

129. Group Frequencies and Analysis sp2 and sp Nitrogen compounds
IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Nitro – Useful, easily incorporated group on aromatic rings, less often encountered on alkyl compounds
Group Frequencies (cm-1):
Stretch (asymm.)
Stretch (sym.)
Aliphatic nitro
Aromatic nitro

130. IR Spectroscopy
Group Frequencies and Analysis
sp2 and sp Nitrogen compounds
Nitro – o-nitrotoluene

131. Group Frequencies and Analysis Sulfur
IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Thiols and Sulfides – Sulfur, due to its large size shifts most observed IR bands to lower frequencies – often out of the observed region
Group Frequencies (cm-1):
S-H
(thiol)
2550
Stretch (sym.)
Unique region of IR spectrum
C-S-C
No useful information

132. IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Thiol (Mercaptan) – 1,2-ethanethiol

133. Group Frequencies and Analysis Sulfur
IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Sulfoxides and Sulfones – Oxidized sulfur, the SO bonds are useful for determining oxidation state, if the presence of sulfur is known
Group Frequencies (cm-1):
SO
1050
Stretch (sym.)
OSO
~1375
~1150
Stretch (asymm.)

134. IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Sulfoxide (Mercaptan) – di-butyl sulfoxide
Be wary of water in the spectrum of sulfoxides

135. IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Sulfone– di-butyl sulfone

136. Group Frequencies and Analysis Sulfur
IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Sulfonic Acids, Sulfonamides and Sulfonates – Sulfur equivalent of the carboxylic acid derivatives; the O or N groups act as we have observed
Group Frequencies (cm-1):
OSO
~1375
~1150
Stretch (asymm.)
Stretch (sym.)
As for sulfones – groups bound to sulfur identify the group, just as with the carboxylic acid derivatives differ from ketones
S-O
(acid & sulfonate)
Stretch
May appear as several bands
O-H & N-H
As for the carboxylic acid derivatives

137. IR Spectroscopy
Group Frequencies and Analysis
Sulfur
Sulfonamides – p-toluenesulfonamide

138. Group Frequencies and Analysis Phosphorus
IR Spectroscopy
Group Frequencies and Analysis
Phosphorus
Phosphines – Phosphorus in its lowest oxidation state – many bands that overlap with other useful regions; exercise caution in interpretation using IR
Group Frequencies (cm-1):
P-H
Stretch (sym.)
Bend
PH2
Bend, two bands
P-CH3
P-CH2-

139. IR Spectroscopy
Group Frequencies and Analysis
Phosphorus
Phosphines – tri-butylphosphine

140. Group Frequencies and Analysis Phosphorus
IR Spectroscopy
Group Frequencies and Analysis
Phosphorus
Phosphine Oxides – More common to observe these phosphorus compounds
Group Frequencies (cm-1):
Phosphate Esters, Acids and Amides – Often encountered in biological systems
PO
Stretch (sym.)
PO
Stretch (sym.)
R-O
Stretch, 1 or 2 band
P-O
Stretch

141. IR Spectroscopy
Group Frequencies and Analysis
Phosphorus
Phosphine Oxides, Phosphate Esters – tri-butylphosphate

142. Group Frequencies and Analysis Halogens
IR Spectroscopy
Group Frequencies and Analysis
Halogens
Fluorides and Chlorides – Smaller halogen bonds to carbon in observed frequency range
Group Frequencies (cm-1):
Bromides and Iodides – Not often obs. Due to low n of C-X stretch
C-F
Stretch (sym.)
Monofluoroalkyl at lower n
Polyfluoroalkyl at upper n
Aryl fluorides up to 1450
C-Cl
Stretch
Different conformers may give split peaks
C-Br
Stretch
Bend is obs. at ~1200
C-I
Bend is obs. at ~1150

143. hn Energy hn hn hn IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
All spectrometers consist of four basic parts that are coupled with all four parts of the spectroscopic process - irradiation, absorption-excitation, re-emission-relaxation and detection.
excited state
Absorption-Excitation: Spectrometer needs to contain the sample
Detection-reemission : Spectrometer needs to detect the photons emitted by the sample and ascertain their energy hn
Relaxation
Energy
rest state
rest state
Irradiation: Spectrometer needs to generate photons hn hn hn

144. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
Those four parts are:
Source/Monochromator
Sample cell
Detector/Amplifier
Output
Dispersive IR spectrometers were the first IR instruments, however their simplicity and longevity allows them to continue in service – for most routine organic analyses their speed and resolution is adequate
For the most part, their design is austere and relies on simple mechanics and optics to generate a spectrum, very similar to simply rotating a glass prism to see different bands of visible light

145. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
Here is a general schematic:

146. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
Source is a heated nichrome wire which produces a broad band continuum of IR light (as heat)
The beam is directed through both the sample and a reference cell
A rapidly rotating sector (beam chopper) continuously switches between directing the two beams to a diffraction grating

147. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
The diffraction grating slowly rotates, such that only one narrow frequency band of IR light is at the proper angle to reach the detector
A simple circuit compares the light from the sample and reference and sends the difference to a chart recorder

148. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
On the older instruments the motor in the chart recorder was synchronized (& calibrated) to the motor on the diffraction grating
Because each spectrum is the result of the tabulation of the spectroscopic process at each frequency individually, it is said to record the spectrum in the frequency domain

149. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Dispersive IR Spectrometers
Advantages – simple, easy to maintain – last the life of the source and moving parts
Disadvantages – to cover the entire IR band of interest to chemists it is necessary to use two diffraction gratings
At high q, the component frequencies are more spread out, so the resulting spectra appear to have various regions expanded or compressed
The limit to resolution is 2-4 cm-1

150. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Fourier Transform IR Spectrometers
FT-IR is the modern state of the art for IR spectroscopy
The system is based on the Michelson interferometer
Laser source IR light is separated by a beam splitter, one component going to a fixed mirror, the other to a moving one and are reflected back to the beam splitter
The beam splitter recombines the two to a pattern of constructive and destructive interferences known as an interferogram – a complex signal, but contains all of the frequencies that make up the IR spectrum

151. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Fourier Transform IR Spectrometers
The resulting signal is essentially a plot of intensity vs. time
Such information if plotted would look like the following:
This is meaningless to a chemist – we need this to be in the frequency domain rather than time….

152. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Fourier Transform IR Spectrometers
By applying a mathematical transform on the signal – a Fourier transform – the resulting frequency domain spectrum can be observed
FT-IRs give three theoretical advantages:
Fellgett’s advantage – every point in the interferogram is information – all wavelenghts are represented
Jacquinot’s advantage – the entire energy of the source is used – increasing signal-to-noise
Conne’s advantage – frequency precision – Dispersive instruments can have errors in the ability to move slits and gratings reproducibly – FTIR is internally referenced from its own beam

153. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Dispersive and Fourier Transform
Fourier Transform IR Spectrometers
Justik’s advantage – does it give me what I need
Single-beam instrument – collect a background (air has IR active molecules!)
Fast – all frequencies are scanned simultaneously
No referencing!
Computer based – scaling and editing of the spectrum to squeeze out the most data; spectra are proportional (no stretching or squeezing of regions), comparison with spectral libraries
Disadvantages – expenisve relative to dispersive instruments, and the components take more expertise and service calls to replace

154. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Sample size – typically the size of the beam – mm’s  mg’s
Non-destructive – sample can be recovered with varying degrees of difficulty
Liquid samples – the easiest IR spectra are those of “neat” liquid samples
Solid samples are too dense for good IR spectra – inter-molecular coupling of vibrational states occurs and peaks are greatly broadened
In the liquid state full 3-D motion is available, and these effects are averaged out and diminished
The thickness of a sample can be decreased to reduce these effects further
 Thin film liquid samples are best!

155. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Liquid samples
Sample cell cannot possess covalent bonds (SiO2, or glass is out)
The most common cell is a pair of large transparent “windows” of inorganic salts
Most common:
NaCl – cheap, transparent from 650 – 4000 cm-1, but fragile
Less common – AgCl, KBr, etc. – if you need transparency below 650, limit is practically 400

156. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Solution samples
One way solids can be handled is as a solution
Key is that the solvent picked will cover the least amount of the spectrum as possible, as it will also be present
Common solvents typically are symmetrical, or have many halogenated bonds – low cm-1: CCl4, CHCl3, CH2Cl2, etc.
The cell in this case is two NaCl (or other) windows with a spacer, the sample is loaded via a syringe into the cell:

157. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Solution samples
A newer method involves the use of a polyethylene matrix, that will hold allow a solution sample to evaporate, leaving small portions of the sample embedded in the matrix
The samples are “liquid-like”
The only interference is that of hydrocarbon

158. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Solid Samples
The most common treatment for solid samples is to “mull” them with thick mineral oil (high MW hydrocarbon) - Nujol®
Just like with the polyethylene cards, the molecules of the sample are held in suspension within the oil matrix
Again, the interference is that of hydrocarbon

159. IR Spectroscopy
Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Solid Samples
The connoisseurs method (with no organic interference) is to press the solid with KBr into a pellet
Under high pressure the KBr liquefies and entraps individual molecules of the sample in the matrix
These spectra are the only spectra of solids that are as interference free as liquids

160. Neat Sample IR Spectroscopy Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Differences in Spectral Appearence
Compare the following three IR spectra of p-cresol
Neat Sample

161. KBr Pellet IR Spectroscopy Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Differences in Spectral Appearence
Compare the following three IR spectra of p-cresol
KBr Pellet

162. CCl4 Solution IR Spectroscopy Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Differences in Spectral Appearence
Compare the following three IR spectra of p-cresol
CCl4 Solution

163. Nujol Mull IR Spectroscopy Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Differences in Spectral Appearence
Compare the following three IR spectra of m-nitroanisole
Nujol Mull

164. KBr Pellet IR Spectroscopy Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Differences in Spectral Appearence
Compare the following three IR spectra of m-nitroanisole
KBr Pellet

165. CCl4 Solution IR Spectroscopy Instrumentation and Experimental Aspects
The IR Spectrometer – Experimental aspects
Differences in Spectral Appearence
Compare the following three IR spectra of m-nitroanisole
CCl4 Solution