Figure 5.

Importance of adequate terminology in NMR analysis. The spectrum of the axial H-9a in (−)-α-santonin (3; 500 MHz, CDCl₃) exemplifies how clear definitions of the terms transitions, resonances, signals, lines, and peaks matter for the interpretation of the observed pattern. Refer to the main text for further explanations. In this example, the number of peaks is due to the very large number and very close proximity of the underlying 155 transitions. This myriad of transitions can be considered as producing 155 lines that are indistinguishable with current instrumentation. Note the differences in the exact positions of the individual peaks of the apparent dt/ddd pattern vs those of the lines in the first-order coupling trees that represent the J-couplings with the geminal H-9b (13.66 Hz), the vicinal H-8a (4.63 Hz) and H-8b (13.16 Hz), and long-range with the angular Me-14 (0.76 Hz). This apparent mismatch demonstrates the subtle but important difference between the first-order assumption of a visual interpretation (phenotypic) and the actual spin parameters determined by QM-based (genotypic) full spin analysis. This case exemplifies why non-QM-based fitting methods (peak deconvolution) and automated methods of multiplicity analysis are bound to yield fundamentally inaccurate results, with the degree of inaccuracy depending on the particular spin system.