Chap 12 - IR

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The Electromagnetic Spectrum

Infrared, ultraviolet, and nuclear magnetic resonance spectroscopies differ from

mass spectrometry in that they are nondestructive and involve the interaction of

molecules with electromagnetic energy rather than with an ionizing source.

Visible light, X-rays, microwaves, radiowaves,..etc. are all different kinds of

electromagnetic radiation. Collectively they make up the electromagnetic spectrum.

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Electromagnetic radiation has dual behavior.

- In some respects it has the properties of a particle(called a photon), yet in other

respects it behaves as an energy wave.

Like all waves electromagnetic radiation is characterized by a wavelength, a frequency

and an amplitude.

Wavelength, (Greek lambda) is the distance from one wave maximum to the

next. Usually given in meters (m) or in nanometers

(nm)

Frequency, (Greek nu) is the number of waves that pass by a fixed point per unit

time. usually given in reciprocal seconds (s-1), or hertz, Hz

(1 Hz =1 s-1)

Amplitude is the height of a wave, measured from midpoint to peak.

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Wavelength x Frequency = Speed of light

(m) x (s-1) = c (m/s)

= c or = c

Electromagnetic energy is transmitted only in discrete amounts called

quanta. The amount of energy, (epsilon), corresponds to 1 quantum ofEnergy (1 photon) of a given frequency, , is expressed by the Plancks

Equation:

= h

= hc

Plancks Equation = energy

h = Plancks constant

= frequency

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Wavenumber, (cm-1) = 1

(cm)

= hc

Most chemists talk about the IR region in terms of wavenumbers.

Wavenumber is inversly proportional to wavelength and reported in reciprocal

centimeters (cm-1)

= h

= hc

High frequencies, high wave numbers and short wavelengths corresponds to highenergy radiation.

Low frequencies, lower wave numbers and long wave lengths corresponds to low

energy radiation.

Plancks Equation

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The Nature of the energy Absorptions by molecules

Molecules can store energy in a variety of ways. They rotate in space, their bonds

vibrate like springs, their electrons can occupy a number of possible molecular

orbitalsetc.

Each of these forms of energy is quantized.

eg: A bond in a molecule can only vibrate at specific energy levels for a particular bond

The bond can only absorb energy, when it is irradiated with a radiation thatmatches the energy gap between its allowed energy levels.

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Although we usually speak of bond lengths as if they are fixed, the numbers given

really are average values.eg: A typical C-H bond with an average bond length of 110 pm is actually vibrating at a

specific frequency, alternately stretching and contracting as if there were a spring

connecting the two atoms.

When a molecule is irradiated with IR radiation, energy is absorbed if the frequency ofthe radiation matches the frequency of a particular vibrational mode of a bond in the

molecule.

- This causes the amplitude of the particular bond stretch or bond bend to increase.

12.6 Infrared Spectroscopy Absorption of IR radiation by molecules causes changes in their vibrational motions.

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These bond stretching and bending vibrations represent the different vibrational

modes available to a molecule.

Some conmmon vibrational modes

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Different kinds of bonds vibrate at different frequencies, so they absorb

different frequencies of IR radiation.

IR spectroscopy distinguishes between different kinds of bonds in a

molecule, so it is possible to determine the functional groups present.

IR Spectrum

In an IR spectrometer, light passes through a sample. Frequencies that match

vibrational frequencies are absorbed, and the remaining light is transmitted to a

detector.

A spectrum plots the amount of transmitted light versus its wave number.

The useful infrared (IR) region for organic chemist is from 4000 cm -1 400 cm-1

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eg: IR spectrum of ethanol

Horizontal axis Wavenumber

Vertical axis percent transmittance.

A transmittance of 100% means that all the energy is passing through

the sample, where as a lower transmittance means that most of the

energy is being absorbed.

Each downward spike corresponds to an energy absorption.

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12.7 Interpreting Infrared Spectra

Complete interpretation of an IR spectrum is difficult because most organic molecules

have dozens of different bond stretching and bending motions, and thus have dozens

of absorptions.

On the other hand, the complexity is useful because an IR spectrum serves as a uniqu

fingerprint of a compound.

If two samples have identical IR spectra, they are almost certainly identicalcompounds.

Fortunately we dont have to interpret an IR spectrum fully to get useful structural

information.

Most functional groups have characteristic IR absorption bands.

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IR spectrum can be divided into two main regions:

1. The functional group region (4000 cm-1 - 1500 cm-1)

Common functional groups give one or two peaks in this region, at

characteristic frequencies.

2. The fingerprint region (1500 cm-1 - 400 cm-1)

This region often contains a complex set of peaks and is unique for each

compound.

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IR absorption trends

The frequency of a bond vibration can be derived from Hookes law, which describes

the motion of a vibrating spring.

Hookes law = k f

m

f = force constant

m = mass

k = constant

The force constant (f) is the strength of the bond (or spring). Stronger the bond,

larger the f and higher the of vibration.

- In general, triple bonds are stronger than double or single bonds between the

same two atoms and have higher frequencies of vibrations (higher wavenumbers):

CC C=C C-C

~2200 cm-1 ~1650 cm-1 ~1200 cm-1

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The mass (m) is the mass of the atoms (or weights). Lighter the

atom, higher the of vibration.

eg: Compare the following bonds. The C-H bond involves the smallest atom (H)

and therefore has the highest wavenumber.

C-H C-D C-O C-Cl

~3000 cm-1 ~2200 cm-1 ~1100 cm-1 ~700 cm-1

Problem:

Choose the bond in each pair that you would expect to have a higher .

1) C=O or C-O 2) C-N or C-H

Hookes law = k f

m

f = force constant

m = mass

k = constant

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Absorptions for bonds to hydrogen always occur on the left side of the spectrum(the high wavenumber region). H has so little mass that H-Z bonds (Where Z = C, O,

and N) vibrate at high frequencies.

Bond strength decreases in going from C C C=C C-C, so the frequency ofvibration decreases that is, the absorptions for these bonds move further to the

right side of the spectrum (low wavenumber region)

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C-H stretching frequencies in alkynes, alkenes, and alkanes

The frequency of the absorption of C-H bonds is a function mostly of the type of

hybridization that is attributed to the bond.

Stronger the bond, higher the vibrational frequency.

Bond C-H =C-H -C-H

Hybridization

typesp -1s sp2 - 1s sp3 - 1s

Strength 506 kJ 444 kJ 422 kJ

IR frequency 3300 cm-1 Above 3000 cm-1 below 3000 cm-1

Bonds with more s character absorb at a higher frequency.

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Alkanes

C-H just below 3000 medium

Alkenes

=C-H just above 3000 medium

C=C around 1650 medium

Alkynes

C-H around 3300 strong

CC around 2200 medium, sharp

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The C=C absorption frequencies increase as alkyl groups are added to a

double bond.

Greater the substitution, stronger the alkene, hence higher the C=C

absorption frequency.

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Alcohols

O-H 3400 3650 strong, broad

C-O 1300 - 1000 strong

b l d

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Carbonyl Compounds

Strong, sharp C=O peak 1800 to 1650 cm1 Exact absorption is characteristic of the type of the carbonyl compound

Carboxylic Acids

C=O 1800 1650 strong


This O-H absorbs broadly, due to strong hydrogen bonding.

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Amides


N-H around 3400 medium,

sometimes appear

Amides


N-H around 3400 medium,

sometimes appear

Amides

C=O 1800 1650 cm-1

N-H around 3400 cm-1 ( sometimes appear as two peaks)

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Esters

C=O 1800 1650

C-O 1300 - 1000

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Aldehydes


C-H 2900 - 2750 weak

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Ketones


Eff t f C j ti th C O t t hi f i

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Effect of Conjugation on the C=O stretching frequencies

The introduction of a C=C bond adjacent to a carbonyl group results in

delocalization of the electrons in the C=O and C=C bonds.

This conjugation increases the single bond character of the C=O and

C=C bonds in the resonance hybrid resulting in a lowering of the

frequencies of carbonyl and double-bond absorption.

Conjugation with triple bonds as well as with aromatic rings have the

same effect.

Examples

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Examples

l

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CN around 2200 medium, sharp

Nitriles

Characteristic IR Absorptions of Some Key Functional Groups

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Functional Group Absorption (cm-1) Intensity Functional Group Absorption (cm-1) Intensity

Alkanes

C-H just below 3000 medium

Carboxylic Acids



Alkenes

=C-H just above 3000 medium

C=C around 1650 medium

Amides


N-H around 3400 medium

Alkynes

C-H around 3300 strong

CC around 2200 medium, sharp

Esters


C-O 1300 - 1000 medium

Alcohols


C-O 1300 - 1000 strong

Aldehydes


C-H 2900 - 2750 weak

Amines

N-H around 3400 medium

C-N around 1100 medium

Ketones


Ethers

C-O 1300 - 1000 medium

Nitriles

CN around 2200 medium, sharp

Aromatic rings

=C-H above 3000 medium

C=C around 1650 weak

around 1500 medium

Nitro

NO2 around 1540 strong

Characteristic IR Absorptions of Some Key Functional Groups

Problems

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Problems

1. What functional groups might the following molecules contain?

a) A compound with a strong absorptions at 1720 cm-1 and

2500 to 3100 cm-1

b) A compound with a strong absorption at 1710 cm-1

2 P f h f ll i d h h

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2. Propose a structure for the following compound that meet the

following descriptions:

C5H8, with a IR absorption at 3300 cm-1

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2. Two IR spectra are shown below. One is the spectrum of cyclohexane, and the other is the

spectrum of cyclohexene. Identify them and explain your answer.

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Chap 12 - IR