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Predicting Vibrational spectra


Infrared Spectroscopy and Raman

This is concerned with the molecular vibrations of the molecule which yields important information about the molecular formula, and the molecular shape. The information can be used in both a qualitative fashion, (e.g. the molecule contains a carbonyl group) or a quantitative fashion (e.g. the molecule has three CO ligands and C3v point group). Below are some inorganic examples.


A vibrational mode is active if there is a change in the dipole moment during the vibration.


A vibrational mode is active if there is a change in the polarizability during the vibration.


Qualitative Considerations
Specific functional groups, or ligands are often found in characteristic regions of the IR spectrum, a few of these are given below. All of the information about characteristic frequencies that you have learnt about in Organic Chemistry courses for identifying carbonyl, O-H, carboxylic acid groups etc is, of course, still relevant. Many ligands contain these groups. Some of the most important characteristic frequencies are given below.

(a) Oxides, Nitrides and Halides

(b) Cyanate, thiocyanate and selenocyanate ligands

(c) Nitrite ligands

(d) Carbonyls and Nitrosyls

(i) Carbonyls

(ii) Nitrosyls


Quantitative Considerations
This predicts the vibrations of specific functional groups, or ligands there are in a molecule.

This is calculated by reduction, which can be performed by eye or the reduction formula. The reduction is performed after calculating the number of bond that remain unshifted by the operations performed on it. These are a sub set of the molecule and at the full molecule.

Examples of representational spectra is available here