Asymmetric NMR spectra of weakly coupled nuclei: A common characteristic in the 13C NMR spectra of [U-13C]-labeled molecules and in natural abundance protoncoupled 13C spectra

✍ Scribed by R. E. London; T. E. Walker

• Publisher: John Wiley and Sons
• Year: 1981
• Tongue: English
• Weight: 576 KB
• Volume: 15
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270150404

✦ Synopsis

Abstract

Typically encountered proton‐decoupled spectra of [U‐^13^C]‐labeled molecules or proton‐coupled spectra of natural abundance or singly labeled molecules contain many nuclei which satisfy the weak coupling approximation. The spectra of such nuclei are frequently highly asymmetric and often appear to exhibit the skewed intensity distribution characteristic of strongly coupled spins. Analysis of typical cases indicates that such effects arise in the presence of at least one moderately strong coupling interaction in the spin system (__J__δν ∼ 1/3), and the apparent intensity asymmetry reflects small differences in the spacing of unresolved components of the observed resonance. This effect is analogous to the case of ‘virtual coupling’ in which weakly coupled spins cannot be analyzed as first order spectra; however; the ABX spin system which generally serves as a model for such systems does not predict the existence of spectral asymmetry for the X resonances. Prediction of this asymmetry requires a spin system of at least four spins with at least ABMX complexity, and can be treated generally using the effective Hamiltonian approach of Poeple and Schaefer.