First figure out to which category the molecule belongs to, eg: AB type, A3B type etc. Then use some symmetry relations to calculate which of the mode is Raman active. There are good books such as Nakamoto's which has listed out large no. of molecules and their Raman and IR active modes. Hope comment will help. GOOD LUCK.
If your molecule has no symmetry element, all modes will be Raman active. (The intensity is another question). If you molecule has symmetry, those vibrations will be Raman active which transform similarly to the xy, yz or xz products. You can use the character tables to see how these products transform using various symmetry elements. Most books on vibrational spectorscopy contain these character tables. The vibrational modes for not too large molecules can be calculated by force field analysis (quantum chemistry).
In general, a vibrational mode is Raman active when almost one of the molecular polarizability tensor components changes during the vibration; in simpler words when the electronic charge density undergoes a sizeable variation. By considering the symmetry properties of a molecule, the vibrations that transform similarly to the products of the x, y or z coordinates (i.e. xx, yy, zz, xy, xz, yz) can be observed in the Raman spectrum. One can deduce this Raman activity by inspection of the character tables usually reported in the vibrational spectroscopy books for each symmetry point group.
If the polarizability of the molecule is not equal to zero, that is, if you have a change in the polarization of the molecule. :)
Polarizibility has unit of volume in terms of gaussian so basically you have to check the particular mode which undergo change in volume,will be raman active.
Raman active modes bolng to the same point group as xy, yz and xz prodcucts. You can find out form the character tables. In low symmetry molecules (C1) all vibrations are Raman active. Sorry - I have just spotted that this information has already been added.
While I can understand what you wish to ask, I find that the question is missing a few words. It refers to modes of motions of a molecule not to the molecule itself. To find the Raman active modes of a molecule, find out the point symmetry group of your molecule, like water, ammonia , benzene molecules, respectively belong to C2v , C3v and D6h point groups. Now, analyse using the group theoretical analysis what motions out of 3N motions of N-atomic molecule belong to which symmetry class. All modes of vibrations that belong to the symmetry class of xx, yy, zz, xy, xz, and yz would be Raman active. This simply means that these modes of vibrations or rotations could be observed in Raman spectrum. Howver, some such motions may not appear in the spectrum in spite of being Raman active until and unless you set very high intensity of exciting laser beam.
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