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O H C
Choose a molecule
carbon dioxide
water
acetylene (ethyne)
> X X X
Carbon dioxide is centrosymmetric.
Therefore it obeys the rule of mutual exclusion:
IR active modes are Raman inactive, and vice versa.
Polarisability Dipole moment q > α q p
Bending mode: 526 cm−1
The polarisability is constant passing
through the equilibrium position.
Therefore this mode is Raman inactive.
The dipole changes sign, so
this mode is IR active.
Symmetric stretch mode: 1480 cm−1
The polarisability changes on passing through the equilibrium position. Therefore this mode is Raman active.
The molecule remains symmetric. Therefore there is no dipole moment and this mode is IR inactive.
Asymmetric stretch mode: 2565 cm−1
The polarisability is constant passing through the equilibrium position. Therefore this mode is Raman inactive.
The dipole changes sign, so this mode is IR active.
> > X X
Water is not centrosymmetric.
Therefore it does not obey the rule of mutual exclusion
Polarisability ellipsoid Dipole moment p q q
Bending mode: 1594 cm−1
The magnitude of the polarisability is not changing much, so this mode is only weakly Raman active.
The dipole changes on passing through the equilibrium position, so this mode is IR active.
Symmetric stretch mode: 3656 cm−1
The polarisability is changing, so this mode is Raman active.
The dipole changes on passing through the equilibrium position, so this mode is IR active.
Asymmetric stretch mode: 3755 cm−1
The magnitude of the polarisability is not changing much, so this mode is only weakly Raman inactive.
The dipole changes on passing through the equilibrium position, so this mode is IR active.
> X X X X X
Acetylene is centrosymmetric.
Therefore it obeys the rule of mutual exclusion:
IR active modes are Raman inactive, and vice versa.
Bending mode: 612 cm−1
The polarisability changes, but only gradually. Therefore this mode isweakly Raman inactive.
The dipole does not change. This mode is IR inactive.
Bending mode: 729 cm−1
The polarisability is constant passing through the equilibrium position. Therefore this mode is Raman inactive.
The dipole changes sign on passing through the equilibrium position, so this mode IR active.
Symmetric stretch mode: 1974 cm−1
The polarisability changes on passing through the equilibrium position. Therefore this mode is Raman active.
The dipole does not change. This mode is IR inactive.
Asymmetric stretch mode: 3287 cm−1
The polarisability is constant passing through the equilibrium position. Therefore this mode is Raman inactive.
The dipole changes sign, so this mode is IR active.
Symmetric stretch mode: 3374 cm−1
The polarisability changes on passing through the equilibrium position. Therefore this mode is Raman active.
The dipole does not change. This mode is IR inactive.