Picking and sorting single-transition NMR lines at two fields: unveiling the pure-shift information

A procedure to construct numerically a pure-shift NMR spectrum is described. The method correlates frequencies of single-transition lines picked from two conventional 1D NMR spectra acquired at two different fields. The resulting 2-field 2D plot synthetically refocuses J-coupling multiplicities along straight lines of slope 1 and intercepts related to the sole chemical shifts. In the general case of overlapping multiplicities, if the order of single transitions is different at the two considered fields, an algorithm to sort the frequencies and establish correct correlations is introduced. In general, given n single-transition frequencies at one field, there exists one of the n! permutations of such frequencies that identifies the ‘decoupled’ correlation with n frequencies at a second field. This synthetic decoupling method is in principle as sensitive and as quantitative as the 1D spectra used for its execution. The resolution is decreased by a factor which depends on the strength of the two fields. The procedure is tested by numerical simulations for weakly- and strongly-coupled 1H spin systems and on experimental data for a sample of cyclosporine A in C6D6. Possible applications on NMR spectra of quadrupolar nuclei in the solid state are briefly discussed.