Molecular vibrations
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
188
2025-08-30
357
Molecular vibrations
Key points: If a molecule has a centre of inversion, none of its modes can be both IR and Raman active; a vibrational mode is IR active if it has the same symmetry as a component of the electric dipole vector; a vibrational mode is Raman active if it has the same symmetry as a component of the molecular polarizability.
A knowledge of the symmetry of a molecule can assist and greatly simplify the analysis of infrared (IR) and Raman spectra. It is convenient to consider two aspects of symmetry. One is the information that can be obtained directly by knowing to which point group a molecule as a whole belongs. The other is the additional information that comes from knowing the symmetry species of each normal mode. All we need to know at this stage is that the absorption of infrared radiation can occur when a vibration results in a change in the electric dipole moment of a molecule; a Raman transition can occur when the polarizability of a molecule changes during a vibration. For a molecule of N atoms there are 3N displacements to consider as the atoms move. For a nonlinear molecule, three of these displacements correspond to translational motion of the molecule as a whole, and three correspond to an overall rotation, leaving 3N – 6 vibrational modes. There is no rotation around the axis if the molecule is linear, so the molecule has only two rotational degrees of freedom instead of three, leaving 3N – 5 vibrational displacements.
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