Multiplet slanting

From the above example, you can infer that the extent to which the intensity ratios depart from first-order (Pascal s triangle) expectations is a function of A 8v/7. If this ratio is large (A 5v/7 > 10), we describe the spin system as weakly coupled, and the resulting multiplets will exhibit essentially first-order intensity ratios (as do the triplets and quartets in Figures 8.1 and 8.13). But as A 8v/J decreases, second-order effects (multiplet slanting and even the appearance of extra lines) become increasingly apparent. Such a spin system is said to be strongly coupled. 

Figure 9.10. Predicted and observed 60-MHz H ABC spectra of compound 9-3. (a) Predicted first order spectrum (b)-(d) multiplet slanting due to second-order effects (e) observed spectrum [From Prediction of the Appearance of Non-First-Order Proton NMR Spectra, by R. S. Macomber, Journal of Chemical Education, 60, 525 (1983). Reprinted by permission.

SLANTING MULTIPLETS AND SECOND-ORDER (STRONG COUPLING) EFFECTS 145... 

As the ratio of A 8v (the difference in chemical shift between two coupled nuclei) to J decreases, the relative intensities of the lines in a multiplet deviate further from first-order (e.g., Pascal triangle) ratios. Inner lines (those facing the coupled multiplet) increase in intensity, while outer lines lose intensity. This slanting of the multiplets is one type of second-order effect. At very small values of A Sv/J, not only may extra lines appear in the multiplets but also apparent line positions and spacings may not equate with true chemical shifts and coupling constants (e.g., deceptive simplicity and virtual coupling). 

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