Infrared Spectrometry (Chapter 16 17) 750-3000nm Region Energy Wavenumber Wavelength (kJ/mol) (cm-1) (um) Near IR 150-50 12,800-4000 0.78-2.5 Mid IR 50-2.5 4000-200 2.5-50 Far IR 2.5-0.1 200-10 50-1000 Energy of IR photon insufficient to cause electronic excitation but can cause vibrational or rotational excitation Molecule electric field(dipole moment)interacts with IR photon electric field (both dynamic) Magnitude of dipole moment determined by (i)charge (ii)separation of charge Vibration or rotation causes varying separation +尺 +)( CEM 333 page 6.1
Infrared Spectrometry (Chapter 16 & 17) ~750-3000 nm Region Energy (kJ/mol) Wavenumber (cm-1) Wavelength (µm) Near IR 150-50 12,800-4000 0.78-2.5 Mid IR 50-2.5 4000-200 2.5-50 Far IR 2.5-0.1 200-10 50-1000 Energy of IR photon insufficient to cause electronic excitation but can cause vibrational or rotational excitation Molecule electric field (dipole moment) interacts with IR photon electric field (both dynamic) Magnitude of dipole moment determined by (i) charge (ii) separation of charge Vibration or rotation causes varying separation + - + - + - CEM 333 page 6.1
Molecule must have change in dipole moment due to vibration or rotation to absorb iR radiation Absorption causes increase in vibration amplitude/rotation frequency Molecules with permanent dipole moments (u)are IR active 0-0 十 No dipole moment HCI.H2O.NO Atoms,O2,H2,Cl2 IR active IR inactive u measured in debye (D) 1D=3.33x10-30Cm 10 D equivalent to +1 and-1 charge separated 1A CEM 333 page 6.2
Molecule must have change in dipole moment due to vibration or rotation to absorb IR radiation Absorption causes increase in vibration amplitude/rotation frequency Molecules with permanent dipole moments (µ) are IR active H - Cl d+ dO - O No dipole moment HCl, H2O, NO Atoms, O2, H2, Cl2 IR active IR inactive µ measured in debye (D) 1 D = 3.33x10-30 C·m 10 D equivalent to +1 and -1 charge separated 1 Å CEM 333 page 6.2
Types of Molecular Vibrations: Stretch Bend change in bond length change in bond angle symmetric scissoring asymmetric wagging rocking twisting/torsion CEM 333 page 6.3
Types of Molecular Vibrations: Stretch Bend change in bond length change in bond angle symmetric scissoring asymmetric wagging rocking twisting/torsion + + - - + ns nas r s w t CEM 333 page 6.3
Only some modes may be IR active: Example CO2 O=C=O linear ○O0 26+ No net dipole moment change 2δ+ Net dipole moment change 28 Net dipole moment change Vs not IR active Vas,bend IR active CEM 333 page 6.4
Only some modes may be IR active: Example CO2 O=C=O linear 2d + d d - - No net dipole moment change 2d + d d - - Net dipole moment change 2d + d d - - Net dipole moment change ns not IR active nas, bend IR active CEM 333 page 6.4
Classical vibrational motion: -V Mass Displacement y m Force required to displace m is F=-k.y Hooke's Law spring constant (N/m) CEM 333 page 6.5
Classical vibrational motion: Mass m -y +y Displacement y Force required to displace m is F = -k× y Hooke's Law spring constant (N/m) CEM 333 page 6.5