Introduction to vibrational spectroscopies

Abstract

Vibrational spectroscopies play, still today, a crucial role in the non-destructive characterization of material having the most varied origins (e.g., environmental, geological, polymeric, artistic, etc.), and, together with UV/VIS spectroscopy, they represent university students' first approach to spectroscopy. Vibrational spectroscopy may be defined as a non-destructive identification tool that measures the vibrational energy in a compound. Each chemical bond corresponds to a specific vibrational energy that can be considered as a distinctive fingerprint, useful to determine compound structures, by comparing it with the fingerprints of known compounds. Several techniques are included under the name of vibrational spectroscopy, but the most important are spectroscopies in the middle and near infrared and Raman. Both mid-infrared and Raman spectroscopies are related to the fundamental vibrations of the molecules, which are then used to determine their structures. Since the vibrational energy levels are unique to each molecule, the infrared and Raman spectra provide a fingerprint of a particular compound. The frequencies of these molecular vibrations depend on the masses of the atoms, their geometric arrangement, and the strength of their chemical bonds. Spectral interpretation thus provides information on the molecular structure, dynamics, and surrounding environment. The aim of this tutorial text is to give a general view of the two main vibrational spectroscopy techniques, namely mid-infrared and Raman spectroscopies. An insight into surface-enhanced Raman spectroscopy, a fundamental technique that aims to overcome the limitations of Raman, is also given. The three techniques are discussed separately, with a brief introductory explanation of the theory behind them, and giving useful practical information about instrumentation, sample preparation, and spectral interpretation. The text can be considered the basis for two or three lectures (4–6 h) in a university course of analytical chemistry/applied spectroscopy.