Question

3. Consider the SF4 molecule.
b) How many stretching vibrations does the molecule have, and what are their symmetries? Which ones appear in an IR spectrum? Which ones appear in a Raman spectrum? Sketch the stretching modes.
c) How many bending vibrations does the molecule have, and what are their symmetries? Which ones appear in an IR spectrum? Which ones appear in a Raman spectrum?

Dont forget to sketch the stretching nodes

Answer 1

SF₄ is a non-linear molecule of trigonal bipyramidal geometry with (C₂V) symmetry.

Total Number of Degrees of freedom = 3N =3*5 = 15

N is the number of atoms.

Translational Degrees of freedom= 3

Rotational Degrees of freedom= 3

Vibrational Degrees of freedom = 15-6 =9

Character table for the C₂V point group

Out of nine, four vibrations in symmetry class A1 (one is active IR and the other three Raman active), one in A2 (Raman active), two in B1 and another two in B2 (in each, one is
IR active and one is Raman active). All in all you should expect six peaks in the
Raman spectrum and three in the infrared (=9).