I mentioned that the benzene fingers are a series of overtone and combination bands, and then spent the rest of the column explaining what those are. In part I of this series (2), we discovered that there is a series of overtone and combination bands between 20 cm -1 that I call the benzene fingers, that are another tool to distinguish substituted benzene rings from each other. For example, both mono- and meta-isomers have the ring bend present and have C-H wagging peaks between 770 and 750 cm -1. However, we discovered that under certain circumstances Table I is not sufficient to distinguish the four structures in question from each other. These peak positions are summarized in Table I. In an earlier installment (1), I introduced the chemical structures and nomenclature for mono- and disubstituted benzene rings, and showed how the presence or absence of the ring bend at 690 ± 10 cm -1 and the position of the out-of-plane aryl C-H wag together can be used to distinguish mono-, ortho-, meta-, and para-isomers from each other. We also review the benzene finger patterns for mono-, ortho-, meta-, and para-substituted benzene rings, and describe an easy mnemonic in which you use your fingers to help you remember the patterns. Here, we show that the benzene fingers are a series of overtone and combination bands that can be used to distinguish substituted benzene rings from each other when other methods do not work. With the theoretical background of benzene analysis laid out in part I of this series, we now know what fundamental, overtone, and combination bands look like.
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