Only nuclei with spin number I ≠ 0 can absorb/ emit electromagnetic radiation. If a nucleus has an even mass A and even charge Z, then the nuclear spin I is zero. Examples are 12C, 16O, and 32S. However, a nucleus with a mass I = n/2, where n is an odd integer, for example, 1H, 13C, 15N, and 31P, in which case it is possible to detect their nuclear magnetic resonance (NMR) signals. The chemical shift of the nucleus is the difference between its resonance frequency and a standard. Usually, this quantity is reported in parts per million (ppm) and given the symbol delta, δ. In NMR spectroscopy as illustrated in Figure 3.1 for the simple compound ethanol, this standard is often tetramethylsilane, Si(CH3)4, abbreviated as TMS. In the ethanol
In the 20th century, as improvements continued in separation techniques and more compounds were isolated from plant and microbial sources, there were quantum leaps in the spectroscopic techniques used to determine chemical structures. An anecdotal story made the rounds in the United Kingdom in the early 1960s, when a Nobel Laureate, sitting on the panel of a candidate’s Ph.D. thesis defense, challenged the student to show he had determined that his isolated chemical compound was pure. All the spectral techniques were described in detail to support the purity and to determine the chemical structure. Alas, the candidate was told to come back and redefend his thesis when he had taken the melting point.