8.20: 1H NMR: Interpreting Distorted and Overlapping Signals
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or roofed towards each other. When Δν/J is less than 10, the spins are said to be strongly coupled, and the spectra show second-order effects.
Peaks may overlap completely and appear deceptively simple, similar to first-order spectra. Second-order effects can also result in shoulders and multiplets that cannot be interpreted. Accordingly, chemical shifts and coupling constants in these spectra are often identified by computer simulation methods. Because Δν increases with spectrometer frequency and J remains constant, second-order effects decrease when spectra are recorded using higher-field instruments.
