Question

How can IR determine the geometry of a triatomic molecule?

Answer 1

Degrees of freedom describe the motion of a molecule in relation to the coordinates (x,y,z). The 3n degrees of freedom also describe the translational, rotational, and vibrational motions of the molecule. There are three degrees of freedom for translational, movement through space, and rotational motion, each for a nonlinear molecule. Therefore, translational and rotational can move and rotate around each of the three Cartesian axes. However, a nonlinear molecule can only rotate around 2 of the Cartesian axes because the rotation about the molecular axis does not represent a change of the nuclear coordinates. If you subtract the translational and rotational degrees of freedom, you obtain the following equations shown below for the degrees of vibrational freedom.

The degrees of vibrational modes for linear molecules can be calculated using the formula:

3n−5

The degrees of freedom for nonlinear molecules can be calculated using the formula:

3n−6

in adition to this they have different kind of stetching in the molecule. Triatomic Linear molecule like C02 will be IR inactive while nonlinear triatomic molecules will IR ACTIVE