1. Vibrational Spectroscopy
Classical mechanics:
n = 1/p (k/2m)1/2 F = kx
n is the frequency of vibration in s-1, m is the mass of the two vibrating objects in kg, and k is the force constant of a connection between the two masses (e.g. a spring) in N m-1. F (N) is the force needed to displace a mass m by a distance x (m).
Various pairs of atoms joined by bonds in a molecule can be treated as diatomic molecules when one of the atoms of the pair is a much lighter than the other and not bonded to any other atom.
n (cm-1) = (k/m)1/2 m = reduced mass m1m2/(m1 + m2)
Only vibrations that cause a change of dipole are IR active. Thus, symmetry plays an important role in IR spectroscopy. Vibrations that are not IR active are Raman active and can be studied by Raman spectroscopy.

4. simple symmetric molecules give simple IR spectra