Proton NMR Revisited: Negative Time Reconstruction Transforms Pure Shift and 2D J ‐Spectroscopy

Proton nuclear magnetic resonance spectra are perennially overcrowded because the signals of chemically different protons span a narrow range of frequencies, and interactions between them yield multiple signals per proton. It has been known for 50 years that separating these two effects, chemical shift and scalar coupling, into different spectral dimensions could greatly improve spectral resolution, and hence facilitate structure elucidation. Experimental implementations to date have failed because they cannot measure the spin behavior for negative times. We show here, for a wide range of samples, that the missing information can be reconstructed from the experimental evidence. This yields both cleanly resolved two‐dimensional spectra, and a highly effective way to obtain simple “pure shift” one‐dimensional proton spectra without homonuclear multiplet structure.