Pulse sequence SEMINA-2 for spectral editing of 13C 13C couplings observed via double-quantum coherence

Abstract

13C satellite spectra observed via a double-quantum (2Q) filter in natural abundance 13C NMR (INADEQUATE) (I) provide a powerful tool in structure elucidation. From this experiment or its two-dimensional version (2-4) the 13C-13C coupling constants and carbon-carbon connectivities for the carbon skeleton of molecular frameworks may be deduced (2-5). However, in the 1D version serious overlap of 13C-13C doublets can make the coupling determination ambiguous even for simple molecules. A recent communication (6) introduced the SEMINA methods for editing of 13C-13C satellite spectra which considerably simplify the assignment of one-bond and long-range 13C-13C couplings. Experimental spectra were shown for the 13 = O”, 180” version of what we would like to call the SEMINA-1 pulse sequence because only one editing step is included. In this communication we describe in detail the 8i, e2 = O”, 180” version of a pulse sequence, SEMINA-2, which contains two editing steps. This sequence, shown in Fig. 1, is a modification of the general sequence proposed in Fig. Id of Ref. (6) especially adapted for the O”, 180” case, where the editing is accompanied by no significant loss in sensitivity compared to the 1D INADEQUATE experiment. Spectral editing of pairs of 13C-13C doublets using the SEMINA-2 pulse sequence shown in Fig. 1 is achieved as follows. After creation of double-quantum coherence (2QC) in the usual way (I), the 2QCs are subjected to a SEMUT sequence with 8, = O”, 180”. In analogy to normal SEMUT (7, 8) the 2QC of a ‘3CH,-‘3CH, fragment is unchanged for 8, = O”, but modulated by (-l),+“’ for 0i = 180”. A 90” pulse transfers the 2QC to observable antiphase magnetization on both carbons, and in SEMINA-1 (6) the FID is acquired with proton decoupling at this point. Addition (subtraction) of the 0, = 0” and 8i = 180” experiments gives a subspectrum containing exclusively the pairs with n + m even (odd). SEMINA-2 includes a refocusing period with a second editing step (0, = O”, 180”) before acquisition, as shown in Fig. 1, and combinations of four subexperiments (0, = O”, 180” and & = O”, 180”) yield four independent subspectra. Further decomposition is possible using other 0 flip angles but this, contrary to the O”, 180” experiment, significantly degrades the sensitivity compared to the INADEQUATE experiment.