Vibrational Assignments in Small Molecules

Abstract

The assignment of the vibrational bands which are observed in the infrared and Raman spectra of small molecules is a relatively simple process for molecules having only three or four atoms and for some larger molecules if they possess sufficient symmetry. However, the complexity of the spectrum increases rapidly with each added atom, and unequivocal assignments are still sought for some molecules with as few as four and five atoms. The object is to identify the vibrational bands which are observed in the infrared and/or Raman spectrum and to associate each one, insofar as possible, with particular vibrational modes or normal coordinates of the molecule. Our primary interest is in identifying the fundamental frequencies, and our attention will be focused in this direction. However, in most spectra many overtone and combination bands are observed and assigned also, although these assignments are frequently very uncertain. The tools which can be brought to bear in making a vibrational assignment include empirical correlations and the group frequency concept, infrared and Raman selection rules, molecular symmetry and group theory, theoretical band shapes for partially resolved bands, frequency shifts with isotopic substitution, frequency calculation with approximate force fields, the polarization characteristics of Raman bands, and (perhaps too often) the investigator’s intuition and prejudice.