combination bands,overtones and fermi resonance

These are used to help and explain and assign peaks in the vibrational spectra that do not correspond to fundamental vibrations.combination bands and overtones have low intensities than fundamental and fermi resonance causes shift in the intensities of peaks with similar energies and identical symmetries.                                                                                                                                                Introduction:fundamental vibrational frequencies of a molecule corresponds to a transition from v=0 to v=1.for a non linear molecule there will be 3n-6  number vibrations while for liner molecule there will be 3n-5 no vibrations because there is one less  rotational degrees of freedom.        following figure shows a diagram for vibrating diatomic molecule.the levels denoted by vibrational quantum numbers v represent the potential energy for the harmonic oscillator.the transition 0 to 1 is fundamental,transitions from 0 to n are  overtones and from 1 to n are hot transitions                          

potential energy diagram for a vibrating diatomic molecule

Fermi resonance:a fermi resonance is shifting of energies and intensities of absorption bands in the infrared or raman spectrum.the phenomenon is explained by the italian physicist enrico fermi.                                                                                                                                                        conditions for fermi resonance:                                                        1.the two vibrations must have same energies.and 2.the transitions must have same energies.           fermi resonance most often occurs between normal and overtone modes.                                                In this molecule transfer its energy from fundamental vibrational level to overtone or combination level.