Accurate determination of small nuclear magnetic resonance coupling constants from decoupling experiments

Abstract

A new technique is described for the accurate determination of small NMR spin coupling constants. The multiplet under investigation is recorded under coupled and decoupled conditions, and then corresponding ordinates of the time-domain signals are divided, leaving only the modulation term, cosⁿ(πJ_ISt) where n is the number of equivalent protons in the irradiated group. Precautions are taken to avoid division by zero or near-zero values. The Fourier transform of the quotient is a single multiplet (1 : 1 doublet, 1 : 2 : 1 triplet, or 1 : 3 : 3 : 1 quartet) giving a direct measure of the coupling constant. The technique is illustrated with reference to the proton–proton couplings in furan-2-aldehyde in various solvents. The reproducibility of the measurements is superior to ±0.04 Hz and the results are in good agreement with those from a related technique where the decoupled multiplet is reconvoluted with a trial coupling J* which is varied to give best fit with the fully coupled multiplet.