factors affecting on vibrational frequencies in IR.

factors affecting on vibrational frequencies in IR.:                                                                        1.coupled vibrations,2.fermi resonance,3.electronic effects and 4.hydrogen bonding.                                                 1.coupled vibrations:an isolated C-H bond has only one stretching vibrational frequency where as methylene group shows two stretching vibrations symmetrical and asymmetrical.                                  assymetric vibrations occur at higher frequencies than symmetric vibrations. these are known as coupled vibrations because these vibrations occur at different frequeencies than that required for an isolated C-H stretching.                                                                                                                         a strong vibrational coupling is present in carboxylic  acid anhydrides in which symmetrical and asymmetrical stretching vibrations appear in the range of 1720-1825/cm.the interaction is effective because of partial double bond due to resonance which keeps system planar for effective coupling.      assymetrical stretching band in acyclic anhydride is more intense while symmetrical stretching band in cyclic anhydride is more intense.                                                                                 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.