Transition energy ranges structure spectroscopy

Vibrational spectroscopy is a powerful tool for the study of molecular structure and dynamics. The typical vibrational frequency range of this spectroscopy is 100-4000 cm, which corresponds to the energy range 0.3-12 kcal/mol. Because the resolution of vibrational spectroscopy is on the order of 5 cm , the band shift on this order corresponds to a 0.02 kcal/mol. Vibrational transitions are correlated with specific vibrational motions by inspection of the transition frequencies. From identification of these fingerprint vibrational modes, conclusions can be drawn on specific structural motifs in the molecules. Vibrational transitions have bandwidths typically smaller (10-20 cm ) than those from electronic transitions (typically 200-2000 cm ), and it is thus less probable that different transition bands overlap in vibrational spectroscopy than in electronic spectroscopy. In addition, small molecular species may always be probed through their vibrations, and electronic transitions. Major disadvantages of vibrational spectroscopy, on the other hand, are the inherent lower cross sections of vibrational transitions and the frequent overlap of the absorption bands with those of the solvent [10]. 

The choice of spectroscopic method is primarily determined by the energy range of the phenomenon to be studied. In Fig. 1.3 the spectral ranges that are of interest in atomic and molecular spectroscopy are shown. The energy ranges for different types of structures and transitions are also indicated. Using the simple relations... 

Section 13 20 IR spectroscopy probes molecular structure by examining transitions between vibrational energy levels using electromagnetic radiation m the 625-4000 cm range The presence or absence of a peak at a charac tenstic frequency tells us whether a certain functional group is present Table 13 4 lists IR absorption frequencies for common structural units... 

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