The QM-based solution encoded in the first-order coupling trees shown in Figure reveals that the frequency differences between individual constituting peaks within the resonance pattern clearly do not match the actual underlying coupling constants (see Section 3.1 regarding the paradigm "Peak Frequency Differences Are Not Coupling Constants"). This is due to the presence of a higher-order spin system. Note that, while the small long-range coupling of 0.53 Hz is not visible as peak separation (aka peak splitting), the noticeable 2-3 Hz peak separation does not have a corresponding ("matching") coupling constant. This behavior has been designated in the early NMR literature as "virtual coupling" , reflecting the fact that peak splitting sometimes cannot be explained by first-order spin-spin coupling rules, and tends to go unnoticed even in prominent compound classes, such as in quinic acid derivatives . The extent of peak splitting in such cases of depends on the higher-order level of the spin system: additional peak separation typically occurs in the 1-4 Hz range, thereby presenting a significant confusion potential for visual interpretation: the observed splittings are "real", but are not due to actual J-coupling and are the result of the exact ("incidental") frequency locations of the lines and underlying QM transitions of the observed resonance.