Question

is IR spectroscopic evidence alone sufficient for functional group identification? What kind of ambiguities can IR spectroscopy evidence have?

Answer 1

Infrared spectroscopy depends on the functional groups absorbing a maximum quantity of a given wavelength of electromagnetic radiation in the infrared region, when the system undergoes a vibrational transition that is accompanied by a change in the dipole moment of the molecule.

Thus, only vibrations of a functional group that involves a change in dipole moment can be identified by IR spectroscopy. Moreover, the peaks in IR spectra tend to be unusually broad in cases of H-bonding or such intermolecular interactions or unusually weak if the vibration is for a highly symmetric molecule. These might mask a particular functional group's characteristic vibration and thereby prevent its identification.

Therefore, IR spectroscopic evidence alone is not sufficient for functional group identification.