Analysis of ABX Spectra in NMR Spectroscopy

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Abstract
ABX spectra are produced by a special type of three-spin system in which two nonequivalent nuclei are strongly coupled and each is weakly coupled to a third nonequivalent nucleus. The strength of the coupling is determined by J/Δ; the magnitude of the coupling constant relative to the difference between the chemical shifts of the nuclei, expressed in hertz. Although spectral changes are continuous as the strength of the coupling varies, spectra in general become quite abnormal at about J/Δ = 0.15, and this is therefore taken as the boundary between strong and weak coupling. (The exact boundary defined by different investigators varies somewhat, but this is not important.) Bernstein et al. [l] have given the notation ABX to this system, indicating with letters of the alphabet the strength of the coupling. Adjacent letters in the alphabet represent strongly coupled nuclei having different chemical shifts and remote letters represent weakly coupled nuclei. Magnetically equivalent nuclei have identical chemical shifts and coupling constants to all other nuclei in the system and are represented by the same letter with a subscript to indicate their number. Chemically equivalent nuclei have the same chemical shifts but different coupling constants to other nuclei in the system, and they are distinguished from magnetically equivalent nuclei by adding primes to the letter symbol.

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