Answer to Question #73326 in Physical Chemistry for Farid Fahim

Question

Dear Sir
Greetings
Please, how to compute the vibration frequency of a molecule of more than two atoms….
The equations?
Regards
Farid Fahim

Answer 1

Answer: The vibrations of a molecule are given by its normal modes and each absorption in a vibrational spectrum corresponds to a normal mode. In general linear molecules have 3N-5 normal modes, where N is the number of atoms. The five remaining degrees of freedom for a linear molecule are three coordinates for the motion of the center of mass (x, y, z) and two rotational angles. Non-linear molecules have three rotational angles, hence 3N-6 normal modes. The normal modes can be calculated using Newton’s equations of motion. Example of this type of calculation is given in the sources section.
Sources:
https://www.colby.edu/chemistry/PChem/notes/NormalModesText.pdf