IR Spectroscopy

Chapter 12 - IR spectroscopy

Infrared spectroscopy(IR spectroscopy)


How to identify a given organic molecule ???A combination of techniques need to be used to determine the molecularstructure of an organic compound. The following gives a list.

Method Information obtained

C, H, N analysis Empirical formula

Mass spectroscopy Molecular mass

UV spectroscopy Conjugation

IR spectroscopy Functional groups

1H and 13C NMR spectroscopy Number and arrangement of atoms

The last three rely on interaction of light with matter


What is the nature of light ?

Electromagnetic spectrum


Light acts as waves


Where, h = Planck’s constant, ν = frequencyc = velocity of lightλ = wavelength

E = hν= hc/λ

Energy (E) of a single photon is given by the following formula

Light also has particle character


• All organic compounds absorb electromagnetic radiation, of only certain wavelengths and transmit the rest.

• When organic molecules absorb energy in the UV-visregion, they undergo electronic transitions (electrons in the ground state go to higher states known as excited state) – (Absorption spectroscopy)

• Photon energies associated with infrared are not large enough to excite electrons, but may induce vibrationalexcitation of covalently bonded atoms and groups.-(Infrared spectroscopy)

Interaction of light with matter

VibrationalTransitions

Electronic Transitions

Shown below is a cartoon representation of light waves of different wavelengthshowing which region causes what transition

Absorption spectroscopy Infrared spectroscopy


IR spectroscopy is an useful tool to identify functional groups in organic molecules

IR spectroscopy is a result of molecular vibrationaltransitions that occur when light interacts with matter

Molecules are always vibrating

For a molecule to be IR active, the vibrations should give rise to a net change in dipole moment

Infrared spectroscopy


Molecular vibrations

• The covalent bonds in molecules are not rigid sticks or rods, but are more like stiff springs that can be stretched and bent.

stretching (symmetric and asymmetric) bond length changes

bending (scissoring, rocking, wagging bond angle changesand twisting)


Cartoon representation of the stretching and bending vibrations

Check this link for animation: http://en.wikipedia.org/wiki/Infrared_spectroscopy


By considering the atoms as point masses, linked by a 'spring' having a force constant f (sometimes denoted as k) and following Hooke's Law, equation shown below can be utilized to approximate the characteristic stretching frequency (in cm-1) of a diatomic molecule with the masses of two atoms being m1 and m2,

where, μ = m1m2/(m1+m2) = reduced mass

c = velocity of light. or

k1/λ =

IR energy in a spectrum is usually measured as wavenumber (cm-1), the inverse of wavelength and proportional to frequency


IR spectra of ethyl alcohol

The above figure shows the range of frequency used by organic chemistsfor structural analysis


2850 - 3000 cm-1

1490 cm-1

n-hexane

Not all vibrations can be assigned

Specific IR absorbed by an organic molecule is related to its structure

Shown below is the IR of a simple alkane. Note the important IR peaks

-C-H

-C-C-


IR of an alkene

Important peaks are shown here


IR of an alkyne


3300 cm-1

3100 cm-1

2850 to2960 cm-1

C-H stretch

1630 cm-1C=C

C∫C2150 cm-1

C-C800-1300 cm-1


The previous slide highlights the differences between the IR spectra ofalkane, alkene and an alkyne.

Note that C-C single bond has lower stretching frequency than a C=C double bond which in turn vibrates at lower energy than CªC triple bond

Other important examples are listed in the following


Note that IR vibration of O-H is greater than N-H which in turn is greater than C-H

2850-3300 cm-1 (Sharp, strong to medium)HC

Differences in the force constants lead to this trend

Hydrocarbons

IR spectra of 3-methyl butamine

-O-H, broad and intense

-N-H, Sharp and medium intensity

IR spectra of n-octanol CH3-(CH2)7-OH

NH2


CarbonylsCarbonyls give a strong, sharp peak in the wavenumber

range 1670 to 1780 cm−1


Amides Carboxylic acid

1670 – 1720 cm-11630 – 1690 cm-1

R NH2

O

R OH

O


Most functional groups absorb at about the same energy and intensity, independent of the molecule they are in

Characteristic higher energy IR absorptions (1500 to 4000 cm-1) can be used to confirm the presence of a functional group in a molecule

IR spectrum has lower energy region (400 to 1500 cm-1) characteristic of molecule as a whole. This region is called fingerprint region

Regions of Infrared spectrum



Some General Trends

Stretching frequencies are higher than correspondingbending frequencies.

Bonds to hydrogen have higher stretching frequencies thanthose to heavier atoms.

Triple bonds have higher stretching frequencies thancorresponding double bonds, which in turn have higherfrequencies than single bonds.


Predict the IR absorptions for the following by using theIR chart

Sample problems

Which IR spectrum corresponds to what molecule ?

IR spectroscopy can be used tomonitor the progress of a reaction

Assume that you are dehydrating 1-methylcyclohexanol to yield 1-methylcyclohexene. How could you use IR spectroscopy to determine when the reaction is complete ?

OH

OH2-

IR Spectroscopy

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