Lecture 4 - Chapter 2-9-09-05

1

CHMBD 449 – Organic Spectral Analysis

Fall 2005

Chapter 2: IR Spectroscopy•Spectroscopic Process• IR Absorption Process•Uses of IR•Covalent bonds

• Vibrational Modes• Absorption Trends

2

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

3

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 0 state, a photon of the same is emitted and detected by the spectrometer; the spectrometer “reports” this information as a spectral band centered at the of the coupling

• The position of the spectral band is dependent on bond strength and atomic size

• The intensity of the peak results from the efficiency of the coupling; e.g. vibrations that have a large change in dipole moment create a larger electrical field with which a photon can couple more efficiently

4

IR Spectroscopy

I. IntroductionG. 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

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

5

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

6

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

7

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

8

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

9

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

2. Mechanical Coupling• For example, the calculated and observed 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

10

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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 v’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

11

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

12

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

13

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

14

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

15

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

16

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

4. Hydrogen Bonding• Some compound, in addition to intermolecular effects for

the monomeric species can form dimers and oligomers which are also observded 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

17

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

18

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

6. Electronic Effects - Inductive• The presence of a halogen on the -carbon of a ketone (or

electron w/d groups) raises the observed frequency for the -bond

• Due to electron w/d the carbon becomes more electron deficient and the -bond compensates by tightening

X

C

O

19

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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.

20

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

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

21

IR Spectroscopy

I. IntroductionG. The IR Spectrum – Factors that affect group frequencies

6. Electronic Effects - Sterics• Consider this example:

• In this case the presence of the methyl group “mis-aligns” 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

22

IR Spectroscopy

Monday – we will finish a few more effects and do instrument design

It looks like we won’t get to group frequencies until late Monday or Wednesday

The next exam would be delayed to compensate

It is your choice for this occurrence whether the exam would be the day after the last workshop on the material or a following Monday

Mass Spectrometry and UV would be shortened to compensate

Aside: Make sure you are using the right print parameters to save ink and preserve clarity of the lecture handouts!!!

23

IR Spectroscopy