IR spectroscopy

Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.

2000200035003500 30003000 25002500 1000100015001500 500500

Wave number, cmWave number, cm-1-1

Infrared Spectrum of HexaneInfrared Spectrum of HexaneInfrared Spectrum of HexaneInfrared Spectrum of Hexane

CHCH33CHCH22CHCH22CHCH22CHCH22CHCH33

C—H stretchingC—H stretching

bendingbending bendingbending

bendingbending

%T

100

Types of Vibration

Stretching Bending

Symmetric

Asymmetric

Scissoring

Rocking

Wagging

Twisting

In plane

Out of plane

TYPES OF MOLECULAR VIBRATION

Symmetric Stretching

AsymmetricStretching

Without change bond angle

In plane

Scissoring Rocking

Out of plane

TwistingWagging

2 )DIPOLE MOMENT

–“A molecule will absorb IR radiation if the change in the vibrational state is associated with the change in the dipole moment of the

molecule”

μ = q x r

A dipole moment arises from a separation of charges in a molecule :

μ =dipole moment )Coulomb ·meters)q =magnitude of charges r =vector going from –ve charge to +ve charge

H Cl

δ+ δ-

r

•A hetronuclear diatomic molecule is composed of 2 diff. atoms. if this atoms exhibit diff. electron withdrawing powers, the e - density will be shifted towards more electronegative atom such molecule are said to

be polar and possesses an electric dipole moment

OR

•“ We can say, dipole moment arise as a consequence of asymmetrical partial charge distribution”

μ = q x r q = magnitude of charge r = Distance between charges e = 1.602 X 10-19 C

Here charge is measured in coulomb & distance in metersSo SI unit of μ = Cm (i.e. coulomb meter)

but for convenience μ is often given in unit “DEBYE” 1D = 3.336X10-30Cm

but atomic charge is q x e

μ = q x e x r

q + q -

δ+ δ- r

If HCl has dipole moment 1.83D bond length And r is 92 pm from eq.

q = 0.41

Then this value indicates that charge in HCl molecule is distributed (asymmetrically) such that Cl atom has effectively gained 0.41 of e- and H atom has lost 0.41of an electron.

In polyatomic molecule, the to total dipole moment is the vector sum of the dipole moment of the individual bond.

In symmetrical molecule such as CCl4 there is no overall dipole moment,althoug C Cl bond is polar

from μ = q x e x r