1 CHEM 430 IR SPECTROSCOPY. I NTRODUCTION The method provides a rapid and simple method for observing the functional group species present in an organic

1FALL 2011

CHEM 430

IR SPECTROSCOPY

Long

LecturePlayDr. Justik

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

Infrared and Raman Spectroscopy 11-1

CHEM 430 – NMR Spectroscopy

2

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.



3

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:



4


The IR Spectroscopic Process.• There are two types of bond vibration:

1. Stretch – Vibration or oscillation along the line of the bond

2. Bend – Vibration or oscillation not along the line of the bond


55

H

H

C

H

H

C

scissor

asymmetric

H

H

CCH

H

CC

H

HCC

H

HCC

symmetric

rock twist wagin plane out of plane


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 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



6


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



77


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 energyis quicker to understand than

Short wavelengths equate higher energy



88


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: 400-4000 cm-1

• The peaks are Gaussian distributions of the average energy of a transition



99


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)



1010

Pote

nti

al En

erg

y

(E)

Interatomic Distance (y)

Remember: E = ½ ky2

where: y is spring displacementk is spring constant


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.



1111

10 pm

154 pm

4o 10 pm


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



1212

Pote

nti

al En

erg

y

(E)

Interatomic Distance (r)

rotational transitions – (in microwave region)

Vibrational transitions, n


The IR Spectroscopic Process.• However, the application of the classical vibrational model fails apart for

two reasons:1. As two nuclei approach one another through bond vibration,

potential energy increases to infinity, as two positive centers begin to repel one another

2. 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



1313


The IR Spectroscopic Process.• Here is the derivation of Hooke’s Law we will apply for IR theory:

• Vibrational frequency given by:

n : frequencyK: force constant – bond strengthm: reduced mass = m1m2/(m1+m2)

• Reduced mass is used, as each atom in the covalent bond oscillates about the center of the two masses



1414


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 1200 cm-1 m = (12 x 12)/(12 + 12) = 6(0.41)

C─H 3000 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)



1515



When greater masses are added, the trend is similar (K’s 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)and a larger atom gives rise to lower wavenumbers (and n and E)



1616



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 ~2100200

C=C ~1650 146

C—C ~120083

Note the good correlation with the heats of formation for each bond!

Stronger bonds give higher wavenumbers (and higher n and E)



1717


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!!!



1818


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



1919


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



2020

21

II. Infrared Group AnalysisA. General

1. The primary use of the IR spectrometer is to detect functional groups 2. 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

3. 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

4. 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

Bonds to H

O-H single bondN-H single bondC-H single bond

Triple bonds

C≡CC≡N

Double bonds

C=OC=NC=C

Single Bonds

C-CC-NC-O

Fingerprint Region

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. IntroductionF. The IR Spectrum

4. 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

5. 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. IntroductionG. 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


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

1. Symmetry2. Mechanical Coupling3. Fermi Resonance4. Hydrogen Bonding5. Ring Strain6. Electronic Effects7. Constitutional Isomerism8. Stereoisomerism9. Conformational Isomerism10. Tautomerism (Dynamic Isomerism)

26

IR Spectroscopy


1. 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 CC bond. No IR band appears on the spectrum

C O

+ - vibrationC O

+ -

C C C Cvibration

27

IR Spectroscopy


1. 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

H3C C C CH3

H3C C CH2C CH3

C C

CH3

CH3

H3C

H3C

C C

H2C

CH3

H3C

H3C

CH3

symmetric

psuedo-symmetric

28

IR Spectroscopy


1. 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

H3C C C CH3

H3C C CH2C CH3

C C

CH3

CH3

H3C

H3C

C C

H2C

CH3

H3C

H3C

CH3

symmetric

psuedo-symmetric

29

IR Spectroscopy


2. 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


2. 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

C C C

H

HH

H

31

IR Spectroscopy


3. 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


3. 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

C

Cl

O

33

IR Spectroscopy


3. 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)

C

Cl

O

34

IR Spectroscopy


3. 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

C

H

O

35

IR Spectroscopy


4. 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

O

H

O

O

CH3

1680 cm-1

O

O

CH3

1724 cm-1

36

IR Spectroscopy


4. 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

Neat liquid

Gas phase

37

IR Spectroscopy


4. 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

O

OH

Monomer

O

OH

O

OH Dimer

O

OH

O OH O O

HOligomer

38

IR Spectroscopy


5. 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:

1815 1775 1750 1715 1705 cm-1

• We will discuss the specific cases for these shifts during our coverage of group frequencies

O OO O O

39

IR Spectroscopy


6. 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

X

C

O

40

IR Spectroscopy


6. 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

O

O

1684 cm-1 1715 cm-1

C=O C=O

C CC

O

C CC

O

vs.

41

IR Spectroscopy


6. Electronic Effects - Resonance• In extended conjugated systems, some resonance

contributors are “out-of-sync” and do not resonate with a group

• Example:

H2N C CH3

O

Strong resonance contributor

vs. N

O

O

CCH3

O

Poor resonance contributor(cannot resonate with C=O)

CH3C

OX X = NH2 CH3 Cl NO2

1677 1687 1692 1700 cm-1

42

IR Spectroscopy


6. 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

O

C=O: 1686 cm-1

O

C=O: 1693 cm-1

CH3

43

IR Spectroscopy

III. Group Frequencies and AnalysisA. Introduction

1. 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)

2. 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-HN-HC-H

C≡CC≡N

C=OC=NC=C

C-CC-NC-OC-X

“Fingerprint Region”

4000 cm-1 2700 cm-1 2000 cm-1 1600 cm-1 400 cm-1

44

IR Spectroscopy

III. Group Frequencies and AnalysisA. Introduction

3. 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)

4. If a molecular formula is available, do an HDI!

5. Many texts list various methods for approaching an IR spectrum; use the method that works best for you and stick to it.

6. 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

IR Spectroscopy

III. Group Frequencies and AnalysisBefore 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):

Bondobserved

n in cm-1 type of vibration Exceptions and things to watch

Scale on bottom summarizes band positions and strengths Strong - Medium - Weak -

46

IR Spectroscopy

III. Group Frequencies and AnalysisB. 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


C-H 3000-2800

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


Alkanes – Dodecane – C12H26

48

IR Spectroscopy


Alkanes – Cyclopentane – C5H10

49

IR Spectroscopy


Alkanes Additional – If the 1400-1350 region is free of interference, the

presence of certain alkyl groups can be discerned:

H

H

CC

Methylene Methyl

Scissor1465

H

H

CC

H

Bendasymm

1450

H

H

CC

H

Bendsymm

1375

usually overlap 1380 1370C

CH3

CH3

gem-dimethyl

13701390C

CH3

CH3

CH3

t -butyl

50

IR Spectroscopy


Alkanes Additional – Example: Compare 2,2-dimethylpentane vs. 2-methylhexane:

vs.

51

IR Spectroscopy


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 3095-3010

Stretch - Diagnostic for unsaturation- may be aromatic as well

=C-H 1000-650

Out-of-plane (oop) bend

- Can be used to determine degree of substitution

C=C 1660-1600

Stretch - Can be reduced by resonance- Symmetrical C=C do not absorb- trans- weaker than cis-

52

IR Spectroscopy


Alkenes – 1-octene – C8H16

Note – you still have alkane present!

53

IR Spectroscopy


Alkenes – trans-4-octene – C8H16

Note – absence of C=C band, shouldering of C-H band

54

IR Spectroscopy


Alkenes – cis-2-pentene – C5H10

Note – shouldering of C-H band

55

IR Spectroscopy


Alkenes – cyclopentene – C5H8

Note – increased complexity due to ring vibrationsH H

56

IR Spectroscopy


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

R

RR

R

R

R

R

RR

R

R

R

R

R

1000 900 800 700

none, with weak C=C

57

IR Spectroscopy


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)

monosubstitued w/conj. group(ex. C=O, CN)

The shifts are similar for 1,1-disubstitued systems

R

1000 900 800 700

overtone usually observed

G

G

58

IR Spectroscopy


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

1646 1611 1566 1656nC=C 1650

59

IR Spectroscopy


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

RRR

R RR

nC=C 1656

nC=C 1566

1788 1883

1641 1675

60

IR Spectroscopy


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 1940 1780 1678 1657 1651

61

IR Spectroscopy


Alkynes General – can be symmetric, psuedo-symmetric or internal – greatly

reducing the number of observed bands


C-H ~3300 Stretch - Diagnostic for terminal alkyne

CC ~2150 Stretch - Can be reduced by resonance-Symmetrical and psuedo-sym. CC do not absorb

C-H 900-700 Bend (Text does not list)

Possible not to observe any bands for the CC system

62

IR Spectroscopy


Alkynes – 1-hexyne – C6H10

Nice terminal, asymmetric, well behaved alkyne

CHC

63

IR Spectroscopy



A not-so-nice, internal, symmetrical alkyne

CC

64

IR Spectroscopy



Nice terminal, asymmetric, well behaved alkyne

CHC

65

IR Spectroscopy


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


-C-H 3050-3010

Stretch Also for alkenes

C-H 900-690 Out of plane (oop) bend Can be used to determine substitution pattern

2000-1667

Overtone and combination bands

If observed, similar too oop

=C-H 1600-1400

Ring stretch – observed as two doublets (1600, 1580, 1500 & 1450)

Greatly dependent on substituents

66

IR Spectroscopy


Mononuclear aromatic rings – toluene – C7H8

Typical mono-substituted (EDG) ring

CH3

67

IR Spectroscopy


Mononuclear aromatic rings – o-xylene – C8H10

Typical ortho-substituted (EDG) ring

CH3

CH3

68

IR Spectroscopy


Mononuclear aromatic rings – m-xylene – C8H10

Typical meta-substituted (EDG) ring

CH3

CH3

69

IR Spectroscopy


Mononuclear aromatic rings – p-xylene – C8H10

Typical para-substituted (EDG) ring

CH3

CH3

70

IR Spectroscopy


Mononuclear aromatic rings – a-methylstyrene – C9H10

Conjugated mono-substituted ring

CCH3

H2C

71

IR Spectroscopy


Mononuclear aromatic ringsSubstitution – 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 2000-1667

72

IR Spectroscopy


Mononuclear aromatic ringsSubstitution –

900 800 700 600

mono

ortho

meta

para

1,2,4

1,2,3

1,3,5

73

IR Spectroscopy


Mononuclear aromatic ringsSubstitution – The aromatic combination and overtone bands are a

set of weak absorptions that occur from 2000-1667. This is often obscured by C=O

mono

ortho

meta

para

1,2,4

1,2,3

1,3,5

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

74

IR Spectroscopy


Polynuclear and Hetero- aromatic ringsGeneral – All bands for these aromatic systems are similar to the

mononuclear systems; shifts should be assumed, and analysis would be case-by-case

N

S

O

N

N

HS

75

IR Spectroscopy

III. Group Frequencies and AnalysisC. 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


O-H (free)

3650-3600

Stretch Seen in dilute solution or gas phase spectra

O-H (H-bond)

3400-3300

Stretch The “classic” H-bonded band, seen in addition to the free band in solution

C-O-H 1440-1220

Bend Often obscured by -CH3 bend

C-O 1260-1000

Stretch Can be used to determine 1o, 2o, 3o or phenolic structure

76

IR Spectroscopy


Alcohols – 1-octanol

Neat liquid sample gives classic spectrum

HO

77

IR Spectroscopy


Alcohols – 1-octanol

Same sample in dilute CCl4 solution (solvent bands deleted for clarity)

HO

78

IR Spectroscopy


Phenols – p-cresol

Presence of aromatic bands, sharper -OH

OH

79

IR Spectroscopy


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 valuephenol 1220

tertiary 1150

secondary 1100

primary1050

OH

OH

OH

OH

HC C

OHnC-O 1070

nC-O 1070

nC-O 1017

nC-O 1060

nC-O 1030

80

IR Spectroscopy


EthersGeneral – like alkynes, the simplicity of the spectra may allow them

to pass unnoticed – deduce from molecular formula if one should be present


C-O 1300-1000

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


Ethers – diispropyl ether

Spectrum dominated by all other functionality

O

82

IR Spectroscopy


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

H2C CH

O R H2C CH

O R

83

IR Spectroscopy


Ethers –Additional Types Epoxides – Most important bands are the ring deformation bands at

nasym 950-815 and nsym 880-750

Weaker “breathing mode” band is present at 1280-1230

Acetals and Ketals – Give four or five unresolved bands in the 1200-1020 region

O

R

O

R

O R

R

R

O

R

O R

H

84

IR Spectroscopy

III. Group Frequencies and AnalysisD. sp3 Nitrogen – Amines

Amines – Once presence is determined, the substitution at nitrogen is easy to determine; only the 3° amine may present a problem


N-H (-NH2)

3650-3600(2 bands)1640-1560

Stretch (sym. and asym.)

Bend

N-H(-NHR)

3400-3300(1 band)1500

Stretch

Bend

For alkyl amines, very weak – for aromatic 2° amines, stronger

N-H ~800 Oop bend

N-N 1350-1000

Stretch Remember 3° amines have no N-H bands

85

IR Spectroscopy


1° Amine – tert-butylamine

Two band –NH2 peak appears as small “w”NH2

86

IR Spectroscopy


2° Amine – dibutylamine

Note weakness of –NH- band (can be mistaken as C=O overtone, if carbonyl is present)

HN

87

IR Spectroscopy


3° Amine – tributylamine

Difficult to discern from alkane – molecular formula for confirmation almost requisite

N

88

IR Spectroscopy


Ammonium Salts

Almost certainly never encountered in neat samples, but an important component of amino acids and many pharmaceuticals

Group FrequenciesN-H 3300-

2600Stretch 1° salts are at the higher n

end of this band, 3° salts at the lower endAdditional band sometimes obs. at 2100

N-H 1600-1500

Bend 1° as two bands (sym. And asymm.), 2° at the upper end of this range, 3° absorbs weakly

89

IR Spectroscopy


Ammonium Salts – anilinium hydrochloride

Spectrum is of a KBr disc sample:

NH H

H

Cl

90

IR Spectroscopy

III. Group Frequencies and AnalysisE. 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

IR Spectroscopy


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)

O

O

O

Cl

O

NH2

O

R

C N O F

ClS

92

IR Spectroscopy


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

O

OO

O

O

O

C N O F

ClS

This inductive effect draws in s electrons from the C=O, which strengthens the p bond – these carbonyls appear at higher n

93

IR Spectroscopy



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

O

NHN

H

O

O

NH

C N O F

ClS

The inductive effect of nitrogen compared to an sp2 carbon is negligible by comparison

94

IR Spectroscopy



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)

O

C N O F

ClS

The inductive effect of nitrogen compared to an sp2 carbon is negligible by comparison

95

IR Spectroscopy



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

ClS

The inductive effect of chlorine will draw s electrons through a-carbon, weakening the C=O s and strengthening the p

96

IR Spectroscopy


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?

C

C

C

O

R

s-cis

C

C

C

R

O

s-trans

97

IR Spectroscopy


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

1) Remove s bond character – p bond becomes more stronger (better overlap) – this is a result inductive w/d

2) Remove p bond character – s bond becomes stronger – ring constraint

O

O

O

NH

O

O

O

O

98

IR Spectroscopy


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

IR Spectroscopy


1. 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

1700-1675

nC=C reduced to 1644-1617

conj. w/Ph

1700-1680

nring 1600-1450

C=O 1815-1705

Decreased ring size raises n

1300-1100

BendC

C

C

O

100

IR Spectroscopy


1. Ketones – 2-hexanone

Typical aliphatic ketone

O

101

IR Spectroscopy


1. Ketones – 4-methylacetophenone

Typical aromatic ketone,

O

102

IR Spectroscopy


1. Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon


conj. w/C=C

1700-1675


conj. w/Ph

1700-1680

nring 1600-1450

C=O 1815-1705

Decreased ring size raises n

1300-1100

BendC

C

C

O

103

IR Spectroscopy


2. Aldehydes – Presence of the unique carbonyl C-H bond differentiates this group from ketones


C=O 1725 Stretch (sym.) n Base, sensitive to change

conj. w/C=C

1700-1680

nC=C reduced to 1640

conj. w/Ph

1700-1660

nring 1600-1450

2820, 2720

Stretch Fermi doublet; Higher nband often obscured by sp3 C-H

R

C

H

O

104

IR Spectroscopy


2. Aldehydes – isovaleraldehyde

Typical aliphatic aldehyde – note appearance of Fermi doublet and C=O overtone

H

C

O

105

IR Spectroscopy


2. Aldehydes – anisaldehyde

Typical aromatic aldehyde, note how C=O obscures the combination and overtone region – oop region would be used to determine substitution

H

O

O

106

IR Spectroscopy


3. 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 1320-1210

Stretch

O-H 3400-2400

Stretch 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


3. Carboxylic Acids – propionic acid

Aliphatic carboxylic acid – neat sample vs. CCl4 solution (right)

108

IR Spectroscopy


3. 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

HO

O

109

IR Spectroscopy


3. 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:

16001400

Stretch (asymm.)Stretch (sym.)

C

O

O

110

IR Spectroscopy


3. Carboxylic Acids - Salts – ammonium benzoate

Here is an example of ammonium and carboxylate moieties:

O

O

N

H

HH

H

111

IR Spectroscopy


3. Carboxylic Acids – Amino Acids – L-alanine

Amino Acids combine the features of carboxylate and ammonium salts:

H3N CH C

CH3

O

O

112

IR Spectroscopy


4. 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


C=O 1735 Stretch (sym.) n Base, sensitive to change

conj. C=C

1735-1715


w/Ph 1735-1715

nring 1600-1450

conj. of sp3 O

1765-1760

1850-1740

nC=O increases with smaller ring

C-O 1300-1000

Stretch, 2 bands

CO

O

113

IR Spectroscopy


4. Esters – methyl butyrate

Simple aliphatic ester

O

O

114

IR Spectroscopy


4. Esters – methyl m-bromobenzoate

Conjugation on the carbonyl end:

Br

O

O

115

IR Spectroscopy


4. Esters – phenyl acetate

Conjugation on the sp3 oxygen end:O

O

116

IR Spectroscopy


5. 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


Can be as low as 1630 w/conj.

N-H ~3300 Stretch Similar to amines, but typically more intense


N-H 1640-1550

Bend

N-H ~800 oop bend

CNH

O

117

IR Spectroscopy


5. Amides – pivalamide

Primary aliphatic amide

NH2

O

118

IR Spectroscopy


5. Amides – 2-pyrrolidone

Cyclic secondary amide - lactam

HN O

119

IR Spectroscopy


6. 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 1830-

1800Stretch (asym.) n Base, sensitive to change

conj. C=C

1778-1740

Stretch (sym.) Two bands of variable relative intensity


C-O 1300-900 Stretch, multiple bands

O OO

120

IR Spectroscopy


6. Anhydrides – iso-butyric anhydride

Typical anhydride

O

O O

121

IR Spectroscopy


7. Acid Halides – Acid bromides and iodides are not often encountered; acid chlorides are the most prevalent (and useful)


C=O 1810-1775

Stretch (sym.) n Base, sensitive to change

conj. w/Ph

add. band

Fermi resonance with combination and overtone region of aromatic ring

C-Cl 730-550 Stretch If below 600, not observed using NaCl windows

C-Br 650-510 Stretch Typically too low to obs.

C-I 600-485 Stretch Typically too low to obs.

122

IR Spectroscopy


7. Acid Halides – propionyl chloride

Overtones of low n peaks can confuse some spectra

123

IR Spectroscopy

III. Group Frequencies and AnalysisF. sp2 and sp Nitrogen compounds

1. 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 CCs.

Group Frequencies (cm-1):CN 2250 Stretch (sym.) n sensitive to change from

conjugation; usually stronger than CC

124

IR Spectroscopy

III. Group Frequencies and AnalysisF. sp2 and sp Nitrogen compounds Carbonyls

1. Nitriles – benzonitrile

125

IR Spectroscopy


2. 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)

1685-1650

Stretch (sym.) n sensitive to change from conjugation; usually stronger than CC

O-H(oxime)

3250-3150

Stretch H-bond effects

N-O(oxime)

965-930 Stretch

126

IR Spectroscopy


2. Imines and Oximes – acetone oxime

127

IR Spectroscopy


3. Isocyanates and Isothiocyanates – Reactive groups, not often observed in routine qualitative IR


N=C=O

~2270 Stretch (sym.) broad n band – coupled vibration

N=C=S

~2125 Stretch (1 or 2 bands) broad n band – coupled vibration

128

IR Spectroscopy


3. Isocyanates and Isothiocyanates– tert-butyl isocyanate

129

IR Spectroscopy


4. Nitro – Useful, easily incorporated group on aromatic rings, less often encountered on alkyl compounds


1600-15301390-1300


Aliphatic nitro

1550-14901355-1315


Aromatic nitro

R N

O

O

130

IR Spectroscopy


4. Nitro – o-nitrotoluene

131

IR Spectroscopy

III. Group Frequencies and AnalysisG. Sulfur

1. 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


1. Thiol (Mercaptan) – 1,2-ethanethiol

133

IR Spectroscopy


2. Sulfoxides and Sulfones – Oxidized sulfur, the SO bonds are useful for determining oxidation state, if the presence of sulfur is known

Group Frequencies (cm-1):SO 1050 Stretch (sym.)

OSO ~1375~1150


134

IR Spectroscopy


2. Sulfoxide (Mercaptan) – di-butyl sulfoxideBe wary of water in the spectrum of sulfoxides

135

IR Spectroscopy


2. Sulfone– di-butyl sulfone

136

IR Spectroscopy


3. Sulfonic Acids, Sulfonamides and Sulfonates – Sulfur equivalent of the carboxylic acid derivatives; the O or N groups act as we have observed


OSO ~1375~1150


As for sulfones – groups bound to sulfur identify the group, just as with the carboxylic acid derivatives differ from ketones

S-O(acid & sulfonate)

1000-650

Stretch May appear as several bands

O-H & N-H

As for the carboxylic acid derivatives

137

IR Spectroscopy


3. Sulfonamides – p-toluenesulfonamide

138

IR Spectroscopy

III. Group Frequencies and AnalysisH. Phosphorus

1. Phosphines – Phosphorus in its lowest oxidation state – many bands that overlap with other useful regions; exercise caution in interpretation using IR


P-H 2320-2270990-885

Stretch (sym.)Bend

PH2 1090-1075840-810

Bend, two bands

P-CH3 1450-13951350-1255

Bend, two bands

P-CH2- 1440-1400

Bend

139

IR Spectroscopy


1. Phosphines – tri-butylphosphine

140

IR Spectroscopy


2. Phosphine Oxides – More common to observe these phosphorus compounds


3. Phosphate Esters, Acids and Amides – Often encountered in biological systems


PO 1210-1140

Stretch (sym.)

PO 1300-1240

Stretch (sym.)

R-O 1088-920 Stretch, 1 or 2 band

P-O 845-725 Stretch

141

IR Spectroscopy


2. Phosphine Oxides, Phosphate Esters – tri-butylphosphate

142

IR Spectroscopy

III. Group Frequencies and AnalysisI. Halogens

1. Fluorides and Chlorides – Smaller halogen bonds to carbon in observed frequency range


• Bromides and Iodides – Not often obs. Due to low n of C-X stretch


C-F 1400-1000

Stretch (sym.) Monofluoroalkyl at lower nPolyfluoroalkyl at upper nAryl fluorides up to 1450

C-Cl 785-540 Stretch Different conformers may give split peaks

C-Br 650-510 Stretch Bend is obs. at ~1200

C-I 600-485 Stretch Bend is obs. at ~1150

143

IR Spectroscopy

II. Instrumentation and Experimental AspectsA. The IR Spectrometer – Dispersive and Fourier Transform

1. 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.

Irradiation: Spectrometer needs to generate photons hn hn hn

Detection-reemission : Spectrometer needs to detect the photons emitted by the sample and ascertain their energy

En

erg

y

Absorption-Excitation: Spectrometer needs to contain the sample

hn

Relaxation

rest state rest state

excited state

144

IR Spectroscopy


1. Dispersive IR Spectrometers• Those four parts are:

1. Source/Monochromator2. Sample cell3. Detector/Amplifier4. 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


1. Dispersive IR Spectrometers• Here is a general schematic:

146

IR Spectroscopy


1. 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


1. 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


1. 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


1. 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


2. Fourier Transform IR Spectrometers• FT-IR is the modern state of the art for IR spectroscopy• The system is based on the Michelson interferometer

o 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

o 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


2. 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


2. 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:1. Fellgett’s advantage – every point in the

interferogram is information – all wavelenghts are represented

2. Jacquinot’s advantage – the entire energy of the source is used – increasing signal-to-noise

3. 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


2. Fourier Transform IR Spectrometers• Justik’s advantage – does it give me what I need

1. Single-beam instrument – collect a background (air has IR active molecules!)

2. Fast – all frequencies are scanned simultaneously

3. No referencing!

4. 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

II. Instrumentation and Experimental AspectsB. The IR Spectrometer – Experimental aspects

1. Sample size – typically the size of the beam – mm’s mg’s

2. Non-destructive – sample can be recovered with varying degrees of difficulty

3. 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


3. 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


4. 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


4. 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


5. 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


5. 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

IR Spectroscopy


6. Differences in Spectral Appearence• Compare the following three IR spectra of p-cresol

Neat Sample

161

IR Spectroscopy



KBr Pellet

162

IR Spectroscopy



CCl4 Solution

163

IR Spectroscopy


6. Differences in Spectral Appearence• Compare the following three IR spectra of m-nitroanisole

Nujol Mull

164

IR Spectroscopy



KBr Pellet

165

IR Spectroscopy



CCl4 Solution

Documents

1 CHEM 430 IR SPECTROSCOPY. I NTRODUCTION The method provides a rapid and simple method for observing the functional group species present in an organic